Pressure relief and structure stability mechanism of hard roof for gob-side entry retaining

Transcription

1 DOI: /s x Pressure relief and structure stability mechanism of hard roof for gob-side entry retaining HAN Chang-liang( 韩昌良 ) 1, ZHANG Nong( 张农 ) 1, 2, LI Bao-yu( 李宝玉 ) 1, SI Guang-yao( 司光耀 ) 3, ZHENG Xi-gui( 郑西贵 ) 1 1. School of Mines, Key Laboratory of Deep Coal Resource Mining of Ministry of Education, China University of Mining and Technology, Xuzhou , China; 2. Hunan Key Laboratory of Safe Mining Techniques of Coal Mines, Hunan University of Science and Technology, Xiangtan , China; 3. Earth Science and Engineering, Imperial College London, London W6 8EP, England Central South University Press and Springer-Verlag Berlin Heidelberg 2015 Abstract: In order to explore the pressure relief and structure stability mechanism of lateral cantilever structure in the stope under the direct coverage of thick hard roof and its impact on the gob-side entry retaining, a lateral cantilever fractured structural mechanical model was established on the basis of clarification for the stress environment of gob-side entry retaining, and the equation of roof given deformation and the balance judgment for fracture block were obtained. The optimal cantilever length was proposed based on the comparison of roof structural characteristics and the stress, deformation law of surrounding rocks under six different cantilever lengths by numerical simulation method. Double stress peaks exist on the sides of gob-side entry retaining and the entry located in the low stress area. The pressure of gob-side entry retaining can be relieved by reducing the cantilever length. However, due to the impact of arch structure of rock beam, unduly short cantilever would result in insufficient pressure relief and unduly long cantilever would bring larger roof stress which results in intense deformation. Therefore, there is optimal cantilever length, which was 7 8 m in this project that enables to achieve the minimum deformation and the most stabilized rock structure of entry retaining. An engineering case of gob-side entry retaining with the direct coverage of 10 m thick hard limestone roof was put forward, and the measured data verified the reasonability of conclusion. Key words: hard roof; pressure relief; cantilever length; double stress peaks; gob-side entry retaining 1 Introduction Gob-side entry retaining locates in the side border of gob and is inevitably impacted by the roof movement during mining [1]. The pressure control for the gob-side entry retaining is mainly to control the movement of main roof and to guarantee the integrity of the immediate roof simultaneously. When the coal seam is covered with thick hard main roof without softer immediate roof, the side wall, coal rib and other structures owned no soft buffer strata [2] and are directly impacted by the main roof pressure, developing strata behaviors significantly. Under this circumstance, it is difficult for the side wall to carry the roof and achieve effective edge support, making them easy to be deformed and fractured during the roof movement and thus losing the stability of entry. Currently, there are a lot of researches dealing with the impact of hard roof on the coal mining [3 6]. Both Engineers practically discovered the characteristics of hard roof of square-easy-collapse [7], which means that the roof will only be collapsed in larger-scale and the roof break suddenly when the working face progressing distance is equal to the working face length, bringing strong disturbance to the stope. WANG et al [8] studied the rheological breaking features of hard roof in gob. HE et al [9 10] studied the formation mechanism of rock burst and believed that the thickness, strength and breaking depth of roof strata were the sensitive factors of dynamic load. HIDALGO and NORDLUND [11] compared the laboratory test result and numerical simulation result, concluding that the laboratory test data could predict the excavation damage process of hard rock. VILLAESCUSA et al [12] studied the method for analyzing the performance quantization of resin anchor for hard roof during underground mining [12]. The Foundation item: Project( ) supported by the National Natural Science Foundation of China; Project (BK ) supported by the Natural Science Foundation of Jiangsu Province of China; Project (2014XT01) supported by the Fundamental Research Funds for the Central Universities, China Received date: ; Accepted date: Corresponding author: ZHANG Nong, Professor, PhD; Tel: ; zhangnong@126.com

