EARTHWORK & FOUNDATIONS
EARTHWORK & FOUNDATIONS
2.1. Classification of soils - The earthwork shall be classified under the following categories and measured separately for each category, unless otherwise specified. The material to be excavated shall be classified as follows: -
2.1.1. Ordinary or soft soil - Generally any soil which yields to ordinary application of pick axes, shovels or any other ordinary digging implements, such as organic soil, turf, gravel, sand, sandy soil, silt, clay, loam, mud, red earth, ‘sudde’, black cotton soil, soft shale, loose moorum and all soils having soil dry density less than 1.80 gm/cc. (IS: 1498-1970) copy enclosed vie Annexure 2-A.1, removal of gravel and/or any modular material having diameter in any one direction not exceeding 75 mm occurring in such strata etc.
2.1.2. Hard and dense soil - All soils classified in soil groups as per IS: 1498-1970 other than what is covered in (a) above; gravel, cobblestone, hard shale, soft Laterite, or any other nodular material having max. diameter in any one direction between 75 mm & 300 mm soft conglomerate, where the stone can be detached from the matrix with pick axes and shovels. This includes soling of roads, paths etc., and hard core, stiff heavy clay, hard shale or compact moorum requiring grafting tool or pick or both and shovel closely applied. Any material, which requires the close application of picks or scarifiers to loosen and not affording resistance to digging greater than the hardest of any soil, mentioned above.
2.1.3. Ordinary or soft rock –
(i) Rock types such as laterites, shales and conglomerates, varieties of limestone and sandstone etc., which may be quarried or split with crow bars, also including any rock which in dry state may be hard, requiring blasting but which, when wet, becomes soft and manageable by means other than blasting ;
(ii) Macadam surfaces such as water bound and bitumen/tar bound; compact moorum or stabilised soil requiring grafting tool or pick or both and shovel, closely applied ;
(iii) Lime concrete, stone masonry in lime mortar and brick work in lime/cement mortar below ground level, reinforced cement concrete which may be broken up with crow bars or picks and stone masonry in cement mortar below ground level; and
(iv) Boulders which do not require blasting having maximum dimension in any direction of more than 300 mm, found lying loose on the surface or embedded in river bed, soil, talus, slope wash and terrace material of dissimilar origin.
Ordinary rock does not require blasting, wedging or similar means. It may be required a split with crow bars or picks. If required blasting may be resorted to, for loosening the materials but this does not be any way entitle the material to be classified as ‘Hard Rock’.
2.1.4. Hard rock - Any rock (excluding Laterite and hard conglomerate) or boulder for the excavation of which the use of mechanical plant and/or blasting is required; reinforced cement concrete (reinforcement cut through but not separated from the concrete) below ground level.
Hard rock requires blasting but where blasting is prohibited for any reason, excavation has to be carried out by chiseling, wedging or any other agreed method.
2.1.5. Marshy soil - This shall include soils like soft clays and peat excavated below the original ground level of marshes and swamps and soils excavated from other areas requiring continuous pumping or bailing out of water.
2.2 Authority for classification - The engineer shall decide the classification of excavation and his decision shall be final and binding on the contractor. Merely the use of explosives in excavation will not be considered, as a reason for higher classification unless blasting is clearly necessary in the opinion of the engineer.
2.3 Types of excavation
2.3.1 Surface excavation - Excavation exceeding 1.5 m in width and 10 sq. m on plan but not exceeding 30 cm in depth in all types of soils and rocks shall be described as surface excavation.
Measurements - The length and breadth shall be measured with steel tape correct to the nearest cm and the area worked to the nearest two places of decimal in square meters.
2.3.2 Rough excavation and filling - Excavation for obtaining earth from borrow pits, cutting hillside slopes etc., shall be described as rough excavation. Wherever filling is to be done, the earth from excavation shall be directly used for filling and no payment for double handling of earth shall be admissible. Filling of excavated earth shall be done as specified, in case of hill side cutting, where the excavated materials are thrown down the hill slopes; payment for filling excavated earth shall not be admissible.
2.3.3. Excavation over area (All kinds of soils) - This shall comprise :a) Excavation exceeding 1.5 m in width and 10 sq. m. on plan and exceeding 30 cm in depth.
b) Excavation for basement, water tanks etc.
c) Excavation in trenches exceeding 1.5 m in width and 10 sq. m. on plan.
2.3.4 Excavation over area (ordinary / hard rock) - This shall comprise:
a) Excavation exceeding 1.5 m in width and 10 sq. m. on plan and exceeding 30 cm in depth, .b) Excavation for basements, water tanks etc, c) Excavation in trenches exceeding 1.5 m in width and 10 sq. m. on plan.
2.3.5 Excavation in trenches for foundations and drains (all kinds of soils) - This shall comprise excavation not exceeding 1.5 m in width or 10 sq. m. on plan and to any depth in trenches (excluding trenches for pipes, cables, conduits etc.
2.3.6 Excavation in trenches for foundation and drains (ordinary / hard rock) - This shall comprise excavation not exceeding 1.5 m in width or 10 sq. m. on plan and to any depth in trenches (excluding trenches for pipes, cables, conduits etc.)
2.3.7 Excavation in trenches for pipes, cables etc. refilling - This shall comprise excavation not exceeding 1.5 mts. In width or 10 sq. m. in plan and to any depth in trenches for pipes, cables etc. and returning the excavated material to fill the trenches after pipes, cables etc. are laid, their joints tested, passed and disposal of surplus excavated material up to 50 m lead.
2.3.8 Width of trench - a) Up to one meter depth, the authorised width of trench for excavation shall be arrived at by adding 25 cm to the external diameter of pipe (not socket/collar) cable, conduit etc. Where a pipe is laid on concrete bed/cushioning layer, the authorised width shall be the external diameter of the pipe (not socket/collar) plus 25 cm or the width of concrete bed/cushioning layer whichever is more.
b) For depths exceeding one meter, an allowance of 5 cm per meter of depth for each side of the trench shall be added to the authorised width (that is external diameter of pipe plus 25 cm) for excavation. This allowance shall apply to the entire depth of the trench. In firm soils the sides of the trenches shall be kept vertical up to a depth of 2 meters from the bottom. For depths greater than 2 meters, the excavation profiles shall be widened by allowing steps of 50 cm on either side after every two meters from bottom.
c) Where more than one pipe, cable, conduit etc. are laid, the diameter shall be reckoned as the horizontal distance from outside to outside of the outermost pipes, cable, conduit etc.
d) Where the soil is soft, loose or slushy, width of trench shall be suitably increased or side sloped or the soil shored up as directed by the engineer. It shall be the responsibility of the contractor to take complete instructions in writing from the engineer regarding increase in the width of trench, sloping or shoring to be done for excavation in soft, loose or slushy soils.
2.4 SPECIFICATIONS FOR PROTECTION DURING EXCAVATION.
Excavation where directed by the engineer shall be securely fenced and provided with proper caution signs, conspicuously displayed during the day and properly illuminated with red lights during the night to avoid accidents.
The contractor shall take adequate protective measures to see that the excavation operations do not damage the adjoining structures or dislocate the services. Water supply pipes, sluice valve chambers, sewerage pipes, manholes, drainage pipes & chambers, communication cables, power supply cables etc. met within the course of excavation shall be properly supported and adequately protected, so that these services remain functional. Excavation shall not be carried out below the foundation level of the adjacent buildings until underpinning; shoring etc. is done as per the directions of the engineer for which payment shall be made separately.
2.5 SPECIFICATIONS FOR BAILING / PUMPING OF WATER
All water that may accumulate in excavation during the progress of work from rains, subsoil water, springs or any other cause shall be bailed, pumped out or otherwise removed. The foundations shall be kept dry during excavation and laying of foundations. Pumping shall be done directly from the foundation trenches or from a sump outside the excavation as necessary in such a manner as to preclude the possibility of movement of water through any fresh concrete or masonry and washing away parts of concrete or mortar. No pumping shall be allowed during laying of concrete or masonry and for a period of at least 24 hours thereafter unless it is done from a suitable sump separated from concrete or masonry by effective means. Pumping shall be done in such a way as not to cause damage to the work or adjoining property by blows subsidence etc. Disposal of water shall not cause inconvenience or nuisance in the area or cause damage to the property and structure nearby.
2.6 SPECIFICATIONS FOR SITE CLEARANCE
Before the earth work is started, the area coming under cutting and filling shall be cleared of shrubs, rank vegetation, grass, brushwood, trees and saplings of girth up to 30 cm measured at a height of one meter above ground level and rubbish removed up to a distance of 50 meters outside the periphery of the area under clearance. The roots of tress and saplings shall be removed to a depth of 60 cm below ground level or 30 cm below formation level or 15 cm below subgrade level, whichever is lower, and the holes, or hollows filled up with the earth, rammed and leveled.
The trees of girth above 30 cm measured at a height of one meter above ground shall be cut only after permission of the engineer is obtained in writing. The roots of tress shall also be removed. Payment for cutting such trees and removing the roots shall be made separately.
Existing Structures and service such as old buildings, culverts, fencing, water supply pipe lines, sewers, power cables, communication cables, drainage pipes, etc. within or adjacent to the area if required to be diverted/removed, shall be diverted/dismantled as per directions of the engineer and payment for such diversion/dismantling works shall be made separately.
In case of archaeological monuments within or adjacent to the area, the contractor shall provide necessary fencing all-round such monuments as per the directions of the engineer and protect the same properly during execution of works. Payment for providing fencing shall be made separately.
2.7. SPECIFICATIONS FOR SETTING OUT AND MAKING PROFILES
A masonry pillar to serve as a bench mark will be erected at a suitable point in the area, which is visible from the largest area. This bench mark shall be constructed as per Fig.1 and connected with the standard bench mark as approved by the engineer. Necessary profiles with strings stretched on pegs, bamboos etc shall be made to indicate the correct formation levels before the work is started. The contractor shall supply labour and material for constructing bench mark, setting and making profiles and connecting bench mark with the standard bench mark at his own cost. The pegs, bamboos etc and the benchmark shall be maintained by the contractor at his own cost during the excavation to check the profiles.
The ground levels shall be taken at 5 to 15 meters intervals (as directed by the engineer) in uniformly sloping ground and at closer intervals where local mounds, pits or undulations are met with. The ground levels shall be recorded in field books and plotted on plans. The plans shall be drawn to a scale of 5 metres to one cm or any other suitable scale decided by the engineer. North direction line and position of benchmark shall invariably be shown on the plans. These plans shall be signed by the contractor and the engineer or their authorised representatives before the earthwork is started. The labour required for taking levels shall be supplied by the contractor at his own cost.
2.8 SPECIFICATIONS FOR EXCAVATION IN ALL KINDS OF SOILS
All excavation operations shall include excavation and ‘getting out’ the excavated materials. In case of excavation for trenches, basements, water tanks etc. ‘getting out’ shall include throwing the excavated materials at a distance of at least one meter or half the depth of excavation, whichever is more, clear off the edge of excavation. In all other cases ‘getting out’ shall include depositing the excavated materials as specified. The subsequent disposal of the excavated material shall be either stated as a separate item or included with the items of excavation stating lead.
During the excavation the natural drainage of the area shall be maintained. Excavation shall be done from top to bottom. Undermining or undercutting shall not be done.
In firm soils, the sides of the trenches shall be kept vertical up to a depth of 2 meters from the bottom. For greater depths, the excavation profiles shall be widened by allowing steps of 50 cms on either side after every 2 meters from the bottom. Alternatively, the excavation can be done so as to give slope of 1: 4 (1 horizontal: 4 vertical). Where the soil is soft, loose or slushy, the width of steps shall be suitably increased or sides sloped or the soil shored up as directed by the engineer. It shall be the responsibility of the contractor to take complete instructions in writing from the engineer regarding the stepping, sloping or shoring to be done for excavation deeper than 2 meters.
The excavation shall be done true to levels, slope, shape and pattern indicated by the engineer. Only the excavation shown on the drawings or as required by the engineer shall be measured and recorded for payment. In case of excavations the excavations as carried out shall be measured but payment restricted to what is permissible as per approved drawings and as directed by the engineer.
