Analysis Study Based On Ultra-Large And Ultra-Deep Foundation Pit Of TOD Engineering

The utilization of underground space in urban areas is increasing gradually,such as Transit-Oriented Development(TOD)projects that integrate stations and cities.The foundation pits for such projects are often ultra-large,ultra-deep,or irregular in shape,leading to increasingly prominent stability issues.To address the stability problems of complex ultralarge,ultra-deep,and irregular-shaped foundation,several composite support methods have been proposed based on traditional foundation support methods.(1)Both N-JET method(grouting pile by ultra-high pressure jet)and dewatering method are aim to seal water and drain.Especially in composite support structures,when the diaphragm wall is cantilever,these methods also are a certain role.However,due to their respective advantages and disadvantages,it is necessary to compare and optimize them comprehensively for different soil properties in different bearing layers and different reinforcement depths of N-JET.Taking a ultra-large and ultra-deep foundation pit in a TOD project as an example,it is effective to compare and analyze the stability of traditional method(internal bracing and dewatering),new method 1(combined the open-cut and top-down methods,only dewatering method)and new method 2(combined the open-cut and top-down methods,local dewatering method and N-JET method)by MIDAS GTS.It was found that the traditional method was unreasonable,and the stability of each support structure item was reliable after adopting new method 1.Eventually,new method 1 was selected in the initial design of this project,which provided practical guidance for this project.(2)Eight numerical simulation models were established to investigate the effectiveness of N-JET method in reinforcing two different types of soil strata serving as bearing layers,with varying depths.Analyzing simulated analysis data from multidimensional aspects to compare the stability of 8 conditions,this study further analyzes the applicable working conditions and optimizes the new method 2.The results showed that: The N-JET method can improve the stability of foundation pits by 20% when the sand soil(fine to medium sand etc.)is bearing layer.For the silt or clay(clayey silt etc.)used as the bearing layer,the recommended reinforcement depth range is 5m-10 m.For sand soil(fine to medium etc.)used as the bearing layer,the recommended reinforcement depth range is 10m-15 m.Additionally,specific recommendations on the depth of N-JET reinforcement are given for improving the stability of dewatering,anchor rods,raft foundations,diaphragm wall,and subway tunnels.(3)Nevertheless,local stability analysis is a supplement research on the stability of each sub-item of the composite support structure.The local instability of the diaphragm wall’s groove is a common problem during the construction of diaphragm walls.For ultra-large,ultra-deep even irregular foundation projects like this,the problem of local instability of the groove is prominent under the conditions of continuous rainfall during the flood season,complex surrounding environment,poor soil quality,complicated water systems.Therefore,composite support method for water-rich sand layer and trenching by main-auxiliary cage method were proposed.A limit equilibrium model of groove in a two-dimensional plane is aim to analyze stability,which calculated safety factor by strength reduction method when applying two different construction methods.Finally,the reliability of the theory and construction methods was verified by comparing the simulation analysis data and detection data.