2 4446 current processing techniques of hard roof in stope mainly include deep- hole directional hydraulic power presplitting [13 15], deep-hole blasting presplitting [16 17], and so on. However, there were few research achievements involving the impact of hard roof on the gob-side entry retaining and the established conclusions believed that the presplitting of hard roof could relieve the entry pressure, and shorter lateral cantilever of the support was advantageous to the stability of entry structure [18]. In fact, arch-like structures are often formed between the broke rock beams [19]. Unduly short cantilever will surely make the block outside the fracture incline to the cantilever and impose load to the cantilever, thus resulting in the insufficient pressure relief. Since the rotation sink of roof couldn t be controlled completely due to the limited load-carrying properties of side support for the entry, the deformation of entry roof was characterized as given deformation. Unduly short cantilever also will cause over large rotation angle, leading to the heavy sink of roof. Viewed from the perspective of roof fracture balanced structure as well as the given deformation characteristic of roof, this work focused on studying the stability mechanism of pressure relief for hard roof and its impact on gob-side entry retaining. It obtained the balance characteristics of side fracture block through theoretical analysis and numerical simulation and carried out a comparative analysis on the stress and deformation law of entry surrounding rock under the condition of six different cantilever lengths, putting forward the optimal cantilever length. Furthermore, a verified engineering of entry retaining with the condition of 10 m thick hard limestone roof was introduced. 2 Lateral cantilever roof structure of stope and its load-imposing effect on entry retaining The roof of gob will deform and fracture after losing the coal restrain, and transfer the load to the surrounding solid coal in which the abutment pressure will form (Fig. 1), while after severe mining disturbance, the bearing capacity of shallow part of coal body beside gob will decline sharply, leading the gob-side entry retaining in decreasing pressure. An abutment pressure zone and a decreasing pressure zone existing respectively from deep to shallow is the typical characteristic of stress environment for gob-side entry retaining. In the broken roof the main roof strata often breaks into regular blocks that extrude and integrate mutually to form the voussoir beam structure and develop A, B and C key blocks in the gob side (Fig. 2). Fig. 1 Stress environment of gob-side entry retaining In most cases, the coal is covered by the immediate roof and main roof strata successively (Fig. 2(a)) and the key block B of main roof can touch the rock, be compacted and stabilized once its rotation angle reached θ 1 with the stress concentration area produced correspondingly in the compacted area. After the roof movement reached stabilization, five stress zones were distributed from the solid coal to the gob successively and the gob-side entry retaining located under the block B, in the area with low stress. The immediate roof plays an important coordination role in the main roof movement, which on one hand, relieved some roof pressure through its fractures closing and lateral deformation and on the other hand, the falling immediate roof in the gob could support the rotating blocks in time to control the rotation angle θ 1 within a smaller range, thus preventing the entry from suffering over large load. The roof movement and pressure behavior will be specific when the coal is covered by thick hard roof directly (Fig. 2(b)). The stiffness of thick hard strata is relatively high and broken blocks are relatively long. Block B constantly imposes pressures to the entry surrounding rock (especially the side wall) during its rotation and subsidence until touches the floor to stabilize. The direct supporting of main roof by side wall may produce seven stress zones in the gob-side. Stress concentration area appears on the wall, which divides the sides of wall into two low stress areas (one internal and one external), and the stress concentration coefficients of the front and back are significantly higher than those of the condition with immediate roof. Since there s no gangue support, the rotation angle θ 2 is far larger than θ 1. The side wall of entry is impossible to control the movement of block B, thus keeping roof in the state of given deformation ultimately. Heavy deformation, with which stabilization of rock can be destroyed, will take place in the entry surrounding rock under the strong roof pressure.

