Instability Mechanism And Reinforcement Design Of Deep Foundation Pit In Soil-rock Dual Stratum

As an important part of engineering construction,foundation pit engineering has always been the research object of scholars,but most of the research objects are limited to soil deep foundation pit or general rock foundation pit,and the research on soil rock combination deep foundation pit is relatively less.Especially in the study of complex deep foundation pit with soil rock binary structure,the research method of inspecting soil foundation pit is often used,which obviously leads to great difference between theory and practice in many aspects,and even leads to great difference in calculation results,especially in the study of instability mechanism.In order to study the deformation evolution characteristics of deep foundation pit in soil rock binary stratum,analyze its instability mechanism,and make reasonable and effective support structure optimization,taking a deep foundation pit in Qingdao as the engineering background,this paper firstly analyzes and deduces the calculation method of lateral pressure and support structure suitable for soil rock binary foundation pit,and makes stability analysis;Then,the field monitoring data are analyzed to obtain the deformation law and characteristics of foundation pit;Finally,the FLAC^3D software is used to conduct dynamic numerical simulation of foundation pit excavation,and the overall deformation characteristics of foundation pit are obtained.By comparing with the field monitoring data,the dynamic process of foundation pit instability is studied.The main contents are as follows:(1)According to the characteristics of soil rock binary deep foundation pit,by improving several conventional foundation pit lateral pressure calculation methods,the calculation results are compared with the actual,and the calculation method suitable for soil rock binary foundation pit is analyzed.The results show that:there is little difference between the conventional calculation methods of lateral pressure of the upper soil of the foundation pit,and the Rankine earth pressure calculation method is in good agreement with the foundation pit studied.When calculating the lateral pressure of rock mass in the lower part of foundation pit,the overlying soil is regarded as uniform load,and the improved calculation method of lateral pressure of rock foundation pit is used;Finally,the resultant force is calculated as the overall lateral pressure of the foundation pit.(2)Based on the field measured data,the analysis and processing are carried out,including the analysis of groundwater level change,the displacement analysis of each part of the foundation pit,the stress and deformation analysis of the supporting structure and the influence of foundation pit excavation on the surrounding buildings.The following points are summarized:except for the east side,the deformation of the other three sides of the foundation pit is small and basically in a stable state;The lateral displacement of foundation pit slope is dominant,while the vertical displacement is relatively small;The influence range of foundation pit deformation is basically within 0.4?0.6 times of excavation depth;The deformation of deep foundation pit is mainly reflected in the soil deformation,and the control of active earth pressure should be considered in the layout of retaining structure.(3)In order to simulate the excavation process of deep foundation pit,FLAC^3Dnumerical simulation software is used.The results show that:the deformation law of soil rock binary deep foundation pit excavation is obviously different from the general soil foundation pit or rock foundation pit,showing a phased,and the deformation is mainly reflected in the overlying soil.The excavation of foundation pit leads to local stress concentration,especially in the lower part of foundation pit.The local stress increases with the excavation of foundation pit,while the stress of overlying soil decreases.From the simulation results of foundation pit excavation and support,it can be seen that with the gradual increase of foundation pit excavation soil displacement,after the construction of retaining structure,although the displacement continues to increase,the speed slows down.The deformation of the foundation pit gradually transforms from the slope to the bottom of the foundation pit,and finally concentrates on the bottom of the foundation pit.Secondly,the displacement of the corner of the foundation pit is relatively small,and the farther away from the corner,the closer to the middle of the foundation pit,the larger the displacement.(4)Through the comprehensive analysis of supporting structure calculation,monitoring data analysis and numerical simulation results,it is considered that the deformation of the east side of the foundation pit is serious and reaches the instability state.The instability reason is that the side pressure of the foundation pit slope caused by over excavation is too large,which makes the retaining structure insufficient to support the deformation of the foundation pit.According to the characteristics of the foundation pit,the reinforcement design is carried out on the east side of the foundation pit.In order to effectively control the deformation,it is mainly to carry out backfill back pressure on the eastern part of the foundation pit,and install anchor rods in each part.