Research On Design Method Of Foundation Pit Support System Of Subway Station Based On The Strata Deformation Control

With the introduction of the "13th Five-Year Plan",China has furtuer increased its investment in infrastructure construction.Urban underground projects have been in a new situation of great development and high standards.In order to better control the safety of underground engineering and its impact on the surrounding environment within a reasonable range during construction process,it is necessary to deeply understand the deformation characteristics,deformation mechanism and control measures of underground engineering in the current environment.Among them,the urban deep foundation pit system is the foundation of all buildings and is an important link in underground engineering.How to establish a safe and reliable foundation pit support system has become a very important research topic in the increasingly severe urban underground environment.This dissertation takes the 25 deep foundation pits of the subway station in Tianjin Metro Line 6 as a research background,the deformation and mechanical characteristics of the foundation pit system are analyzed in detail by mathematical statistics.Under the conditions of Tianjin strata,deformation characteristics of retaining wall and ground surface of braced foundation pit are obtained,and then give the corresponding control standards.The influence of the support stiffness of the foundation pit,the stiffness of the internal support and the surrounding strata conditions on the deformation of the foundation pit are studied by means of the combination of numerical simulation and field measurement statistics.The primary and secondary relationship of the influencing factors and the optimal parameters combination are obtained.Secondly,based on the analysis of the above influencing factors,the expressing method of the stiffness of the comprehensive supporting system is proposed,which can well reflect the deformation law of the foundation pit system and provide the basis for the design of the supporting structure of the foundation pit system.The active earth pressure behind foundation pit during excavation is the key to the deformation of foundation pit.Based on the deformation pattern of foundation pit obtained in the previous study,considering the excavation of foundation pit,the flexible deformation of retaining wall,the friction between wall and soil,the non-limit state of soil and soil arching effect,to obtain an improved theoretical model of active earth pressure.Finally,the design method of foundation pit support system based on deformation control is proposed.Based on the limit analysis method,the development of stress and strain of soil and the elastic deformation energy stored in the supporting structure of the foundation pit during construction are considered.According to the plastic deformation mechanism,the relationship between the deformation control of any point behind the wall and the deformation of the retaining structure of the foundation pit can be obtained.Whether or not the energy conservation equation is satisfied,it is judged whether the design scheme of the supporting structure satisfies the requirements of the control standard,and suggestions are provided for the design of step-by-step excavation of the internal-supported foundation pit.The main research contents include:(1)The basic deformation pattern and deformation characteristics of excavation are obtained through the case study of 25 deep foundation pits in station of Tianjin metro line 6.The deformation of the foundation pit retaining structure is mainly convex type,and the deformation of the ground surface behind the wall is mainly concave type.The maximum lateral displacement ?hm of the retaining structure is no more than 0.120%H,H is the excavation depth of the foundation pit,and the depth of the maximum lateral displacement Hhm is in the range of(0.75H±8m).The maximum settlement ?vm of ground surface behind the wall is no more than 0.15%H,which is about 1.13 of the maximum lateral displacement ?hm.The influence of ground surface settlement behind the wall is similar to the depth of the retaining wall.(2)The main and secondary influential factors that affect the deformation of the retaining structure of the foundation pit and the settlement of the ground behind the wall are studied by the combination of numerical simulation and measured data.The stiffness of the comprehensive support system is obtained.The factors affecting the maximum horizontal displacement of the retaining structure and the settlement of the ground surface behind the wall are respectively the internal friction angle of the soil,the soil weight and the vertical number of the internal support.Based on the stiffness of the Clough system,a comprehensive stiffness expression of the support system considering the surrounding structures of the foundation pit,the inner support and the soil condition after wall is proposed.Based on the undrained shear strength transfer coefficient of soil surrounding the foundation pit,the stiffness of retaining structure is combined with the stiffness of the soil behind the foundation wall.According to the deformation control standard of retaining structure and the strength of soil layer,improve the system to optimize.The stiffness of the modified dimensionless integrated support system can effectively reduce the discreteness of the deformation.The maximum deformation of the retaining structure of the pit decreases obviously with the increase of the stiffness of the modified dimensionless integrated support system.(3)The active earth pressure calculation model that can comprehensively consider the construction process,the depth of foundation pit,the flexible deformation of the retaining structure,the friction effect between the wall and soil,the non-limit state of soil behind the retaining wall and the soil arching effect is established.The excavation of the foundation pit causes disturbance to the soil behind the wall.The inclination angle of the slip surface behind the wall ? decreases with the increase of excavation depth H and the friction angle between the wall and soil ?.And ? increases with the increase of the inner friction angle ?.The active earth pressure coefficient Kaw decreases with the increase of ?,soil cohesion c and ratio of maximum displacement to limit displacement Smax/Sa.Kaw increases with the maximum displacement depth ratio H/h,increases with a smaller depth,and decreases with a larger buried depth.Considering the excavation depth of foundation pit,the influence of wall friction angle 8 and the friction angle ? between soil layers,the active earth presssure ?aw are non-linearly distributed of "great in the middle and small in both ends".The higher of the maximum lateral displacement,the larger of ? and ?,the more pronounced of the nonlinear distribution.?aw decreases with increasing of c and Smax/Sa,and is less affected by Hlh.The resultant active earth pressure Eaw decreases with increasing of ?,?,c and Smax/Sa,and is less affected by Hlh.The point of active earth pressure resultant Haw increases with increasing of ? and ?.Haw decrease with the increase of c and Smax/Sa and slightly increases with H/h.(4)Introducting the deformation of foundation pit and the non-limit state of soil into the limit analysis method,a mobilized strength design method which called MSD method is obtained.Compared with the existing design method,the impact of the construction process and coordinated deformation with soil of the foundation pit can be considered.In this paper,the MSD method is improved to make it suitable for the analysis of the internal support combined with the flexible retaining wall in the foundation pit project,and a dynamic design method for the foundation pit support system based on the formation deformation control is proposed.Based on the traditional MSD method and engineering practice experience,the incremental deformation mechanism is modified,and a cosine function pattern with changed wave peak is proposed.The improved incremental influence range is closer to the actual measurement result than the original MSD method.By introducing the internal support compressive deformation energy in the energy conservation equation and the influence of the bending deformation energy of the retaining structure above the excavation face,combined with the plastic deformation mechanism of the foundation pit system,the deformation of the foundation pit enclosure structure can be controlled by the formation deformation.Judge whether the supporting structure design scheme meets the deformation control standard.By comparing the calculated results with the traditional MSD method,the vertical elastic foundation beam method and the measured values,it shows that the prediction effect is pretty well by using the improved method of this paper than the others.It shows that the improved method has a better applicability.The research results provide a basis for the design of foundation pit engineering.