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.