In case of excavation for foundations in trenches or over areas, the bed of excavation shall be to the correct level or slope and consolidated by watering and ramming. If the excavation for foundation is done to a depth greater than that shown in the drawings or as required by the engineer, the excess depth shall be made good by the contractor at his own cost with the concrete of the mix used for leveling/bed concrete for foundations. Soft/ defective spots at the bed of foundations shall be dug out and filled with concrete (to be paid separately) as directed by the engineer.
While carrying out the excavation for drain work, care shall be taken to cut the side and bottom to the required shape, slope and gradient. The surface shall then be properly dressed. If the excavation is done to a depth greater than that shown on the drawing or as required by the engineer, the excess depth shall be made good by the Contractor at his own cost with stiff clay puddle at places where the drains are required to be pitched and with ordinary earth, properly watered and rammed, where the drains are not required to be pitched. In case the drain is required is to be pitched, the back filling with clay puddle, if required, shall be done simultaneously as the pitching work proceeds. The brick pitched storm water drains should be avoided as far as possible in filled-up areas and loose soils.
In all other cases, where the excavation is taken deeper by the contractor, it shall be brought to the required level by the contractor at his own cost by filling in with earth duly watered, consolidated and rammed.
2.9 SPECIFICATIONS FOR FILLING EXCAVATED EARTH IN FOUNDATION TRENCH, IN PLINTH AND UNDER FLOOR ETC.
2.9.1 Earth - The earth used for filling shall be free from salts, organic or other deleterious matter. Highly expansive soils like black cotton soil shall not be used, unless so specified. All clods of earth exceeding 50 mm shall be broken or removed. Earth obtained from borrow pits and surplus earth from excavation, if any, shall be directly used for filling and double handling avoided.
2.9.2 Filling sides of trenches - As soon as the work in foundation has been completed and measured, the space around the foundation masonry in trenches shall be cleared of all debris, brickbats, etc., and filled with earth in layers not exceeding 250 mm, each layer being watered, rammed and compacted before the succeeding one is laid. Earth shall be rammed with iron rammer where feasible and with the butt ends of crowbar where rammer cannot be used.
2.9.3 Filling plinth, under floor and hard standing etc - Filling shall be started from the lowest level in regular horizontal layers each not exceeding 250 mm in depth. Each layer shall be compacted by ramming with rammers of 7 to 10 kg weight. Earth filling shall be adequately watered for achieving maximum compaction.
2.9.4 Filling at the junction of the floors with walls and columns, shall be done with care to ensure good compaction. The top surface of the filling shall be neatly dressed level or to a slope or grade as directed.
2.9.5 Filling in large floors, like factory floors, hangars, hard standing, etc., where indicated, each layer of earth filling shall be compacted by mechanical means such as sheep foot roller or by hand roller or by power roller to 90 to 95 per cent of standard Proctor’s density under optimum moisture conditions.
2.9.6 Filling in trenches for pipes, drains, cables, etc.
Earth used for filling shall be free from salts, organic or other deleterious matter. All clods of earth exceeding 50 mm shall be broken or removed. Unless otherwise indicated, where the excavated material is mostly rock, the rock fragment shall be broken into pieces not bigger than 150 mm size and mixed with fine material consisting of decomposed rock moorum or earth as available, so as to fill up the voids as for as possible and then the mixture used for filling.
Filling in trenches for pipes and drains shall be commenced only after the joints of pipes and drains have been tested and passed by the engineer in writing.
Where the trenches are excavated in soil, the filling shall be done with earth on both the sides simultaneously and on top of pipes in layers not exceeding 250 mm thick, watered, rammed and compacted; taking care that no damage is caused to the pipe below.
In case of excavation in rock, the filling up to a depth of 300 mm above the crown of pipe shall be done with fine material such as earth, moorum, or pulverized decomposed rock according to the availability at site, in the same manner as described for trenches excavated in soil. The remaining filling shall be done with rock fragments mixed with fine material as available to fill up the voids, watered, rammed and compacted, in layers not exceeding 250 mm thick. Particular care shall be taken in back filling to avoid future troubles from bursts and leakage due to differential settlement.
2.10 SPECIFICATIONS FOR MOORUM AND SAND FILLING IN FOUNDATIONS, PLINTH AND IN FLOORS
2.10.1 Moorum - Moorum shall be obtained from approved pits and quarries of disintegrated rocks, which contain silicon material, and natural mixture of clay of calcareous origin. These shall not contain any admixture of ordinary earth. Size of moorum shall vary from dust to 40 mm gauge.
2.10.2 Sand - Sand shall be clean, free from dust, organic and other extraneous matter. It shall not contain more than 5 percent of clay/silt.
2.10.3 Shingle - Shingle shall be clean and free from foreign matter and obtained from river or canal beds. Shingle of all in size ranging from 40 mm down to 4.75 mm gauge shall contain a sufficient proportion of fine material to fill all interstices and ensure binding when consolidated.
2.10.4 Filling - Filling shall be done in a manner similar to earth filling in plinth except that thickness of individual layer shall not exceed 15 cm. Shingle or ballast filling shall be binded with earth before ramming/consolidation. The surface of the compacted moorum, sand or shingle shall be dressed to the required level, grade or slope. In the case of moorum and sand filling, surface shall be flooded with water for at least 24 hours, surface allowed to dry and then compacted and graded.
2.10.5. When the filling in floors etc., has nearly dried, any developing cracks shall be tapped and a thin layer of the same material as used for filling and earth in case of shingle filling shall be spread over the surface evenly and tapped in.
220.127.116.11 The length and breadth of excavation or filling shall be measured with a steel tape correct to the nearest cm. The depth of cutting or height of filling shall be measured, correct to 5mm, by recording levels before the start of the work and after the completion of the work. The cubical contents shall be worked out to the nearest two places of decimal in cubic meters.
18.104.22.168 In case of the ground is fairly uniform and where the site is not required to be leveled, the engineer may permit the measurements of depth of cutting or height of filling with steel tape, correct to the nearest cm. In case of borrow pits, diagonal ridges, cross ridges or dead men, the position of which shall be fixed by the engineer, shall be left by the contractor to permit accurate measurements being taken with steel tape on the completion of the work. Deduction of such ridges and dead men shall be made from the measurements unless the same are required to be removed later on and earth so removed is utilised in the work. In the later case nothing extra will be paid for their removal as subsequent operation.
22.214.171.124 Where ordinary rock and hard rock is mixed, the measurement of the excavation shall be made. The two kinds of rock shall be stacked separately and measured in stacks. The net quantity of the two kinds of rocks shall be arrived at by applying deduction of 50% to allow for voids in stacks. If the sum of net quantity of two kinds of rocks exceeds the total quantity for each type of rock shall be worked out from the total quantity in the ratio of net quantities in stack measurements of the two types of rocks. If in the opinion of the engineer stacking is not feasible, the quantity of ordinary and hard rock shall be worked out by means of cross-sectional measurements.
126.96.36.199 Where soil, ordinary rock and hard rock are mixed, the measurements for the entire excavation shall be made. Excavated materials comprising hard rock and ordinary rock shall be stacked separately, measured, and each reduced by 50% to allow for voids to arrive at the quantity payable under hard rock and ordinary rock. The difference between the entire excavation and the sum of the quantities payable under hard rock and ordinary rock shall be paid for as excavation in soil or hard soil as the case may be.
188.8.131.52 Where it is not possible or convenient to measure the depth of cutting by recording levels, quantity of excavation shall be worked out from filling. The actual measurements of the fill shall be calculated by taking levels of the original ground before start of the work after site clearance and after compaction of the fill as specified and the quantity of earth work so computed shall be reduced by 10% in case of consolidation is done by heavy mechanical machinery to arrive at the net quantity of excavation for payment. No such deduction shall, however, be made in case of consolidation by heavy mechanical at optimum moisture content, or when the consolidated filling is in confined situations such as under floors.
2.10.7. Rates - Rates for earthwork shall include the following;
a) Excavation and depositing excavated material as specified.
b) Handling of antiquities and useful material as specified.
c) Protection as specified.
d) Site clearance as specified.
e) Setting out and making profiles as specified.
f) Forming (or leaving) dead men or ‘Tell Tales’ in borrow pits and their removal after measurements.
g) Bailing out or pumping over water from excavations.
h) Initial lead of 50 m and lift 1.5 m.
i) Blasting operations for having rock as specified.
2.11. SPECIFICATIONS FOR PLANKING AND STRUTTING
2.11.1 When the depth of trench in soft / l loose soil exceeds 2 metres, stepping sloping / or planking and strutting of sides shall be done. In case of loose and slushy soils, the depths at which these precautions are to be taken shall be determined by the engineer according to the nature of soil.
Planking and strutting shall be ‘close’ or ‘open’ depending on the nature of soil and the depth of trench. The type of planking and strutting shall be determined by the engineer. It shall be the responsibility of the contractor to take all necessary steps to prevent the sides of trenches from collapse. Engineer should take guidance from IS: 3764 for designing the
shoring and strutting arrangements for specifying the profile of excavation.
2.11.2 Close planking and strutting
Close planking and strutting shall be done by completely covering the sides of the trench generally with short upright, members called ‘poling boards’. These shall be 250 x 38 mm in section or as directed by the engineer.
The boards shall generally be placed in position vertically in pairs, one boards on either side of cutting. These shall be kept apart by horizontal walling of strong wood at a maximum spacing of 1.2 metres cross strutted with ballies, or as directed by engineer. The length and diameter of the ballies strut depends upon the width of the trench.
Where the soil is very soft and loose, the boards shall be placed horizontally against the sides of the excavation and supported by vertical ‘walling’, which shall be strutted to similar timber pieces on the opposite face of the trench. The lowest boards supporting the sides shall be taken in the ground for a minimum depth of 75 mm. No portion of the vertical side of the trench shall remain exposed.
The withdrawal of the timber members shall be done very carefully to prevent collapse of the trench. It shall be started at one end and proceeded systematically to the other end. Concrete or masonry shall not be damaged while removing the planks. No claim shall be entertained for any timber, which cannot be withdrawn and is lost or buried, unless required by the engineer to be left permanently in position.
2.11.3 Open planking and strutting - In case of open planking and strutting, the entire surface of the side of the trench is not required to be covered. The vertical boards, 250 mm wide & 38 mm thick, shall be spaced sufficiently apart to leave unsupported strips of 50 cm average width. The detailed arrangement size of the timber and the distances apart shall be subject to the approval of the engineer. In all other respects, SPECIFICATIONS for close planking and strutting shall apply to open planking and strutting.
2.11.4 Measurements - The dimensions shall be measured correct to the nearest cm and the area of the face supported shall be worked out in square meters correct to the two places of decimal.
Works shall be grouped according to the following,
(1) Depth not exceeding 1.5 m.
(2) Depth exceeding 1.5 m in stages of 1.5 m.
Planking and strutting to the following shall be measured separately:
(2) Areas – the description shall include use and waste of raking shores.
(3) Shafts, walls, cesspits, manholes and the like.
(4) Where tightly driven close butt joined sheeting is necessary as in case of running sand the item shall be measured separately and packing of cavities behind sheeting with suitable material included with the item.
(5) Planking and strutting required to be left permanently in position shall be measured separately.
2.11.5 Rates - Rates shall include use and waste of all necessary timber work as mentioned above including fixing maintenance and subsequent removal.
2.12. SPECIFICATIONS FOR EXCAVATION IN WATER, MUD OR FOUL POSITION
2.12.1 All water that may accumulate in excavations during the progress of the work from springs, tidal or river seepage, broken water mains or drains (not due to the negligence of the contractor), and seepage from subsoil aquifer shall be bailed, pumped out or otherwise removed. The contractor shall take adequate measures for bailing and / or pumping out water from excavations and construct diversion channels, bunds, sumps, coffer dams etc. as may be required. Pumping shall be done directly from the foundation trenches or from a sump out side the excavation in such a manner as to preclude the possibility of movement of water through any fresh concrete or mortar. During laying of concrete or masonry and for a period of at least 24 hours thereafter, pumping shall be done from a suitable sump separated from concrete or masonry by effective means.
2.12.2 Capacity and number of pumps, location at which the pumps are to be installed, pumping hours etc. shall be decided from time to time in consultation with the engineer.
2.12.3 Pumping shall be done in such a way as not to cause damage to the work or adjoining property by subsidence etc. Disposal of water shall not cause inconvenience or nuisance in the area or cause damage to the property and structure nearby.
