Study On The Mechanism Of Karst Collapse Induced By Underground Engineering Construction In Chongqing

Karst collapse is a phenomenon that there are open type karst cave in the lavacrevices, and its surface rocks, overlying soils under natural or human factors, fall downand formed collapse trap in the surface. Karst collapse is the largest and widest, highestfrequency of occurrence, most risk factors in all collapses, it also has a strong disguiseand sudden and so on. Inevitably, when the underground construction through Karstlandform areas, its process of construction can very easily induced karst collapse.Predisposing factors of karst collapse include many aspects, so it has a variety collapsemodes, its formation mechanism must consider all aspects of the force. Take extensivepumping groundwater in the underground construction process to alter thehydrodynamic conditions as the main entry point, study the formation and mechanismof karst collapse through Platts theory and ultimate balance theory, use modelexperiments simulate the process of collapse caused by water level depression and therelation of vacuum negative pressure with water level. Main research findings are asfollows:（1）For karst collapse caused by water level depression happened in undergroundengineering construction, analysis its development process, according to the break ofsolid which cover the cave, collapse state, divided into overall collapsed type and soilhole gradually spread collapsed type, established stability criterion of overall collapsedtype collapse, and detailed analyze the calculation process of collapse criterion fortypical cylindrical cover soil.（2）Necessary conditions for forming solid hole, effect factors of solid holeformation and growth, formation mechanism of solid hole and its initial conditions,mechanical mechanism，detailed expound the development of solid hole from over foursides, and through control the speed of pumping/drainage and the groundwater level tocontrol total displacement, thus to control the starting of soil hole development.Analyzed the change of collapse criterion when groundwater level at different locationsin cover soil, obtain the direct effects of forming karst collapse from properties of soiland vacuum negative pressure generated or not when groundwater level drop.（3）Take the karst collapse occurred in Chongqing yujiawan Reservoir as anexample for subsidence calculation, establish mechanical model of cover soil in karstcollapse area, analyze whether it can develop a solid hole or not, through calculating,the thickness(h) of cover solid all can meet conditions on h≥2h’. Use collapse criterionformula of soil hole gradually spread collapsed type, according to the actual field data of four collapses at Chongqing yujiawan reservoir, take parameters into formula tocalculate, get the coefficient of determination are less than1, that is soil subsidence，andverified the accuracy of collapse criterion.（4）Based on the physical model of karst collapse, precipitate experimental devicefor model of karst collapse induced by water level drop, implemented in total of16kinds conditions of water level drop collapse experiment under four different water headin1m,3m,6m,9m and4species of soil thickness in5cm,10cm,15cm,20cm, obtainthe data of vacuum negative pressure and collapse pit dimension during the experiment.Through observing and analyzing the process of experiment, obtain the process ofoverall collapsed type and soil hole gradually spread collapsed type karst collapse, thechanges of vacuum negative pressure along with thickness ratio and different waterlevel combinations, the changes of collapse pit size along with thickness ratio anddifferent water level combinations, and compare analyzed the measured and calculatedvalue of vacuum negative pressure which produced in different water levelcombinations.