Research On Disturbance Aging Characteristics Of Huge Pit Excavation In Soft Soil Layer And Its Evaluation Methods

The paper focuses on the key techniques and theoretical issues of deep foundation pit excavation in soft soil layer.Based on the predecessors' research results,the paper carries out detailed researches on a series of important engineering techniques and methods,such as spatial distribution and time effect characteristics of the disturbance displacement caused by excavation,key mechanics problems in large foundation pit engineering and its practical calculation methods,real-time and fast back-analysis methods of the equivalent mechanical parameters in foundation pit construction based on the monitoring results,calculation methods of the long-term disturbance of displacement considering the soil's creep characteristics,prediction of the long-term stability of the foundation pit,environmental risks assessment and underground pipelines protection,etc.A lot of innovative and practical research results are obtained.(1)Based on the analysis of former researches along with the systematic study on FEM numerical models,the spatial Gauss distribution function of the settlement displacement and horizontal displacement caused by excavation in soft soil layer is established.Besides,parameter sensitivity analysis of the distribution function is conducted with an overall consideration of the physical and mechanical properties of soil,the geometric conditions of the foundation pit,the type of the building envelopes,the parameters of the envelopes and supports,the excavation methods,etc.,from which the relations between various factors are obtained.Combining the basic FEM model with the parameter sensitivity correction,good prediction results are acquired compared with the measured results,which provides a reliable support for risk prediction of displacement and technical methods in practical foundation pit excavation engineering.(2)To study the characteristics of the deformation and internal force of the diaphragm wall,the paper proposes a new analytic methods based on the measured results of the support force,which can simulate the load applied from the supports to the diaphragm wall and the equivalent elastic support from the soil to the diaphragm in real time during excavation process.The method can well reflect the real effect of the excavation,which provides a scientific and useful way to analyze the stability of the diaphragm wall in deep foundation pit excavation engineering in soft soil.(3)To analyze the disturbance of soil displacement and stress characteristics of the underground pipelines,the paper proposes space foundation beam method and extended 3D foundation beam method for stability analysis of underground pipelines based on the measured results of the soil displacement.The method considers various kinds of complicated loads,such as the bending,torsion,shear,axial tension and compression,and even extrusion during excavation process.It has the following characteristics:(a)The parameters are calculated through the monitoring displacement values,and the human and environment factors are considered while neglecting excavation construction order and its supporting conditions;(b)Considering the excavation disturbance influence on the mechanical characteristics of the underground pipelines;(c)Displacement distribution is applied as the conditions of the pipeline elastic foundation model.Neither the complex calculation nor the analysis of the various influencing factors are needed;(d)Establishing a soil foundation model with the monitoring displacement distribution at different times,which can indirectly consider the effect of the soil's rheological properties on the pipelines.The method is practical in calculating and evaluating the stability of pipelines in excavation engineering under arbitrary complex environmental conditions.(4)Considering the large geometric scale,complex soil layer,multiple maintenance structures and complicated interactions,miscellaneous excavation ways and construction orders in large deep foundation pit engineering in soft soil layer,the optimization identification method based on the theory of Nelder-Mead optimization inversion analysis and FEM software is established.It can perform the iterative inversion analysis under the control of initial parameter values and the initial objective function values,from which calculated results qualified for the termination conditions can be obtained.Case study shows that the prediction results of the displacement could match well with the measured results.(5)According to the similar model test and theoretical analysis,the time dependent components of the disturbance displacement(including the unloading level of soil,the change of the construction period,the creeping displacement caused by the soil rheology)and the mechanical mechanism caused by excavation in soft soil layer are revealed.Then the time function of the displacement caused by unloading,rheology and their combined action are established respectively.Considering the superposition and coupling of the unloading and creeping,a unified displacement time inflection function for the whole process of the foundation pit construction is given.(6)Combing the creep function conversion method with the 3D FEM simulation,a practical method to effectively predict the long-term displacement of soil around the foundation pit under the rheology is established.The calculated results have good agreement with the test results of engineering practice in Shanghai and Ningbo,which provides a reliable way to deal with similar projects.(7)In order to solve the anisotropy soil foundation problems,pipeline hanging and local reinforcement problems,the assessment method of the pipeline deformation,the judgment method of the separation of soils and pipelines,the safety evaluation methods of pipelines based on elastic foundation model and FEM simulation respectively are created based on the extended 3D foundation beam method.Besides,pipeline stability evaluation system and its calculation methods under excavation conditions are established in terms of critical displacement and pipeline strength,which is quite applicable in engineering.