08-FOUNDATIONS



FOUNDATIONS (BRIDGES)

1. GENERAL

Quality aspects in the construction of foundation for bridge structures are discussed in this section. The work may be carried out in accordance with the details shown in the Drawings and conforming to the requirements stipulated in IRC: 78 and MoRTH Specifications as applicable or as directed by the Engineer. The materials shall comply with Section 3300 of this Manual.

2. OPEN FOUNDATION

Open foundations are provided where the foundations can be laid in a stratum which is in erodible or where the extent of scour of bed is reliably known and the work is constructed with plain or reinforced concrete.

2.1 Workmanship

  1. Excavation for laying the foundation shall be carried out in accordance with Section 3100 of this Manual. The bottom 300 mm of excavation may preferably be done just before laying the levelling course below foundation.
  2. In the event of excavation having been made deeper than that shown in the Drawing or as ordered by the Engineer, the extra depth shall be made up with M10 concrete in case of foundation resting on soil and with concrete of the same grade as that of the foundation, in case of foundation resting on rock. This shall be done at the cost of the Contractor unless specified in the Contract.
  3. Open foundations shall be constructed in dry conditions and the Contractor shall provide for adequate dewatering arrangements, wherever required, to the satisfaction of the Engineer.
  4. For foundation resting on soil, a levelling course of M10 concrete of minimum thickness 100 mm or as specified in the Drawing shall be provided above the natural ground to support the foundation concrete. No construction joint shall be provided in the levelling course.
  5. For foundations resting on rock, the rock surface shall be cleaned of any loose material and then levelled with a layer of concrete of the same grade as that of the foundation, so as to provide an even surface. In case of sloping rock profile, the rock surface can be prepared by providing suitable benching.
  6. Levels of the surface shall be taken at suitable intervals to check the precision of work.
  7. Side formwork shall be used for foundation concrete work. When concrete is laid in slope without top formwork, the slump of the concrete shall be carefully maintained to ensure that compaction is possible without slippage of freshly placed concrete down the slope.
  8. Foundation concrete of required dimensions and shape shall be laid continuously up to the location of construction joint shown in the Drawing or as directed by the Engineer.
  9. Formwork shall not be removed earlier than 24 hours after placing of concrete. Where formwork has been provided for slanting surface of the footing, the same shall be removed as soon as concrete has hardened.
  10. Curing of concrete shall be carried out by wetting of formwork before removal. After its removal, curing shall be as given in Section 3300 of this Manual. Alternately curing may also be carried out by laying not less than 100 mm thickness of loose moistened sand, free from clods or gravel, over the concrete. The sand shall be kept continuously moist for a period of 7 days. Before backfilling is commenced, the loose sand shall be removed and disposed of or as directed by the Engineer.
  11. In situations where foundations cannot be laid dry, concrete shall be laid under water only by tremie and no pumping of water shall be permitted from the time of placing of concrete up to 24 hours after placement.
  12. Where blasting is required, it shall be carried out in accordance with Section 3100 of this Manual.
  13. All spaces excavated and not occupied by the foundations or other permanent works shall be backfilled with earth up to surface of surrounding ground with sufficient allowance for settlement. Backfilling shall be in accordance with Section 3100 of this Manual.
  14. In case of rock excavation, the backfill up to 1.5 m in soft rock or 0.6 m in hard rock, the annular space around the foundation shall be filled with M15 concrete up to the top level of rock.

If the depth of backfill required is more than 1.5 m in soft rock or 0.6 m in hard rock above the foundation level, then backfill shall be made up to this level by M15 concrete and the portion above shall be filled up with a leaner concrete or with boulders grouted with cement.                                                                                                                                                                                                                                                                                            

 
  1. Fig. : Open foundation on sloping rock profile.
  2. For guidance regarding safety precautions to be taken IS: 3764 may be referred to.                            

2.2 Tolerances

The following tolerances are permitted for open foundations.

a) Variation in dimensions : +50 mm/-10 mm.

b) Misplacement from specified position in plan : ±15 mm.

c) Surface unevenness measured with 3 m straight edge: ±5 mm.

d) Variation of levels at the top : ±25 mm.

