Bearing Behaviour of Composite Foundation with Rigid-Flexible and Long-Short Piles under Embankment Load

In order to study the bearing characteristics of the composite foundation with rigid-flexible and long-short piles under embankment loading, a field test of a foundation improved by the capped pre-stressed thin-wall concrete (PTC) piles and soil cement deep mixed (CDM) columns was conducted in a bridgehead transition section. The variation laws of pile-soil stress ratio, load sharing ratio and pile-soil differential settlement were analyzed. Furthermore, the embankment load transfer characteristics of long-short-pile composite foundation were numerically simulated. The test and simulation results indicate that soil pressure between piles and stress on the top of CDM pile increase slowly with the increase of embankment load, while the pressure on the top of PTC pile cap increases faster. Due to the effect of soil arching caused by differential settlements between the piles and the soil, a large part of embankment load is shifted to the rigid-long piles. And the maximum values of stress ratio between rigid-long pile as well as flexible-short pile and soil are 7.5 and 2.1, respectively. The location of the neutral point of rigid-long pile shaft gradually ascends up during the embankment filling and soil consolidation due to the existence of short piles, and the pile-top pressure and pile shaft friction resistance of rigid-long pile decrease. The change of short pile length makes no difference to embankment settlement when it becomes larger than a certain value. Therefore, the length of short pile can be designed to be the critical value that satisfies the requirement of ground bearing capacity.