Study on Cushion Effect of Cement-Mixing-Pile Composite Foundation

Shenzhen-Zhongshan Link was a super infrastructure project in China, in which the deep cement-mixed piles (DCM piles) were first widely used to improved the foundation of the immersed tunnel. Due to a lack of knowledge about settlement laws of composite foundation composed of the gravel cushion, the block-stone-vibrated cushion and DCM plies, to study the effect of cushion on settlement of composite foundation of DCM plies, based on the static load test on site, the influence of thickness, moduli of the gravel cushion and the block-stone-vibrated thickness on the settlement of pile tops and the total settlement of composite foundation were analyzed by numerical simulations. The results indicated that the flexible cushion under the rigid foundation could make the foundation flexible from rigid, and the cushion could adjust the loads shared by piles and soil, so that loads shared by piles became smaller and loads borne by soil were larger, which leaded to the settlement of pile tops decreased and the total settlement of composite foundation increased. With the decrease of cushion stiffness (increasing thicknesses or decreasing moduli), the adjusting effect of loads shared by piles and soil would not maintain continuous growth. With the increase of total settlement, the bearing capacity of composite foundation almost remained stable. To improve the bearing capacity of composite foundation and control settlement, combined with the actual project, it was suggested that the thickness of the gravel cushion should range from 0.4 to 0.8 m, the modulus should range from 10 to 20 MPa, and the block-stone-vibrated thickness should range from 0.7 to 1.1 m.