| The coal and rock structural characteristics are very complex in China, thick and soft roof compose a larger proportion in roadway roof, especially in Shanxi, Anhui, Henan province. These kinds of soft roofs mainly are carbon mudstone, sandy mudstone, carbon shale or siltstone. Their compressive strengths are 5 ~ 40 MPa, the thickness is almost or more than 8 m.The excavation and maintaining of roadways with thick soft roof is costly each year. During the usage of the roadway, as results of the huge deformation of floor rock, roof rock and side roadway rock, the support ability of anchoring seam would decrease or even failure, then cause the whole anchoring seam shear failure or even collapse. How prevent roof collapse and maintain the stability of anchoring seam is the key technique for developing anchor bolt support system. Therefore, a systematic theoretical and in situ study of mechanism and prevention methods of thick and soft roof disaster is necessary.In order to make clear the mechanism of thick and soft roof disaster and the prevention methods, based on the geological conditions of coal roadways with thick and soft roof, with the research methods of in situ test, laboratory testing, numerical simulation, theoretical analysis and in situ application, we systematically studied the deformation and fracture characteristics of thick and soft roof, the geological mechanics of roof, the cracks and energy evolution in roof, the mechanism of roof disaster, the influence factors of roadway stability, the ground control techniques and the roadway safety evaluation system. Lastly, all of the research results were applied in mining project. These results are presented as follows.(1)The deformation and fracture characteristics of thick and soft roofThrough the in situ testing and mining pressure monitoring in 15107 tailegate of Huangyanhui coal mine and 2927 headgate of Luling coal mine respectively, we found the main deformation and fracture characteristics are:(1)The stability of roof is low, and the deformation value of roof mainly more than 200 mm, some even more than 1000 mm and cause the roof overall subsidence.(2)In situ investigation found that rock bolt and cable would decrease corresponding to roof sink, and fracture depth is longer than the length of bolt and cable. These demonstrate that the support structure formulated by cable face challenges in roadways with thick and soft roof.(3)The fracture zone is larger and the stability is lower in side roadway with soft roof than those with hard roof. In roadways with soft roof, the phenomenon of rock bag, collapse of roadway side and hole formulated in side roadway are more obvious, which cause the anchor force of rock bolts in roadway side decrease sharply.(4)The deformation value varies with different location in roadway, the variation has relation to the thickness of roof, strength of coal and rock, the cross-section of roadway, in situ stress and support system.(2) The fracture evolution and disaster mechanism of thick and soft roof(1)With the discrete element simulation software(UDEC5.0), we studied the impacts of the thickness of roof, strength of roof, the depth of roadway, horizontal stress and geological joint on fracture evolution in roof. The results show that with the increase of the thickness of roof, the fracture zone in roof would extend, however, the extended range of fracture zone would become static when the roof thickness pass a critical value, the fracture zone is respond to the cross-section of roadway, character of rock and in situ stress. The height of fracture zone is about 5 m and without huge variation when the thickness of roof is 7m, 9m or 12 m. The density of fracture increases with the decrease of roof strength, but the variation of fracture density is small, which means that the fracture extension in roof depends on the mechanical property of roof and is not sensitive to roof strength.(2)The crack initiation, extension and connection in rock would cause rock failure, in order to study the damage characters of rock, the roof of roadway is treat as a combination of damage body Da and viscous body ηb. through treating rock bolt as elastic-plastic model and treating cable as elastic model, we constructed the roof support model and calculated the deformation value of roof with bolt and cable support system:The cable would break as the deformation value of roof surpass the deformation value of pre-tightened cable, the total deformation value of roof after cable breakage is calculated by the follow formula.(3)The damage procedure of thick and soft roof could be summarized as follow: the fracture propagate and strength decrease in roof, the support strength of bolt support body dropâ†'the rock seam of bolt support body would separate from each otherâ†'the rock seam of cable support body would separate from each other, the force in cable increase sharplyâ†'under the impacts of mining induced stress and water in roof, the subsindence of roof increase significantly and the plastic zone extended dramatically, which may cause extensive roof collapse disaster.(3)The key influence factors of the stability in thick and soft roof(1)With the orthogonal experimental method and FLAC5.0 numerical simulation software, we studied the sensibility of the thickness of roof, strength of roof, the support strength of the roof, the support strength of the rib, the width of roadway and the strength of coal seam, the results demonstrate that: from the roof deformation view, strength of roof, support strength of the roof and the width of roadway are category â… for roof deformation aspect, these factors have high influences on roof deformation; the thickness of roof is category â…¡, the support strength of the rib and the strength of coal seam are category â…£. The rank of the sensibility of all factors is as follows: the strength of roof â†'the width of roadway â†'the support strength of the roof â†'the thickness of roof â†'the support strength of the rib â†'the strength of coal seam. From the rib deformation view, the support strength of the rib and the strength of coal are category â… , and the other factors are category â…£. The rank of the sensibility of all factors is as follows: the strength of coal seam â†'the support strength of the rib â†'the thickness of roof â†'the strength of roof â†'the support strength of the roof â†'the width of roadway(2) Among the category â… factors, we can change the support strength of the roof by altering the length of bolt, distance between each bolt, preload force of bolt, length of cable, distance between each cable and preload force of cable. We designed 25 testing programs to numerically simulate the impacts of the above 6 factors, and the sensibility of these factors are obtained by using FLAC5.0 simulation software. Distance between each cable>preload force of bolt>preload force of cable>the length of bolt = distance between each bolt>the length of cable. The value between the maximum and the minimum value of the influence of the distance between each cable is larger than the other factors, which means that the distance between each cable is more sensible than the other five factors with respect to the stability of roof, and the length of cable is the most unsensible factor among the 6 factors. In order to prevent the thick and soft roof collapse disaster, we should first determine reasonable distance between each cable and the preload force of cable on the basis of roadway geological condition, and then design the length of bolt, the distance between each bolt and the length of cable secondly.