| Rockburst is a dynamic disaster of ground stress which usually occurs in mining workface or roadway with the characters of sudden elastic energy release, non-linear dynamics, emergentness and serious damage. The geological structures and mining engineering which would lead to energy concentration and its unsteady release, combined with the variation of the geological factors and mining techniques according to time and space sequence, are the main causes of rockburst.Coal seam with high dip angle has more difficulties during excavation, the angle of coal seam distributed in Urumchi in the north-east part of China is about 87°. In the southern of Wudong coal mine, B3+6 and B1+2 coal seam has high dip angle, the upper coal seam was first exploited with fully mechanized top coal caving technique and then the lower seam. With the depth of 350 m, a large number of rockbursts occurred in the roadway, and the characteristics of these rockbursts have relationship with the geological conditions and mining techniques. In order to understand fully of the mechanism of rockburst and design a scientific support technique for roadway, we analyzed the characters and influence factors of rockburst occurred in high-angle seam and the distribution of ground stress field and energy field during mining activities by theoretical analysis, numerical simulation on the basis of in situ monitoring data. The results provide seasonable data for studying the mechanism and types of rockbursts in high-angle coal group in Wudong coal mine. The traditional rockburst prevention method in roadway, such as rock bolts, rock cable and U-shape steel, is insufficient to cope with the rockburst disaster in high-angle roadway. Therefore, Pro. He put forward a new type of bolt with the qualities of constant resistance and large deformation(CRLD). In this paper, we tested the mechanical properties of the bolt and obtained its static tension quality and dynamic impact quality. Based on the current studies of roadway support technique, the principles of rockburst prevention design procedure in roadway and of the support technique with constant resistance and large deformation(CRLD) bolt was summarized, and the results were verified through theoretical and numerical methods. The main research results are as follow.Firstly, we analyzed the technical problems faced by the Wudong coal mine with the tectonic condition of high-angle coal seam, and summarized the research results of rockburst mechanism in this mine zone based on which new rockburst mechanism was proposed. The technical difficult of rockburst prevention in roadway was discussed and the current rockburt prevention methods in roadway were analyzed. It is notify that the traditional rockburt prevention technique cannot meet the demand of support system in roadway with rockburst risk. As the ground rock would deformation seriously under the impact of dynamic load with the current support technique, we proposed a new deformation control support technique with constant resistance and large deformation(CRLD) bolt on the basis of large deformation control support theory including high pre-loading support theory, large deformation for decreasing stress theory and constant resistance for absorbing elastic energy theory.The geological conditions of Wudong coal mine, especially the high-angle coal seam zone surrounded by tectonic structures, were analyzed. What is more, we further investigated the mining techniques in this coal mine. With the method of hollow body stress release, we tested the in situ stress in the mining zone and analyzed the distribute rules and impacts on rock of the in situ stress field. The results show that in this mining zone the in situ stress field satisfy the relation of σH>σh>σv. The horizontal stress mainly caused by geological structures has more influences than the vertical stress. The angle between the horizontal stress and the roadway axis is about 82° which has serious impacts on roadway and rock pillar between two seam. The mechanical parameters and burst liability of B3+6 coal and its roof and floor were obtained by laboratory testing method which demonstrate that the strength of B3+6 coal sample is higher, and the coal seam’s bust liability is medium; the roof and floor of the seam mainly are hard rock and have medium bust liability.We systematically analyzed the time and space character of three rockburst disasters occurred in southern part of the Wudong coal mine, and obtained the occurrence types of rockbursts as well as its impact factors. With the numerical simulation results, we noticed that the stress concentration zone mainly distribute from 30 m to 70 m under the B6 coal seam, and the highest stress concentration zone is around the rock strata 50 m lower than the coal seam and the stress concentration factor is between 2.9 and 3.1. The main reason of stress concentration the dip deformation of the roof strata of B6 seam under the comprehensive influences of unbalance horizontal stress and gravity stress which cause huge squeezing stress on the strata lower than the exploiting seam. The roof has a thickness of 350 m which could cause a stress concentration zone easily. Additionally, in the B3+6 pillar zone, the pillar would dip forward the gob of B1+2 under the impacts of unbalance horizontal stress and gravity stress, these would cause bending force in the floor strata and further leading fractures in the floor, the fractures would lead to decrease of the pillar and cause a stress concentration zone on the other side of the pillar. Based on the systematic analysis of the numerical data and in situ monitoring data as well as the deformation occurrence of coal and rock, we divided the rockbursts occurred in Wudong coal mine into two types: 1, rockburst caused by high stress in strength and thick roof; 2, rockburst caused by moment force in floor strata. These could be verified by the energy theory and mechanic theory.In order to obtain the structural and technical characters of the constant resistance and large deformation(CRLD) bolt, we tested the deformation property under static tensile load. The results demonstrate that the constant resistant and large deformation(CRLD) bolt could resistant large deformation and maintain steady resistance force at the same time, the support load is equal to the designed load. The curve of the static load test could be divided into three phases: load increase phase, constant load phase and load decrease phase. The load increase phase and constant load phase are the same as the ideal plastic curve which means the constant resistance and large deformation(CRLD) bolt has satisfied mechanical mechanics. Additionally, we tested the dynamic property of the constant resistance and large deformation(CRLD) bolt under dynamic load produced by freely falling heavy punch. The results show that the constant resistance and large deformation(CRLD) bolt could maintain the steady support force under the dynamic force. During the dynamic load procedure, the constant resistance and large deformation(CRLD) bolt could maintain a constant force by adjusting the constant force model which could absorbing the dynamic energy and weakening the dynamic load on the bolt body. The average resistant force of each dynamic load is equal to the designed value. These indicate that the constant resistance and large deformation(CRLD) bolt could prevent the rockburst damage reasonably. Based on the characters of rockburst and the damage of ground rock, the rockburst prevention support system has the properties of high support strength, deformation for releasing stress, constant resistant force and absorbing energy. In order to meet the demand of rockburst prevention and constant resistant bolt, the rockburst prevention mechanism of the constant resistance and large deformation(CRLD) bolt was discussed from three aspects pre-load and high support strength, large deformation and absorbing.The significance and necessity of rockburst prevention support technique, as well as the materials of support system are analyzed. The principles of rockburst prevention are as follow: 1, rockburst prevention; 2, pre-estimation; 3, large deformation for absorbing energy; 4, key part support; 5, reinforce the whole support system. The constant resistance and large deformation(CRLD) was proposed to prevent the rockburst, and the auto –couple mechanism of CRLD bolt was discussed on the basis of couple support mechanism.Based on the geological conditions and rockburst occurrence in B3 roadway of +450 mining level in Wudong coal mine, the constant resistance and large deformation(CRLD) bolt was used to design the support system for rockburst prevention. The comprehensive support system comprising large section, 20 t constant resistance and large deformation(CRLD), bar-mat reinforcement, steel belt, 35 t constant resistance and large deformation(CRLD) cable, 35 t constant resistance and large deformation(CRLD) cable in the lower corner of the roadway, was formulated. According to the energy balance theory, the adaptation of the constant resistance and large deformation(CRLD) bolt was discussed. The results show that support system consist of the constant resistance and large deformation(CRLD) bolt could resist 3.2 level rockburst occurred 20 m away the support material. With the numerical simulation method, a dynamic load was applied in the ground rock with low depth, and the deformation velocity is equal to the energy theory results. What is more, constant resistance and large deformation support system and the deformation of surrounding rock are both within the design scope of large deformation. The constant resistance and large deformation(CRLD) bolt could resist the deformation which demonstrate the CRLD bolt could meet the rockburst prevention demand under dynamic load. |
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