3. PILE FOUNDATION

Piles transmit the load of a structure to competent sub-surface strata by the resistance developed from bearing at the toe or skin friction along the surface or both. The piles shall be required to carry vertical downward or upward (uplift) loads and lateral loads.

3.1 Requirements/Workmanship

  1. For construction of piles, guidance shall be taken from IS: 2911 (With latest amendments) subject to limitations/stipulations given in IRC: 78 and MoRTH  Specifications.
  2. For both precast and cast-in-situ piles, the grade of concrete, minimum cement content, water to cement (w/c) ratio and slump at the time of placement shall be as per table.

Table: Requirements for concrete for pile foundation

Item

 Bored cast in –

situ piles

Precast piles

Grade of concrete

M35

M35

Minimum cement content

400 kg/m3

400 kg/m3

Maximum w/c ratio

0.4

0.4

Slump

150 - 200

50 - 75

 

  1. High  alumina  cements  (quick  setting  cement)  shall  not  be  used  in  marine  conditions.
  2. When both chlorides and sulphates are present, in soil or ground water, sulphate resistant cement shall not be used. For improving resistance against the penetration of chlorides and sulphates, mineral admixtures such as fly-ash, silica fumes as per IRC: 112 shall be used.

3.2 Precast concrete piles

Manufacture and driving of precast/pre-stressed concrete piles shall conform to the guidelines contained in IS: 2911, IRC: 78 and MoRTH Specifications.

3.3 Bored cast-in-situ concrete piles

  1. Boring shall be carried out using rotary equipment. Percussion type of equipment shall be used only if approved by the Engineer. The capacity of rig shall be adequate so as to reach the required depth.
  2. A continuous record shall be kept by the Engineer as to the volume of concrete placed in relation to the length of pile that is cast.
  3. Defective piles shall be removed or left in place as judged convenient without affecting performance of adjacent piles or pile cap. Additional piles shall be provided to replace defective piles.
  4. Uncased cast in-situ piles shall not be allowed where mud flow conditions exist. The drilling mud such as bentonite suspension shall be maintained at a level sufficiently above the surrounding ground water level throughout the boring operation.
  5. Chiselling shall be carried out within the borehole in case of any underground obstruction or hard strata with the approval of the Engineer.
  6. In case pile is to be socketed into medium or hard rock, the equipment mobilised shall have adequate capability to do so up to the required socket length as per the Drawing below.

                     a) Normal pile                                               b) Rock socketed pile

Fig.: Founding of bored piles in hard rock

  1. Prior to the placing of the reinforcement cage, the pile shaft shall be cleaned of all loose materials. Inadequate cleaning of the base can lead to formation of a soft base or soft toe which may result in reduction of load bearing capacity of the pile.
  2.  Pile reinforcements as shown in the Drawing shall be tied in place to form a cage which is lowered into the pile shaft. Reinforcement at the bottom shall not be provided with L-bends as these may interfere with cleaning of the pile base.
  3. Where concrete is placed in dry and a casing is present, the top 3 m of the pile shall be compacted using internal vibrators. Where mud flow conditions exist, the casing of cast in-situ piles shall not be allowed to be withdrawn.
  4. Concrete shall be properly graded, shall be self compacting and shall not get mixed with soil, excess water, or other extraneous matter. Care shall be taken in silty clays and other soils which have the tendency to squeeze into the newly deposited concrete and cause necking. Sufficient head of green concrete shall be maintained to prevent inflow of soil or water into the concrete.
  5. The slump of concrete shall be in the range between 100 mm to 180 mm depending on the manner of concreting. 

Table: Stump of concrete for pile Foundation

Slump (mm)

Condition of concreting

Minimum

Maximum

 

100

180

- Pouring into water free unlined bore.

- Reinforcement is not spaced widely.

150

180

- Concrete is to be placed under water

or drilling mud by tremie.

