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.
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.
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.