Abstract: Residual stress along piles has a great influence on bearing capacity of pipe pile jacked into red clay with an upper soft layer and a lower harder layer. Using self-developed equipment, indoor model tests are conducted to measure close-end single pipe pile and pile group jacked into layered red clay foundation. The variation of residual stress of single pile and pile group and their time effect with three kinds of diameters are analyzed. The results indicate that the residual stress along pile shaft increases first and then decreases from top of the pile to the bottom. The distribution of residual stress shows a broken line. For a typical red clay foundation with a hard top layer soil and soft lower layer, the residual stresses at pile shaft and pile tip are much smaller when the bearing stratum is hard plastic red clay layer or the plastic layer. However, the residual stress will increase when the piles puncture into better bearing layer. In the early stage of completing jacking single pile, the time effect of residual stress at pile shaft performs significantly. The stress value decreases, and exponentially decays to a lower stable value after the single pile being re-pressed as the rest period keeps going. If residual stress being ignored, the pressure value at pile shaft gotten by static load tests decreases. In this test, more than 7.97% of pile side friction above neutral surface is predicted, while less than 2.29% of pile side friction and 2.29% of pile tip resistance below neutral plane is predicted.

Key words: red clay foundation, jacked pipe pile, residual stress, pile group, time effect