2.12.4 To prevent slipping of sides, planking and strutting may also be done with the approval of the engineer.
2.12.5 Classification - The earth work for various classification of soil shall be categorised as under:
a) Work in or under water and / or liquid mud - Excavation, where water is met with from any of the sources as specified shall fall in this category. Steady water level in the trial pits before the commencement of bailing or pumping operations shall be the sub-soil water level in that area.
b) Work in or under foul position - Excavation, where sewage, sewage gases or foul conditions are met with from any sources, shall fall in this category. Decision of the engineer whether the work is in foul position or not, shall be final.
2.12.6 Measurements - The unit, namely, meter depth shall be the depth measured from the level of foul position/ sub-soil water level and up to the centre of gravity of the cross sectional area of excavation actually done in the conditions classified. Meter depth shall be
reckoned correct to 0.1 m., 0.05 or more shall be taken as 0.1 m and less than 0.05 m ignored. The extra percentage rate is applicable in respect of each item but the measurements shall be limited only to the quantities of earth work actually executed in the conditions classified.
Pumping or bailing out water met within excavations from the sources as specified where envisaged and specifically ordered in writing by the engineer shall be measured separately and paid. Quantity of water shall be recorded in kilolitres correct to two places of decimal. This payment shall be in addition to the payment under respective items of earthwork and shall be admissible only when pumping or bailing out water has been specifically ordered by the engineer in writing.
Planking and strutting or any other protection work done with the approval of the engineer to keep the trenches dry and / or to save the foundations against damage by erosion or rise in water levels shall be measured and paid for separately.
Bailing or pumping out water accumulated in excavation, due to rains is included under respective items of earthwork and is not to be paid separately.
2.11.7 Rates - The rates for respective items described above shall include cost of all the operations as may be applicable.
2.13. SPEICIFICATIONS FOR SURFACE DRESSING OF GROUND
2.13.1 Surface dressing shall include cutting and filling up to a depth of 15 cm and clearing of shrubs, rank vegetation, grass, brushwood, trees and saplings of girth up to 30 cm measured at a height of one meter above the ground level and the removal of rubbish and other excavated material upto a distance of 50 meters outside the periphery of the area under surface dressing. High portions of the ground shall be cut down and hollows depressions filled up to the required level with the excavated earth so as to given an even, neat and tidy look.
2.13.2 Measurements - Length and breadth of the dressed ground shall be measured correct to the nearest cm and the area worked out in square meters correct to two places of decimal.
2.13.3 Rates - The rates shall include cost of labour involved in all the operations described above.
2.14. SPEICIFICATIONS FOR JUNGLE CLEARANCE
2.14.1 Jungle clearance shall comprise uprooting of rank vegetation, grass, brushwood, shrubs, stumps, trees and saplings of girth up to 30 cm measured at a height of one meter above the ground level. Where only clearance of grass is involved it shall be measured and paid for separately.
2.14.2 Uprooting of vegetation - The roots of trees and saplings shall be removed to a depth of 60 cm below ground level or 30 cm below formation level or 15 cm below subgrade level, whichever is lower. All holes or hollows formed due to removal of roots shall be filled up with earth rammed and leveled. Trees, shrubs, poles, fences, signs, monuments, pipe lines, cables etc. within or adjacent to the area which are not required to be disturbed during jungle clearance shall be properly protected by the contractor at his own cost and nothing extra shall be payable.
2.14.3 Stacking and disposal - All useful materials obtained from clearing and grubbing operation shall be stacked in the manner as directed by the engineer. Trunks and branches of trees shall be cleared of limbs and tops and stacked neatly at places indicated by the engineer. The materials shall be the property of the Government. All unserviceable materials, which in the opinion of the engineer cannot be used or auctioned, shall be removed up to a distance of 50 m outside the periphery of the area under clearance. It shall be ensured by the contractor that unserviceable materials are disposed off in such a manner that there is no likelihood of getting mixed up with the materials meant for construction.
2.14.4 Clearance of grass - Clearing and grubbing operations involving only the clearance of grass shall be measured and paid for separately and shall include removal of rubbish up to a distance of 50 m outside the periphery of the area under clearance.
2.14.5 Measurements - The length and breadth shall be measured correct to the nearest cm and the area worked out in square meters correct to two places of decimal.
2.14.6. Rates - The rates include cost of all the operations described above.
Note: Jungle clearance and clearance of grass are not payable separately for the earthwork.
2.15. SPEICIFICATIONS FOR FELLING TREES
2.15.1 Felling - While clearing jungle, growth trees above 30 cm girth (measured at a height of one metre above ground level) to be cut, shall be approved by the engineer and then marked at site. Felling trees shall include taking out roots up to 60 m below ground level or 30 cm below formation level or 15 cm below sub-grade level, whichever is lower.
All excavations below general ground level arising out of the removal of trees, stumps etc. shall be filled with suitable material in 20 cm layers and compacted thoroughly so that the surfaces at these points conform to the surrounding area. The trunks and branches of trees shall be cleared of limbs and tops and cut into suitable pieces as directed by the engineer.
2.15.2. Stacking and disposal - Wood, branches, twigs of trees and other useful material shall be the property of the Government. The serviceable materials shall be stacked in the manner as directed by the engineer up to a lead of 50 m.
All unserviceable material, which in the opinion of engineer cannot be used or auctioned shall be removed from the area and disposed off as per the directions of the engineer. Care shall be taken to see that unsuitable waste materials are disposed off in such a manner that there is no likelihood of these getting mixed up with the materials meant for construction.
2.15.3. Measurements - Cutting of trees above 30 cm in girth (measured at a height of one metre above ground level) shall be measured in numbers according to the sizes given below
a) Beyond 30 cm girth, up to and including 60 cm girth.
b) Beyond 60 cm girth, up to and including 120 cm girth.
c) Beyond 120 cm girth, up to and including 240 cm girth.
d) Above 240 cm girth.
2.15.4. Rate - The rate includes the cost involved in all the operations described above. The contract unit rate for cutting trees above 30 cm in girth shall include removal of stumps as well.
2.16. SPECIFICATIONS FOR EXCAVATION IN ORDINARY / HARD ROCK
All excavation operations shall include excavation and ‘getting out’ the excavated matter. In case of excavation for trenches, basements, water tanks etc. ‘getting out’ shall include throwing the excavated materials at a distance of at least one meter or half of depth of excavation, whichever is more, clear off the edge of excavation. In all other cases ‘getting out’ shall include depositing the excavated materials as specified. The subsequent disposal of the excavated material shall be either stated as a separate item or included with the item of excavation stating lead.
During the excavation, the natural drainage of the area shall be maintained. Excavation shall be done from top to bottom. Undermining or under cutting shall not be done.
Where hard rock is met with and blasting operations are considered necessary, the contractor shall obtain the approval of the engineer in writing for resorting to the blasting operations. Blasting operations shall be done as specified and chiseling shall be done to obtain correct levels, slopes, shape and pattern of excavation as per the drawings or as required by the engineer and nothing extra shall be payable for chiseling.
Where blasting operations are prohibited or are not practicable, excavation in hard rock shall be done by chiseling or by such other methods prescribed by engineer.
In ordinary rock excavation shall be carried out by crowbars, pick axes or pneumatic drills and blasting operation shall not be generally adopted. Where blasting operations are not prohibited and it is practicable to resort to blasting for excavation in ordinary rock, contractor may do so with the permission of the engineer in writing but nothing extra shall be paid for this blasting.
If the excavation for foundations or drains is done to a depth greater than that shown in the drawings or as required by the engineer, the excess depth shall be made good by the contractor at his own cost with the concrete for foundations. Soft/defective spots at the bed of foundations shall be dug out and filled with concrete (to be paid separately) as directed by the engineer.
In all other cases where the excavation is taken deeper by the contractor, it shall be brought to the required level by the contractor at his own cost by filling with earth duly watered, consolidated and rammed.
In case the excavation is done wider than that shown on the drawings or as required by the engineer, filling wherever required on this account shall be done by the contractor at his own cost.
Only the excavation shown on the drawings or as required by the engineer shall be measured and recorded for payment except in case of hard rock, where blasting operations have been resorted to, excavation shall be measured to the actual levels, provided the engineer is satisfied that the contractor has not gone deeper than what was unavoidable.
2.17. SPECIFICATIONS FOR BLASTING
2.17.1 Where hard rock is met with and blasting operations are considered necessary, the contractor shall obtain the approval of the engineer in writing for resorting to blasting operation.
Note: In ordinary rock blasting operations shall not be generally adopted. However, the contractor may resort to blasting with the permission of the engineer, but nothing extra shall be paid for such blasting operations.
The contractor shall obtain license from the competent authority for undertaking blasting work as well as for obtaining and storing the explosive as per the Explosive Act, 1884 as amended upto date and Explosive Rules, 1983. The contractor shall purchase the explosive fuses, detonators, etc. only from a licensed dealer. Transportation and storage of explosive at site shall conform to the aforesaid Explosive Act and Explosive Rules. The contractor shall be responsible for the safe custody and proper accounting of the explosive materials. Fuses and detonators shall be stored separately and away from the explosives. The engineer or his authorised representative shall have the right to check the contractor’s store and account of explosives. The contractor shall provide necessary facilities for this.
The contractor shall be responsible for any damage arising out of accident to workmen, public or property due to storage, transportation and use of explosive during blasting operation.
2.17.2 Explosives: An explosive is a solid or liquid substance or a mixture of substances which on application of a suitable stimulus is converted in a very short time interval into other more stable substances, largely or entirely gaseous, with the development of heat and high pressure.
2.17.3 Classification of explosives
184.108.40.206 The explosives can be classified into the following types:
a) Low (or deflagrating) explosives;
b) High explosives: (i) Primary and (ii) Secondary.
220.127.116.11 Low explosives were the earliest to be developed. These lead to an explosion which is really a rapid form of combustion in which the particles burn at their surfaces and expose more and more of the bulk until all has been consumed. Such an explosion is called deflagration and the reaction in this case moves slower than the speed of sound. Typical examples of this category are the gun powder, propellants in ammunition, rocket propellants and pyrotechnics.
18.104.22.168 High explosives, depending on their composition, explode at velocities of 1500 – 8000 m/s and produce large volumes of gases at considerable heat at extremely high pressures. High explosives themselves may be further subdivided into primary explosives and secondary explosives. Primary explosives are characterised by their sensitiveness to stimuli like mechanical shock, spark or flame, the application of which will take explosive compounds from state of deflagration to detonation easily. Examples of these explosives are Mercury Fulminate, Lead Azide, Lead Styphnate, Tetrazene and other mixtures. These explosives are used as initiating charges in the initiating devices such as detonators. Secondary explosives are capable of detonation only under the influence of a shock wave, normally generated by the detonation of primary explosives. Secondary explosives of this type are military explosives like TNT, RDX, PETN, Tetryl and other combinations of these and industrial explosives like nitro-glycerin, emulsion, slurries and water gels. These explosives are normally set off with suitable initiating devices like detonators or detonating cords. The explosive needing another high explosive for initiation is called blasting agent such as ANFO, some slurries, some emulsions and mixtures of emulsions and ANFO.
2.17.4 Nitro-glycerin based explosives - Nitro-glycerin (NG) has been in use for a long time as the most important sensitiser for commercial explosives. It is, converted into a more convenient gelatinous (plastic) solid by the addition of 8% guncotton or nitro-cellulose to form Blasting Gelatine or by absorbing it in Kieselghur to give straight dynamic (containing about 75% NG), by admixture with other explosive agents and additives to form other types of dynamites. The properties of nitroglycerine, and the way in which it is mixed with other ingredients, determines the type of explosive produced.
2.17.5 Ammonium nitrate dry mixes
Ammonium nitrate (AN) and fuel oil (FO) mixtures, known as ANFO, were introduced for blasting operations in mid 1950s. Ammonium nitrate in a proper form when mixed with carbonaceous or combustible material in appropriate proportion forms a blasting agent. Although many forms of AN could be used with a solid or liquid fuel to form a blasting agent, the porous prilled forms are preferred for ANFO.
AN is stable at ambient temperatures, but can absorb moisture from the atmosphere. To minimise moisture absorption and caking, the prills are lightly coated with anti-caking agents. Proper mixing of AN and FO is important for predictable explosive performance.
