Study On Influence And Control Technology Of Concealed Karst Stratum On Stability Of Foundation Pit Of Semi-covered Excavation Station

This paper takes the Qiligou Station of Xuzhou Metro Line 2 as the engineering background.The geological conditions in the study area are complex.There are large ranges of moderately weathered limestone in the underlying strata,and the karst is moderately developed.Especially the hidden karst will affect the deformation and stability of the foundation pit.Because the station nears the flow of people and traffic,buildings,so the foundation pit excavation using semi-covered excavation.The construction method of the foundation pit adopts a new technology of semi-covered excavation.In view of the above situation,the main research objects are karst and foundation pits.The theoretical analysis,numerical simulation and measured data analysis methods are used to analyze the influence of hidden karst on the stability of semi-covered excavation pits.Through the collation of domestic and foreign literature,the research status of karst disasters and control technology and the research status of foundation pit stability are expounded,which provides a theoretical basis for this paper.Through the analysis of the drilling and geophysical data of the station,the karst development and distribution characteristics are summarized.Combined with the foundation pit design scheme,the stability analysis of the large-diameter caves R1,R19 and R26 found in the study area is carried out,and qualitative and quantitative methods are used for analysis.It can be judged that the cavity R1 is unstable and R19 and R26 are stable.Combined with the engineering characteristics and difficulties of the semi-covered excavation excavation of the foundation pit in the study area,the factors affecting the stability of the foundation pit are analyzed.The MIDAS software was used to simulate the influence of karst caves on the excavation of semi-cover excavation pits.Considering the semi-cover excavation construction process of foundation pits,the numerical analysis of the foundation pits before and after the karst treatment was selected.Before and after the karst treatment,the vertical displacement of the support structure at the local location of the karst cave increased from 15.8mm to 18 mm,which increased by 2.2mm;the horizontal displacement of the karst hole to the overall support structure was from 23.8mm to18.6mm,and the overall reduction was 5.2mm;The vertical displacement of the pile increases from 5.2mm to 6.8mm by 1.6mm;the maximum settlement of the foundation pit is from 4.9mm to 1.6mm,which is reduced by 67.3%.The simulation results are analyzed.The existence of the cave will affect the stability of the foundation pit,and the cavern needs to be treated in a reasonable way.There are 50 monitoring points on the top of the support structure in Qiligou.There are 50 structural deformation holes and 154 ground settlement monitoring points.These monitoring points are used for on-site monitoring of the foundation pits.In the process of increasing the excavation depth of the foundation pit,the vertical displacement LZX of the column firstincreases to a certain value and then becomes slow,the maximum value is 6.98mm;the horizontal displacement of the horizontal displacement of the pile body ZQT first increases and then decreases to a maximum of 22.5mm and the maximum point of horizontal deformation of the pile moves continuously downward with the increase of depth;the closer the surface settlement DBC monitoring point is to the support pile,the larger the ground settlement value is1.5mm.When the value increases to a certain value,the settlement begins to decrease.According to the unstable caves existing in the Qiligou station,the treatment methods and treatment principles of the karst in the actual project are analyzed,and the karst is treated by grouting reinforcement.Comparing the measured data with the simulated data,the simulation results are the same as the deformation results of the measured results.Therefore,the numerical simulation method can be used to preliminarily evaluate the influence of the cave on the stability of the half-cover excavation pit,and provide guidance for taking reasonable engineering measures.