3 4447 Fig. 2 Structure and stress distribution of gob-side fractured roof: (a) Condition with immediate roof (I In situ stress area; II Front stress concentration area; III Low stress area; IV Back stress concentration area; V Compaction stress area); (b) Condition with direct coverage of thick hard roof (I In situ stress area; II Front stress concentration area; III Internal low stress area; IV Stress concentrated area on walls; V External low stress area; VI Back stress concentration area; VII Compaction stress area) 3 Mechanism of pressure relief and fracture stabilization for roof presplitting 3.1 Pressure relief mechanism of roof presplitting With respect to the condition of thick hard roof (Fig. 2(b)), since the primary strata fracture is relative thick and the load on the block B is very large, it is easy for block B to slide down and cause strong pressure and extremely large impact on the under entry. However, the length of block B can be shortened significantly by the roof processing with blasting presplitting within a certain range of outside wall of gob-side entry retaining. After presplitting, block C will lose the lateral restrain and collapse firstly to form the under layer, whereas block B rotates and sinks in later period and integrates with C mutually on the fracture to form stabilized hinge-like structure. The stress concentration degree will be decreased and the given deformation of roof will be reduced through the optimization of hanging block length, thus relieving the deformation of entry surrounding rock effectively. After mining, the roof strata will experience the progress of mutual separation and mutual lamination successively, while the key blocks of main roof strata all separate before forming the voussoir beam structure rather than moving in mutual compaction and integration all the time. At this moment, the key block B is in the state of a cantilever structure. Block B loses the lateral restrain and imposes all load to the below rock, during which the gob-side entry retaining suffers the maximum pressure. The key strata theory deemed that the soft strata above the key strata can t form structure and its weight is imposed on the key strata. According to the balance equation, it can be known that the relationship between the wall pressure σ and roof cantilever L is h L i i (1) B 2x 2w B) ( 0 3 where γ i and h i are the volume force and thickness of main roof as well as its above soft strata, respectively; L is the length of main roof cantilever; B is the width of side wall of entry; x 0 is the horizontal distance of main roof from the fracture line within the coal to the entry

4 4448 and can be determined through Eq. (2); w is the width of entry retaining. x 0 m 2 tan φ 0 C0 kh tan φ0 ln C p 0 x tan φ0 (2) where λ is the lateral pressure coefficient; φ 0 and C 0 are the internal friction angle and cohesion of the interface between coal seam and roof as well as floor strata; k is the stress concentration coefficient; γ is the average volume weight of overlying strata; H is the cover depth of coal seam; p x is the supporting strength of coal rib. Since the main roof bears the weights of several soft strata, it will surely rotate and sink until the outside cantilever touches a stable supporting point once the length of the cantilever reaches certain extent. Such sink brings the under entry given deformation. The mechanical model was established on this basis, as shown in Fig.3. The mutual compaction of block B and block C enables block B to reach balance in advance without touching the floor strata and produces spatial distance s with the floor. Analyzing its structural relationship, it can be known the maximum roof subsidence c on side wall is ( x w B)( m s) c 0 (3) L where m is the mining height. If the side wall is filled with paste, it will often produce certain insufficient roof-touching height δ. Therefore, the roof subsidence on wall is composed of the insufficient roof-touching height δ and wall compaction e, that is, c=δ+e. Combined with the Hooke s law, it can be known that the wall pressure σ under the state of given deformation is E[( x 0 w B)( m s) L ] L( m ) where E is the elastic modulus of wall. (4) Fig. 3 Mechanical model of gob-side entry retaining under hard roof Based on the above analysis and viewed from the perspective of roof load imposing, the longer the cantilever, the higher the stress that the wall has to bear. For guaranteeing the wall stress smaller than its strength, the cantilever length must be smaller than certain value, with which the upper limit of cantilever length can be determined. Viewed