| In recent years,dual influences from surrounding sensitive environment and complex boundary conditions have led to new challenges for traditional deep foundation pit design,construction,and monitoring technology in the context of comprehensive development and deep integration of urban underground space.The irregular plane shape of the foundation pit or synchronous construction of multiple foundation pits will contribute to the above mentioned complex and sensitive boundary conditions.This further causes multiple and multi-directional disturbances to the soil under multi-sided excavation unloading,resulting in a significant environmental deformation superposition and posing a potential threat to adjacent facilities.Therefore,it is of great significance to explore the disturbance mechanism of the soil and the superposition mechanism of ground deformation under multi-sided excavation unloading conditions.In this paper,taking the soil unloading problem caused by multi-sided foundation pit as the breakthrough point,the stress field and displacement field in Shanghai soft soil under multi-sided excavation unloading are systematically studied,and the analysis approach of existing building response is proposed.Accordingly,deformation control measures are put forward,which provide theoretical support and technical means for practical foundation pit engineering under complex and sensitive boundary conditions.The main contents and conclusions of this thesis include:(1)Based on the similarity theory,the multi-sided excavation unloading model test in soft clay is designed and implemented.The development and spatial distribution characteristics of horizontal stress of soil induced by multi-sided excavation are investigated,and the superposition effect of ground surface settlement and deformation characteristics of deep soil are also studied.The disturbance mechanism of multi-sided excavation unloading is revealed,and the main factors that affect the soil disturbance are identified,which provides the internal basis for subsequent complex conditions and parametric analysis.In addition,the influence of support axial force adjustment on the foundation pit deformation is discussed by developing a single-point and multi-point freely adjustable support system.(2)Based on the small-strain characteristics of soft soils,the numerical models for two-sided,three-sided and four-sided excavation unloading are established,and the spatial distribution characteristics of soil stress field among pits,the changing process of three-dimensional stress state in Cartesian coordinate system and stress path evolution law of soil have been investigated.The influences of excavation sequence,pit spacing,geometrical asymmetry,and soft soil thickness on the stress state and stress paths of soil are analyzed,and two typical stress paths and multiple derived stress paths of multi-sided excavation unloading are summarized.(3)From the perspective of the interaction of foundation pits and the evolution of soil stress path,this paper analyzes the spatial distribution of and deformation characteristics of foundation soil displacement field among pits caused by multi-sided excavation,and investigates the influence of different factors on ground deformation.Thus,the superposition mechanism of ground deformation under the condition of multisided excavation unloading is revealed.By using the Gaussian distribution function,the empirical formula of ground settlement behind a single foundation pit is obtained.According to the superposition principle and the interaction mechanism of foundation pits,a practical method for predicting ground surface settlement between pits is proposed.(4)Considering the soil disturbance mechanism and deformation superposition characteristics under the multi-sided excavation unloading condition,the deformation control indexes of existing structures subjected to multi-sided excavation unloading are generalized and summarized,which match the structure mechanical property and deformation behavior.From the aspects of excavation sequence optimization,partition excavation,axial force adjustment of servo support and deformation isolation respectively,deformation control measures for multi-sided excavation unloading are analyzed,and their effectiveness is verified.Finally,a comprehensive deformation control procedure including exploration,design,construction,and monitoring is suggested along with a control method that combines active and passive measures.(5)The main findings are applied to the super-large deep foundation pit project of Dongjiadu Financial City in Huangpu District,Shanghai,which verifies the reasonability of practical method for predicting ground surface settlement and the effectiveness of deformation control method for multi-sided excavation unloading.The approach of establishing local 3D numerical models including important protected objects and 2D numerical models of typical sections could avoid some disadvantages of integral three-dimensional calculation models,such as excessive computing resources consumption and time consumption,unclear main contradictions and so on.This is also beneficial to accurately grasp and identify the main risks in the construction of the complex foundation pit group in advance,which is especially important for devising targeted protection schemes.Compared with other cases in Shanghai soft soil area,it shows that the lateral displacement of the retaining wall in this project stays in a reasonable range,and the protective building is in a safe and controllable state.The effectiveness and reasonability of the comprehensive deformation control method and technical measures combining active and passive methods are verified,and the corresponding optimization suggestions are also given.The main innovations of this thesis are as follows:(1)By adopting the developed single-point and multi-point freely adjustable support system,the similar model test of multi-sided excavation unloading is carried out.The characteristics of stress field and displacement field of soil between pits during excavation are analyzed,and the disturbance mechanism of multi-sided excavation unloading to the ground is revealed.(2)By means of the three-dimensional numerical simulation method considering small strain stiffness characteristics of soft soil,the evolution of soil stress path and the superposition law of ground deformation under the excavation unloading conditions of two-sided,three-sided and four-sided are analyzed,and a practical calculation method for predicting ground surface settlement between pits under multi-sided unloading is proposed.(3)Based on the characteristics of unloading disturbance deformation and stress path evolution of multi-sided excavation,the control effects of partition excavation,axial force adjustment of servo support and deformation isolation are analyzed.Moreover,it is further verified by field data,which provides theoretical support for engineering optimization. |
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