Experimental Study On Negative Pressure Induced By Groundwater Fall

Ground collapse disaster is one of the main geological disasters in my country.Among them,karst ground collapse is the type of collapse with the highest frequency,widest distribution,most complex collapse mechanism,and most severe impact.It is characterized by sudden occurrence,complex prediction,and difficult governance.There are many inducing factors for karst ground collapse,but the influence of groundwater activity is the most obvious.At present,the development and utilization of water resources in our country are becoming more frequent,and the phenomenon of mining and over-exploitation of groundwater is becoming more and more serious.As a result,the number of ground collapses caused by this has increased dramatically.The influence of "vacuum negative pressure" induced by the rapid fall of groundwater is an important indicator for studying the cause mechanism and treatment technology of such ground collapse.This paper takes the rapid fall of groundwater as the starting point,based on the limit equilibrium theory and vacuum absorption theory to study the development process and mechanical mechanism of this type of ground collapse,and uses the cavern negative pressure test device to simulate the rapid fall of groundwater.Through the quantitative and qualitative analysis of the landing height and soil parameters,the main research results are as follows:(1)According to the collapse evolution process of ground collapse induced by groundwater fall,it is defined as the collapse caused by the direct action of external force.And established the collapse discrimination formula.It is analyzed that the main influencing factors that affect the negative pressure in the underground cave are:groundwater drop height,soil compaction,soil moisture content,gravel gradation and soil-rock ratio.(2)Using the underground cavern negative pressure test device,the working conditions are formulated with a soil-rock ratio of 0.1 to 0.4 and 5 kinds of gravel gradation combinations,precipitation tests at different heights are carried out,and the negative pressure change curve is obtained.The negative pressure change law is divided into three stages: the pressure steep drop stage,the pressure slowly rise stage,and the pressure rapid rise stage.(3)Through the comparative analysis of the negative pressure change curve and the negative pressure peak,it can be seen that the negative pressure peak in the underground cave is positively related to the groundwater falling height,but there is a limit to the negative pressure peak,which depends on the upper overburden soil being subjected to negative pressure The shear resistance and collapse resistance and the degree of cracking of the soil body during the action of pulling and tearing;The soil-to-rock ratio and gravel gradation of the upper overburden soil affect the peak size of negative pressure in the cavern by affecting the porosity and shear resistance,collapse resistance and cracking capabilities of the soil;when the soil-to-rock ratio is about 0.3,the weakening effect of the peak value is the smallest,and the negative pressure peak generated is greater;when the soil-rock ratio is between 0.1 and 0.4,the more uniform the gravel gradation,the greater the weakening effect of the negative pressure and the smaller the negative pressure peak,this phenomenon is more obvious when the soil-rock ratio is higher.(4)Through the comparative analysis of the moisture content of the upper overburden soil before and after the precipitation test,it is proved that the suction and erosion of the negative pressure accelerate the water infiltration in the soil.(5)According to the negative pressure peak table and the influence law of the soil-rock ratio and the gravel gradation on the negative pressure peak,summarize the table of weakening degree about the negative pressure peak between the soil-rock ratio and gravel grading and established the negative pressure experience formula.And put forward this kind of ground collapse monitoring and early warning system and prevention ideas.