Early Warning And Assessment Of Cover-Collapse Sinkhole Induced By Urban Construction In Karst Area

At present,the demand of human society for urban development is becoming more and more intense.The geological environment problems caused by urban engineering construction aren't been neglected.The geological environment in karst area is complex and weak,so environmental effects should be payed more attentionduring urban construction,especially,the karst ground collapse.This paper takes Jinshazhou Island as the research area.The main purpose is to identify hidden danger and evaluate on the ground collapse which induced by the disturbance of underground water flow field during urban construction in covered karst area,to solvepotential threat of the ground collapse and provide theoretical support for disaster prevention and reduction.Jinshazhou Island,located at the border of Guangzhou and Foshan,began large-scale urban development and construction since 2006.At meantime,ground collapse events were caused by tunnel construction and deep foundation excavation.Since July 2007,ground collapse had occurred in 6 areas,causing a series of problems such as road interruption,building cracking and school relocation.Jinshazhou Island is located at the Guanghua basin,which is in the north of the Pearl River Delta.It is a typical covered karst area in south of China and one of areas where the economy develops fastest and the geological environment is transformed most strongly by human beings.The urban engineering construction in this area is faced with serious karst geological problems.So,it is a typical and representative research site.The main contents and results include:(1)On the basis of a full analysis of the environmental geological conditions,and history and trend ofhuman activities and geological disasters,the study on the mechanism of the ground collapse disaster in covered karst induced by urban constructionis carried out.To determine geological conditions(internal factors)and trigger conditions(external factors).Using mechanical analysis model of rock and soil and fluid,finite difference numerical simulationmodel and bottomfrictionphysical model,the deformation and failure law of soil with different covered strata is studied,and the determination method of covered soil stability and the groundwater dynamic criterion which induces ground collapse are discussed.(2)On the basis of summarizing and analyzing the hidden danger identification marks and detection methods of the karst ground collapse,a comprehensive detection system is formed by joint ground geophysical prospecting,engineering drilling and borehole geophysical prospecting.The refined detection method system of karst formcan be realized,including 3D form detection ofsoil holebased on GPR,form detection of karst bedrock surface based on micro detection,form detection of karst fissure at hole wall/hole circumference based on drilling ultrasonic/drilling radar,and form detection of karst cave based on sonarlogging technology.The refined evaluation and research method system of the karst rate can be realized,including karst rate evaluating methodsof wall-hole based on ultrasonic imaging in borehole,cross-hole based on cross-hole radar tomography,and circum-hole based on radar reflection imaging.(3)On the basis of division of engineering geological strata,the 3D model of it can be builded by GOCAD software,which is effective to reflect the geological structure and engineering characteristics in the covered karst area.At meantime,by 3D attribute modeling function of GOCAD software,the scattered geological detecting data is statistically analyzed,the 3D distribution of the bad soil(soft soil and sand soil)and karst cave is simulated.Thereby,the identification and speculation of the key geological elementswhich affect the karst ground collapse can be realized.(4)On the basis of 3D geological model,the study on urban karst ground collapse susceptibility is carried on.In order to preventing and controlling the karst ground collapse in covered karst area,the evaluation index system is established.Resorting to attribute computation function of the GOCAD software,getting the weight of factor set by chromatography,the karst ground collapse susceptibility is evaluated by comprehensive index evaluation model and fuzzy comprehensive evaluation model respectively to guide urban engineering construction.(5)On basis of detection data and groundwater level monitoring data,the hydrogeological conditions are analyzed,the numerical model of groundwater seepage field is established,and the risk assessment method of karst ground collapse disturbed by groundwater engineering is studied.The 3D numerical modeling method in karst water-bearing media which is anisotropic is discussed,including regional anisotropy aquifer simulation based on grid rotation processing method and strong runoff zone simulation based on unit seepage calculation module.The monitoring data are used to correct the model parameters to obtain the optimal value,and then the groundwater seepage field under the influence of engineering construction is simulated,and the warning threshold is compared,finally the risk areas of karst ground collapse under the disturbance of groundwater engineering are divided.