CM桩复合地基桩土应力比及垫层效应现场试验研究

Field test study of pile-soil stress ratio and cushion effect of CM-piles composite foundation

通过现场4桩大压板静载试验,布置土压力盒,测定并分析C桩(刚性桩)桩顶、M桩(亚刚性桩)桩顶和桩间土体应力,计算桩(C桩+M桩)土荷载分担比,分析CM桩复合地基静载试验过程中桩、土的受力特性。通过设置不同垫层厚度,对比大压板试验P-S曲线,结合单桩静载试验,分析CM桩复合地基垫层的破坏模式及垫层效应。试验结果表明,CM桩复合地基工作时C桩桩顶应力集中现象明显,C桩桩土应力比大于10,M桩桩土应力比小于10;随着试验加载,桩土应力比逐渐增大,C桩桩土应力比增大幅度大于M桩;在复合地基设计荷载条件下,桩土荷载分担比接近6:3:1;将M桩和桩间土体视为一整体,即地基处理后相对于C桩的"桩间土",则桩土荷载分担比为6:4,设计较为合理;随着垫层厚度增大,CM桩复合地基沉降值增大;当C桩成桩质量较好时,垫层破坏模式为C桩桩顶刺入破坏,试验时应合理选取褥垫层的材料和厚度。

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.