Some of the nitrogen from the ammonium nitrate combines with this excess oxygen to form nitrous oxide which, upon exposure to normal atmosphere, forms NO2, an extremely toxic gas. An oxygen balanced mixture, thus, maximises energy release while minimising the formation of toxic gases.
The blast hole diameter has a pronounced effect on the VOD (Velocity Of Detonation) of the ANFO. As the diameter increases so does the VOD. But the energy yield does not vary with blast hole diameter. Well mixed loose-poured ANFO can be used successfully in blast hole diameters down to about 25 mm.
Although ANFO mixtures are explosives, they are relatively insensitive and unless suitably primed, reliable detonation in large blast holes will not occur. In general, the primer should have a high VOD and the maximum possible diameter. Needless to mention, the primer should be in intimate contact with the ANFO.
Lack of water resistance is the major limitation and disadvantage of ANFO.
2.17.6 Slurry explosives
Slurry explosives were first developed as a result of attempts to waterproof, improve density and strength of ammonium nitrate.
A slurry is a mixture of nitrates such as ammonium nitrate and sodium nitrate, a fuel sensitiser, either explosive or non-explosive, and varying amounts of water. Although they contain large amounts of ammonium nitrate, slurries are made water resistant through the use of gums, waxes, and cross linking agents. Most commonly used fuel sensitisers are carbonaceous fuels, aluminium, and amine nitrates. They are sensitised by air bubbles which are entrapped by churning the mixture. Even when none of the ingredients are in themselves explosive substances and it is only in the final stages of production that the compositions acquire explosive characteristics.
Slurries could be manufactured without some of hazards usually present during explosives manufacture, and the user no longer suffers the discomfort of headache associated with nitro-glycerine based explosives. Except for their excellent water resistant and higher density and bulk strength slurries are similar in many ways to dry blasting agents. Good oxygen balance, decreased particle size and increased density, increased charge diameter, good confinement, and coupling and adequate priming all increase their efficiency.
Most slurry depends on the entrapped air for their sensitivity. If this air is removed from slurry through pressurisation from adjacent blast, prolonged periods of time in the boreholes, or prolonged storage, the slurry may become desensitised. To overcome this problem, ‘Perlite’ or Microballons are added.
2.17.7 Emulsion explosives
An emulsion is a two phased system in which an inner or dispersed phase is distributed in an outer or continuous phase. In simpler terms an emulsion is a mixture of two liquids that do not dissolve in one another. This unique feature coupled with the fact that minute size of the nitrate solution droplets are tightly compacted within the continuous fuel phase results in good intimacy between the oxidiser and fuel and increased reaction efficiency compared to other systems.
The emulsion matrix is obtained by emulsification of two immiscible liquids. By the process of emulsification two types of emulsions are obtained, one is oil-in-water and other is water-in-oil. Water is in discrete dispersed phase in water-in-oil emulsion in the form of the droplets dispersed in continuous phase, which is oil phase whereas in oil-in-water emulsion, the reverse is true.
The emulsion explosives have the oxygen donor consisting of nitrates and perchlorates in an aqueous solution. The water phase is in the form of the small droplets, in the continuous oil phase, which constitutes the fuel. The fuel consists of mixtures of waxes and oil. The explosive in this case is sensitised by gas bubbles in the form of microspheres. Additional strength can be achieved by the addition of fuels such as aluminium powder.
2.17.8 Explosives for specialized blasting operations
22.214.171.124 Pipe charges - While carrying out blasting which requires minimum damage to the remaining rock in which development of cracks needs to be reduced, then one way of achieving this is the use of pipe charges which are explosives having lesser diameter (11 to 19 mm) than the borehole. The reduced diameter thus provides decoupling between the hole walls and the explosives and thus the peak pressure of the liberated gases reduces, causing reduced crushing and the intensity of cracks developed in the rock around the blast hole gets reduced.
Pipe charges in rigid plastic tubes are used which can be screwed together by means of extension sleeves. The cartridge dimensions range between 25 mm and 50 mm in diameter and 600 and 700 mm in length.
Some examples of pipe charges are K-pipe of Finland, Gurit of Nitro Nobel of Sweden. These types of explosives are used in the last rows of a blast while carrying out smooth blasting operation.
Comparatively weaker explosives are also used in many situations to blast very weak rocks. Polystyrene granules have been mixed with ANFO to obtain reduced energy explosives thus the crushing of rock and intensity and extent of cracking get reduced.
2.17.9 Explosives properties - Each explosive has certain specific characteristics or properties. Some of the principal properties of explosives are: detonation velocity, strength, density, detonation pressure, water resistance; sensitivity; safety in handling; storage qualities; sensitiveness and fumes.
2.17.10 Detonation velocity - The detonation velocity is a measure of the speed at which the detonation wave travels through a column of explosives. Many factors affect the detonation velocity such as explosive type, diameter, confinement, temperature and priming.
The detonation velocities of commercial explosives range from about 1500 m/s to more than 6700 m/s. Velocity of detonation (VOD) for common explosives falls within the range of 3000 to 5000 m/s. Every explosive has an ultimate or ideal velocity, steady-state velocity of the explosive.
Depending upon the type of explosive up to a certain diameter the velocity of detonation of an explosive is influenced by the diameter. In general, the larger the diameter the higher the velocity until the steady state velocity of the explosive is reached.
Every explosive also has a ‘critical diameter’ which is the minimum diameter at which the detonation process, once initiated, will support itself in the column. In diameters smaller than the ‘critical’ the detonation of the explosive will not be supported and will be extinguished.
Generally, the greater the confinement of an explosive, the higher is the detonation velocity. This is particularly true for products such as ANFO and some watergels in small diameter boreholes.
It is essential that adequate priming is ensured so that the explosive may reach its maximum velocity as quickly as possible. Inadequate priming can result in the failure of the explosive to detonate, a slow build-up to final velocity, or a low order detonation.
2.17.11 Energy / strength - A large number of tests and various calculations have been made which refer to energy content to predict the performance of explosives. However, the term ‘strength’ traditionally associated with the ‘strength ratings’ of different dynamite grades, has little correlation with the effectiveness of an explosive in blasting and has no meaningful relation to modern commercial products like ANFO, emulsions or water gels.
Explosive energy ratings - The nitro-glycerine or straight dynamites are rated according to percentage by weight of nitro-glycerine they contain. A 60% straight dynamite contains 60% of nitro-glycerine, the 30% grace strength contains 30% nitro-glycerine, etc. An erroneous concept is that the actual blasting power developed by the different grades is in direct proportion to the strength ratings. Such simple ratios unfortunately do not exist because nitro-glycerine is not the only energy-producing ingredient in their formulation.
Performance of an explosive - is not determined simply by knowing the total energy released by the explosive. It depends also upon the rate of energy release and how effectively the energy is utilised in fragmenting, and moving the material being blasted. Both the explosive properties and the properties of the material being blasted influence the effectiveness of an explosive.
Under water tests - Some of the current tests or calculations to measure and characterise an explosive energy are the various underwater tests, the pressure time measuring technique in the rock, and the theoretical calculation techniques.
The density of explosives is expressed in g/cm3. Densities of commonly used explosives are:
ANFO free poured
ANFO charged pneumatically from pressure vessel
up to0.95 g/cm3
Watergels and emulsions
0.80 to 1.50 g/cm3
Semigelatine and gelatine
1.0 to 1.60 g/cm3
In general commercial water gels, emulsions and nitro-glycerine based explosives commonly used are in the range of 1.10 to 1.35 g/cm3. The prime purpose in varying the density of commercial explosives is to enable the total energy charge in a borehole to meet particular field conditions.
2.17.13 Detonation pressure and explosion pressure - The detonation pressure is the pressure in the shock zone ahead of the reaction zone. When an explosive detonates, tremendous pressure is released, practically instantaneously, in a shock wave, which exists for only a fraction of second at any given place. The sudden pressure thus created will shatter rather than displace objects. The detonation pressure is a function of the density, the detonation velocity, and the particle velocity of the explosive. For condensed explosives, the particle velocity is about ¼ of the detonation velocity. The detonation pressure can be approximated as follows:
P = 2.5 r.V2 x 10 -6 Where P = Detonation pressure (kilo bars), r = Density (g/cm3) and
V = Velocity of detonation (m/s).
The detonation pressure is important in that it is related to the stress level in the material to be blasted which may be an important factor in fragmentation. It is also important in priming for effective and reliable initiation that the detonation pressure of the primer exceeds the detonation pressure of the main explosive charge.
The detonation pressure is different from the explosion pressure, which is the pressure after adiabatic expansion back to the original explosive volume. The explosion pressure is theoretically about 45% of the detonation pressure.
2.17.14 Water resistance - The ability of an explosive to withstand water penetration is termed as water resistance. Water resistance is generally expressed as the number of hours a product may be submerged in static water and still be detonated reliably. Explosives penetrated by water have their efficiency impaired first and, upon prolonged exposure or in severe water conditions, they may be desensitised to a point where they will not detonate.
Commercial explosives vary widely in their ability to resist the effect of water penetration. Ammonium nitrate/fuel oil has no inherent water resistance. If ANFO is poured into water filled drill holes, it will quickly desensitise. Packaged ANFO products, if used in wet work, depend entirely on their packaging to resist water penetration. Slurries, emulsions and many nitro-glycerine based products have good water resistance.
2.17.5 Sensitivity - Sensitivity is the measure of ease of initiation. There are numerous measures of sensitivity, including cap sensitivity, drop tests, friction test and others. These tests are often carried out to measure an explosive’s ease of initiation through accidental means and thereby measure safety in handling.
Cap sensitivity not only characterises an explosive’s ease of initiation with a blasting cap, but also is used to classify products. Either a No. 6 strength or No. 8 strength detonator is used as standard by the explosive industry.
Normally the explosive is initiated by the use of a detonator but some explosives need more powerful initiation. As an example it can be mentioned of ANFO and some slurry explosives which are normally initiated by using some cap sensitive explosive primers or special primer consisting of an explosive with a high detonation velocity.
2.17.16 Safety in handling
Safety in handling is very important and one obvious requirement of an explosive is that it can be transported, stored and used under normal conditions without any risk to people who carrying out blasting operation. Explosives are subjected to many tests before they are approved for use. Explosives, regardless of their degree of safety, should never be abused in any way.
2.17.17. Storage qualities - Most explosives are perishable, and both climate and magazine conditions are factors of great importance in their storage. Very old stock of explosives should not be used.
2.17.18. Sensitiveness - Sensitiveness of an explosive is a measure of its propagating ability. NG-based explosives and some slurries are specified in terms of gap sensitivity.
An explosive with high flash-over tendency can cause flash-over between adjacent drill holes if the holes are closely spaced. Particularly in the case of rock types which have many cracks, and under moist conditions, there is a risk of flashover.
2.17.19 Fumes - The gases resulting from the detonation of commercial explosives are principally carbon dioxide, nitrogen and water vapour, all are non-toxic in the ordinary sense. However, poisonous or toxic gases including carbon monoxide and nitrogen oxides also result from detonation. These toxic gases are called fumes.
The composition of an explosive is said to be balanced when the oxygen contained by the ingredients combines, with the carbon and hydrogen content to form carbon dioxide and water. If there is insufficient oxygen (a negative oxygen balance), the tendency to form carbon monoxide is increased. If there is an excess of oxygen (a positive oxygen balance), oxides of nitrogen are formed.
In open blasting operation fumes cause little concern if they can be quickly dispersed by air movement, but in underground work the type and amount of explosive, the blast conditions, ventilation and other factors must be considered.
2.17.20 Initiation systems - Commercial explosives (and blasting agents) are designed to be relatively stable for safe usage, transport, storage and manufacture. A powerful localised shock or detonation is required to initiate commercial explosives. There are basically two methods of initiation: electrical and non-electrical.
126.96.36.199 Electrical initiation systems - An electrical initiation utilises an electrical power source with an associated circuit to convey the impulse to the electric detonator which in turn fires and initiates the explosive charge.
