| The hydrological and geological conditions are unique in Lanzhou and the subway construction is in its early stage.Moreover,support design,field monitoring and numerical simulation is inadequate.Because foundation pit design has grown from strength control to deformation control,the research on the stability and deformation characteristic of deep pit for subway tunnel in Lanzhou is very significant.Based on typical deep foundation pit excavation of Lanzhou Metro Line 1 Ma Tan Station,combined with theoretical analysis,three-dimensional numerical simulation and field monitoring,the F-SPW7.0 software is adopted to imitate the supporting project of bored piles and steel support,and analyze the change rules of the internal forces and displacement of support structure.Meanwhile,the three-dimensional finite element model is established to simulate and analyze the process of foundation pit excavation by using the MIDAS/GTS NX analytical software for geotechnical engineering,and the deformation characteristics of bending moment,shearing force,the horizontal displacement of pile,the surface subsidence and the basement upheaval,triangle method,index method and parabolic method are used to predict and analyze the ground settlement of Ma Tan Station.The main conclusions in this paper are as follows:(1)F-SPW7.0 software is applied to simulate the process of the deep foundation excavation,the results shows that the displacement of the whole support structures increases as the depth increase during excavation,when digging down 16.76 meters the displacement of retaining structure mutation occurred.Both bending moment and shear force increases with increasing depth of foundation digging,the bending moment slightly decreases under the condition of bracing.The type of support structure has a certain constraint on the surrounding wall.Calculation results show that the overall stability,anti overturning stability and stability of the foundation pit of the station are satisfied with the design and specification requirements.(2)It indicated that there were some cracks or joints in the contact surface by analyzing the mutation displacement of supporting structure of foundation pit.Under the condition of the foundation pit precipitation,the maximum earth pressure was always 254.52 k N/m in the layer that it was made up of pebble soil of the Holocene artificial fill(Q4m1),alluvial(Q4a1)sandy pebble soil and lower Pleistocene alluvial(Q1a1)pebble soil in the process of excavation of foundation pit to basement.(3)Through comparative analysis simulation results and actual monitoring data,it indicated that the surface subsidence was small by MIDAS/GTS NX software simulation and the simulated data by F-SPW7.0 software parabolic method was coincided with the measured data,and it was less than the warning value 30mm;The change of the horizontal shift of foundation ditch pile was obvious at the initial stage of excavation,and the horizontal displacement of pile tended to be stable and slightly reduced because of the embedded depth o f 8 meters when excavated to basement;The maximum uplift amount of the excavation closure basement was 73 mm,so it was considered to set the limit pile in the center of the basement to reduce the uplift amount.(4)Using MIDAS/GTS NX numerical method to simulate the bracing process of foundation pit,the results show that the largest part of the steel support force is located in the third support.So the third support should be appropriate to add the treatment;by the changes of bending moment,It can be seen that the maximum value of the excavation to the base moment is concentrated in the lower part of the foundation pit.(5)The calculation of rain falls about the foundation pit of the Ma Tan station shows that the surface water collection and treatment measures should be taken into account for the larger area and the deeper foundation pit.Phreatic water of Ma Tan station is the quaternary loose layer Pore diving,and the aquifer is mainly composed of Quaternary sand and gravel layers.The use of pipe wells to extract the layer of the hole diving to ensure that the construction of the foundation is not affected by groundwater,drainage wells are placed around the pit to form a closed precipitation,control the central water depth of the foundation pit,to reap the requirements of reducing the groundwater level.As the groundwater diving sources are mainly from the Yellow River water,upstream groundwater,precipitation and other supplies,therefore,suitable spacing arrangement must be used around the fo undation pit. |
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