03-BUILDING FOUNDATIONS



FOUNDATIONS

Foundations are broadly classified into shallow and deep foundations.

In areas which have already been developed, advantage shall be taken of existing local knowledge, records of trial pits, bore holes, etc., in the vicinity, and the behavior of existing structures, particularly those of a nature similar to that of the proposed structure. In such cases, exploration may be limited to checking that the expected soil conditions are those as in the neighborhood.

If the existing information is not sufficient or is inconclusive the site shall be explored in detail in accordance with the principles set in IS: 1892 so as to obtain the knowledge of the type, uniformity, consistency, thickness, sequence and dip of the strata and of the ground water conditions.

SHALLOW FOUNDATION

The shallow foundation shall be of three types:

  1. Spread foundation which includes isolated and combined footing.
  2. Strip foundation for masonry wall as well as a row of columns.
  3. Raft foundation, laid normally to a depth of 3 m.

1. Materials

Generally building foundations are constructed with stone masonry or concrete. The quality aspects of materials are given in sections of concrete and masonry.

2. Workmanship

  1. The excavation for foundation shall be carried out as per the Drawing and section 200 of this manual.
  2. After excavation, the bottom of the trench shall be cleared of all loose soil and rubbish and shall be levelled, wherever necessary. The bed shall be wetted and compacted by heavy rammers to an even surface.
  3. Excavation in clay or other soils that are liable to be affected by exposure to atmosphere shall be concreted as soon as they are dug.
  4. Alternatively the bottom of the excavation shall be protected immediately by 8 cm thick layer of cement concrete not leaner than mix 1:4:8 over which shall come the foundation concrete.
  5. The backfilling of the trench shall be done with care so as not to disturb the constructed foundation, and shall be compacted in layers not exceeding 15 cm thick with minimum quantity of water sprinkled for ensuring proper compaction.
  6. Masonry shall be constructed over the base concrete after curing the base of concrete at least for 3 days. Before laying concrete, the bed of the foundation pit/trench should be thoroughly compacted by manual ramming.

DEEP FOUNDATIONS

Where the bearing soil is either loose or soft, the load from the structure has to be transferred to deeper firm strata. These foundations are classified as deep foundations. Pile foundation is the most commonly used deep foundation for buildings.

1. Pile foundation

  1. Setting out of the main grid lines shall be done. The installation of marker pins at pile positions shall be located from the main grid lines of the proposed structure. Before installation of the pile, the pile position relative to the main grid lines shall be got verified by the Engineer.
  2. The proposed sequence and timing for boring piles should be submitted to the Engineer for avoiding damage to adjacent piles.
  3. The work shall carry out in such a manner and at such times as to minimise noise, vibration and other disturbance in order to comply with current environmental legislation.
  4. The damage will be, or is likely to be, caused to services or adjacent structures, shall submit to the Engineer the proposals for making preconstruction surveys, monitoring movements or vibrations, and minimizing or avoiding such damage.

1.1 Bored cast-in-situ piles

For construction of piles, guidance shall be taken from relevant Part/Section of IS:2911. The following points shall be kept in mind while bored cast-in-situ piles are installed.

1.2 Under-reamed piles

  1. The construction of under-reamed piles shall be as per the specifications in the Drawings or as per the stipulations given in IS: 2911(Part 3).
  2. In order to achieve proper under-reamed bulb, the depth of bore hole should be checked before starting under-reaming. It should also be checked during under-reaming and any extra soil at the bottom of bore hole removed by auger before reinserting the under-reaming tool.
  3. The completion of desired under-reamed bulb is ascertained by  i) the vertical movement of the handle, and  ii) when no further soil is cut.
  4. In double or multi-under-reamed piles, boring is first completed to the depth required for the first (top) under-ream only and after completing the under- reaming, boring is extended further for the second under-ream and the process is repeated.
  5. Under-reamed piles may be constructed by selecting suitable installation techniques at a given site depending on subsoil strata conditions.