188.8.131.52 Detonators - All detonators consist of a metal tube or shell 6.5 to 7.5 mm in diameter (outer) of varying length. Normally detonator shells are made of aluminium. At the closed end of the tube an explosive charge of either a single initiating explosive (as mercury fulminate) or a combination of secondary explosive (base charge) and an initiating explosive charge (top charge) is placed. The charges are compacted to give the desired strength and also ensure that they do not fall out while handling. The quantity of charge used must be adequate to reliably initiate the high explosive. Either No. 6 strength or No. 8 strength detonators are used. No. 6 strength detonator was originally the strength obtained from 1 g of a mixture containing mercury fulminate and 20% potassium chlorate. No. 8 strength detonators have explosive mixture equivalent to 2 g of 80% of mercury fulminate and 20% potassium chlorate. Detonators are classified as plain detonators or electric detonators. In the electric detonators (Fig. 2), the electrical current to the detonator is supplied from the power source through the circuit wiring to the detonator by means of two leg-wires that are internally connected by a small length of high resistance bridge wire. The electrical energy is converted into heat energy on passing the firing current through bridge wire. The heat energy ignites the pyrotechnic that surrounds the bridge wire on the match head assembly. The resulting flash or flame ignites initiating charge or the delay element, these in turn set off base charge. A rubber or neoprene plug seals the opening and only the leg wires pass through the plug. This prevents contamination by foreign material or water. Electrical detonators may again be classified as instantaneous and delay detonators.
184.108.40.206 Instantaneous detonators - Instantaneous detonators fire within a few millisecond (less than 5 ms) after they receive the current. These are used when all the holes are to be fired simultaneously.
220.127.116.11 Delay detonators - In delay detonators, a delay element is inserted between the electric fuse head and the ASA/PETN charge in the detonator (Fig 3). This delay element consists of a column of slow burning composition. The length and composition determine the amount of delay-time introduced into the detonator.
18.104.22.168. Basic delay series – There are two types.
(1). Half-second interval between successive numbers (also called long delay detonators);
(2). Millisecond interval between successive numbers (also called short delay detonators).
The long delay detonators are used mainly in tunneling and shaft sinking. The short or millisecond delay detonators are the most commonly used delays.
22.214.171.124 Electric blasting circuits - In order to fire electric detonators, they must be connected together in a firing circuit and energised by a power source. There are three types of blasting circuits: series parallel and series/parallel.
126.96.36.199 Safety fuse and plain detonators - Safety fuse has a core consisting of granulated gunpowder. The continuous core is covered with molten bitumen and in some kinds of fuse an extra costing of high polymeric material is given to make it highly waterproof.
The most important requirement of safety fuse is that it should have a uniform rate of burning. Fuse should be test burned periodically so that the blaster can keep a record of its actual burning rate.
Plain or ordinary detonator is the earliest of the modern blasting detonators which provide non-electric method of initiating explosive charges, when used in conjunction with safety fuse. The detonator contains two types of charges (sometime three types); the igniting charges and the base charge. The igniting charge ensures flame pickup from the safety fuse, which in turn detonates the base charge and thus detonates the explosive charge being primed with the detonator.
188.8.131.52 Detonating cord initiation- As an alternative to electric blasting, detonating cord initiation has been used for many years. It is mainly used in multihole blasting and when detonated, has the initiating energy of a detonator at all points. Generally in mining practice, one line of detonating fuse is used as a trunkline, from which a number of branches are drawn each of which leads into a hole containing explosive to be blasted.
The most commonly used detonating cord has an explosive charge of normally 10 g of PETN per meter run. The diameter of this cord is nominally 4.65 mm and it has a breaking strength in excess of 100 kg.
184.108.40.206. Delay connectors - Millisecond delay surface connectors are used for delaying detonating cord blasts. To place a delay between two holes, the trunkline between holes is cut and the ends are joined with a delay connector. Detonating Relays, consist of a long aluminium tube with two mini delay detonators on either side and having an attenuator in the centre. Openings at either end are provided to receive and crimp the cut ends of the detonating fuse. At each end of the element is an opening into which a loop of trunkline can be inserted. A tapered pin is used to lock the trunkline cord into place. Delays of 25 ms and 50 ms are available.
2.17.21. Non-electric systems (shock tube system). – There are three types.
1) Nonel, 2) Raydet and 3) Exel.
220.127.116.11. The Nonel system - Nonel is the common trade name of a series of blast initiation accessories developed by Nitro Nobel of Sweden, which uses shock tube principle. The system is based on a plastic tube, the inside of which is coated with a reactive substance that maintains the propagation of a shock wave at a rate of approximately 2000 m/s. This shock wave has sufficient energy to initiate the primary explosive or delay element in a detonator. Since the reaction leaves the tube intact, there is no lateral shock effect and the tube acts merely as a signal conductor.
The Nonel tube is made of flexible plastic with an outside diameter of 3 mm and inside diameter of 1.5 mm. In its standard form it is transparent. The tube is crimped to a delay detonator.
Electrical methods provide a relatively accurate method of initiating a blast circuit that may readily be tested prior to initiation. Primary disadvantages however are that electrical detonators are more susceptible to accidental initiation by heat, impact, friction or extraneous current from static, stray current, lightning or radio waves.
Non-electrical methods, on the other hand are relatively insensitive to premature or accidental initiation from these cases noise from detonating cord surface lines. The introduction of the shock tube system has eliminated some of these problems.
18.104.22.168. Blasting accessories
Power source (exploders, sequential blaster or appropriate mains firing instrument).;
- Blasting circuit testers;
- Non-metallic measuring tapes equipped with lead or non-sparking weights;
- Lowering ropes;
- Non-sparking lowering and retrieving hooks;
- Tamping poles (wooden or non-sparking);
- Blasting knives
- Connecting wire (new not reclaimed);
- Firing cable
Exploders are machines, which provide the required electrical power to fire a series of electrical detonators. Usually there are two types of exploders; generator type and condenser discharge type. The capacity of the exploder must be 1.5 to 2 times the needed capacity.
The blasting galvanometer is used only to check the circuit resistance, whereas a blasting multimeter can be used to check resistance, ac and dc voltage, stray currents, and current leakage. Only a meter specifically designed for blasting should be used to check blasting circuits. The output of such meters is limited to 0.05 amp, which will not initiate an electric blasting cap. Electric blasting caps should not be used in the presence of stray currents of 0.05 amp or more.
22.214.171.124. Raydet - Non-electric devices made by IDL Chemicals are now available in India. Raydet like NONEL system consists of a plastic tube carrying a very small quantity of explosive material on its inner surface. A high strength \no.8 instantaneous or delay detonator is crimped to one end of the ray tube. When initiated a low order shock wave travels through the tube and reliably initiates the detonator. It can be initiated by a detonator or detonating cord. A tag indicates the delay number of Raydet and a tape fastening the tube in a coil indicates the tube length.
126.96.36.199. Exel shock tube - This is the single layer shock tube initiation system developed by ICI Explosives. Recently ICI had conducted trials with this system and are planning to introduce this product in a big way in our country.
The tubing is made from readily available polyfin polymer blends. By a dynamic melt flow process, the surface properties are enhanced, which enable a thin film of explosive dust to adhere to the inner wall. It has four times the tensile strength and four times the abrasion resistance of regular shock tube. This is achieved by cold draw/orientation process.
EXEL detonator assemblies have been used in a variety of operations, and have proved to be safe and reliable under a variety of loading conditions.
188.8.131.52. Exel detonator - These are short delay and long delay series of detonators made with fixed tube lengths. These detonators look and function similar to electric delay detonators. The detonator is inserted in the explosive to make a primer which goes down-the-hole. On the surface, shock tube tails can be connected as per field requirement. The easiest way of making surface connections is to connect all the tails with a detonating cord trunkline. Detonating relays are inserted in the surface hookup to provide a combination of surface and down-the-hole delays.
184.108.40.206. Exel trunkline - Exel Trunkline when used with detonating cord on the surface creates an air blast. Compared to the detonating cord, shock tube detonators are almost noiseless. Surface hookup with shock tubes would provide scope for great reduction in the air blast. This requirement is fulfilled by "EXEL" Trunkline delay detonators (TLD). The "EXEL" TLD detonator comprises of a specified length of tube crimped to a delay detonator which is enclosed in a special plastic block. Each TLD block can hold and initiate upto 5 "EXEL" tubes.
220.127.116.11. Exel Handidet - It combines a short delay detonator and TLD into one single unit. In this one end of the tube is crimped to a short delay detonator (200 to 500 ms) and the other end has short delay detonator (25 to 50 ms) enclosed in a TLD block. During application, the longer delay detonator goes into the explosive to make a primer and on the surface TLD block is used for making connections.
2.17.22. Blasting operations - General - Blasting operations shall be carried out under the supervision of a responsible authorised agent of the contractor (referred subsequently as agent only), during specified hours as approved in writing by the engineer or his authorised agent. The agent shall be conversant with the rules of blasting. In case of blasting with dynamite or any other high explosive, the position of all the bore holes to be drilled shall be marked in circles with white paint. These shall be inspected by the contractor’s agent. Boreholes shall be of a size that the cartridge can easily pass down. After the drilling operation, the agent shall inspect the holes to ensure that drilling has been done only at the marked locations and no extra hole has been drilled. The agent shall then prepare the necessary charge separately for each borehole. The boreholes shall be thoroughly cleaned before a cartridge is inserted. Only cylindrical wooden tamping rods shall be used for tamping. Metal rods or rods having pointed ends shall never be used for tamping. One cartridge shall be placed in the bore hole and gently pressed but not rammed down. Other cartridges shall then be added as may be required to make up the necessary charge for the borehole. The top most cartridges shall be connected to the detonator which shall in turn be connected to the safety fuses of required length. All fuses shall be cut to the length required before being inserted into the holes. Joints in fuses shall be avoided. Where joints are unavoidable, a semi-circular nitch shall be cut in one piece of fuse about 2 cm deep from the end and the end of other piece inserted into the nitch. The two pieces shall then be wrapped together with string. All joints exposed to dampness shall be wrapped with rubber tape.
The maximum of eight boreholes shall be loaded and fired at one occasion. The charges shall be fired successively and not simultaneously. Immediately before firing, warning shall be given and the agent shall see that all persons have retired to a place of safety. The safety fuses of the charged holes shall be ignited in the presence of the agent, who shall see that all the fuses are properly ignited.
Careful count shall be kept by the agent and others of each blast as it explodes. In case all the charged bore holes have exploded, the agent shall inspect the site soon after the blast but in case of misfire the agent shall inspect the site after half an hour and mark red crosses (X) over the holes which have not exploded. During this interval of half an hour, nobody shall approach the misfired holes. No driller shall work near such bore until either of the following operations has been done by the agent for the misfired boreholes.
a) The contractor’s agent shall very carefully (when the tamping is of damp clay) extract the tamping with a wooden scraper and withdraw the fuse, primer and detonator. After this a fresh detonator, primer and fuse shall be placed in the misfired hole, and fired, or
b) The holes shall be cleaned for 30 cm of tamping and its direction ascertained by placing a stick in the hole. Another hole shall then be drilled 15 cm away and parallel to it. This hole shall be charged and fired. The misfired holes shall also explode alongwith the new one.
Before leaving the site of work, the agent of one shift shall inform the other agent relieving him for the next shift, of any case of misfire and each such location shall be jointly inspected and the action to be taken in the matter shall be explained to the relieving agent.
The engineer shall also be informed by the agent of all cases of misfires, their causes and steps taken in that connection.
2.17.23. General precautions - For the safety of person’s red flags shall be prominently displayed around the area where blasting operations are to be carried out. All the workers at site, except those who actually ignite the fuse, shall withdraw to a safe distance of at least 200 metres from the blasting site. Audio warning by blowing whistle shall be given before igniting the fuse.
Blasting work shall be done under careful supervision and trained personnel shall be employed. Blasting shall not be done within 200 metres of an existing structure, unless specifically permitted by the engineer in writing.
All procedures and safety precautions for the use of explosives drilling and loading of explosives before and after short firing and disposal of explosives shall be taken by the contractor as detailed in IS : 4081, safety code for blasting and related drilling operation (Annexure 2-A.2).
2.17.24. Precautions against misfire - The safety fuse shall be cut in an oblique direction with a knife. All saw dust shall be cleared from inside of the detonator. This can be done by blowing down the detonator and tapping the open end. No tools shall be inserted into the detonator for this purpose.
If there is water present or if the bore hole is damp, the junction of the fuse and detonator shall be made water tight by means of tough grease or any other suitable material.
