General
The pile load test is intended to validate the computed
capacity for a pile foundation and also to provide information for the improvement
of design rational. Therefore, a test to pile failure or soil/ pile failure
should be conducted in lieu of testing to a specified load of termination.
Data from a test should not be used to lengthen or shorten
piles to an extent that their new capacities will vary more than 10 percent
from the test load. Finally, if the pile tests are used to project pile
capacity for tip elevations other than those tested, caution should be
exercised.
In a complex or layered foundation, selecting a tip
elevation for the service piles different from the test piles may possibly
change the pile capacity to values other than those projected by the test. As
an example, shortening the service piles may place the tips above a firm
bearing stratum into a soft clay layer.
In addition to a loss in bearing capacity, this clay layer
may consolidate over time and cause a transfer of the pile load to another
stratum. Lengthening the service piles may cause similar problems and actually
reduce the load capacity of the service piles if the tips are placed below a
firm bearing stratum.
Also, extending tips deeper into a firmer bearing may cause
driving problems requiring the use of jetting, predrilling, etc. These
techniques could significantly alter the load capacity of the service piles
relative to the values revealed by the test pile program. A pile load testing
program ideally begins with the driving of probe piles (piles driven at
selected locations with a primary intention of gaining driving information) to
gain knowledge regarding installation, concentrating their location in any
suspect or highly variable areas of the foundation strata.
Test piles are selected from among the probe piles based
upon evaluation of the driving information. The probe and test piles should be
driven and tested in advance of the construction contract to allow hammer
selection testing and to allow final selection of the pile length. Upon
completion of the testing program, the probe/test piles should be extracted and
inspected.
The test piles, selected from among the probe piles driven,
should be those driven with the hammer
selected for production pile driving if at all possible. In
some cases different hammers will produce piles of different ultimate capacity.
Additionally, use of the production hammer will allow a correlation between
blow count and pile capacity which will be helpful during production pile
driving.
The pile driving analyzer should be used wherever possible
in conjunction with the probe/test piles. This will allow the pile driving
analyzer results to be correlated with the static tests, and greater reliance
can be placed upon future results when using the analyzer for verifying the
driving system efficiency, capacity, and pile integrity for production piles.
Safety Factor for Design
It is normal to apply safety factors to the ultimate load
predicted, theoretically or from field load tests. These safety factors should
be selected judiciously, depending upon a number of factors, including the
consequences of failure and the amount of knowledge designers have gained
relative to the subsurface conditions, loading conditions, life of the
structure, etc.
Basis for Tests
A pile loading test is warranted if a sufficient number of
production piles are to be driven and if a reduced factor of safety (increased
allowable capacity) will result in a sufficient shortening of the piles so that
a potential net cost savings will result. This is based upon the assumption
that when a test pile is not used, a higher safety factor is required than when
test piles are used.
If very few piles are required, longer piles as required by
the higher factor of safety (3.0) may be less expensive than performing a pile
load test, reducing the factor of safety to 2.0, and using shorter piles.
Pile load tests should also be performed if the structure
will be subjected to very high loads, cyclic loads of an unusual nature, or
where highly variable soil conditions exist. Special pile load tests should be
performed to determine soil parameters used in design when the structure is
subject to large dynamic loads, such as large reciprocating machinery,
earthquakes, etc.
Test Location
The pile load test should be conducted near the base of the
structure with the excavation as nearly complete as possible. If the pile load
test cannot be performed with the excavation completed, it will be necessary to
evaluate and compensate for the additional soil confining pressure that existed
during the load test.
Note that casing off soils that will later be excavated does
not provide a solution to this problem. Test piles should be located so that
they can be incorporated into the final work as service piles if practical.
Cautions
A poorly performed pile load test may be worse than having
no test at all. All phases of testing and data collection should be monitored
by an engineer familiar with the project and pile load test procedures and
interpretation. In highly stratified soils where some pile-tip capacity is used
in design computations, care should be taken to keep at least 5 feet or 8 pile
tip diameters of embedment into the bearing stratum.
Similarly, the tip should be seated a minimum of 5 feet or 8
pile tip diameters above the bottom of the bearing stratum. The driving records
of any piles driven should be used to evaluate driveability of the production
piles, considering the possibility of soil densification.
In clay formations, where the piles may tend to creep under
load, add in holding periods for the load test and make sure that the load on
the pile is held constant during the holding period.
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