01 Jul 97
b. Limitations of proof tests. Many load tests
but the remaining shaft could be sound and adequately
supported by the soil.
performed today are "proof" tests, which are designed to
prove that the pile can safely hold the design load or to
c. Load tests. The only positive way to prove the
determine the design load. Proof tests do not determine the
integrity of a suspected drilled shaft is to perform a load
ultimate capacity so that the pile is often designed to
test. Drilled shafts are often constructed in relatively large
support a higher load than necessary and can cause
sizes and load tests are often not economically feasible.
foundation costs to be greater than necessary. Proof tests
Replacing a suspected drilled shaft is often more
are not adequate when the soil strength may deteriorate
economical than performing the load test.
with time such as from frequent cyclic loads in some soils.
Coral sands, for example, can cause cementation that can
(1) Application. Load tests as described in paragraph 4,
degrade from cyclic loads.
Chapter 6, shall be performed for drilled shafts when
c. Selecting and timing load tests. Load tests are
economically feasible such as for large projects. Results of
load tests can be used to reduce the FS from 3 to 2 and can
always technically desirable, but not always economically
increase the economy of the foundation when performed
feasible because they are expensive. These tests are most
frequently performed to assist in the design of major
structures with large numbers of piles where changes in
(2) Preload. An alternative to load tests is to construct
length, size, and type of pile and installation method can
the superstructure and to preload the structure to determine
provide significant cost savings. The costs of load tests
the integrity of the foundation. This test must be halted
should be compared with potential savings when using
immediately if one or more drilled shafts show more
reduced safety factors permitted with the tests. Factors to
settlement than is anticipated.
be considered before considering load test are:
4. Load Tests
(1) Significance of structure. The type and significance
of a structure could offset the added cost of load tests for a
Field load tests determine the axial and lateral load capacity
complex foundation when the consequences of failure
as a function of displacements for applied structural loads
would be catastrophic.
to prove that the tested pile or drilled shaft can support the
design loads within tolerable settlements. Load tests are
(2) Soil condition. Some subsurface investigations may
also used to verify capacity calculations and structural
indicate unusual or highly variable soils that are difficult to
integrity using static equations and soil parameters. Soil
parameters can be determined by laboratory and in situ
tests, wave equation and pile driving analysis, and from
(3) Availability of test site. Testing should not interfere
previous experience. Load tests consist of applying static
with construction. Load tests should be conducted early
loads in increments and measuring the resulting pile
after the site is prepared and made accessible. The
movements. Some aspects of load tests that need to be
contractor must wait for results before methods and
equipment can be determined and materials can be ordered.
Advantages of completing the testing program prior to
a. Categories of load tests. Types of load tests
construction include discovery of potential and resolution of
performed are proof tests, tests conducted to failure without
problems, determination of the optimum installation
procedure, determination of the appropriate type, length and
instrumentation. Proof tests are not conducted to a bearing
size of the piles. Disadvantages include increased design
capacity failure of the pile or drilled shaft but usually to
time to allow for load tests and testing conditions and data
twice the design load. Tests conducted to failure without
extrated from a test site used in the design may not simulate
actual construction conditions such as excavation,
do not indicate the separate components of capacity of end
groundwater, and fill. Problems may also occur if different
bearing Qbu and skin resistance Qu . Tests with internal
contractors and/or equipment are used during construction.
instrumentation, such as strain gauges mounted on
reinforcement bars of drilled shafts or mounted inside of
(4) Location. Test piles should be located near soil test
pipe piles, will determine the distribution of load carried by
borings and installed piezometers.
skin friction as a function of depth and will also determine
the end-bearing capacity when conducted to failure.
(5) Timing. Load tests of driven piles should be
performed after 1 or more days have elapsed to allow