The detonator shall be inserted into the cartridge so that about one-third of the copper tube is left exposed outside the explosive. The safety fuse just above the detonator shall be securely tied in position in the cartridge. Water proof fuse only shall be used in the damp bore hole or when water is present in the bore hole.
If a misfire has been found to be due to defective fuse, detonator or dynamite, the entire consignment from which the fuse, detonator or dynamite was taken shall be got inspected by the engineer or his authorised representative before resuming the blasting or returning the consignment.
2.18. SPECIFICATIONS FOR CONTROLLED BLASTING
2.18.1. The objectives of controlled blasting techniques include
a) Minimization of over break and/or fracturing of rock beyond the designed boundary of excavation so as to achieve smooth post blast surface
b) Control of fly rock and/or ground vibration within permissible limits and
c) To serve both the above purposes
2.18.2. Techniques for minimising rock damage - The main purpose of controlled blasting is to minimise fracturing and loosening of the rock mass beyond the predetermined excavation line/profile. The objective is normally achieved by minimising and judicious use of explosives in the blast holes. Several controlled blasting techniques such as line drilling, pre-splitting, smooth blasting and cushion blasting are used to achieve the mentioned objectives.
2.18.3. Line drilling - For many years line drilling was the only technique used for over break control. In line drilling, a single row of closely spaced, unloaded, small diameter holes is drilled along the excavation line. This provides a plane of weakness to which the primary blast can break and to some extent reflects the shock waves created by the blast, reducing the shattering and stressing in the finished wall. Line drilling is best suited to homogenous formations where bedding planes, joints, and seams are at a minimum. Line drilling has very limited application. The only place where it is applicable is in areas where even the light explosive loads associated with other controlled blasting techniques may cause damage beyond the excavation limit, or where line drilling is used between loaded holes to promote shearing and guide the pre-split line.
2.18.4. Pre-splitting – Pre-splitting involves a single row of holes drilled along the excavation line. Pre-split in the rock forms a discontinuous zone which minimises or eliminates over break from the subsequent primary blast and produces a smooth, finished rock wall. Pre slitting is also used to reduce ground vibration in some critical cases.
2.18.5. Smooth blasting - Smooth blasting sometimes referred to as contour blasting or perimeter blasting. This method is widely accepted for controlling over break in canal, underground headings and slopes. In smooth blasting the holes are drilled along the excavation limits, lightly loaded with well distributed charges, and fired last in the blasting round. The holes for smooth blasting should be fired instantaneously or with minimum delay to achieve a shearing action and smooth walls with minimum over break.
Smooth blasting and pre-splitting techniques differ mainly from the line drilling principle in that some or all of the holes are loaded with relatively light, well-distributed charges of explosives. The fact that the firing of these charges tends to crack or split the rock between the holes permits wider hole spacing than when line drilling. Consequently drilling costs are reduced and in many cases better control of over break is obtained.
2.18.6. Cushion blasting - Cushion blasting sometimes referred to as trimming. Like smooth wall blasting, a single row of holes is drilled along the excavation line, loaded with light, well distributed charges, and fired after the main excavation is removed. In cushion blasting, the charged holes are further decoupled by reducing the diameter or using stemming material of crushed stone or sand to provide crushing effect. This “cushions” the shock from the finished wall as the holes are detonated and minimises the stresses and fractures in the finished wall. This technique is rarely used today because the reduction in decoupling could be achieved by the use of small diameter explosive cartridges which serves the same purpose. The holes are blasted using the last delay number in the same blasting round preferably with jumping delay of 50 mts.
The suitable parameters for controlled blasting are established through trial blasts. Usually it needs to establish the optimum hole spacing and the charge per hole.
2.18.7. Ground vibration - When an explosive charge is detonated inside a blast hole it is instantly converted into hot gases and the expanding gases exert intense pressure on the blast hole walls. A high intensity shock wave travels through the rock mass which attenuates sharply with distance. Simultaneously the rock around the blast hole up to twice the radius of the original hole gets completely crushed. Expanding gases continue to work on the rock, extending the cracks and moving the rock outward and upward. A major portion of the explosive energy passes beyond the fractured zone in the form of elastic ground vibrations. As seismic waves travel through the rock mass, they generate particle motions which are termed as ground vibrations. The velocity of oscillation of rock particles is called “particle velocity” and its maximum value is called “peak particle velocity (PPV)”. Internationally, peak particle velocity is used to express the intensity of ground vibrations from blasting. Damage caused by ground vibration is dependent on PPV and the frequency of the ground motion.
Even though, the use of explosives has unwanted side effect in the form of vibration, explosives provide an inexpensive source of energy for rock excavation in mining and civil engineering projects.
Vibrations from blasting are of transient nature but the disturbance may result in permanent damage to property/structure. Most of vibration associated problems may be expressed as two separate questions:
1) What level of vibration will be produced by the proposed excavation activities? This will depend on the method of excavation and the seismic propagation characteristics of the site.
2) What is the acceptable level of vibration in terms of particle velocity or displacement or acceleration? This will depend on the type of structures at risk.
The answers to both of these questions are site–specific and need trial blasts for vibration measurements as part of the site investigation program.
2.18.8. Principle factors affecting vibration - When a blast is fired, the vibration level is controlled by two principal factors, distance and charge size. Obviously, it is safer to be far away from a blast than to be near it. Equally obvious is that a large explosive charge is more dangerous than a small charge.
2.18.9. Safe limits of vibration -Various codes and standards have been prescribed for ground vibration limits in different countries. Some have so little that urban blasting is prohibited altogether while others have more than the regularly allowed 50 mm/s maximum peak particle velocity at high excitation frequencies. The recent trend is to refer to the frequency of the ground motion. Low frequency waves cause more damage to structures particularly in case of multi-storied buildings. For low frequency ground vibration, the safe level of vibration from this curve is 12.5 mm/s.
2.18.10. Vibration control procedures - The most common method of controlling ground vibration is by minimising the charge weight per delay. Delay blasting permits to divide total charge into smaller charges, which are detonated in a predetermined sequence at specified intervals. Blasting without delay or sufficient delay numbers increases ground vibrations due to increase in maximum charge per delay.
The vibration can be significantly reduced by optimising blast design parameters. It is suggested to establish optimum burden, hole spacing, powder factor and hookup to control vibration.
2.19. SPECIFICATIONS FOR FLY ROCK CONTROL.
The rock fragments ejected from the blast called “fly rock” is a serious hazard of blasting operations, particularly when the blast is conducted in the vicinity of village and structures. The factors which influence the fly rock distance include:
- Height of stemming column in the blast holes and type/quality of stemming material.
- Irregular shape of free face
- Excessive large burden or blasting without free face
- Muffling of the blast area and the muffling material type.
- Scattering and overlapping of delay timings of the delay detonators/relays.
- Presence of water in blast holes.
The first four parameters can be controlled by properly designing the blasting pattern whereas the last two parameters are not easily controllable.
Fly rock can be controlled by proper blast design and by muffling/covering. From the experience it is found that unless blast design is proper, muffling will not be effective. Proper blast design and accurate implementation of the blast are the two areas of fundamental concern for controlling the fly rock. The third important parameter is understanding the local geology and adjusting the explosive charge with regard to the geological features.
The reliable and effective method of controlling fly rock fragments from the mouth of the blast holes (vertical fly rock on the rear side) is the height of stemming column. It has been observed that the fly rock, particularly towards the rear side, was effectively controlled by maintaining the height of stemming column in all holes greater than the burden. The height of stemming column should be 1.2 to 1.5 times the burden.
A good stemming material should retain borehole pressure till the burden rock starts to move. Dry angular material under the effect of the impulsive gas pressure tends to form a compaction arch which locks into the wall of a blast hole, thus increases its resistance to ejection. In general, drill cutting is a better stemming material as compared to sand and should be preferred except in case of watery holes. In case of watery holes only sand free of clayey materials should be used as stemming material.
If fly rock is originating from the face and flying far distances, it could be an indication that too little burden is used or that mud seams or other geological discontinuities are prevalent. Most fly rock however, is not produced from the face. It is produced from the top.
If fly rock towards the free face side is also to be contained then the blasting should be done using the technique of buffer blasting along with muffling. Buffer blasting is a technique in which a buffer of blasted rock of 4 to 6 m thick should be left against the next round of blast.
Muffling or covering of holes including entire area to be blasted is one of the most common methods to contain the distance of travel of flying fragments particularly when blasting is done within the danger zone as specified by DGMS.
In mining blasts, the most common practice is covering the blast using wire mesh of 50 mm x 50 mm to 75 mm x 75 mm. Gunny bags and cartridge empty boxes 4 to 5 numbers are filled with sand or drill cuttings are placed over the wire mesh. Sometimes the entire area to be blasted is covered by old belt conveyors over the wire net which was found to be more effective as compared to wire nets alone. Gunny bags filled with sand, free of pebbles, weighing at least 30 to 40 kg should be placed over the belt conveyor which is placed over the wire nets at an interval of 2 m between and within the rows. This method will contain the vertical fly to a great extent.
Flying fragments is excessive when blasting is done in shallow holes and where bench height or hole depth is less than two times the burden. Therefore for controlling flyrock, the bench height must be greater than two times the burden and preferably three times the burden. The fly rock is also excessive in watery holes.
2.20. SPECIFICATIONS FOR ROCK BREAKING WITHOUT EXPLOSIVES
2.20.1. One of the alternatives to explosives for rock breaking is to use of chemical compounds near very sensitive structures where blasting is not permitted. These chemical compounds are non-explosive in nature since they create only cracks in the rock without any violent reaction. These are basically high expansive cements which when mixed with proper proportion of water and poured in drill holes generate high lateral pressure, exceeding 300 kg/cm2 in about 6 to 8 hours which result in breaking of rock without generating any dust, fume, noise, fly rock and vibrations. However, such non-explosive chemicals should only be used under the expert supervision to optimise various parameters. The potential areas where it can be gainfully used include secondary breakage of boulders, pre-splitting, smooth blasting, controlled breakage, pit slope stabilisation and production of dimension stone. However, the cost of breakage by non explosive chemicals is anticipated to be 6-8 times more than explosives.
Another alternative is to go for hydraulic splitters and hydraulic hammers. However, mechanical breakage is costly because the initial capital cost of the equipment.
2.20.2. Measurements and rates - Measurements and Rates shall be as in 2.10.6, 2.10.7 and Annexure 2-A.7 [Extract of IS: 1200 (Part 1)]
2.21.1 Pre-construction- After the excavation is done, construction of foundations should be undertaken. However, certain pre-construction activities should be completed before commencement of work. These are described below.
Drainage - If the site of the building is such that water would drain towards it, land may be dressed or drains laid to divert the water away from the site.
Setting out - Generally the site shall be leveled before the layout of foundations is set out. In case of sloping terrain, care shall be taken to ensure that the dimensions should be set out with Theodolites in case of important and intricate structures where the length of site exceeds 16 m. In other cases these should be set out by measurement of sides. In rectangular or square setting out, diagonals shall be checked to ensure accuracy.
The setting out of wall shall be facilitated by permanent row of pillars, parallel to and at a suitable distance beyond the Periphery to and at a suitable distance beyond the periphery of the building. The pillars shall be located at junctions of cross walls with the peripheral line of pillars. The centre lines of the cross walls shall be extended and permanently connected to the tops of corresponding pillars.
The datum lines parallel to and at a known distance fixed from the centre lines of external walls should also be permanently set on the rows of pillars to serve as checks on the accuracy of the work as it proceeds.
The tops of pillars shall be at the same level and preferably at plinth or floor level. The pillars shall be of sizes not less than one brick wide and shall be embedded sufficiently deep into the ground so that they are not disturbed.
2.21.2. Protection of excavation - Protection of excavation during construction of shoring, timbering, dewatering operations, etc, shall be ensured.
After excavation, the bottom of the trench shall be cleared of loose soil and rubbish and shall be leveled, where necessary.
Excavations, in clay or other soils, that are likely to be effected by exposure of atmosphere, shall be concreted as soon they are dug. Alternatively, the bottom of the excavation shall be protected immediately by 8 cm thick layer of cement concrete not leaner than mix 1:5:10; the foundation concrete should then be placed on this. Or in order to obtain a dry hard bottom, the last stretch of excavation of about 10 cm shall be removed just before concreting.