Bored cast in-situ piles in soils which are stable, may often be installed with only a small liner length at the top. A minimum of 2 m length of top of bore shall invariably be provided with casing to ensure against loose soil falling into the bore.

Permanent steel liner shall be provided at least up to maximum scour level. The minimum thickness of steel liner shall be 6 mm.

Permanent steel liner shall be provided for the required depth of the pile in the following situations:

 i) The surrounding soil is marine clay.

ii) Soft soil is present.

iii) Under ground water flow is predominant to cause loss of cement.

iv) Surrounding soil has sulphate content equal to or more than 1%.

v) Surrounding water has sulphate content equal to or more than 2500 ppm.

vi) Leakage of sewage is expected.

  • Where bored cast in-situ piles are used in soils liable to flow, the bottom of the casing shall be kept sufficiently in advance of the boring tool to prevent the entry of soil into the casing leading to formation of cavities and settlements in the adjoining ground. The water level in the casing should generally be maintained at the natural ground water level for the same reasons.
  • The diameter of the boreholes shall not be more than the inside diameter of the liner when the liners are installed before boring. When the liners are installed after boring, the diameter of the boreholes shall not be more than the outside diameter of liner +2 mm, unless otherwise approved by Engineer.
  • The concrete should invariably be poured through a tremie with a funnel, so that the concrete can be properly deposited in the hole without segregation.
  • It is desirable that the concrete above cut-off level is removed before the concrete is set. This may be done manually or by specially made bailer or other device. Such removal of concrete helps in preventing the damage of the good concrete below the cut-off level, which results from chipping by percussion method. The removal of concrete shall be within ±25 mm from the specified cut off level, preferably on the minus side.

3.4 Pile cap

  1. In marine conditions or areas exposed to harmful chemicals, the pile cap shall be protected with a coating such as bituminous based coal tar, epoxy or epoxy based coating or with suitable anti-corrosive paint.
  2. Concrete with high alumina cement, shall not be used in marine environment.

3.5 Tolerances

The following tolerances are permitted for pile foundation as per table.

Sl. No.

Type of pile

Limits prescribed

I.

Precast Concrete piles:

 

 

a)

Variation in cross-sectional dimensions

±5 mm

 

b)

Variation in length

±25 mm

 

c)

Surface irregularities measured with 3 m straight edge

5 mm

 

d)

Bow for total length of pile in mm

1 mm/m length of pile

limited to 20 mm

II.

Driven piles:

 

 

a)

Variation in cross-sectional dimensions

+50 mm/-10 mm

 

b)

Variation from vertical for vertical piles

1 in 150

 

c)

For vertical piles deviation at piling platform level

75 mm

 

d)

Variation of level of top of piles

±25 mm

III.

Bored piles:

 

 

a)

Variation in cross-sectional dimensions

+50 mm/-10 mm

 

b)

Variation from vertical for vertical piles

1 in 150

 

c)

For vertical piles deviation at piling platform level

75 mm

 

d)

Variation of level of top of piles

±25 mm

IV.

Pile caps

 

 

a)

Variation in dimensions

+50 mm/-10 mm.

 

b)

Misplacement from specified position in plan

15 mm

 

c)

Surface irregularities measured with 3 mm straight edge

5 mm

 

d)

Variation of level of top of piles

±25 mm

3.6. Testing of piles

   3.6.1 Initial load test on test pile

  1. Test piles which are used to arrive at the load carrying capacity shall not be incorporated in the structure.
  2. All test piles shall be installed with the same type of equipment that is proposed to be used for piling in the actual structure.
  3. Test piles which are not to be incorporated in the completed structure shall be removed to at least 600 mm below the proposed soffit level of pile cap and the remaining hole so formed shall be backfilled with earth or other suitable material.
  4. The procedure for testing of pile shall be as per IS: 2911 (Part 4).

