Study On Law Of Ground Subsidence In Deep Mining Under Extremely Thick Unconsolidated Formation

Based on the research background of deep mining under extremely thick unconsolidated formation of Xieqiao coal mine,the law of overlying strata movement and surface subsidence in deep mining under extremely thick unconsolidated formation was studied by the means of theoretical analysis,discrete element numerical simulation and grey correlation method.The results show that the law of surface subsidence in deep mining under extremely thick unconsolidated formation is similar to that under general geological and mining conditions.However,due to the existence of the extremely thick unconsolidated formation and the special mechanical environment of deep mining,the movement and deformation of the surface show a special law.The creep function of rock layer with time under high in-situ stress was derived from the creep function of rock stratum under low ground stress.A two-medium coupling model of discrete medium in lower bedrock and random medium in upper unconsolidated formation was established,and the model was used to predict the ground subsidence in deep mining under extremely thick unconsolidated formation.Combining with the measured data of surface subsidence in Xieqiao Coal Mine,the rationality of the prediction was verified.Taking unconsolidated formation thickness,mining depth,mining thickness and soil-rock ratio as variables respectively,the effects of four factors on surface subsidence in deep mining under extremely thick unconsolidated formation were studied.The results show that:1)When the mining depth is constant,the surface subsidence increases with the increase of the thickness of unconsolidated formation,which is less than 200 m.The horizontal deformation value of the surface increases with the increase of the thickness of loose layer.When the thickness of the loose layer is larger than 200 m,the surface subsidence will be alleviated,and the horizontal deformation will not increase anymore.When the thickness of the loose layer is thin,the loading effect of the lower bedrock is larger,and the migration of the strata caused by mining can easily convey the loose layer.This causes the loose layer to be compressed and deformed to a large extent,resulting in the larger subsidence value of the surface.With the increase of the thickness of loose layer,the overall loading of overburden becomes smaller,and the deformation and failure of bedrock caused by mining is smaller.When the damage propagates into the loose layer,the thicker loose layer can alleviate and absorb the rock movement,thus making the movement and deformation that can reach the surface smaller.This can reflect in the decrease of the subsidence value of the surface.Because the loose layer structure is soft and the ability to resist deformation is low,the larger the thickness of loose layer is,the greater the tensile deformation of the surface occurs,and the convergence range of the subsidence basin will also expand accordingly.However,because of the increase of the thickness of loose layer,the migration of rock layer propagating to the surface decreases gradually.When the horizontal deformation of the local surface reaches a certain value,the deformation value of the rock layer will not continue to increase with the increase of the thickness of the loose layer.2)The surface subsidence is negatively related to the mining depth.The larger the mining depth,the smaller the surface subsidence value is.When the mining depth increases,the surface horizontal displacement shows a tendency of decreasing.Under certain condition of thickness of the loose layer,the stope stress increases with the increase of mining depth,and the damage degree of the mining field increases gradually.During the gradual upward propagation,the mining subsidence is limited by the gradual thickening of the bedrock.The bending deformation of the bedrock will prevent the propagation of the damage,and finally,the vertical displacement and horizontal displacement of the surface are reduced.3)The subsidence and horizontal displacement are proportional to the mining thickness.Both of them increase with the increase of mining thickness.Mining thickness determines the size of extraction space,and the increase of mining thickness increases mining space.Under the condition of extremely thick loose layer and deep mining,the mining area is in the state of high in-situ stress,and the overburden strata are destroyed by the action of high stress.The increasing mining space provides enough space for the collapse of overburden strata.Therefore the larger the mining thickness,the more intensive the mining is.Due to the existence of the extremely thick loose layer,the gradually increasing damage spreads to the surface of the earth through the loose layer,causing subsidence settlement of the surface and increase of horizontal displacement.4)The surface subsidence value and the surface horizontal displacement show the opposite trend with the increase of the ratio of soil to rock.The surface subsidence value decreases first and then increases while the horizontal movement of the surface increases first and then decreases with the increase of the ratio of soil to rock.The ratio of soil to rock reflects the composition of overlying strata.When the ratio of soil to rock increases gradually(the thickness of loose layer increases and the thickness of bedrock decreases),the mining failure of stope decreases gradually.The mining failure is not easy to spread to the surface of the earth,so the subsidence value of the surface is small.At this time,the failure degree of stope is larger,and the spreading range of rock fracture is also large.This reflects the large horizontal movement range of the surface.When the ratio of soil to rock increases,mining deformation is easier to spread to the surface of the surface,so the value of surface subsidence increases gradually.The larger the ratio of soil to rock,the smaller the failure degree of stope and the smaller the failure range of strata is.The convergent range of surface subsidence basin will also become smaller.By using the method of grey correlation analysis,the correlation degree of loose layer thickness,mining depth,mining thickness and soil-rock ratio with surface subsidence and horizontal displacement was studied.The mining depth is the most important factor that affects the maximum subsidence value of the surface,that is,the mining depth is the main control factor of the surface subsidence.The mining thickness is the most important factor affecting the horizontal movement value of the surface.Mining thickness is the main control factor of surface horizontal movement.