Is the PDA test procedure different for a test run to failure than for a test run to a lesser proof load?
A static load test is generally conducted to failure or up to the design load times the safety factor, whichever comes first. In static “proof tests” the pile may be loaded to a lesser load. In dynamic tests, the PDA and CAPWAP® always give the capacity activated by the impact. In general, if the permanent movement (set) of the pile for the impact is greater than 2 or 3 mm, then it is thought that the dynamic test has fully mobilized the pile capacity (corresponding to a static test run to failure). Several reference papers suggest this criterion. Another criterion is described in:
Rausche, F., Likins, G. E., Hussein, M.H., February, 2008. Analysis of Post-Installation Dynamic Load Test Data for Capacity Evaluation of Deep Foundations. From Research to Practice in Geotechnical Engineering, Geotechnical Publication No.180, Laier, J. E., Crapps, D. K., and Hussein, M. H., Eds, American Society of Civil Engineers: Reston, Virginia; 312-330.
For testing drilled shafts, many engineers use a series of a few impacts with increasing drop heights. This results in the first blows at low drop height having very small set per blow and thus not activating the full capacity (see published papers addressing this topic here). These initial impacts may be thought of as a “proof test”, although usually the dynamic test continues with higher drop heights in order to reach the full capacity which corresponds to the failure load condition (unless the test is discontinued due to stresses so high that further increase in drop height risks damage to the shaft).