Soil Tests Required For Deep Foundations

1. While the composition and depth of the bearing layer for shallow foundations may vary from one site to another, most pile foundations in a locally encounter similar deposits. Since pile capacity based on soil parameters is not as reliable from load tests, as a first step it is essential to obtain full information on the type, size, length and capacity of piles (including details of load – settlement graph) generally adopted in the locality. Correlation of soil characteristics (from soil investigation reports) and corresponding load tests (from actual projects constructed) is essential to decide the type of soil tests to be preformed and to make a reasonable recommendation for the type, size, length and capacity of piles since most formulae are empirical.

2. If information about piles in the locality are not available or reliable, it may be necessary to drive a test pile and correlate with soil data.

 

3. Standard penetration test (SPT) to determine the cohesion (and consequently the adhesion) to determine the angle of friction (and consequently the angle of friction between soil and the pile and also the point of resistance) for each soil stratum of cohesion less soil of clip_image001 soil.

 

4. Static cone penetration test (CPT) to determine the cohesion (and subsequently the adhesion) for soft cohesive soils and to check with SPT result for fine to medium sands. Hence for strata encountering both cohesive and cohesion less soils, both SPT and CPT tests are required.

 

5. Vane shear test for impervious clayey soils.

 

6. Un-drained triaxial shear strength of undisturbed soil samples (obtained with thin walled tube samplers) to determine cohesion (c) and angle of internal friction (clip_image002) for clayey soils (since graphs for correlations were developed based on un-drained shear parameter). In case of driven piles proposed for stiff clays, it is necessary to check with the c and clip_image002[1] from remoulded samples also. Drained shear strength parameters are also determined to represent in-situ condition of soil at end of construction phase.

 

7. Self boring pressure meter test to determine modulus of sub-grade reaction for horizontal deflection for granular soils, very stiff cohesive soils, soft rock and weathered or jointed rock.

 

8. Ground water condition and permeability of soil influence the choice of pile type to be recommended. Hence the level at which water in the bore hole remains are noted in the bore logs. Since permeability of clay is very low, it takes several days for water in the drill hole to rise upto ground water table.

 

Ground water samples need to be tested to consider the possible chemical effects on concrete and the reinforcement. Result of the cone penetration test for the same soil show substantial scatter. Hence, they need to be checked with supplementary information from other exploration methods.

 

Pressure meters are used to estimate the in-situ modulus of elasticity for soil in lateral direction. Unless the soil is isotropic, the same value cannot be adopted for the vertical direction.

Source: constructor.org