from the perspective of given deformation, the longer the cantilever, is the smaller the given deformation will be and the lower the compaction the wall will produce. The compaction is proportional to the load bearing. It is necessary to control the compaction for the purpose of preventing the wall from being destroyed, thus the lower limit of cantilever length can be determined. This reveals that cantilever length has a reasonable interval and both unduly long and unduly short cantilever are disadvantageous for entry. Further presentation is provided in view of Fig. 4. When the roof break line is near to the wall (Fig. 4(a)), it is obviously unnecessary for block B to rotate until reaching the floor rock, but block C will quickly contact with block B and mutual extrude after rotary sink and impose large force to block B, finally developing the arch structure. After the structure reaches balance, blocks B and C achieve similar rotation (shown by the rotation angles θ 2 and 2 ) and impose pressure to the entry retaining together in the three-hinged arch structure. Under this circumstance, the pressure relief efficiency of roof presplitting decreases significantly. However, when the roof break line is selected in the appropriate position (Fig. 4(b)), the outside block B sinks greatly and block C has almost become horizontal when it reaches stabilization, which not only can stop the further rotation of block B by imposing larger horizontal force to it, but also can produce certain vertical support force. Therefore, the wall only needs to bear a part of weight of block B at most, which shows obvious pressure relief efficiency. 3.2 Stabilization mechanism of fracture surface Crush circle, crack circle and vibration circle can be produced successively surrounding the blasting hole under the shock wave formed by presplitting blasting. The former two play critical role in the formation of fracture surface on rock beam. Their radii can be determined through Eqs. (5) and (6) [20]: R 0 bp r mc p( Cp a) (5) mcp( Cp a) R 1 R0 (6) brc where R 0 and R 1 are the radii of crush circle and fracture circle, respectively; r is the radius of blasting hole; a and b are the rock-related constants determined by the experiment, valuing 3.50 and 1.43 for limestone respectively; P is the initial pressure of shock wave on the hole wall; ρ m is the density of primary rock; C p is the elastic P-wave velocity in the rock; R c is the compression strength of rock; α is the relationship between damping exponent of stress wave and impedance of rock wave, α= ρ m C p.

5 4449 Fig. 4 Impact of presplitting line position on roof structure and entry retaining: (a) Over narrow space for roof break line; (b) Reasonable position of roof presplitting line The contact points between blocks are in plastic state due to the existence of crack circle and the block state after rotation is shown in Fig.5. The friction on occlusal spots is opposite to the slide direction of block, which stops the rock slide. According to the equilibrium principle of voussoir beam, to guarantee the structure keeping stable, it is necessary to satisfy that: Q T tan (7) B where Q B and T are vertical force and horizontal force between blocks respectively and φ is the friction angle between blocks. Fig. 5 Force analysis of key blocks It can be deduced from the equilibrium principle of three-hinged arch that: h m L 2R 0 1 tan 2 (8) where h is the thickness of main roof. The spatial distance s between block B and floor strata can be computed according to the geometrical relationship of block structure: 2 s h ( h m) 4R0 ( L R0 ) cos2 (9) Equation (8) is the condition for ensuring stability between key blocks. It can be known that under given thickness of main roof and mining height as well as small radius of blasting crush circle, the length of key block B determines whether it will lose stability. The longer the block B is, the stronger the stability of the structure will be. If the key block B falls for losing stability, the given deformation to the below rock will be increased correspondingly, which results in the more concentrated stress on surrounding rock in entry retaining. Therefore, it is beneficial for the stabilization of surrounding rock structure in entry retaining by increasing the length of key block appropriately. 4 Impacts of cantilever length on gob-side entry retaining 4.1 Model establishment and experimental method A numerical analysis was carried out using the calculation software UDEC (Universal Distinct Element Code) based on discrete element method in order to further reveal the impacts of gob side cantilever on the surrounding rock stability, stress and deformation of gob-side entry retaining. The established model is of 150 m (Width) 60 m (Height) with its two sides and bottom edge using horizontal displacement restrain and displacement restrain respectively. A vertical stress of p=

