| The study area is located in the northeast of Shenzhen,belonging to the coastal area of low mountains and hills,with an area of 844.07 km ~2,The area where it is located is the largest area in Shenzhen.Its special geological conditions have created a large karst development zone.The area where the study area is located has strong karst development,more complex distribution and frequent karst collapse events.With the increasing intensity of underground engineering construction in urban karst areas,the risk of karst collapse is also increasing.The disturbance to the base soil layer during tunnel construction and the change of groundwater runoff conditions will cause water level fluctuation,which may drain the karst cave water and take away the filling,destroy the stress state of the karst cave,cause the karst cave to collapse,and then cause the ground to collapse;When the tunnel construction is completed,there is a risk of karst collapse,which affects the stability and safety of the surrounding soil and the tunnel,and seriously endangers the driving and personal safety.Taking an intercity underground railway project as the background,this paper selects the tunnel engineering survey section from Ruyi road to Longcheng Avenue,which is about 1.7km long,as the research object,uses the geophysical method to explore the development of karst in this area,analyzes the development characteristics of karst combined with the data of karst caves exposed by boreholes,and uses the analytic hierarchy process to evaluate the risk of tunnel karst collapse in this section,Finally,the typical sections in high-risk areas are selected to establish a numerical model to analyze the influence of tunnel excavation in karst areas on rock and soil displacement,overall tunnel displacement and stress field.The research results provide support and reference for railway survey,design,construction and safe operation in the study area,and provide scientific reference for karst collapse prevention,urban construction and underground engineering planning in similar areas.Results of this paper:(1)Through the analysis of the survey data of the research area and the surrounding projects,the basic geological data such as topography,stratigraphic lithology,geological structure and hydrogeological conditions in Long gang District were obtained.According to the engineering geological conditions of the study area,the location,scale,morphology and filling characteristics of the underground cave were discovered by combining geophysical seismic CT method and drilling.(2)The development characteristics and spatial distribution of tunnel karst are revealed.It was found that there were 106 engineering drilling holes in the study area(1),60 drill holes involving caves,244 caves were exposed,and the cave roof was buried at a depth of 8.4-45.66 m,with an average of 25.48m;the height of a single cave was 0.4-24.29 m,and the average height was 2.5m.(2)According to the drilling hole,the degree of cave filling is revealed: there are 193 full fillings,42 semi-filling,and 9 non-filling.(3)The use of geophysical exploration method to explore 363 caves,underground caverns buried depth mostly concentrated between 20 m ～40m underground;the height of the cave is 1m smaller accounted for 7.16%,a total of 26;the height of between 1m and 3m of the cave accounted for 30.58%,a total of 111;the height of between 3～ 5m of the cave accounted for 26.17%,there were 95;the height of between 5m ～7m of the cave accounted for 20.39%,a total of 74,the height of the cave is greater than 7m,57,accounting for 15.7%.(4)The spatial relationship between the tunnel and the cave,the geophysical results revealed that the 363 caves are located at the top,excavation range and bottom of the tunnel,and there are 65 caves distributed at the top of the tunnel,accounting for17.91% of the total;there are 129 caves in the tunnel excavation range,accounting for 35.53%of the total;there are 169 caves at the bottom of the tunnel,accounting for 46.56%.(3)By analyzing the relationship between the cover layer conditions,karst development characteristics,hydrological conditions and the hazards caused by tunnel construction in the research area,the fourth series soil layer thickness,soil structure,cave filling,cave point density,linear karst rate,the plane area of a single cave,the hole encounter rate of drilling,the groundwater burial depth,and the distance from groundwater to the soluble rock were selected,a total of 9 index factors were selected,and the risk evaluation model of tunnel karst collapse was established by the hierarchical analysis method,and the natural breakpoint method was adopted to divide it into high,medium and low risk levels.The total area of the high-risk zone was 28644.8m~2,accounting for 33.7% of the total study area,the area of the medium-risk zone was 36890.88 m~2,accounting for 43.4% of the total area,and the size of the low-risk zone was19464.32 m~2,accounting for 22.9%.(4)Combined with the risk assessment and zoning of tunnel karst collapse,taking the typical road section of the high-risk area as the research object,a three-dimensional tunnel shield model is established,the influence of karst on tunnel construction is analyzed,and the displacement of the top and bottom of the tunnel,the overall displacement of the tunnel and the degree of influence by the stress field are analyzed according to different situations such as the spatial distance between the cave and the tunnel H and the degree of filling of the cave.The results show that under a certain diameter of the cave,the tunnel excavation process is affected by the upper cave,and the displacement of the geotechnical layer of the tunnel vault gradually decreases with the increase of H,but the stress field of the rock and soil body does not change significantly;the karst cave filling has little impact on the displacement of the geotechnical layer around the tunnel.The cave at the bottom of the tunnel,the closer the cave is to the tunnel,the greater the impact on the displacement of the tunnel,the surrounding soil layer and the stress field,with the increase of H,the degree of influence gradually decreases and tends to stabilize,while the degree of cave filling has less impact on the whole. |
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