Geotechnical Data and Pile Design - Offshore Structures: Design, Construction and Maintenance - page 251

The designer should be aware of the many limitations of such comparisons
when making use of the results. Limitations of particular importance include:
1. There is considerable uncertainty in the determination of both predicted
capacities and measured capacities. For example, determination of the pre-
dicted capacities is very sensitive to the selection of the undrained shear
strength profile, which itself is subject to considerable uncertainty. The mea-
sured capacities are also subject to interpretation as well as possible mea-
surement errors.
2. Conditions under which pile load tests are conducted generally vary signif-
icantly from design loads and field conditions. One clear limitation is the
limited number of tests on deeply embedded, large-diameter, high-capacity
piles. Generally, pile load tests have capacities that are 10% or less of the
prototype capacities. Briaud et al. (1984) mentioned that another factor is
that the rate of loading and the cyclic load history are usually not well repre-
sented in load tests. According to Clarke (1993) , pile load tests are often
conducted before full set-up occurs, for practical reasons. Furthermore,
pile-tip conditions (closed versus open-ended) may differ from offshore
piles.

3. In most of the studies, an attempt has been made to eliminate the factors that

are thought to be significantly affected by extraneous conditions in load test-
ing, such as protrusions on the exterior of the pile shaft (weld beads, cover
plates, etc.), installation effects (jetting, drilled out plugs, etc.), and artesian
conditions, but it is not possible to be absolutely certain in all cases.

The tests most relevant to offshore applications have all been conducted in
the United States or in Europe. As regional geology and particularly operating
experience are considered very important in foundation design, care should be
exercised in applying these results to other regions of the world. In addition, the
designer should note that certain important tests in silty clays of low plasticity
indicate overprediction of frictional resistance by Equations (4.14) and (4.15) .
The reason for the overprediction is not well understood and has been an
area of active research. The designer is thus cautioned that pile design for
soils of this type should be given special consideration.
Additional considerations that apply to drilled and grouted piles are dis-
cussed by Kraft and Lyons (1974) and O

Neill and Hassan (1994) .
Pile load tests are commonly used as a basis for determining pile load-
movement characteristics. In clay, the ultimate capacity of the pile, as shown
in Figure 4.7 , reaches a maximum value at some movement, beyond which
there is a gradual drop to a residual value.
The frictional resistance increases rapidly and reaches a maximum value at a
very small displacement, referred to as the critical movement. However, the point
resistance continues to increase beyond this critical movement and tends to reach
a maximum value at a relatively larger movement. This maximum value is
referred to as the end-bearing capacity.

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