6 MPa was applied to its upper edge. Mohr-Coulomb model was selected for its material. The simulated strata conditions and parameters are shown in Fig. 6 and Table 1, respectively. The gob-side entry retaining is of 4.8 m (W) 2.5 m (H), which was supported by lengthening anchorage with bolts and cable. The side wall with 2.4 m width, was filled with paste-like materials. The immediate roof of the coal seam was 10 m hard limestone and the roof presplitting was simulated by preset notches on different roof positions. Considering the impact of construction factors and blasting on the entry retaining, the cantilever length shall be larger than 0 m. Six plans were designed with the cantilever length ranging from 2.5 m to 15 m, increasing 2.5 m one time. 4.2 State of surrounding rock motion After the coal mining, the states of surrounding rock motion at gob-side entry retaining under three different cantilever lengths was selected as shown in Fig. 7. When the cantilever was 2.5 m long, the roof was cut directly near the outer edge of wall. The outside blocks couldn t fall completely due to the impact of cut space, which finally mutually combined with inside blocks and imposed some pressure on the inside blocks, leading to the insufficient roof pressure relief. When the cantilever was 7.5 m long, the outside blocks subsided completely to become the supporting point of inside blocks and bore some pressure of inside blocks. In this case the roof produced the minimum given deformation to entry retaining. When the cantilever was 15 m long, the inside blocks achieved enough rotation and sank to touch the floor strata, while outside blocks failed to support the inside blocks, thus weakening the pressure relief efficiency significantly. 4.3 Impact of cantilever on roof stress A horizontal monitoring line was set in the roof limestone 0.25 m away from the coal seam and the vertical stress was output, as shown in Fig. 8. The Fig. 6 Numerical simulation model Table 1 Strata parameters of numerical simulation Bulk Shear Density/ Lithology (kg m 3 modulus/ modulus/ ) GPa GPa Cohesion/ MPa Internal friction angle/( ) Joint normal stiffness/ (GPa m 1 ) Joint shear stiffness/ (GPa m 1 ) Joint cohesion/ MPa Joint friction angle/( ) Overburden Limestone Coal Sandy mudstone Mudstone Fig. 7 Surrounding rock motion state and displacement distribution of gob-side entry retaining: (a) 2.5 m long cantilever; (b) 7.5 m long cantilever; (c) 15 m long cantilever

7 4451 original rock stress in the position was 5.64 MPa. An abutment stress zone existed in both sides of entry retaining above the solid coal and the wall respectively, forming the double stress peaks with the entry retaining locating in the zone of low stress between the two peaks. The peak of coal side was between 8 10 m long from the entry coal rib, which increased with the increasing of the cantilever length within the range of m and m. It increased more under the later condition. For instance, when the cantilever was 2.5 m long, the abutment pressure coefficient was 1.96, but the coefficient was 2.44 when the cantilever was 7.5 m long, and was 2.57 when the cantilever was 15 m long. The roof stress apparently had been transferred to the deep part for the peak value of 7.5 m was greater than that of 10 m and 12.5 m. The peak of wall side located at the position of 0.5 m away from the wall rib, which decreased with the increasing of cantilever length. For example, when the cantilever was 2.5 m long, the abutment pressure coefficient was 1.73, which decreased to 1.39 at 12.5 m and further to 1.06 at 15 m. This indicated that with the increasing of roof sink, the wall performance weakened continuously and its control ability to roof decreased continuously. However, when the cantilever was 7.5 m long, the abutment pressure coefficient was 1.85, in good bearing state. The abutment stress is the comprehensive reflection of roof load and the bearing capacity of below rock. The peak value and peak difference between the double stress peaks reflected the coordination between coal seam and wall load bearing. The peak difference between the double stress peaks valued 3.36 MPa under 7.5 m long cantilever, achieving the best coordination in all plans. under different cantilever lengths were obtained, as shown in Fig. 9. With the increasing of cantilever