The refilling of excavation shall be done with care so as not to disturb the just constructed foundation. The backfill should be carried out evenly on both sides of the wall. The fill shall be compacted in layers not exceeding 20 cm thick, with sprinkling of just enough water necessary for proper compaction.
2.21.3. Types of foundations
(1). Shallow foundations - These cover such types of foundations in which load transfer is primarily through shear resistance to the soil and are normally laid to a depth of 3 m.
The various types of shallow foundations are as under
a) Spread or Pad (see IS: 1080-1986),
b) Strip (see IS: 1080-1986),
c) Raft [see IS: 2950(Part 1)-1981], and
d) Ring and Shell (see IS: 11089-1984 and IS: 9456-1980).
(2). Deep foundations - These foundations are generally in the form of piles, caissons, and diaphragm walls, used separately or in combination to transmit the loads to a deeper load bearing strata. The transfer of load may be through friction, end bearing or a combination of both.
The various types of deep foundations are as under
a) Pile foundations
1) Driven cast in-situ – see IS: 2911 (Part 1/Sec 1)-1979
2) Bored cast in-situ – see IS: 2911 (Part 1/Sec 2)-1979
3) Driven precast – see IS: 2911 (Part 1/Sec 3)-1979
4) Bored precast – see IS: 2911 (Part 1/Sec 4)-1984
5) Timber – see IS: 2911 (Part 2)-1980
6) Under-reamed – see IS: 2911 (Part 3)-1980
b) Diaphragm walls – see IS: 9556-1980
c) Combined foundations – Two or more of the above
2.21.4. Foundations for special structures-These include foundations for machines, towers, etc.
Machine foundations are subject to vibrations. Manufacturer’s instructions, if any, may be followed. Indian standards cover machinery foundations for reciprocating type [see IS: 2974 (Part 1)-1982]; Impact type [see IS: 2974 (Part 2)-1980], Rotary type [see IS: 2974 (Part 3)-1992 and IS: 2974 (Part 4)-1979]; Impact type (other than Hammer) [see IS: 2974 (Part 5)-1987}.
Tower foundations are covered by IS: 4091-1979 for steel towers and by IS: 11233-1985 for Radar, Antenna, Microwave and TV towers.
2.21.5. Construction of foundations
Shallow foundations - In shallow foundations, generally, masonry and/or concrete, plain and reinforced, are used. The procedure for masonry and concrete foundations shall be the same as described in masonry and concrete work.
Pile foundations – Under-reamed piles, though listed under deep foundations also are used for foundations up to 3 m depth.
Under-reamed foundations - Under reamed piles are of bored cast in-situ and bored compaction concrete types having one or more bulbs formed by suitably enlarging the bore hole of the pile stem. With the provision of bulbs, substantial bearing or anchorage is available.
These piles find application in widely varying situations in different types of soils where foundations are required to be taken to a certain depth in view of considerations like the need, (1). To avoid the undesirable effect of seasonal moisture content changes as in expansive soils, (2). To reach firm strata, (3) To obtain adequate capacity for downward, upward and lateral loads and moments; and (4) To take the foundations below scour level. Under reamed piles may also be used in situations where vibrations and noise caused during construction of piles are to be avoided.
The provision of bulbs is of advantage in under reamed piles to resist uplift as they can be used as anchors; increased bearing surface also becomes available.
a) Cement –The cement used shall conform to the requirements of IS: 269-1989 or IS: 455 1989 or IS: 8041-1990 or IS: 6909-1990 or IS: 1489 (Parts 1 and 2)-1991 or IS 12269-1987.
b) Steel – Reinforcement steel shall conform to IS: 432 (Part 1)-1982 ; or IS: 1786-1985 ; or IS 2062 : 1992. For under reamed bored compaction piles, the reinforcement cage shall be prepared by welding the hoop bars to withstand the stresses during compaction process.
c) Concrete - Consistency of concrete for cast in-situ piles shall be suitable to the method of installation of piles.
Concrete shall be so designed or chosen as to have a homogenous mix. Slump of concrete shall range between 100 mm and 150 mm for concreting in water – free unlined bore holes. For concreting by Tremie, a slump of 150 mm to 200 mm shall be used. In case of Tremie concreting of piles of smaller diameter and depth up to 10 m, the minimum cement content should be 350 kg/m3 of concrete. For piles of larger diameter and/or deeper piles, the minimum cement content should be 400 kg/m3 of concrete. In case the piles are subsequently exposed to water or incase piling is done under water or drilling mud is used in methods other than Tremie, 10 percent extra cement shall be used over and above that required for the grade of concrete at specified slump. For making concrete, aggregate, as described in IS 456: 2000 shall be used. For Tremie concreting, aggregates having nominal size more than 20 mm should not be used. For bored compaction piles Rapid Hardening Cement conforming to IS 8041: 1990 shall not be used.
2.21.7. Equipment - Normally the equipment required for manual operations are, a) auger; b) under-reamer; c) boring guide; and d) accessories.
For piles of deeper and larger size greater than 30 cm and a portable tripod hoist with manually operated winch is required.
For piles in high ground water table and unstable soil conditions, boring and under-reaming shall be carried out using suitable equipment. Tremie pipe shall be used for concreting.
For compact piles, the additional equipment required are drop weight for driving the core assembly and pipe or solid core.
2.21.8. Construction - a) Bore holes may be made by earth augers. In case of manual boring, an auger boring guide shall be used to keep the bores vertical or at the desired inclination and in position. After the bore is made to the required depth, enlarging of the bore shall be carried out by means of an under-reaming tool. b) Drilling mud may be used for boring and under-reaming in a site with high water table. Bentonite may be used. c) To avoid irregular shape and widening of bore hole in very loose strata at top, a casing pipe of suitable length may be temporarily used. d) For better under-reamed piles, the reinforcement cage should be placed guiding it by a chute or any other means.
e) In order to achieve proper under-reamed bulb, the depth of bore hole should be checked before starting under-reaming. It should also b checked during under reaming; any extra soil at the bottom of bore hole shall be removed by auger before re-inserting the under reaming tool. f) The completion of the desired under reamed bulb is ascertained by vertical movement of the handle and when no further soil is cut. g) In multi-under-reamed piles, the boring is first completed to the depth required for the first (top) bulb only and after completing under reaming bulb, the boring is extended further down to the second bulb and so on.
h) The piles shall be installed as correctly as possible, both at the correct location and truly vertical (or at the specified batter). Piles shall not deviate by more than 75 mm or one quarter the stem diameter whichever is less; for piles of diameter more than 600 mm, the deviation may be 75 mm or 10 percent of the stem diameter. i) Concreting shall be done as soon as possible after completing the bore. The bore hole full of drilling mud should be concreted between 12 to 24 h depending on the stability of the hole. j) The method of concreting should be such that the entire volume of the pile bore is filled up without formation of voids and/or mixing of soil and drilling fluid in the concrete.
For placing concrete in pile bores, funnel should be used.
In the empty bore holes for under reamed piles a small quantity of concrete is poured to give about a 100-mm layer of concrete at the bottom. Reinforcement is lowered next and positioned correctly. The concrete is poured to fill the bore hole. Care shall be taken that soil is not scrapped from sides if rotting is done for compaction. Vibrators shall not be used.
If the subsoil water level is confined to the bucket length portion at the toe, the seepage is low and the water should be bailed out before commencing concreting.
In case the pile bore is stabilized with drilling mud or by maintaining water head within the bore hole, the bottom of bore hole shall be carefully cleaned by flushing it with fresh drilling mud and the pile bore be checked before concreting.
Concreting shall be done by the Tremie method. The Tremie should have a valve at the bottom and lowered with the valve closed at the start and filled up with concrete. The valve is them opened to permit concrete which permits the upward displacement of drilling mud. The pouring should be continuous and the Tremie is gradually lifted up such that the pipe opening remains always in the concrete. In the final stage the quantity of concrete shall be enough so that on the final withdrawal some concrete spills on the ground .
Notes- 1), All Tremie tubes should be cleaned before and after use.
2). The Tremie pipe should always penetrate well into the concrete with an adequate margin of safety against withdrawal of the pipe.
3). The Tremie method shall not be changed for a given pile, to prevent the laitance from being entrapped in the pile.
4). In the case of withdrawal of a pile accidentally or to remove a choke, the Tremie may be reintroduced in a manner to prevent fragmentation of laitance or scum lying on the top of the concrete deposited already in the bore.
5). In the exceptional case of interruption of concreting, which can be resumed in one or two hours, the Tremie shall not be taken out of concrete. Instead it shall be raised or lowered slowly, from time to time, to prevent the concrete around the Tremie from setting. Concreting should be resumed by introducing a little richer mix of concrete with a slump of about 200-mm for easy displacement of the partly set concrete. If the concreting cannot be resumed before the final set of concrete, the pile may be rejected or used with modifications.
k) In inclined piles, concreting should be done through a chute or by remie method.
l) A bored compaction pile is one in which the compaction of surrounding ground as well as fresh concrete in the bore is simultaneously accomplished. In under reamed bore compaction piles, the pile shall be filled up with concrete, without placing reinforcement. Immediately, the core assembly shall be driven and extra concrete shall be poured in simultaneously to keep the level of concrete up to ground level. If a hollow driving pump is used in core assembly, the pipe shall be withdrawn after filling it with fresh concrete.
m) The top of the concrete pile shall be brought above the cut-off level to permit removal of all laitance and weak concrete before capping and to ensure good concrete at the cut-off level for proper embedment into the pile cap.
n) Where cut-off level is less than 1.5 m below working level, concrete shall be cast to minimum of 300 mm above cut-off level, for every excess of 0.3 m over 1.5 m, additional of 50 mm shall be cast over and above 300 mm.
When Tremie method is employed, it shall be cast to the piling platform level to permit overflow of concrete for visual inspection or to a minimum of 1 m above cut-off level.
When the cut-off level is below the ground water level, there is a need to maintain a pressure on the unset concrete equal to or greater than the water-pressure and a length of extra concrete above the cut-off level may be permitted to provide this.
o) When defective piles are formed, they shall be removed or left in places whichever is convenient, without affecting the performance of adjacent piles or the cap as a whole.
Any deviation beyond permissible limits from the designed location, alignment or load capacity of any pile shall be note d and adequate measures to taken well before the concreting of the pile cap and plinth beam.
The pile should project 50 mm into the cap concrete.
2.22. Precast piles
2.22.1. Bored precast piles - Bored precast concrete piles are constructed in a casting yard and subsequently lowered into pre-bored holes and the space grouted.
As far as possible, in-situ extensions shall be avoided.
The casting yard should be well drained.
As far as possible, longitudinal reinforcement shall be in one length. In case joints are needed, they should be staggered.
The hoops and links for reinforcement shall fit tightly against the longitudinal bars and be bound to them by welding or by tying with binding wire, the free ends of which should be turned into the interior of the pile. The longitudinal bars may be held apart by temporary or permanent spreader forks not more than 1.5 m apart. The reinforcement shall be checked for tightness and position immediately before concreting.
After casting the piles, they shall be stored as described in section 0.
Bored precast piles shall be constructed by suitable choice of boring and installation techniques depending on detailed information about the subsoil conditions. The bottom end of the pile shall have proper arrangements for cleaning and grouting. Piles shall be installed as vertically as possible according to the drawings, or to the specified batter. The deviation from pacified alignment shall be as permitted for under-reamed piles in 3.3.
Cement and sand (1:2) grout mixed with water in a high-speed colloidal mixer is fed to the pile with grout pump of suitable capacity to the central duct through a manifold. Temporary casing used here shall be removed in stages with the rise of level of grout. The grout should be leveled off at the top. The strength of the grout shall be at least equal to the strength of the surrounding soil.
Where a pile is to have another length cast on to it before or during placing, the longitudinal reinforcement should be welded with full penetration butt welding, after the concrete at the top of the pile should be cut-off to expose not less than 200 mm of the bars. Bars may be lapped if it is not possible to undertake butt welding with an overlap of 40 times the dia of bar.
2.22.2 Driven precast piles - Driven precast piles transmit the load of the structure by resistance developed either at the tip or by end bearing or along the shaft by friction or by both. They are cast in a yard and subsequently driven into the ground with or without jetting. These piles find wide application for structures, such as, wharves, jetties, etc, or where conditions are unfavorable for use of cast in-situ piles.
