Research On Damage And Deformation Mechanism Of Overburden With Gently Inclined Inner And Siltstone High And Steep Slope

Western Guizhou is the area with the most concentrated coal resources and the largest production capacity,but it is also the area with the most developed high and steep slopes and the most widely distributed sandstone and mudstone in Guizhou.In recent years,there have been many mining geological disasters in this area,with high and steep mining slopes and gentle dip and siltstone as the main lithology.At the same time,there are a large number of high and steep mining slopes with similar deformation characteristics,which pose a great threat to people's lives and property.Based on the analysis of slope structure,mining characteristics and deformation characteristics,a representative slope engineering geological model is established.The stress distribution characteristics and damage evolution rules of overburden in goaf are revealed by using finite difference numerical simulation method and triaxial stress path test Based on a series of discrete element numerical simulation tests,the laws of overburden damage and slope deformation under the control of key goaf parameters are revealed.Finally,based on the above results,the mechanism of overburden damage and slope deformation are systematically revealed.And the application suggestions are given.The main results are obtained as follows:(1)Based on the summary and analysis of 5 typical high and steep mining slopes in the study area: basic geological conditions and slope structure characteristics,coal mining characteristics and slope deformation and failure characteristics,a representative engineering geological model of high and steep siltstone slope with gentle dip is established.It is proposed that the position relationship between goaf and slope shoulder and slope toe is the key factor to control the large deformation of slope,and the relationship between them is also analyzed.There are mainly 4 basic models:the goaf only crosses the toe,the goaf crosses both the toe and the shoulder,the goaf only crosses the shoulder,and the goaf is far away from the toe of the shoulder.(2)Through FLAC numerical simulation test,physical simulation test and dynamic mechanical calculation model considering surface characteristics,the stress characteristics of overburden in goaf of 4 basic models are revealed.The results show that the change of gravity center of overburden caused by the relative position between goaf and slope,and the different damage inside and outside overburden caused by mining sequence are the main factors leading to the significant difference of stress inside and outside goaf.The horizontal distance from the toe to the shoulder of the high and steep siltstone slope with gentle dip and the fitting result of the vertical height difference show an increasing exponential function,which leads to an exponential increase in the vertical load of the overburden from the toe to the shoulder.The stress distribution laws of overburden rock in goaf of three common basic models are obtained:(1)when the shoulder and toe are located above the goaf at the same time,there is a maximum stress difference inside and outside the goaf,and the inner boundary is significantly larger than the outer boundary;(2)when only the shoulder is located above the goaf,the extreme stress value of the outer boundary is larger than the inner boundary;(3)when the goaf is far away from the shoulder and toe,the stress characteristics of the inner and outer sides are basically similar.Considering the horizontal pushing and overburden gravity,the overburden of goaf in the same coal seam shows different confining pressure characteristics from outside to inside of the slope.(3)Through the 36 siltstone standard samples test with 4 confining pressures and3 different stress paths,the damage characteristics and laws of overburden in goaf under different stress environments are revealed,and the rock damage constitutive model based on acoustic emission parameters and statistical damage constitutive model based on Weibull distribution are established.The research results show that: under low confining pressure,the failure of siltstone in the center and boundary of goaf is mainly brittle and splitting failure,and it is easier to be compacted by self weight after failure than fragmentation.Under high confining pressure,the siltstone at the above position has flat and obvious shear plane after failure,which has good integrity and is easy to form overhead structure after failure.The acoustic emission parameter model of siltstone can reflect the whole process of stress-strain state of rock.Its application in PFC bond model correction shows that it has certain advantages in reflecting the accelerated damage of rock due to mining,but it has shortcomings in reflecting the micro fracture characteristics of siltstone.(4)Through a series of stress path tests,3 large-scale physical simulation tests with one kind of key parameters of goaf and 20 numerical simulation tests with two kinds of key parameters of goaf are considered,and the law and mechanism of overburden damage in goaf of high and steep siltstone slope with gentle dip are revealed.The results show that: when the slope shoulder and toe are located above the goaf at the same time and only the slope shoulder is located above the goaf,the overburden damage law presents a similar "sinusoidal function" curve;the former has the maximum damage value,the latter has the maximum damage range,and there is a damage accumulation and release process.When the goaf is far away from the shoulder and toe of the slope,the law of overburden damage will be affected by the width of the goaf.When the width is small,it shows a similar "sinusoidal function" curve;when the width is large,it causes the overall collapse of the overburden,it shows a "hyperbolic" curve.The results show that there is a linear relationship between the slope damage range and the increase of the goaf width;when the goaf crosses the slope shoulder and the slope shoulder,with the increase of the goaf width,the middle peak of the damage factor contour of the overburden turns from the inside to the outside,and the overburden shows the characteristics of deformation to the outside of the slope;the overburden on the inner boundary can be divided into compression damage area,stability area and tension fracture damage area from bottom to top,and its outside is all the overburden strong deformation area.(5)Based on the results of physical simulation test and numerical simulation test,the control mechanism of horizontal displacement on large deformation of the high and steep siltstone slope with gentle dip is proposed,and the distribution law of horizontal displacement on the slope is proposed.The results show that the high and steep slope with free surface can produce large horizontal displacement,which is an important reason for the large deformation of the slope.The horizontal displacement can be divided into three sections from the inside to the outside of the slope: the characteristics of the horizontal displacement from the inside to the shoulder of the slope conform to the increasing "exponential function" distribution,the characteristics of the horizontal displacement from the shoulder to the toe of the slope conform to the "quadratic function" distribution,and the characteristics of the horizontal displacement from the toe to the front of the slope conform to the decreasing "exponential function" distribution.(6)Based on the results of physical simulation test and numerical simulation test,the slope deformation mechanism of different goaf and slope position models of high and steep siltstone slope with gentle dip is revealed,and three typical failure modes are proposed.The results show that: when the shoulder and toe of the slope are located above the goaf at the same time,the deformation can be divided into tension affected area,collapse toppling deformation area and push deformation area from the inside to the outside,and the deformation failure mode is collapse toppling.Only when the shoulder is above the goaf,the deformation can be divided into tension compression affected area,collapse fragmentation area and push deformation area from inside to outside,and the deformation failure mode is collapse slip.When the outer boundary of the goaf is far away from the toe of the slope and only the toe of the slope is above the goaf,the deformation and failure mode is collapse and push.With the increase of goaf width,the deformation degree of slope will be strengthened and the deformation failure mode will be changed.