The Use of Superposition for Evaluating Pile Capacity

Abstract

Evaluating static pile capacity by dynamic testing has become routine procedure in contemporary foundation engineering practice worldwide. Testing is typically performed during initial pile driving, and/or during restrike some time after installation. Situations limiting the accurate assessment of static pile capacity by dynamic methods include: (1) the pile capacity changes due to the time-dependent characteristics of the supporting soils and/or rock, and (2) the hammer energy is insufficient to fully mobilize all soil/rock resistance forces present during the test. This paper presents discussions on the fundamental mechanics of these two limitations and. proposes the use of superposition of dynamic test results for evaluating total pile capacity. A case history where both a significant increase in pile capacity occurred after the end of initial driving, and insufficient hammer energy limited the activation of full resistance during restrike is presented that demonstrates the applicability of the proposed approach. Dynamic test results performed with end of initial driving (EOID) and beginning of restrike (BOR) data are provided along with full scale static loading test results. Using superposition by combining the pile end bearing resistance from EOID and shaft resistance from BOR dynamic test results produced excellent correlations with the static loading test results including the ultimate pile capacity as well as the pile top load-movement relationship.