Pile foundations shall be designed in such a way that the load of supports can be transmitted to the soil without any soil failure and without causing settlement as may result in structural damage. It shall withstand all loads (vertical, axial, or otherwise) and moments to be transmitted to the soil.
When working near existing structures care shall be taken to avoid damage to such structures. In case of deep excavations adjacent to piles, proper shoring or other suitable arrangements be provided against lateral movement of soil stratum or releasing the confined soil stress. [For guidance, see IS: 2974 (Part 1)-1982 for effect of vibrations due to reciprocating machines].
The casting yard for all concrete piles shall be so arranged that they can be lifted directly into the piling area. The yard shall have a well drained surface to prevent excessive or uneven settlement during manufacturing and curing.
As far as possible longitudinal reinforcement shall be in one length. In case joints are needed they shall be butt welded and staggered.
The hoops and links reinforcement shall fit rightly against the longitudinal bar and bound to them by mild steel wire or by welding. The bars may be held apart by spreader forks not more than 1.5 m apart. The reinforcement shall be checked for tightness and position before concreting.
After casting the piles, it shall be cured and stored.
Any type of hammer provided they penetrate to the prescribed depth or attain the specific resistance without being damaged may drive the piles. Any change in the rate of penetration, which cannot be ascribed to normal changes in the nature of the ground, should be noted and cause ascertained if possible before driving is continued.
The head of the precast pile should be protected with packing of resilient material.
Piles should be installed as accurately as possible in a pile group. The sequence of installation of pile shall be from centre to the periphery of the group or one side to the other.
For details of manufacture of piles, pile driving, etc, reference may be made to IS 2911 (Part 1/Sec 3)-1979.
2.22.3 Cast in-situ piles
Cast in-situ driven piles transmit load to the soil by resistance developed by the toe of the pile or by end bearing or by friction along their surface or by both.
(1) Materials – (see in 3.3.1)
(2) Equipment – Among the commonly used plants, tools and accessories the suitability depends on subsoil conditions, manner of operation, etc., and some commonly used equipment are:
a) Dolly – A cushion or hardwood or suitable material placed on top of the casing to receive hammer blows;
b) Drop hammer – Hammer (rams or monkey) raised by a winch and allowed to fall under gravity;
c) Single or double acting hammer – A hammer operated by steam or compressed air;
d) Kentledge – Dead weight used for applying a test load to a pile; and
e) Pile rig – A fabricated movable steel frame.
a) Concrete – The minimum slump should be 100 mm when the concrete in the pile is not compacted, and shall not in any case be more than 180 mm.
b) Control of alignment – Piles shall be installed as accurately as possible according to the drawings. Permitted deviations shall be as per 3.3.3.
c) Sequence of piling – In a pile group, the sequence of installation of piles shall normally be from centre to the periphery of the group.
No adjacent pile should be driven until the concrete in the pile under construction has set; otherwise the pile may be damaged. The damage is greater in piles driven in compact soils than in loose soils.
In loose sandy soils compaction will increase as the piles are driven. Therefore the order of installing such a pile should be so chosen as to avoid creating a compacted block in the ground, which would prevent further piles being driven.
Similar precautions should be taken in stiff clayey soils and compact sand layers; driving the piles from centre outwards can do this.
However in very soft soils, the driving of piles should be from outside to centre, so that soil is prevented from floating out during driving of piles.
The casing may be jetted out by means of water without impairing the bearing capacity of the pile, stability of the soil and safety of adjoining structure.
The cut off level, formation of laitance, etc shall be dealt with as in 3.3.3.
Defective piles shall be dealt with as in 3.3.3.
2.22.4. Bored piles - The bored cast in-situ piles, of less than 2 500 mm transmit the load to soil by resistance developed either at the tip by end bearing or along the shaft by friction or by both.
(1) Bored cast in-situ piles may be driven by suitable choice of installation techniques; the manner of soil stabilization, that is, using of casing and/or use of drilling mud; manner of concreting, etc. Sufficient information on sub soil conditions is essential to predetermine the details of installation techniques.
(2) Piles shall be installed as accurately as possible as per drawings. Great care shall be taken in installing single pile or a group of two piles. Any deviation from designed location, alignment or load capacity of any pile shall be noted and adequate measures taken well before the concreting of the pile cap and plinth beam.
(3) A minimum length of 1 m of temporary casing shall be inserted in each bored pile. Additional length of temporary casing may be used depending on the conditions of strata, ground water level, etc. Drilling mud of suitable consistency may be used instead of temporary casings to stabilize sides of holes. For marine locations, the piles may be formed with permanent casing (liner).
(4) In case the bored pile is stabilized by drilling mud or by maintaining water heads in the hole, the bottom of the hole shall be cleaned carefully before concreting work is taken up. Flushing of holes before concreting with fresh drilling fluid/mud is preferred.
(5) The specific gravity of the drilling mud shall be consistent. For this periodic samples shall be taken and tested. Concreting shall not be taken up when the specific gravity is more than 1.2. Concreting shall be done by Tremie method in all such cases. The slurry should be maintained at 1.5 m above ground water level if casing is not used.
(6) Concreting may be done by Tremie method or by the use of specially designed underwater placer to permit deposition of concrete in successive layers without permitting the concrete to fall through free water.
(7) Convenience of installation may be taken into account while determining the sequence of piling in a group.
(8) The top of concrete in a pile shall be brought above cut-off level to permit removal of laitance and weak concrete before capping and to ensure good concrete at the cut-off level for proper embedment into the pile cap.
(9) In case defective piles are formed, they shall be removed or left in places whichever is convenient affecting the performance of the adjacent piles or the cap as a whole. Additional piles shall replace them.
(10) Pneumatic tools shall not be used for chipping until seven days after pile casting. Manual chipping of pile top may be permitted after 3 days of casting the pile.
(11) After concreting the actual quantity of concrete shall be compared with the average obtained from actual observations in the case of few piles cast initially. If the quantity is found to be considerably less, special investigations shall be conducted and appropriate measures taken.
2.22.5. Timber piles - Timber piles find extensive use for compaction of soils and also for supporting as well as protecting water front structures. The choice of use of timber piles shall be mainly governed by the site conditions, particularly water table conditions. They are comparatively light for their strength and are easily handled. However, they will not withstand as hard driving as steel or concrete piles. Timber has to be selected carefully and treated as durability and performance would considerably depend upon the quality of the material and freedom from natural defects.
Class of piles - Depending upon the use, piles shall be classified as Class A and Class B.
(1) Class A – For railway and highway bridges, trestles, docks and warehouses. The butt diameter or sides of square shall not be less than 30 cm.
(2) Class B – For foundation work other than specified in Class A and temporary work. Piles used for compaction of ground shall not be less than 100 mm in diameter or side in case of square piles.
Timber species - The species of timber shall conform to IS 3629: 1986. The length of the individual pile shall be specified length ±30 cm for long and ± 60 cm for lengths above 12 m. In case of round piles, the ratio of heartwood diameter to the pile butt diameter shall not be less than 0.8. Both the ends shall be sawn at right angles to the length of the pile and trimming the knots and limbs shall make surface. The timber shall be treated as per IS 401: 1982 on timber preservation.
Control of pile drives - The piles in each bent of a pile shall be selected for uniformity in size to facilitate placing of bracing members.
The pile tip shall be pointed (unless driving is in wholly soft strata) in the form of truncated cone or a pyramid having the end 25 cm2 to 40 cm2 in area and the length shall be 1 ½ to 2 times the diameter or side of a square.
f the driving is to be done in hard material such as stiff clay, gravels, etc, metal shoes of approved design shall be attached to the tip.
To prevent splitting and reduce brooming, the head of the pile should be hooped with a suitable ring or wrapped with wires. The heads of piles shall be further protected by the provision of cushion blocks.
If the piles are required to be formed from two or more lengths, the butting surfaces shall be cut square to ensure contact over the whole cross section of the pile. A thin steel plate placed between the butting surfaces will reduce the tendency to brooming.
The pieces should also be secured with steel tubes or steel flats. Splices in the middle of the pile should be avoided. If it is necessary to obtain increase in size and length of pile by building up sections, the joints should be staggered and the timber members connected by means of bolts or screws. Piles shall be installed as accurately as possible according to drawings.
In a pile group, the sequence of installation of piles shall normally be from center to periphery of the group or from one side to the other. Adjacent piles shall not be damaged when driving a pile; the danger is greater in compact soils than in loose soils.
Driving piles in loose sand tends to compact the sand, which in turn increases the skin friction for friction piles. Therefore the order of installing of such a pile group should avoid creating a compact block of sand pile into which further piles cannot be driven.
Similar precautions have to be taken in case piles have to be driven into stiff clay or compact sand layers. This may be overcome by driving piles from the centre to the periphery or by beginning at a selected edge or working across the group. In case of very soft soils, driving may have to proceed from outside to inside, so that soil is retained from flowing and during operation.
Jetting of cases by means of water shall be carried out if required in such a manner as not to impair the bearing capacity of piles already in place, the stability of the soil or the safety or any adjacent buildings.
Defective piles shall either be removed or left in place as is convenient without affecting the performance of the adjacent piles or the cap as a whole. Additional piles shall be provided to replace the defective piles.
Any sudden change in the rate of penetration, which cannot be ascribed to the nature of ground, shall be noted and its cause ascertained, if possible, before driving is continued.
Handling of piles - Care shall be taken to see that the piles are sufficient numbers of points, properly located to prevent damage due to excessive bending.
Treated piles shall be handled with hemp or manila rope slings or other means of support that will not damage the surface of the wood. c) Dropping, brushing, breaking of fibres and penetrating the surface shall be avoided. d) Sharp pointed tools shall not be used for handling or turning them in leads.
Minor abrasions of the surface of treated piles below cut-off level in the portions, which are to be remaining permanently under water, shall be permitted. Surface of treated piles below cut-off shall not be disturbed by boring holes or driving nails to support temporary material or staging.
Load test on piles - Shall be done as prescribed in IS: 2911 (Part 4)-1985.
2.23. Machine foundations
2.23.1. General - Machine foundations are specialized structures, according to type of machines, namely, rotary, impact, reciprocating, etc. however, a few criteria for construction are listed below.
2.23.2. Criteria for construction
Concrete - The concrete used shall be controlled concrete. The grade of concrete shall be between M15 to M20 for block foundations and M20 for formed foundations. A slump of 50 mm to 80 mm is allowable. The concrete used is of plastic consistency without excessive water. The water cement ratio shall not exceed 0.45, which shall be maintained throughout the concreting of foundation.
Continuous concreting shall be done as far as possible for the entire block, eaving provisions for grouting.
All areas under and adjacent to the foundation shall be well cleaned prior to pouring of concrete. The surfaces except the pockets for grout shall be made rough so as to secure good bond with fresh cement. Cement grout with non-shrinkable additive shall be used where structurally required.
All elements of foundation shall be provided both at top and bottom by two-way reinforcement. Reinforcement shall be provided along the surface in case of block foundations. The amount of reinforcement shall vary between 25 to 50 kg/m3 of concrete as the case may be. The minimum dia shall be 12 mm and maximum shall be 20 mm in order to take care of shrinkage. Concrete cover shall be 75 mm at bottom, 50 mm on sides and 40 mm on top. The finished surface of the foundation shall be leveled before installing the machine. The foundation bolts shall be properly anchored.
Construction joints should be avoided. If needed the plane of the joint shall be horizontal.
The requirements of a construction joint are:
a) Embed dowels of 12 mm to 16 mm dia at 60 mm centers to a depth of at least 30 cm depth; and
b) Before laying fresh concrete, the previously laid surfaces shall be cleaned and roughened and covered by a rich layer of 1:2 cement grout 20 mm thick, concrete should be placed not later than 2 h after the grout is laid.
2.24. Miscellaneous - For field testing of soils reference may be made to SP: 36 (Part 2)-1988.
Some information choice and characteristics of foundations is given in Annexure 2-A.4 Information on improvement of weak soils to carry more loads is given in Annexure 2-A.5.
2.25. Special Structures - Requirements of foundations for special structures shall be as per design and drawings and any requirements specified. By and large IS: 456-2000 may be followed.