Deformation Behavior Evolution And Collapse Mechanism Of Karst Covers Under Water-air Interaction In Karst Area

Karst collapse is one of the hot topics for scholars.Deep valleys are distributed in Southwest of China Karst Mountains.The well-off rainfall,fast recharge and discharge cycle of groundwater leading to a high degree growth of karst.Fracture type and pipe type-karst pipes are common in these regions,so the hydrodynamic is complex and changeable.karst collapse has great influence on the highway or other traffic hub,such as route selection,city construction,it threatens construction safety and people lives.In prone or high incidence areas of karst collapse,the geological structure of the cover-layer is changeable,and the pressure of water and air is always abrupt.The deformation and evolution process of the cover is complicated under the water and gas interaction.Based on the study of“a model test study on the process and mechanism of karst collapse triggered by pumping water?NO.SKLGP2014Z014?and National Science Fund of china"double medium groundwater level fluctuation and air flow mutual drive and its supporting capillary?NO.41502244?",considering the ground water dynamic conditions to trigger the karst collapse as the mainly subject of this research.In this study,physical model tests,numerical simulation and cellular automaton of karst collapse was adopted,and focuses on the study water-air interaction that deduced the karst cover-layer deformation,and the instability and collapse of several typical karst capping structures under some hydrodynamic conditions in which refers to nature.The mechanism and the deformation behavior evolution of cover under certain groundwater conditions were studied.The research and research progress achieved are as follows:Based on the geological conditions of cover,several kinds of karst collapse area geological characteristic are summarized and analyzed,and several typical cover geological structure models are constructed,mainly considered the relationship between the cover-layer and the groundwater.Permeable monolayer cover is mainly caused by erosion with the change of rainfall or groundwater level,blocking monolayer is mainly controlled by the change of air pressure in the relatively closed space under water level.The main factors that caused collapse of double layer which have permeable upper and water blocking below is the gradual change or sudden change of pressure made collapse,but for blocking upper and permeable below,like this kind of double layer,the deformation evolution and collapse is complicated.Erosion and vaccum suction exist simultaneously.Using the physical models which was founded in our own laboratory,deformation and collapse evolution of the cover tests under the decrease or increase in water level in the cavity was carried out,the object of tests study is water blocking monolayer cover.Double cover-layer which permeable upper and water blocking below this type of cover-layer,its deformation and collapse tests were also in progress.The gradual change and catastrophe law of the lower cave in the change process by the changes of groundwater level under monolayer are monitored,and the influence of the pressure change on the stability of the cover is discussed.After that,the deformation behavior and internal mechanism of the cover are analyzed,and the appearance of the vacuum suction and the positive pressure air explosion is reproduced.Negative pressure in the air zone and negative pressure in the cavern are monitored during the process of rate of decrease in water level in the seep layer.The influence process of the vacuum suction force combined with the seepage deformation on the stability of the cover was discussed,and the mechanism of the deformation of the cover was analyzed.Combined with the physical simulation test results and the related engineering examples,the FLAC^3D is used to calculate the cover deformation of monolayer water cap and rate of decrease in water level of double cover-layer.The numerical results can basically coincide with the physical model test results,both blocking monolayer cover and double cover-layer water blocking cover,under the smaller rate of decrease in water level can cause deformation of the cover,but the vacuum suction force produced by larger rate of decrease in water level which can destroy the structure of cover-layer.In the study,it is the first time that Cellular Automata was adopted to simulate karst collapse,there may be set a precedent to karst collapse study.The object oriented method was used to construct the cellular space,and the CA-Collapse model was established,and the karst collapse under certain hydrodynamic conditions was preliminarily discussed,which provided a new idea for the study to simulate and predict karst collapse.