   3.6.2 Routine load test on working pile

  1. Pile load test for vertical load capacity of pile to be conducted as per Section 300 in Part A of this Manual.
  2. Lateral load tests shall be carried out for ensuring the lateral load capacity of the piles. The test procedure shall be carried out as per IS: 2911 (Part 4). However the permissible deflection shall be as per IRC: 78.
  3. The minimum number of piles to be tested shall be as per table in cases where it is not specified in the Drawing. The test pile shall be one of the working piles as decided by the Engineer.
  4. In particular cases where upper part of pile is likely to be exposed later due to scour, then the capacity contributed by that portion of the pile during load test, shall be accounted for.
  5. Results of routine load tests done for reconfirm the allowable loads shall not be used for upward revision of design capacity of piles.

Table:  Minimum number of tests

Total number of piles

for the bridge

Minimum number of test piles

Up to 50

2

50-150

3

Beyond 150

2% of total piles (fractional number

rounded to next higher integer number)

 While conducting routine test on one of the piles belonging to a pile group, if the pile capacity is found to be deficient an overload up to 10% of the capacity may be allowed.

 3.6.3 Dynamic pile load test

  1. For a quick assessment of pile capacity, strain dynamic tests as prescribed in IRC: 78 may be conducted after establishing co-relation using the results of load tests.
  2. The results of strain dynamic tests shall not be used for upward revision of design pile capacity.
  3. If there exists any doubt about the quality of concrete and construction defects like voids, discontinuities etc, the Engineer shall insist to conduct pile integrity tests as per the guidelines given in IRC: 78.

 3.6.4 Pile integrity test

  1. Pile integrity test is a non-destructive integrity test method for pile foundation.
  2. The method evaluates continuity of the pile shaft and provides information on any potential defects due to honeycombs, necking, cross sectional reduction, potential bulbs, sudden changes in soil stratum, concrete quality in terms of wave speed etc.
  3. The test procedure is standardised as per ASTM D 5882.
  4. The number of tests shall be decided by the Engineer.

4.  WELL FOUNDATION

Well foundations shall be constructed taking it down to the founding level by sinking through all kinds of substrata, plugging the bottom, filling the well with approved material, plugging the top and providing a well cap, in accordance with the details shown in the Drawings and as per the Specifications, or as directed by the Engineer.

Typical cross sectional details of well foundation resting on sandy soil and hard rock are shown in figure.

b) Resting on sloping rock surface                                                                  

a) Resting on sandy strata            

c) Resting on levelled rock surface

Fig. : Founding details of well foundation.

4.1 Sinking of well

  1. In case blasting is anticipated for facilitating sinking through difficult strata, special protective/strengthening measures for the curb and steining of the well will be required.
  2. Pneumatic sinking may have to be resorted to in cases where there are obstacles such as tree trunks, large sized boulders etc., which cannot be removed by open dredging.
  3. Reference points shall be accurately fixed and it shall be away from the zone of blow-ups or possible settlements which may result from well sinking operations and shall be connected to the permanent stations with the base line on the banks. Individual wells shall be marked with reference to these stations. A temporary benchmark shall be established in addition to this near the well foundation.
  4. For wells in dry area, the bed may be prepared by excavating the soil up to 1.5m, followed by levelling and dressing before placing the cutting edge. For wells in water, an earthen island up to 5 m maximum height shall be constructed for laying the cutting edge and well curb. For locations where soil is too weak to sustain earthen island, floating caissons may have to be adopted.
  5. In earthen islands, a working space of at least 2.0 m all around the steining of the well shall be available. The earthen island shall be held in position and protected against scour by means of wooden ballies properly braced or sheet piles.

4.2 Cutting edge

  1. In V-shaped cutting edge, the inclined plate should meet the vertical plate in such a way that full strength connection by welding is feasible.
  2. The parts of cutting edge shall be erected on level firm ground about 300 mm above prevalent water level. Steel sections shall not be heated and forced into shape. However, "V" cuts may be made in the horizontal portion, uniformly throughout the length, to facilitate cold bending. Joints in the lengths of structural sections, unless otherwise specified, shall be fillet welded using single cover plate to ensure the requisite strength of the original section.

