Comparison Between Model and Full Scale Pile Capacity Gain in the Boston Area

Piles driven into low permeability soils gain capacity over time. This capacity gain, often called "set-up" or "freeze", is believed to be controlled by the mechanisms of effective stress increase due to the dissipation of excess pore pressures built up during driving and stress independent phenomena such as strength increase due to thixotropic bonding. Practical efforts have been recently made to quantify time dependent pile capacity and develop a methodology of incorporating it into pile design by utilizing insitu testing. The following paper examines the use of the Multiple Deployment Model Pile (MDMP) and a full-scale instrumented pre-stressed concrete pile to examine time dependent pile capacity gain in Boston Blue Clay. Normalized relationships were used to examine the overall and segmental pile capacity gain for both the MDMP and the full scale pile and correlations are made to normalized excess pore pressure dissipation. The results of the MDMP and full scale instrumented pile compare favorably with previously compiled pile capacity gain and excess pore pressure measurements.