DEEP FOUNDATIONS

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.

TYPES OF DEEP FOUNDATION

Deep foundations are required to carry loads from a structure through weak compressible soils or fills on to stronger and less compressible soils or rocks at depth, or for functional reasons. These foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface.

The types of deep foundations in general use are as follows:

1) Basements

2) Buoyancy rafts (hollow box foundations)

3) Caissons

4) Cylinders

5) Shaft foundations

6) Piles

Basement foundations:

These are hollow substructures designed to provide working or storage space below ground level. The structural design is governed by their functional requirements rather than from considerations of the most efficient method of resisting external earth and hydrostatic pressures. They are constructed in place in open excavations.

Buoyancy rafts (hollow box foundations)

Buoyancy rafts are hollow substructures designed to provide a buoyant or semi-buoyant substructure beneath which the net loading on the soil is reduced to the desired low intensity. Buoyancy rafts can be designed to be sunk as caissons, they can also be constructed in place in open excavations.

Caissons foundations:

Caissons are hollow substructures designed to be constructed on or near the surface and then sunk as a single unit to their required level.

Cylinders:

Cylinders are small single-cell caissons.

Shaft foundations:

Shaft foundations are constructed within deep excavations supported by lining constructed in place and subsequently filled with concrete or other pre-fabricated load-bearing units.

Pile foundations:

Pile foundations are relatively long and slender members constructed by driving preformed units to the desired founding level, or by driving or drilling-in tubes to the required depth – the tubes being filled with concrete before or during withdrawal or by drilling unlined or wholly or partly lined boreholes which are then filled with concrete.

The soil tests required for deep foundations are:

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  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 () 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  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.