4.3 Well curb

  1. Concreting of the well curb shall be done in one continuous operation.
  2. The formwork on outer face of curb may be removed within 24 hours after concreting while that on inner face shall be removed only after 72 hours.
  3. In case blasting is anticipated, the inner face of the well curb shall be protected with steel plates of thickness not less than 10 mm up to the top of the well curb.
  4. If considered necessary, the inner face of steining may also be protected with steel plates of 6 mm thickness up to a height of 3 m above the top of the well curb or as specified by the Engineer.

4.4 Well steining

  1. Concreting of steining may be carried out in subsequent lifts of about 2 m to 2.5 m. Attempts should be made to minimize the number of construction joints. The concreting layers shall be limited to 450 mm restricting the free fall of concrete to not more than 1.5 m. Laitance formed at the top surface of a lift  shall be removed to expose coarse aggregates before setting of concrete at the proposed construction joint. As far as possible, construction joints shall not be kept at the location of laps in the vertical steining bars.
  2. The steining of the well shall be built in one straight line from bottom to top such that if the well is titled, the next lift of steining will be aligned in the direction of the tilt. After sinking of a stage is complete, damaged portions, if any, of steining at top of the previous stage shall be properly repaired before constructing the next stage.

4.5 Well sinking

  1. The well shall be sunk true and vertical through all types of strata. No well shall be permitted to be placed in a pre-dredged hole. Sinking or loading of the well with kentledge shall be commenced only after the steining has been cured for at least 48 hours or as specified in the Drawings.
  2. The well shall be sunk by excavating the material uniformly from inside the dredge hole. Use of water jetting, explosives and divers may be adopted for sinking of wells through difficult strata, with prior approval of the Engineer. Dewatering shall be done when well is to be founded in rock.
  3. Complete history of sinking of each well giving details of concreting, sinking and problems met, if any, shall be maintained in the format given in MoRTH Specifications.
  4. Dewatering of the well shall not be carried out if sand blows are expected. Sand blow can often be minimised by keeping the level of water inside the well higher than the water table and also by adding heavy kentledge.
  5. Kentledge shall be placed in an orderly and safe manner on the loading platform and in such a way that it does not interfere with the excavation of the material from inside the dredge hole and also does not in any way damage the steining of the well.
  6. Water jetting can be used to facilitate sinking of wells through clay/hard strata. For carrying out water jetting, the required number of steel pipes of 40 mm to 50 mm diameter shall be embedded in the steining of the well, spaced evenly around its periphery. The diameter of the nozzle shall be 6 mm. The steel pipe shall be kept about 1 m above the top of each lift of steining, so that it can be extended by means of suitable couplers before the next lift of steining is cast.
  7. When water jetting is to be adopted, the Contractor shall furnish a method statement for approval of the Engineer covering all aspects of the work including the number, capacity and location of the high pressure pumps and other ancillaries required for executing the work.
  8. For use of jack down method of sinking, the Contractor shall furnish a method statement for approval of the Engineer, giving full details of construction of ground anchors, fabrication of pressurising girder, type, number and capacity of jacks to be used, method of dredging and application of jack down force and all other relevant aspects for proper execution of the work.
  9. All safety precautions shall be taken as per IS: 4081 and/or IS: 4138. Protection of the bottom portion of the well shall be done as per MoRTH Specifications.
  10. Blasting of any sort shall be done only with prior permission and in the presence of the Engineer. Blasting shall not be done before the concrete in the steining has hardened sufficiently and more than 7 days old. Use of large charges, 0.7 kg or above may not be allowed, except under expert direction and with the permission of the Engineer.
  11. The pattern of charges may be suitably arranged with delay detonators so as to reduce the number of charges fired at a time. The burden of the charge may be limited to 1 m and spacing of holes may normally be kept as 0.5 m to 0.6 m.
  12. If rock blasting is to be done for seating of the well, the damage caused by flying debris should be minimised by covering blasting holes with rubber mats before detonating the charge.
  13. Separate high pressure connection shall be made for use of pneumatic tools. Electric lights where provided shall be at 50 volts (maximum).
  14. Pneumatic sinking shall be limited to a depth of 30 m below ground level.
  15. A pair of wells close to each other has a tendency to come closer while sinking. Timber struts may be introduced in between the steining of these wells to prevent such movement.
  16. Tilts occurring in a well during sinking in dipping rocky strata can be controlled by suitably supporting the curb.
  17. Diameter of sump made in hard rock shall be 1.5 m to 2 m less than that of the dredge hole.
  18. Before concreting the bottom plug, it shall be ensured that the inside faces of curb and steining have been cleaned thoroughly.
  19. Any dewatering required, shall only be done 7 days after casting of bottom plug.
  20. The soundness of the bottom plug may be tested by dewatering the well to a level 5 m below the surrounding water level and checking the rise of water.
  21. If necessary, filling inside the well shall commence 7 days after laying of bottom plug with sand or excavated material free from organic matter.
  22. Well cap concreting shall be carried out in dry condition. A properly designed false steining may be provided if required, to ensure that the well cap is laid in dry condition.

