Time-Dependent Capacity of Driven Displacement Piles in Sand

Abstract

A time-dependent capacity increase has often been observed for driven displacement piles in sand. Long-term bearing capacity increase after dissipation of excess pore water pressure is referred to as set-up. Significant increases in axial capacity have been reported where the rate of set-up varied between 15 and 75% per log cycle of time.

More economic foundation designs are feasible when set-up is incorporated in pile design methods. The objective of this thesis is to expand knowledge on time-dependent capacity of driven displacement piles and make way for a pile design method that incorporates time dependency of pile bearing capacity.

The time dependency of virgin pile capacity has been assessed in a database study and a scaled test program. 4 scaled closed-ended steel piles were driven in a man-made sand deposit. The virgin capacity of a pile was assessed 2, 16, 31 and 70 days after installation. The 3 m long, 0.15 m diameter piles were statically tested in compression. Strain was measured at multiple locations along the shaft during the load tests. The applied load and pile displacement were measured at the pile head.

The contribution of the pile shaft and pile tip was determined from the strain distribution over the pile length at failure. A trend for capacity increase with time has been found in the database study and the test program. A capacity increase of 13% per log cycle of time was found for the closed-ended piles tested. Both an increase in shaft and tip resistance was found with respect to the reference capacity determined 2 days after installation. The shaft capacity increased on average 18.5% per log cycle of time and an average increase of 9% per log cycle of time has been found for the pile tip capacity. The measured capacity was compared with the calculated capacity determined by the Dutch design code. Considering the measured capacity normalized by the calculated capacity the influence of time is even stronger on the tip resistance than on the shaft resistance for the piles tested. As discussed in the report, there are some uncertainties with the interpretation of the strain distributions over the pile shafts.

The accuracy of the CPT-based design method available in the Dutch design code, method Koppejan, has been assessed from the pile test results in the database. The results were compared with other design methods available. The accuracy of all methods was low. The calculated capacity exceeded the measured capacity up to a factor 3.

Time-dependent capacity increase is a potential mechanism to incorporate in designs of axially loaded driven displacement piles. A trend for time-dependent increase of virgin capacity was found for all the test results considered in literature as well as for the test program conducted. However, the rate of capacity increase was highly variable. More research is required on the mechanism(s) for a better understanding of time-dependent capacity increase. Understanding of the mechanisms will also help understand what factors, and how these factors, influence the rate of increase.