Soil tests required for deep foundation to ensure the bearing capacity of the soil to support the loads from deep foundation. Deep foundations are those where the depth of foundation is generally greater than two times of width of footing (D = 2B).
Deep foundations are required due to various reasons. Read here about why deep foundations are required and types of deep foundations. Types of soil tests for deep foundations are discussed.
Soil tests required for deep foundations
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 performed and to make a reasonable recommendation for the type, size, length and capacity of piles since most formulae are empirical.
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. Generally, the following tests may be required to obtain certain required data:
1. Direct shear test
It is conducted to estimated effective friction angle of cohesionless soil. Nonetheless, there are correlations which are developed to evaluate effective friction angle of cohesionless soil. for more information about shear strength test of soil click here.
2. Standard penetration test (SPT)
It is performed 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 soil.
3. Static cone penetration test (CPT)
This test conducted 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.
4. Vane shear test
it is used to estimated undrained in-situ shear strength of impervious clayey soils.
5. Undrained triaxial shear strength
It is carried out on undisturbed soil samples (obtained with thin walled tube samplers) to determine cohesion () and angle of internal friction () for clayey soils. Procedures for conducting this test is provided by ASTM D 2850-03, 2004.
In case of driven piles proposed for stiff clays, it is necessary to check with the () and () from re-moulded samples. Drained shear strength parameters are also determined to represent in-situ condition of soil at end of construction phase. Details of this test procedure can be found in ASTM D4767-02, 2004.
6. Pressure meter test
It is performed to determine the stress- strain curve of horizontal loading. The equipment is either inserted into pre-drilled borehole or a self-boring pressure meter is used. Unless the soil is isotropic, the same value cannot be adopted for the vertical direction. This test is performed specifically for piles that subjected to lateral loading.
7. Ground water condition and permeability of soil
Ground water condition and soil permeability 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.