Abstract: Based on the concept of settlement controlling design, the composite piled foundation of raft, cushion and scattered piles has been applied to the ground treatment of Beijing-Shanghai high-speed railway ballast track, in order to achieve the goals of effectively reducing the post-construction settlement and make full use of the natural foundation bearing capacity. The field test of CFG(cement fly-ash gravel) composite piled foundation has been carried out to explore settlement controlling mechanism of the foundation reinforcement technology. Through long-term observation of foundation settlement, the pressure of pile and soil, raft pressure at the top and bottom during the process of high-speed railway filling, standing and preloading and unloading; the variation laws of the embankment settlement , pile-soil stress ratio and load sharing ratio, and the variation of raft pressure with the changes of both the filling height of the embankment and the consolidation time are analyzed. The results indicate that the soil between piles shares large loading pressure in the early filling, which resulted in differential settlement between piles and soil; and then raft can significantly improve the bearing capacity of piles and stress concentration degree of pile top under the regulating role of the cushion; and the settlement of foundation soil occurs mainly in the reinforcement area. Measured results provide experimental basis for revealing the bearing mechanism and deformation characteristics of composite pile foundation, and guide the optimization of design parameters of high-speed railway CFG composite piled foundation.

Key words: high-speed railway, composite pile foundation for controlling settlement, field test, settlement characteristics, bearing mechanism