Recent field study on the behaviour of jacked piles

Abstract

In recent years jacked steel H-piles have been introduced into Hong Kong as an alternative to driven piles. In Hong Kong practice, termination of pile driving is often based on dynamic formula and pile tips are normally embedded in a stratum with SPT-N values exceeding 200. On the other hand, installation of jacked piles ceases when the rate of penetration is less than some prescribed termination criterion and jacked piles are normally embedded in layers with SPT-N values of less than 160. As such bearing stratum is less stiff, the load transfer mechanism and capacity of jacked piles could be very different from driven piles and experience from the latter may not be directly applicable. To investigate the behaviour of such piles, a comprehensive study was conducted. In the study, six piles were instrumented with strain gauges and load tested to failure so allowing their load transfer mechanism to be studied. During installation the development of pore pressure around three of the test piles was monitored by electronic piezometers. Different criteria were adopted for determining the termination of jacking. It was found that the test piles passed the same acceptance criteria specified in the building code for driven piles. Shaft friction in the test piles contributed to over 80% of the piles' ultimate capacity. The pore pressure was found to dissipate quickly after jacked pile installation.