(4) The control techniques of roadways with thick soft roof(1)Based on the theory of energy balance, three technical methods were proposed for preventing the thick and soft roof disaster. Firstly, by optimizing the layout of roadway, the roadway would not locate in the stress concentration area and the strain energy accumulated in ground rock would decrease. Secondly, improving the adaptation ability of support system to the deformation of ground rock and preventing the failure of support system caused by high stress. Thirdly, for decreasing the load force on support body, a kind of soft structure was formulated in roadway which could consume elastic energy.(2)The development and innovation of bolt support technology are still the key way to control thick and soft roof collapse disaster. The basic theory is to prevent the progressive failure of roof and the main principle is to reinforce the strength of support body in roof rock. As to guarantee the safety of roof and restrict the deformation of coal and roof in support body, bolts were used to connect separate rock seams in roof into a thick and stable composite beam structure. At the same time, lots of bolts would formulate a strong anchor in the roof in which the strength of rock would increase and the extension of fracture would decrease, what is more, the anchor could provide support force for rock seam outside the anchor area. Lastly, reinforcement of the support body in ribs and roof should be taken out at the same time on the basis of the strengthen measurements implemented in roof rock.(3)As to solve the problems in support system with U-shape steel used in thick and soft roof, a cooperative control method combined the energy releasing drill-hole and leg-lock bolt with U-shape steel was put forward. Then, we analyzed the influence of the length of leg-lock bolt, the preload of leg-lock bolt, the diameter of energy releasing drill hole, the length of energy releasing drill hole on the cooperative control system. The results show that the parameters of energy releasing drill hole have higher impacts on the deformation of roadway than other factors, and the length of the leg-lock bolt has the smallest impact on the deformation of roadway.(5) The safety evolution system of thick and soft roof in roadway(1)As the diversity and complexity of support geological condition and deformation and fracture characters of roadway, six indicators in terms of quality of roof rock, quality of bolt, cable and U-shape steel, the geological condition of roadway, mining activity, strength of support system and layout of roadway were analyzed comprehensively. Additionally, catastrophe progression method was applied to scientifically and reasonably classify the support difficulty of thick and soft roof, the support difficultly in roof was divided into four levels: â… stand for easy support condition, â…¡ stand for medium support condition, â…¢ stand for hard support condition, â…£ stand for worst support condition. Lastly, the corresponding control methods for the different support conditions were proposed.(2)In order to fully estimate the safety condition of roadway with thick and soft roof, ten indicators, such as rock separate indicator, rock deformation indicator, the support structure load indicator and broken rock zone indicator, was put forward. What is more, we proposed the analytic hierarchy process to evaluate the safety of roadway comprehensively. On the basis of multi-parameter monitoring method, we classified the stability of roadway into stable, medium stable, medium unstable and unstable. Additionally, prevention techniques for the four different roadway was proposed to guarantee the safety of the roadway.(6) The in situ prevention project of classical thick and soft roof15111 tailegate in Huangyanhui mine and 2927 headgate in Luling mine were choose as the application in situ project as to verify the liability of comprehensive control method combing the bolt and U-shape steel in thick and soft roof.(1)Firstly, the geological condition of the 15111 tailegate was analyzed, and base on that the roadway was divided into good geological condition zone and geological fracture zone. The catastrophe indicator of the good geological condition zone is 0.843 which means the support difficulty is hard(â…¢). Correspondingly, the catastrophe indicator of the geological fracture zone is 0.918 which stands for worst support condition(â…£). Different prevent methods were implemented in different zones and the monitoring data of the ground pressure and deformation demonstrate that the roof would become stable gradually 2 or 3 months after excavation in good geological condition zone, the deformation of roof is 50~120 mm and the rib is 350~370 mm.For the geological fracture zone, bolts in ribs were replaced by cables which have larger diameter but short length, and strength bolts were applied in upper and low angle of the roadway. The analysis of the monitoring data find that the period of the impact of excavation is about 60 to 80 days, the deformation of roof is from 70 mm to 150 mm, and of the rib is form 600 mm to 800 mm, and the floor is about 500 mm. All of these mean the control method could prevent the roof deformation reasonably, the deformation of ribs, however, is high, especially the fracture propagation in coal pillar is heavy. The air leakage from coal pillar to adjacent gob in geological fracture zone is more than 200 m3, which demonstrates that methods should be taken to strengthen the air flow monitor in roadway and reduce the air leakage to gob. Regulating the air pressure and plugging the fractures in coal pillar are among the measurements to control air leakage.(2)The catastrophe indicator of the 2927 headgate in Luling coal mine is 0.969 which stands for the worst support condition(â…£). Control methods combining the energy releasing drill hole, leg-lock bolts and U-shape steel are implemented in roadway. The ground pressure monitoring data show that the average deformation velocity of floor decrease from 2.0 mm/d to 0.98 mm/d, and the average deformation velocity of the ribs drop from 2.64 mm/d to 1.86 mm/d, the average deformation velocity of the roof, however, is remain at about 0.36 mm/d, the section of the roadway could satisfy the mining engineer demands. |
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