4.6 Precautions during well sinking

The following precautions shall be taken during sinking of well foundation:

  1. When clear distance between wells is less than the diameter of wells, sinking shall be taken up on all wells and they shall be sunk alternately so that the sinking proceeds uniformly. The difference in the levels does not exceed half the clear gap between them. Plugging of all the wells shall be done together.
  2. The type of soils obtained during the well sinking should be compared with bore chart so as to take prompt decisions.
  3. Before seasonal floods, all wells on which sinking is in progress shall be sunk to sufficient depths below the designed scour level. Further, they shall be temporarily filled and plugged so that they do not suffer any tilt or shift during the floods.
  4. All necessary precautions shall be taken against any possible damage to the foundations of existing structures in the vicinity of the wells, prior to commencement of dredging from inside the well.
  5. Very deep sump shall not be made below the well curb, as it entails risk of jumping (sudden sinking) of the well. The depth of sump shall be generally limited to one-sixth of the outer diameter/least lateral dimension of the well in plan. Normally the depth of sump shall not exceed 3.0 m below the level of the cutting edge unless otherwise specially permitted by the Engineer.
  6. In case a well sinks suddenly with a jerk, the steining of the well shall be examined to the g) Dewatering shall be avoided if sand blows are expected. Any equipment and men working inside the well shall be brought out of the well as soon as there are any indications of a sand-blow.
  7. Sand blowing in wells can often be minimised by keeping the level of water inside the well higher than the water table and also by adding heavy kentledge.
  8. In soft strata prone to settlement/creep, the construction of the abutment wells shall be taken up only after the approach embankment for a sufficient distance near the abutment, has been completed.

4.7 Tolerances

  1. The permissible tilt and shift shall not exceed 1 (horizontal) in 80 (vertical) and the shift at the well base shall not be more than 150 mm in any resultant direction.
  2. For the purpose of measuring the tilts along the two axes of the bridge, reduced level of the marks painted on the surface of the steining of the well shall be taken. For determination of shift, locations of the ends of the two diameters shall be precisely measured along the two axes, with reference to fixed reference points.
  3. For the well steining and well cap, the permissible tolerances shall be as given in table.

      Table: Permissible tolerances for well foundation

a)

Variation in dimension

+50 mm/-10 mm.

b)

Misplacement from specified position in plan

15 mm.

c)

Surface unevenness measured with 3 m straight edge

5 mm.

d)

Variation of level at the top

±25 mm.

 

5. SAMPLING AND TESTING

The materials and work shall be tested in accordance with the relevant sections of this Manual and shall meet the prescribed standards of acceptance.

RELATED CODES:

1.IRC: 78-Standard specification and code of practice for road bridges: Foundations and substructures.

2.IS: 2911-Code of practice for design and construction of pile foundations.

     (Part-1/Sec. 2)-Bored cast in situ piles.

     (Part-4)-Load test on piles.

3.ASTM D 5882-Standard test method for low strain integrity testing of piles.

4.MoRTH-Specifications for Road and Bridge works (5th revision).

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