Stability Analysis Of Surrounding Rock During Re-mining In High-risk Damage Area Of Small Coal Mine

At present,due to various factors,there will be a large number of failure areas in the mining process of small coal mines,and the pressure in these failure areas is larger and more complex than that in the conventional goaf.In the process of remining in the damaged area of small coal mine,when the working face passes through the damaged area,there are a series of problems,such as the intense mine pressure and the instability of coal pillar between the damaged area and the damaged area.Therefore,this paper takes B909 fully-mechanized caved working face of Pingshuo No.2 well as the engineering background,theoretically analyzes the problems of roof fracture and coal pillar instability.By means of the similar simulation test,the rock failure characteristics of the roof of the working face and the coal pillar between the failure areas during the whole process of the reworking working face passing through the failure area are obtained.For the damaged area,the changes of displacement,stress and plastic zone before and after filling were analyzed by 3DEC numerical simulation.Specific research results are as follows:(1)The analysis of roof fracture shows that with the decrease of the distance between the working face and the failure area,the upper surrounding rock of the failure area is prone to fracture in advance of the working face under the action of mine pressure,and the super-long block is formed.However,the existence of failure zone makes the failure zone insufficient to support the upper strata,which leads to the super-long rock mass is easy to slip and fall instability.The stability of coal pillar with different widths is simulated by FLAC^3D,and the results show that with the increase of coal pillar width,the stress value and the plastic zone area decrease gradually.(2)According to the results of similar simulation,the model is in a stable state when the working face is far from the failure zone without filling.However,when the working face advances 100 m,the overlying strata on the model roof collapse in a large area with a collapse height of 30 m.When the working face advances 120 m,the coal pillar slips and becomes unstable.After the coal pillars between the failure zones are mined,the caving range of overlying strata is further enlarged,and the caving height reaches 50 m at this time.When the working face is 24 m away from the failure zone,the whole model caving shape presents an asymmetric trapezoidal shape,and the caving height increases to 60 m.However,when the working face is 42 m away from the failure zone,the caving height rises to 75 m.After pre-filling the failure area,at the initial stage of excavation,the model remains in a stable state,the overburden collapsed in a large area with a collapse height of 30 m after 120 m advance of the working face.Meanwhile,the cracks continued to spread around.After the working face advances 148 m,the collapse of the overlying strata at the top becomes more severe,and the collapse height reaches 45 m.When the working face is 36 m away from the failure area,the caving height is further increased to 55 m.When the working face is 42 m away from the failure zone,the caving height increases to 60 m.(3)The numerical simulation results show that the roof fracture occurs after the working face advances 40 m,and the cracks gradually extend upward to the overlying strata.Then,with the increase of the advancing distance,the roof caving continuously,and the failure zone of surrounding rock gradually presents an asymmetric trapezoidal distribution.The horizontal displacement of surrounding rock increases from 22 cm to 266 cm in the process of propulsion.According to the stress distribution of rock strata,when the working face advances40 m,the arch stress distribution of 4MPa appears in the coal strata in front of the working face.With the increase of the working face distance,the stress arch range decreases but the stress value increases.At this time,the stress value suffered by the failure area is relatively small,which is 1MPa.When the working face is advancing 120 m,the coal pillar between the failure zone is subjected to the stress value of 22.5MPa and the stress concentration coefficient is 5.62.At this time,the stress value above the failure zone is 5MPa and the stress concentration coefficient is 1.25.When the working face passes through the failure zone,the stress mainly concentrates on the roof in front of the working face.(4)After filling in the failure zone,during the process of working face advancing to the failure zone,the strata failure zone extends from the roof to the overlying strata and gradually presents an irregular trapezoidal distribution.When the working face advances 40 m,the roof fracture occurs,and then the rock caving becomes more and more serious with the working face advancing.The horizontal displacement of surrounding rock increases from 21 cm to 264 cm in the process of working face advancing.According to the distribution of rock strata stress,when the working face advances 40 m,the stress is mainly concentrated on the top of the coal pillar between the unmined coal seam in front of the working face and the failure area,and the stress value is 3.5MPa,while the stress value on the top of the failure area is 1MPa.When the working face advanced to the coal pillar between the failure zone,the concentrated stress on the coal pillar was 21.5MPa,and the stress concentration coefficient was 5.36.At this time,the top of the right failure zone was subjected to a stress value of 2.5MPa.Then,with the working face gradually moving away from the failure zone,the stress mainly concentrates on the rock strata in front of the working face.