| In recent years,as the high-speed railway construction network in the Yangtze River Delta region becomes more and more dense,the new cities along the high-speed railway develop rapidly.At present,the underground space development in new city construction is getting stronger and stronger,and a large number of subway stations and deep foundation pits of buildings adjacent to high-speed railway bridges have appeared one after another.Due to the high speed of high-speed railway operation and high requirements for track stability,the multi-source plus unloading environment caused by the foundation pit construction is likely to have adverse effects on the pile foundations of the adjacent high-speed railway bridges.Therefore,it has become an urgent problem to carry out the research on the deformation stress and load-bearing performance weakening of the adjacent pile foundation induced by the multi-source loading and unloading environment in the soft soil area of Yangtze River Delta.This paper takes the Yangtze River diffuse phase soft soil area in Jiangbei,Nanjing as the background area for research.Based on the summary analysis of the nature of soft soil layer in Jiangbei New Area and its parameters,a three-dimensional fluid-solid coupling finite element analysis model is established to study the deformation force and load transfer law of single pile and group pile and its bearing performance variation mechanism under the multi-source plus unloading environment of proximity foundation pit in deep soft soil area.The optimal scheme of isolation pile protection under typical Yangtze River floodplain phase soft soil geological conditions is proposed by applying orthogonal test combined with single factor test method,and the protection effect of the scheme is verified by using the example of high speed railway bridge adjacent to the deep foundation pit of subway.The main research works and results are as follows:(1)The statistics and analysis of physical and mechanical parameters of soft soils in Yangtze River floodplain phase in Jiangbei New Area were carried out,and the correlation between physical and mechanical parameters of soft soils in the area was studied.The results show that the soft soils in the area have the properties of high water content,high compressibility,high sensitivity,low density,low strength and low permeability;the correlation analysis reveals that there are good correlations between natural water content(w)and pore ratio(e),heaviness(γ)and pore ratio(e),plasticity index(I_P)and liquid limit(W_L),and the regression equation is established to provide reference for subsequent projects in the area.(2)ABAQUS was used to establish the three-dimensional fluid-structure coupled finite element analysis models of single pile and group pile(3×3)under two typical pit multi-source loading and unloading modes(precipitation excavation,precipitation excavation and pile loading),and studied the deformation and load transfer law of single and group pile under different engineering conditions(different precipitation excavation stages,distance between pile and pit,load safety factor,soft soil thickness,pile loading form,pile loading size,pile loading sequence,pile loading distance)in various loading and unloading modes.The deformation force and load transfer law of single and group piles under different engineering conditions(different precipitation stage,pile and pit distance,load safety factor,soft soil thickness,pile form,pile size,pile sequence,pile distance),and the weakness of pile performance caused by various working conditions were evaluated by vertical load bearing performance loss rate.The results show that under the multi-source loading and unloading environment of the foundation pit,the load transfer along the pile body from top to bottom will occur for each pile of the adjacent single pile or group pile,and the magnitude of load transfer is closely related to the depth of precipitation excavation,pile distance,soft soil thickness,pile load and other engineering conditions;the redistribution of the top load will occur for each pile of the group pile under the multi-source loading and unloading environment,and the direction and magnitude of load redistribution are affected by the bearing distance,load safety factor,soft soil thickness and pile load and other conditions.The direction and magnitude of load redistribution are influenced by the bearing distance,load safety factor,soft soil thickness and pile load,etc.Under the multi-source loading and unloading environment of the foundation pit,the vertical bearing capacity of both single pile and group pile will be lost,and the size of vertical bearing capacity loss rate is related to the depth of precipitation excavation,pile distance,load safety factor,soft soil thickness and pile load,etc.(3)A two-stage analysis method based on Vlazov two-parameter foundation is used to derive the differential equation of single pile deflection under the action of multi-source loading and unloading,and the differential equation is solved by using the difference method to obtain the distribution of single pile lateral displacement under multi-source loading and unloading.(4)A model of isolation pile protection for group piles in adjacent pits under typical soft ground conditions in Yangtze River floodplains is established,and the influence of isolation pile length,isolation pile spacing,distance between isolation piles and pits(protected piles)and elastic modulus of isolation piles on the isolation control effect is studied by orthogonal test analysis.On this basis,the optimal solution of isolation pile setting in this ground condition is proposed by multi-group single-factor test:the isolation pile length should be 1.25 times the excavation depth of the foundation pit,and the isolation pile should be placed as close as possible to the protected pile,the isolation pile spacing should be set within 1～2m,and the low stiffness pile can also be used as isolation pile.The results show that the reasonable isolation pile setting can effectively control the lateral movement of the high speed railway bridge adjacent to the precipitation excavation within the range required by the specification. |
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