Ha Qi High-speed Railway Passenger Line CFG Pile - Raft Composite Foundation Study

Cement Fly-ash Gravel (CFG) pile-raft, a new foundation reinforcement technology developed by China in the late1980s, possesses the following advantages: higher bearing capacity, low cost, the capability of controlling the subgrade settlement and shortening the construction period. CFG raft has been widely used in buildings’ foundation construction nowadays. CFG’s application in subgrade construction has been increasing over the past years, however, its theoretical study is still insufficient in serving engineering practice. Moreover, its method in calculating subgrade settlement and bearing capacity needs improving. Based on the research on Daqing Railway section of Harbin-Tsitsihar High-Speed Railway, this paper analyzes CFG pile-raft with field test, theoretical calculation and numerical simulation.Main conclusions of this paper are as follows:1、The final settlement at the subgrade preloading stage is higher than that at the subgrade filling stage, while the settlement rate at the subgrade filling stage is higher than that at the subgrade preloading stage. After the stack preloading stage, the settlement development on both sides of the subgrade can be split into three phases: the rapid growth phase, the slow growth phase and the stable subsidence phase.2、This paper, by comparing the measured value of subgrade settlement of Daqing Railway section of Harbin-Tsitsihar High-Speed Railway with the calculated value acquired through the method by Code for Design of Building Foundation, gives correction coefficient. Moreover, by introducing the average coefficient of additional stress under the trapezoidal load proposed by Guo Zaixu, this paper revises the settlement calculation method provided by Code for Design of Building Foundation and establishes a calculation formula suitable for the settlement of CFG pile-raft for high-speed railways.3、Through field test data, this paper concludes that CFG pile bearing capacity of single pile and pile body completeness all meet the design requirements. Meanwhile, this paper determines the characteristic value of bearing capacity of composite foundation. By referring to field test data provided by related literature, this paper amends the bearing capacity calculation formula on composite foundation proposed by Niu Zhirong, and proposes the bearing capacity calculation for CFG pile-net composite foundation under the high-speed railway. On this basis, this paper sets up a bearing capacity calculation formula for CFG pile-raft composite foundation of high-speed railway.4、By the numerical simulation on CFG pile-raft composite foundation through FEM software ABAQUS, the basic laws of stress field and displacement field of CFG pile composite foundation are acquired. By calculating the subgrade settlement, and analyzing and comparing the calculated value with the measured value, the reliability of numerical simulation is proved. By calculating the foundation’s bearing capacity, and comparing the calculated value with the result acquired through the method provided by this paper, it is found that those two results are similar. Moreover, this paper analyzes the effects of different pile length, replacement rate, thickness of mattress layer on settlement and bearing capacity of the CFG pile-raft composite foundation. It can provide the reference to the theoretical calculation and optimization design of CFG pile-raft composite foundation.