Analysis On Pore Water Pressure In Deep Soft Soil Area Of Hangzhou-Ningbo Passenger Railway Reinforced By Pipe Pile And Raft Foundation

In deep soft soil areas, higher excess pore water pressure would be induced instantaneously when installing the pre-stressed pipe pile by hammering method and some serious troubles maybe caused to projects. At the same time, the generation and dissipation of excess pore water pressure have significance for pile foundation bearing capacity and soil’s reconsolidation. Based on the monitoring materials of Shangyu North Station of Hangzhou-Ningbo passenger railway, whose subgrade is reinforced by pre-stressed pipe piles and raft foundation in deep soft soil, classic theory, in-situ test and numerical simulation are used in the paper to research the space distribution, influence area and dissipation law of excess pore water pressure in some construction stages. The main research contents and conclusions are as follows:(1) Based on the research results both in domestic and abroad, the technical route was determined. On integration of engineering geology condition and design scheme, typical region and sections were selected, and then test scheme was made.(2) Through monitoring of pore water pressure at three monitoring sections when pipe piles were installing, the paper analyzed change rules and influence scope of excess pore water pressure in radial and deep direction. A reasonable sequence of pile-driving was educed by the comparison among continuous beating, segmental construction and skipping method. The study is important to the information construction. Also, it provided references to construction organization of the pre-stressed pipe pile at similar geological conditions.(3) By comprehensive comparison of several calculation theory of excess pore water pressure based on cavity expansion method, the calculation principle, method, merits, demerits and application scope were analyzed, and an improved calculation method was obtained. Comparing with the actual measurements, it was verified that the improved method could applied to the research of excess pore water pressure caused by continuous beating pipe piles.(4) When piling works completed, during the period of filling, surcharge preloading, filling construction of surface layer of subgrade and the period of installation and construction of floating slab,the long-term monitoring were carried out, which lasted about23months. The variation of excess pore water pressure with time was obtained and it provided important experience and first-hand observed data to the further theoretical research and mechanism discussion.(5) Numerical analysis models of monitoring section were built by finite element analysis software. Change rules of excess pore water pressure in radial and deep direction were obtained when driving the pile, and dissipation law of excess pore water pressure was also acquired after the pile was driven. Various influence factors of excess pore water pressure were considered when driving piles. Among the influence factors, large deformation analysis, soil’s cohesion, friction angle and dilation angle have strong impacts on excess pore water pressure, and the influence degree of elastic modulus, poisson ratio and friction coefficient are small.