Volume 23, Issue 3 (September 2000)
Estimation of Post-Driving Residual Stresses Along Driven Piles in Sand
Based on both field data collected from engineering literature and measurements made using model piles, a correlation for the residual pressure at the pile tip point is presented. The resulting correlation suggested that pile flexibility is a key parameter controlling the magnitude of the residual point pressure, qres. The calculated residual point pressure values for the database piles using the developed correlation were compared with the measured values and with those calculated using Briaud and Tucker's (1984) method. The comparison indicated that predictions made using the proposed method were closer to the measurements than those obtained using Briaud and Tucker's (1984) method.
In the present study, the residual load distribution along the lower portion of the pile (the bottom 10m) was modeled as a parabola. In the upper portion of the pile, and upon the removal of the driving force, it is assumed that the upward movement of the pile is enough to reverse the direction of the unit shaft resistance from an upward direction during driving to a downward direction, with a magnitude controlled by pile flexibility as well as the available shaft friction along the upper portion of the pile. As a consequence of this approach of modeling residual load distribution, the results indicate that residual load distribution peaks at a distance (less than 0.3L) above the pile tip point. This distance (measured from the pile tip) decreases with increasing pile flexibility, and for flexible pile, residual shear stresses may act in the downward direction all the way down to the pile tip. Also, the method of analysis adopted in this paper indicates that in some cases the magnitude of the peak residual load may exceed 50% of the measured ultimate uplift load. This magnitude is significant and leads to the prudent conclusion that any design method for an axially loaded pile in sand using pile load test data as a database should consider the existence of residual loads when pile load test data are interpreted.