DHARMPILE FOUNDATIONS 669Engineering News formula and the Danish formula have gained popularity owing
to their simplicity.- If the pile is driven into saturated loose sand and silt, liquefaction might result,
reducing the pile capacity. - Dynamic pile driving formulae are not considered to be applicable to piles driven
into cohesive soils. This is because there may be apparent increase in driving resist-
ance due to the development of pore pressures, while there may be a tendency for it
to decrease later on due to dissipation of pore pressure. - Pile driving in cohesive soils disturbs the soil structure and consequently the resist-
ance tends to decrease due to remoulding in sensitive clays although there might be
some regaining of strength with passage of time, due to thixotropy.
Thus, it is evident that some degree of caution and judgement are called for in the use of
these formulae.
16.5.3Load Test on Pile
Load test on a pile is one of the best methods of determining the load-carrying capacity of a
pile. It may be conducted on a driven pile or cast-in-situ pile, on a working pile or a test pile,
and on a single pile or a group of piles. A working pile is one which forms part of the founda-
tion, while a test pile is one which is used primarily to check estimated capacities (as predeter-
mined by other methods).
The aim of a pile load test is invariably to determine the vertical load capacity; however,
in certain special cases the test may be used to obtain the uplift capacity or lateral load capac-
ity. Load test on a pile group is expensive and may be conducted only in the case of important
projects.
Both cohesive and cohesionless soils will have their properties altered by pile driving.
In clays, the disturbance causes remoulding and consequent loss of strength. With passage of
time, much of the original strength will be regained. The effect of pile driving in sand is to
create a temporary condition wherein extra resistance is developed, which is lost later by
stress relaxation. Hence, the test should be conducted only after a lapse of a few weeks in clays
and at least a few days in sands, in order that the results obtained be more meaningful for
design.
Load may be applied by using a hydraulic jack against a supported platform (Fig. 16.6a),
or against a reaction girder secured to anchor piles (Fig. 16.6b). Sometimes a proving ring is
preferred for better accuracy in obtaining the load. Instead of reaction loading, gravity loading
may also be used; but the former is given better uniformity in loading. Measurement for pile
settlement is related to a fixed reference mark. The support for the reference mark has to be
located outside the zone that could be affected by pile movements.
The most common procedure is the test in which the load is maintained slowly. About
five to eight equal increments are used until the load reaches about double the design value.
Time-settlement data are recorded for each load increment. Each increment is maintained
until the rate of settlement becomes a value less than 0.25 mm per hour. The final load is
maintained for 24 hours.