length, the surrounding rock deformation formed two peaks successively at m and m of cantilever length. The second peak was larger than the first one with valley value between these two peaks where better surrounding rock deformation achieved than other conditions. For instance, the first peak of coal rib deformation appeared at 5 m cantilever when deformed 760 mm, and the second peak appeared at 12.5 m cantilever when deformed 817 mm, while the valley appeared at 7.5 m cantilever when deformed 744 mm. This verified the result of theoretical analysis, that is, the surrounding rock is easy to be extruded by outside rock block under unduly short cantilever and thus bear some stress of outside rock block, while it will bear too much load under unduly long cantilever. The reasonable cantilever length lies between them and 7 8 m is the best. Fig. 9 Deformation distributions of entry-retaining under different cantilever lengths 5 Engineering verification Fig. 8 Roof stress distribution of entry-retaining under different cantilever lengths 4.4 Impact of cantilever on surrounding rock deformation The maximum deformation of entry retaining was monitored with partial cross section method and the maximum deformation distributions of surrounding rock 5.1 Engineering overview The average mining depth of working face in Fenghuangshan Mine was 208 m. The 15# coal seam, whose average dip angle was 5, Protogyakonov s coefficient of strength f ranges from 2 to 4 and volume weight was 1.5 t/m 3, is being mined. The immediate roof was K2 limestone with an average thickness of 10 m and average compression strength of MPa, identified as hard and stable roof. The floor rock was mudstone or aluminum mudstone, known as soft strata. The distribution for roof and floor strata of the coal seam is listed in Table 2. The working face was designed with a strike length of 783 m and an inclination length of 176 m. During the recovery period, gob-side entry retaining was implemented in the rail entry. The working face applied the comprehensive mechanization mining, and the gob-side

8 4452 Table 2 Strata distribution of working face Order Lithology Thickness/m Lithology description 3 Limestone Sandy mudstone K2 limestone # coal seam Mudstone Aluminum mudstone Limestone 9.62 Dark grey and contains calcite Grey black and contains pyrite. It will be phase changed to fine sandstone locally Dark grey. Dense and hard. Contain phytolith and flint Submetallic luster. Contain bright coal mainly as well as one layer of 0.45 m dirt band Black and contains phytolith Light grey-grey black and contains pyrite Grey. Rich fracture. Filled with mud wall of entry retaining was filled with special concrete mechanically pumped by high-power filling pump, as shown in Fig. 10. The filling frame support (hereinafter referred as frame) was selected at the face end region near the entry retaining. Two groups of chock-shield hydraulic supports were installed in the front part of frame, behind of which an additional ancillary shoring was installed. A pulling jack was installed back to the support base, which was connected with the rear filling formwork. The wall side of entry retaining was filled with paste-like filling materials composed of cement, coal ash, sandstone, compound additives and water, whose 28 d measured strength could reach 22 MPa. The material was transported to the stock ground near the working face after being prepared on the ground and then mixed with water evenly in the filling pump before being sent to the filling frame through the pump. Waiting for a certain time after accomplishing the filling, Fig. 10 Filling sketch of gob-side entry retaining the filling wall would demould the support through pulling the jack. The processing for cutting coal could be implemented normally during the time waiting for demoulding. New filling space will be formed by moving the frame forward to continue the next filling and mining cycle. 5.2 Presplitting scheme roof caving Based on the above studies, presplitting blasting for the roof should be used ahead of the working face to shorten the lateral cantilever, aiming to reduce the impacts of the hard roof. The blasting range on the roof strata was determined at the position of 10 m vertical above the coal seam according to the abovementioned analysis, and a group of entry retaining-exclusive presplitting blasting holes was set every 30 m, four holes (A, B, C and D) in sector distribution for each group (Fig. 11). The ZDY1300 hydraulic drill with d50 mm drill stem diameter, 75 mm broach diameter and mm drill hole diameter was applied for drilling. The parameters of blasting hole are presented in Table 3. The 2# mining emulsion explosive (d60 mm 500 mm/ cartridge and 1.4 kg/cartridge) was used. The roof cutting range was m in vertical height and m in inclination depth. The explosive was installed and Fig. 11 Plane graph of presplitting blasting holes distribution

