Abstract: In order to realize the scientific and reasonable design and construction of composite foundation of grid-shaped CDM(cement deep mixing) piles with high-replacement rate in sand foundation, we carried out the static load tests on the composite foundation of grid-shaped CDM piles, Xinglong hydro-junction in Hanjiang River basin in Hubei provinceis taken as example. Result shows that 1) the P-ΔS/ΔP(pressure vs. variation rate of settlement with pressure) curve of CDM piles can be divided into three sections, that is, the line segment of pile shaft resistance, the adjustment and change section of pile tip resistance and the failure section of pile tip resistance. 2) The curve of P-ΔS/ΔPcould better reflect the rigid variation and working behavior of the composite foundation of CDM piles than the curve of P-S(pressure vs. settlement). Moreover, the pile shaft resistance can be fully developed, which has mouth-shaped layout with high-replacement rate in sand foundation. The piles in grid shape are typical frictional end bearing piles as the limit bearing capacity of pile is mainly affected by the residual strength of bottom sandy soil in compacted state after shear failure.

Key words: grid-shaped CDM piles, sandy soil foundation, static load test, stiffness of pile foundation, pile shaft resistance