TOLERANCES

  1. Greater care shall be exercised in respect of installation of single pile or piles in two pile groups. Unless otherwise specified in the Drawing:
  1. For vertical piles a deviation of 1.5 percent should not normally be exceeded although in special cases a closer tolerance may be necessary.
  2. Piles shall not deviate more than 75 mm or one-tenth of the diameter whichever is more in case of piles having diameter more than 600 mm from their designed positions at the working level of the piling rig.
  3. In the case of a single pile in a column positional tolerance shall not be more than 50 mm (100 mm in case of piles having diameter more than 600 mm).
  1. In case of piles deviating beyond these limits and to such an extent that the resulting eccentricity cannot be taken care of by a redesign of the pile cap of pile ties, the piles should be replaced or supplemented by one or more additional piles.
  2. For piles cast in dry bores using temporary casing and without the use of a permanent lining, pile heads shall be cast to a level above the specified cut-off so that, after trimming, a sound concrete connection with the pile can be made.
  3. The casting level shall be within the tolerance above the cut-off level shown in table 300-1, but shall not be above the original ground level.
  4. No pile shall be cast with its head below standing water level unless approved measures are taken to prevent inflow of water causing segregation of the concrete as temporary casing is extracted, and, where approved by the Engineer, the groundwater level for each pile shall be treated as the cut-off level for the purpose of calculating tolerance.
  5. For piles cast in dry bores within permanent lining tubes or permanent casings, or where their cut-off levels are in stable ground below the base of any casing used, pile heads shall be cast to a level above the specified cut-off so that, after trimming, a sound concrete connection with the pile can be made. The casting level shall be within the tolerance above the cut-off level shown in the table, but shall not be above the original ground level.

Casting Tolerance above cut-off level for piles

Cut-off distance below  commencing surface (H) 

Casting tolerance above cut-off level in metres

0.15 m to 10 m 0.3 + H/12 + C/8

g. For piles cast under water or drilling fluid, the pile heads shall be cast to a level above the specified cut-off so that, after trimming to remove all debris and contaminated concrete, a sound concrete connection with the pile can be made. The casting level shall be within the tolerance above the cut-off level shown in table above, but shall not be above the commencing surface level. Cut-off levels may be specified below the standing groundwater level, and where this condition applies, the bore hole fluid level shall not be reduced below the standing groundwater level until the concrete has set.

TESTING OF PILES

  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.
  1. 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.

Load Test

The testing shall be done as per the procedure laid down in IS: 2911- Part4.

The number of tests may generally be 0.5% of the total number of piles required. The number of the test may be increased up to 2% in a particular case depending upon nature, type of structure and strata condition. However in the case of sandy or clayey strata at least one pile shall be tested.

RELATED CODES:

1. IS:1080-Design and construction of shallow foundations in soils. (Other than raft, ring and shell)

2. IS: 1888-Method of load test on soils.

3. IS: 1904-Design and construction of foundations in soils: General  requirements.

4. IS: 2911-Design and construction of pile foundation.

   (Part 1/Sec.1) Driven cast-in-situ concrete piles.

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

   (Part 1/Sec.3) Driven cast-in-situ concrete piles.

   (Part 1/Sec.4) Bored precast concrete piles.

    (Part 4)Load test on piles.

5. IS:2950-Design and construction of raft foundation.

6. IS:6403-Determination of bearing capacity of shallow foundations.

7. IS: 8009-Calculation of settlement of foundations.

     (Part 1)-Shallow foundations subjected to static symmetrical vertical loads.

     (Part 2)-Deep foundation subjected to static symmetrical vertical loads.

8. IS: 9456-Design and construction of conical and hyperbolic. Paraboloidal  types of Shell foundations.

9. IS: 9527-Design and construction of caissons.

10. IS: 9556-Design and construction of diaphragm wall.

11. IS: 11089-Design and construction of ring foundation.

12. IS: 13094-Selection of ground improvement techniques for foundation in weak soils - guidelines.

13. IS: 14893-Non-destructive integrity testing of piles (NDT) - Guidelines.

14. IS: 15284-Design and construction for ground improvement- guidelines.

   (Part 1)-Stone columns.

   (Part 2)-Vertical drains

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