Research On The Mechanism And Technology Of Roof Control Of Gob-Side Entry Retaining With Heap Spraying Concrete

Gob-side entry retaining has technical advantages such as no coal pillars left,more resources extracted,less excavation of tunnels for dual use,and the implementation of Y-shaped ventilation,which is beneficial for gas management.It is in line with the concept of green mining and has become the development direction of mining deployment in underground mines.How to efficiently maintain the roof is the key and difficult point of the goaf retaining technology,and the key to this technology is the support beside the roadway.The side support of the gob-side entry retaining should not only meet the requirements of bearing capacity,but also adapt to the complex and ever-changing geological conditions and rapidly advancing coal mining face.With the development of surrounding rock control theory and technology,non pillar mining along goaf entry is widely used in China.However,due to the complex and ever-changing geological conditions underground,there are still some problems in the control of the roof of gob-side entry retaining,mainly reflected in the concept of surrounding rock control,the adaptability of the process to environmental changes,and the integration of construction efficiency and working face advancement speed.Therefore,for the non pillar mining of goaf retaining tunnels that require supporting walls along the roadway,the theory and technology of roof control,the supporting materials and bearing structures of the roadway,construction techniques and methods are still important issues that need to be solved in the promotion and application process of gob-side entry retaining.Based on the demand traction of non pillar mining along goaf retaining roadway,comprehensive application of research methods such as theoretical analysis,indoor experiments,numerical simulation,similar material simulation,and industrial experiments has revealed the mechanism of roadway side support based on the characteristics of roof activity area;A safe and efficient method of constructing supporting structures through heap spraying has been proposed,and tunnel side support materials with rapid resistance increase in the initial stage and high strength stability in the later stage have been developed;Studied the structural characteristics of the roadside support body,and constructed a roof mechanical model for roadside support and temporary maintenance inside the roadway;Based on the key technology of heap spraying,the roof control process for gob-side entry retaining is provided,forming a theoretical,material,structural,and technological system for the roof control of heap spraying wall retaining tunnels along the goaf,and carrying out engineering practice.The main work and research achievements of the paper are:（1）Through the analysis of roof activity patterns and stability,it has been found that the structural form of the roof of gob-side entry retaining is significantly affected by the basic roof fracture rock block B.The main forms of instability of the roof of gob-side entry retaining are cutting,rotating,breaking,and caving.Combining the FLAC^3Dnumerical simulation method,considering the strength effect and resistance increasing effect of roadway side support under roof control,the influence mechanism of early initial support strength and later final strength of roadway side support on the safety control of goaf retaining roadway roof is elucidated;On this basis,the migration activity of the roof of gob-side entry retaining is divided into development area,active area,and stable area in sequence,and the mechanism of the roadway side support function that is coordinated and coupled with the bearing performance,compression deformation,and roof rock layer activity process is revealed.This lays a theoretical foundation for the selection of roadway side support materials and the design of structural parameters.（2）A safe and efficient method for constructing support structures on the side of roadways using heap spraying has been proposed.Its characteristics are that the construction does not require formwork and vibration,and the all-round roof connection is not limited by the cross-section and size of the roadway;We have developed roadway side support materials that can adapt to the characteristics of roof movement and mining pressure patterns of gob-side entry retaining.A study was conducted on the SEM microstructure,strength characteristics,and failure characteristics of reactor spray specimens.The strength growth process was divided into three stages:rapid growth in the initial stage（aged 0-14 days）,sustained growth in the middle stage（aged 14-28 days）,and stable growth in the later stage（aged>28 days）.The appropriate shrinkage of the support body on the side of the pile spraying roadway can adapt to the"given deformation"of the roof.The rapid increase in resistance in the initial stage can provide support for the roof as early as possible,and the high-strength stability in the later stage can resist the dynamic pressure effects such as roof breakage and collapse,meeting the load and position requirements of roof control,and providing material support for the roof control of gob-side entry retaining.（3）By installing roof anchor cables and bottom anchor bolts above and below the support body on the side of the roadway,a"roof-wall-floor"integrated support system for the side of the roadway is formed,which suppresses the rotational instability or sliding deformation of the supporting body on the side of the roadway.Based on the system stiffness,the cross-sectional shape of the roadside supporting body is optimized as a right angled trapezoid,to prevent the roadside supporting body from drilling to the bottom and weaken the impact of the floor plate on the support system stiffness;Based on the construction of reinforcement,steel mesh is installed on both sides of the roadside supporting body and tension steel bars are installed inside,which improves the bearing strength and shear deformation resistance of the supporting structure.Through similar simulation experiments,the overlying rock structure of the structure was mastered,and a mechanical model of the roadway side support and temporary maintenance inside the roadway was constructed,providing a design basis for the roadway side supporting structure and parameters for roof control.（4）A jet motion model of the spraying material was constructed,and the motion process of the spraying material from leaving the nozzle to reaching the sprayed surface was inverted.The process of self compacting and mold free forming of the spraying structure was divided into two stages:high-speed jet impact and impact response.Based on the fluid dynamics method,a supplementary air joint is set up in the middle of the conveying pipeline and connected to a high-pressure air source to overcome the resistance along the route and achieve the effect of relay long-distance conveying,achieving efficient construction of roadside support bodies.Through the analysis of the hydration reaction of the pile spraying material,it is clear that the dust generated by the heap spraying concrete is mainly the non wetting cement;By adding water in stages at a distance of 3-5 meters from the nozzle,the material is mixed and wetted in advance,achieving"wet spraying of dry materials".The overall process for controlling the roof of gob-side entry retaining is provided from the aspects of reinforcement of the roadway roof,construction process,load-bearing performance of the roadside wall,and supporting structure,providing technical and technological support for engineering practice.（5）In response to the mining engineering conditions of the material transportation roadway in the 11212-2 fully mechanized top coal caving working face of Xin’an Coal Mine,Jizhong Energy Fengfeng Group,roof control parameters were provided from three aspects:reinforcement of anchor cables in the roadway,stacking and spraying concrete walls next to the roadway,and initial temporary maintenance of the retained roadway.Industrial experiments were conducted on retaining the roadway along the goaf,achieving good application results,and verifying the feasibility and reliability of the method of stacking and spraying walls for retaining the roadway along the goaf and its roof control technology.This provides a new technical method for achieving goaf retaining in mines with complex geological conditions in China.