Study On Settlement And Load Sharing Properties Of CFG Pile Composite Foundation Under The Flexible Load In BeiJing-ShangHai High-speed Railway

CFG pile composite foundation has been widely used in treatment of soft foundations of such high-speed railways as Beijing-Tianjin Intercity Rail Transit, Wuhan-Guangzhou passenger transport line and Beijing-Shanghai high-speed railway with 350km/h. Being one of soft foundation treatment means, CFG pile composite foundation has such characteristics of less settlement, high bearing capacity, fast stability and short-period project, etc. Tremendous economic and social benefits have been achieved. However, the design method of CFG pile composite foundation and computing method of bearing capacity are all based on the civil buildings in standards. It usually has great differences with the results of which are used in the computing of high-speed railway composite foundation under flexible load. In view of the immaturity of the action mechanism and computing theory of CFG pile composite foundation under flexible load, the settlements and stress characteristics of pile-raft and the pile-net composite foundation, the fundamental principal and methods of flexible load test and three-dimensional numerical model and optimization method of CFG pile composite foundation in Beijing-Shanghai high-speed railway are studied deeply in this paper through on-site testing, theoretical analysis, numerical analysis and optimization tools. Corresponding study achievements are presented as follows:(1) The settlement of pile-cap-net composite foundation is more than that of pile-raft composite foundation during the building of embankment. However, the stress and load sharing ratio of pile and soil of the former are more obviously smaller than the latter, as well as economical cost is also lower than the latter.(2) The computing method of CFG pile composite foundation bearing capacity of high-speed railway is firstly put forward by means of flexible load test. The test uses rubber plank to simulate the flexible load acting to the pile and soil of composite foundation during building process with the flexible load test equipments developed by ourselves.(3) The load developing coefficient of soil is the first to arrive at 1.0 with the increase of load through on-site flexible test of CFG single pile composite foundation. The results prove that the soil body is destroyed more rapidly than composite foundation under flexible load which is different with rigid foundation.(4) The influence of cushion on the settlement and deformation, load sharing behaviors and developing coefficient of pile and soil between piles are studied through CFG pile composite foundation flexible load test with different cushion thickness. It is found that the bearing capacity is more suitable for playing when the thickness of cushion is 20cm.(5) Three dimensional numerical model of CFG pile group composite foundation is established through finite element analysis software. And the influence of different pile (cap) strength, pile diameter, pile length, pile spacing, pile cap diameter and cushion thickness on the deformation and force characteristics of CFG pile composite foundation are studied after then.(6) The principle of optimal design and ideas of CFG pile composite foundation under flexible load is proposed. And the design of the Beijing-Shanghai high-speed railway is optimized in accordance with post-construction settlement of subgrade and the utilization requirements of pile caps and soil between piles.