9 4453 blasted when the coal rib of face was 5 m away from the bottom of blasting hole. 5.3 Analysis for impacts of cantilever on entry retaining The cantilever lengths at different positions of entry were calculated according to the geometrical parameters of splitting line, and site test was carried out to test the surrounding rock deformation and roof crush. The observation sites are shown in Fig. 11. The roof cantilever length was larger than 9 m at observation site 1, about 7 m at observation site 2 and about 4 m at observation site 3. The peeped situation from the observation holes on the roof of gob-side entry is shown in Fig. 12. When the cantilever length was larger than 9 m, the roof produced large bending moment driven by the cantilever, while the original fractures inside the rock expanded and secondary fractures were developed simultaneously, forming obvious crush and vertical or horizontal fractures (Figs. 12(a, b)), which decreased the load bearing capacity and stability of roof. When the cantilever was about 7 m long, blocks formed by the Table 3 Presplitting blasting holes parameters Hole Diameter/ mm Loaded length/m Explosive load/kg Sealing length/m Roof cutting depth/m Roof cutting height/m A B C D Fig. 12 Comparison on peeped situations of roof of gob-side entry: (a, b) 9 m long cantilever; (c, d) 7 m long cantilever; (e, f) 4 m long cantilever

10 4454 by the presplitting could establish relative stable structure and the bending moment produced by strata was relative small, relieving internal stress and enabling to maintain higher integral stability (Figs. 12(c) and (d)). When the cantilever was about 4 m long, the block outside the fracture surface rotated and sank to fall on the inside block, imposing certain load to the inside block and thus resulting in the insufficient pressure relief. Under this circumstance, clear fractures were still developed inside the rock (Figs. 12(e, f)). Periodic changing was witnessed in surrounding rock deformation of entry retaining through the field observation, which found basically accordance with the distribution of cantilever lengths. The deformations of entry retaining under different cantilever lengths are shown in Table 4, revealing that the minimum deformation is produced under 7 m long cantilever, followed by that under 4 m long cantilever. However, the maximum deformation was produced when the cantilever was longer than 9 m. When fractures were produced on side wall, taking the wall rib for instance, the deformation under 7 m cantilever decreased by 8.6% and 16.9% respectively compared with later two conditions. Table 4 Comparison on deformation of entry retaining under different cantilever lengths Cantilever Deformation/mm length/m Roof Floor Coal rib Wall rib Conclusions 1) Under the direct coverage of thick hard roof, there s double stress peaks existing on two sides of gob-side entry retaining and the coal side achieved higher peak than wall side. The entry lies in the low stress zone between these two peaks. 2) The surrounding rock of entry retaining is influenced by the given deformation of roof. The rock beam of main roof will get larger rotation angle and given deformation without the gangue supporting during its sinking period. 3) The pressure of gob-side entry retaining can be relieved by shortening the roof lateral cantilever through presplitting blasting. The pressure relief efficiency is determined by the balanced structure of blocks after breakage. If the cantilever is too short, the block outside the fracture surface will incline to and will contact with the inside block to form arch structure and then impose pressures to the entry retaining together, thus resulting in the insufficient pressure relief. However, if the cantilever is too long, the roof will suffer unduly large pressure, thus making the surrounding rock of entry retaining deform significantly and lose stability. There s an optimal cantilever length. 