Numerical Simulation Of Deep Excavations In Shanghai Soft Clays,Based On Small Strain Shanghai Model

Excavation and construction of deep foundation pits in soft soil regions often disturb the surrounding environment.Therefore,it is imperative to evaluate the environmental impacts of deep excavation before the start of the project.Since the deformations in the stratum around the foundation pit are in the small strain range,the high-precision numerical evaluation must adopt a constitutive model that considers the small strain stiffness.Based on the unified Shanghai model,this paper carries out numerical analyses of deep excavation and discusses the influence of deep excavations on the surrounding environment.Firstly,the small strain Shanghai model is introduced,which can accurately describe the mechanical properties of Shanghai soft clays under small strain and large strain.It is then used to carry out numerical simulations separately for the foundation pit adjacent to the subway tunnel and the foundation pit adjacent to the historical protection building.The results of the numerical simulations are validated,and the effectiveness of relevant construction measures are discussed.The main results of this paper are as follows:(1)The user-defined soil model of the small strain Shanghai model is compiled.The model is explained in detail,and its implications are investigated.Triaxial tests and a simple deep excavation are simulated,and their results are verified to prove the effectiveness of the user-defined soil model.Compared with the Shanghai model without considering the small strain stiffness,the simulation results of small strain Shanghai model in the form of wall displacement and ground settlement caused by deep excavation are more realistic,showing that the new constitutive model is suitable for numerical analysis of deformations linked with deep foundation pit excavation.(2)In the numerical simulation of multi zoned deep excavation in the vicinity of metro tunnels,typical cross-sections are selected to establish the finite element models.The simulation results are compared with the on-site monitoring results,indicating the maximum horizontal displacement of the underground diaphragm wall,the location of the maximum horizontal displacement,and the surrounding ground settlement changes are very close to the monitoring results.Also,the final simulated value of the adjacent tunnel’s deformation is close to the monitoring value.Further analysis of the variation of soil shear strain proves the importance of considering small strain stiffness.The effectiveness of multi zoned excavation in reducing the impact on the surrounding environment is discussed.(3)In the numerical simulation of the deep excavation adjacent to the historical protection building,the simulated results are compared with the on-site monitoring results to verify the constitutive model’s validity.Through comparative calculations,it is revealed that the existence of adjacent buildings increased the deformation of the underground diaphragm wall and the ground settlement.In summary,the small strain Shanghai model can be applied to the numerical simulation of deep excavations in soft clays.It can effectively and accurately predict the deformation of the deep excavation retaining structure and the surrounding ground settlement.It should be further applied in the future.