Soil Tests Required for Shallow and Raft Foundations

Soil Tests for Shallow and Raft Foundations

Soil tests required to determine safe bearing capacity of shallow foundations and raft foundations are discussed here. These tests are as per IS 6403 – 1981.

Apart from ascertaining the highest level ever reached by the groundwater table and tests for classification of soil as per IS 1498 – 1970 based on grain size analysis as per IS 2720 (Part –IV)– 1985, index properties of soil as per IS 2720 (Part-V) – 1985, the following tests are required to determine safe bearing capacity based on shear strength consideration:

1. Standard penetration test as per IS 2131 – 1991 for coarse grained / fine grained cohesionless soils with semi-pervious clayey soils (i.e.  soils with clay upto 30%).

2. Direct shear test (controlled strain) as per IS 2720 (Part – 13) – 1986. Consolidated undrained tests for cohesive and for  soils and consolidated drained tests for cohesion less soils. The results may be compared with standard penetration test / static cone penetration test results.

Since there is escape of pore water during box shear, partial drainage vitiates the consolidated undrained test. Hence this test is not exact for semi-pervious soils such as clayey sands / silts (i.e. with clay more than 15% but less than 30%). For such soils, triaxial tests are required if shear strength is critical criterion.

3. Static cone penetration test as per IS 4968 (Part -3) – 1976 for foundations on non-stiff clayey soils such as fine grained soils (i.e. more than 50% passing through 75 micron sieve). In fine and medium coarse sands such tests are done for correlation with standard penetration test and to indicate soil profiles at intermediate points.

4. Unconfined compressive strength test as per IS 2720 (Part-10) – 1973 for highly cohesive clays except soft / sensitive clays.

5. Vane shear tests for impervious clayey soils except stiff or fissured clays.

6. Triaxial shear tests for predominantly cohesive soils. If shear strength is likely to be critical.

Soil Tests for Shallow Foundations

Tests required to determine allowable bearing pressure for shallow foundations on settlement consideration:

1. Standard penetration test as stated above.

2. Consolidation test as per IS 2720 (Part-15) if the settlement of clayey layer /layers calculated on the basis of liquid limit and in-situ void ratio indicates that settlement may be critical. Consolidation test is not required if the superimposed load on foundation soil is likely to be less than pre-consolidation pressure (assessed from liquidity index and sensitivity or from un-confined compressive strength and plasticity index).

3. Plate load tests as per IS 1888 – 1982 for cohesionless soils and  soils where neither standard penetration test or consolidation test is appropriate such as for fissured clay / rock, clay with boulders etc..

Soil Tests Required for Raft Foundations

(As per Para 3 of IS 2950 (Part-1) – 1981.

Apart from other tests for shallow foundations, the following soil tests are required especially for raft foundations:

1. Static cone penetration test as per IS 4968 (Part-3) – 1976 for cohesionless soils to determine modulus of elasticity as per IS 1888 – 1982.

2. Standard penetration test as per IS 2131 – 1981 for cohesionless soils and  soils to determine modulus of sub-grade reaction.

3. Unconfined compressive strength test as per 2720 (Part -10) – 1973 for saturated but no pre-consolidated cohesive soil to determine modulus of sub-grade reaction.

4. As specified in IS 2950 (Part -1) – 1981¸ plate load test as per IS 1888 – 1982 where tests at Sl. No. – 1 to 3 above are not appropriate such as for fissured clays / clay boulders.

5. In case of deep basements in pervious soils, permeability is determined from pumping test. This is required to analyze stability of deep excavation and to design appropriate dewatering system.