4) In this project case, the broken blocks can establish stable structure and the surrounding rock of entry retaining achieves the minimum deformation when the cantilever length is controlled at 7.5 m. Under this circumstance, the double stress peaks on two sides of entry retaining are relatively coordinated. The entry deformation reaches peaks respectively when the cantilever length lies within m and m with the later one higher than the former one. 5) Field observation indicated that surrounding rock of entry retaining produced periodic deformation. The deformation reaches the maximum and roof suffers heaviest damage when the cantilever length is larger than 9 m, followed by that of 4 m long cantilever when the roof is destroyed obviously. It achieves the minimum deformation and roof keeps basically integrate when the cantilever is 7 m long. A good accordance is found among the theoretical analysis, numerical simulation and site measurement. References [1] SUN Heng-hu, ZHAO Bing-li. Theory and practice of gob-side entry retaining [M]. Beijing: China Coal Industry Publishing House, (in Chinese) [2] ZHANG Nong, YUAN Liang, HAN Chang-liang, XUE Jun-hua, KAN Jia-guang. Stability and deformation of surrounding rock in pillarless gob-side entry retaining [J]. Safety Science, 2012, 50(4): [3] PAN Yue, WANG Zhi-qiang, LI Ai-wu. Analytic solutions of deflection, bending moment and energy change of tight roof of advanced working surface during initial fracture [J]. Chinese Journal of Rock Mechanics and Engineering, 2012, 31(1): (in Chinese) [4] LIU Chuan-xiao. Numerical simulation of moving features of hard roof with three-dimensional discrete element method and nonlinear dynamic analysis [J]. Rock and Soil Mechanics, 2005, 26(5): (in Chinese) [5] WU Li-xin, QIAN Ming-gao, WANG Jin-zhuang. The influence of a thick hard rock stratum on underground mining subsidence [J]. International Journal of Rock Mechanics and Mining Sciences, 1997, 34(2): [6] DUAN Hong-fei, JIANG Zhen-quan, ZHU Shu-yun, SUN Qiang, LIU De-qian, YANG Wei-feng. Centrifuge model tests on rock brusting induced by great depth highly stressed roof strata of weak structural plane [J]. Journal of Central South University: Science and Technology, 2011, 42(9): (in Chinese) [7] QIAN Ming-gao, SHI Ping-wu, XU Jia-lin. Mine pressure and strata control [M]. Xuzhou: China University of Mining and Technology Press, (in Chinese) [8] WANG Jin-an, LI Da-zhong, SHANG Xin-chun. Creep failure of roof stratum above mined-out area [J]. Rock Mechanics and Rock Engineering, 2012, 45(4): [9] HE Jiang, DOU Lin-ming, CAO An-ye, GONG Si-yuan, LÜ

11 4455 Jian-wei. Rock burst induced by roof breakage and its prevention [J]. Journal of Central South University of Technology, 2012, 19(4): [10] HE Jiang, DOU Lin-ming. Gradient principle of horizontal stress inducing rock burst in coal mine [J]. Journal of Central South University of Technology, 2012, 19(10): [11] HIDALGO K P, NORDLUND E. Failure process analysis of spalling failure-comparison of laboratory test and numerical modelling data[j]. Tunnelling and Underground Space Technology, 2012, 32: [12] VILLAESCUSA E, VARDEN R, HASSELL R. Quantifying the performance of resin anchored rock bolts in the Australian underground hard rock mining industry [J]. International Journal of Rock Mechanics and Mining Sciences, 2008, 45(1): [13] HE Hu, DOU Lin-ming, FAN Jun, DU Tao-tao, SUN Xing-lin. Deep-hole directional fracturing of thick hard roof for rockburst prevention [J]. Tunnelling and Underground Space Technology, 2012, 32: [14] FAN Jun, DOU Lin-ming, HE Hu, DU Tao-tao, ZHANG Shi-bin, GUI Bing, SUN Xing-lin. Directional hydraulic fracturing to control hard-roof rockburst in coal mines [J]. International Journal of Mining Science and Technology, 2012, 22(2): [15] HOSSAIN M M, RAHMAN M K. Numerical simulation of complex fracture growth during tight reservoir stimulation by hydraulic fracturing [J]. Journal of Petroleum Science and Engineering, 2008, 60(2): [16] WANG Kai, KANG Tian-he, LI Hai-tao, HAN Wen-mei. Study of control caving methods and reasonable hanging roof length on hard roof [J]. Chinese Journal of Rock Mechanics and Engineering, 2009, 28(11): (in Chinese) [17] ZHANG Jie. Study on forced roof caving to hard roof by deep-hole pre-blasting in shallow coal seam [J]. Journal of Mining & Safety Engineering, 2012, 29(3): (in Chinese) [18] WANG Ju-guang, WANG Gang. Discussion on gateway retained along goaf technology with roof breaking and pressure releasing [J]. Coal Engineering, 2012(1): (in Chinese) [19] QIAN Ming-gao, MIAO Xie-xing. The oretical analysis on the structural form and stability of overlying strata in longwall mining [J]. Chinese Journal of Rock Mechanical and Engineering, 1995, 14(2): (in Chinese) [20] XU Ying, LIU Yong-sheng, FU Ju-gen, ZONG Qi. The blasting grouting in weak-layer zones: theory and practice [M]. Hefei: University of Science and Technology of China Press, 2008: (in Chinese) (Edited by YANG Hua)