Abstract: During the four piles static load field test, soil pressure boxes are arranged. With the testing of stress on the top of C-piles namely the rigid piles, the top of M-piles namely the semirigid piles, and soil between piles, the piles-soil stress ratio and load sharing ratio are calculated. Then, the stress state of piles and soil during the CM-piles composite foundation static load test is analyzed. Besides, combining the large plate test P-S curve after setting different thicknesses of cushion layer with the single C-pile static load test, the failure mode and cushion effect are analyzed. The test results are as follows. First, when the CM-piles composite foundation is working, C-pile top stress concentration phenomenon is obvious as the pile-soil stress ratio of C-pile are more than 10, while that of M-pile are less than 10. As the test loading, piles-soil stress ratio increasing gradually, increasing rate of pile-soil stress ratio of C-pile is larger than that of M-pile. Second, under the conditions of design load of CM-piles composite foundation, load sharing ratio among C-piles, M-piles and soil is around 6:3:1. Considering M-piles and soil between piles as a whole, namely, after the foundation treatment, it is more reasonable for the soil between piles compared to C-piles to have a pile-soil load sharing ratio of 6:4. Third, as the thickness of cushion layer increases, the CM-piles composite foundation settlement values increase. While the quality of C-pile is better and cushion failure mode is pierced by C-pile, the material and thickness of cushion layer should be properly selected.

Key words: CM-piles composite foundation, field test, piles-soil stress ratio, load sharing ratio, cushion effect