Weakly Permeable Overburden Of Water-gas Pressure To Collapse Mechanism Of The Karst Collapse Physical Model Research

Weakly permeable cover has a poor effect of waterproof and breathable, and the relatively closed space is formed with its underlying karst space. In the relatively closed environment of karst voids, when groundwater level under the overburden vacuum suction erosion and air explosion become the main mechanism of karst collapse. Researchers on the study of karst collapse are mainly composed of underground erosion mechanism, but often neglect the influence of water-gas pressure (vacuum suction erosion and air explosion). Few scholars devoted to the mechanism of vacuum suction erosion and air explosion for physical testing simulation.On the basis of summary of karst collapse research both at home and abroad, analyzing the study of karst collapse formation conditions, factors of collapse, and the collapse of a typical generalized model of geological, clay layer is selected as karst overburden. Physical simulation experiment was carried out for the process of vacuum suction erosion and air explosion. Respectively to simulate the deformation law of overburden when the overburden thickness, overburden density and water supply (drainage) conditions are not the same, getting the following conclusion:(1)when groundwater table declining, overburden breaks out the vacuum suction collapse while the summation of overburden’s dead weight and vacuum suction erosion pressure is greater than the shear stress of overburden itself. When groundwater table rising, overburden breaks out the air explosion collapse while the difference value between the pressure inside dissolving cavity and overburden’s dead weight is over the shear stress of overburden.(2) When the density of overburden and the pressure inside of dissolving cavity are under the same changing condition, simulating the overburden deformation rule under different overburden thickness with a low vacuum-compression can reach the following conclusion:the thicker the overburden thickness is, the less the displacement deformation would be, and the overburden would be more stable.(3) When the thickness of overburden and the pressure inside of dissolving cavity are under the same changing condition, simulating the overburden deformation rule under different density can reach the following conclusion:when the vacuum-compression in the dissolving cavity is low, the loose overburden is more stable than the thick one.(4) When the thicknessand density of overburden are the same, simulating the overburden deformation rule under different vacuum-compression can reach the following conclusion:the higher vacuum-compression is, the greater deformation it happens, the more unstable the overburden is; when the water supply rate reaches 6cm/min, overburden occurs karst air explosion, soil of overburden spurt to all direction with the explosive noise.(5) After the analysis to the test data, it can draw such conclusion:the critical deformation value that overburden occurs air explosion is 0.724cm.Using Bolzmann equation to match the relation curve betweencavity plus vacuum-compressionand overburden displacement formation, draws such conclusion:the critical cavity vacuum-compression value is 4342Pa, the predicting critical minus pressure value which leading to the karst collapse is 5420Pa.(6) After force analysis to the overburden, obtains the smallest safe thickness d which keeps overburden stable, under this test condition, when groundwater table rising, the relation curve between safe thickness d and cavity plus pressure P’can be expressed as d≥ 1.15 × 10-3P’; when the water table declining, the relation curve between safe thickness D and cavity vacuum minus pressure can be expressed as D≥ 9.17 × 10-4P.