Table of Content

15 July 2016, Volume 33 Issue 4

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Research on permeability evolution law of goaf and pressure-relief mining effect

TU Shihao，ZHANG Cun，YANG Guanyu，BAI Qingsheng，YAN Ruilong

2016, 33(4):  571-577. 

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In order to study the permeability evolution law and distribution characteristics of goaf during protective seam mining and its pressure-relief mining effect, a calculation model to calculate gas flow rate of drilling well extracting gas from goaf and pressure-relieved seams has been simplified. This model has made full use of gas extraction monitoring data of the working face 1242(1) of a coal mine in Huainan city. Combined with Darcy’s law, the equivalent permeability of goaf has been obtained. Meanwhile, by fitting gas extraction volumes from protected seam, the decline coefficient of gas extraction has been obtained. A decrease multiples of decline coefficient has been introduced to reflect the pressure-relief performance of protective seam mining. The results have shown that: 1) The permeability evolution of goaf has experienced three stages including sharp rise stage, the decline stage, and the stable stage. Longwall face advancing distances of the first two stages are 19.24 m and 186.2 m respectively. The permeability in the middle of the compacted goaf is much smaller than the permeability on the edge of goaf, and presents O shape trend after longwall face mining. 2) Gas extraction volume from protected seam has also experienced three stages including sharp rise stage, the decline stage and the stable stage. The time of the first two stages are 10 d and 125 d. 3) Decrease multiples of decline coefficient of 1^# drilling well is as high as 7.7, which has indicated that the protective seam mining has great pressure-relief effect.

Research on the upper protective coal seam mining effect induced by coal thickness and interburden rock properties

CHEN Yanlong，WU Haoshuai，ZHANG Mingwei，WU Yu，ZHANG Houquan，ZHANG Guimin

2016, 33(4):  578-584. 

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Based on numerical simulation, a new model has been established on the effectiveness of protective mining due to the influence of the mining thickness of the upper protective coal seam, the thickness of the protected coal seam, and interburden rock strata properties. The results have shown that if the mining thickness of the upper protective coal seam increases but still is less than the occurrence thickness of the lower protected coal seam, or if the occurrence thickness of the protected coal seam increases but the mining thickness of the upper protective coal seam remains the same, the protective effects will increase gradually. But once the mining thickness of the upper protective coal seam is larger than the occurrence thickness of the protected coal seam, the protective effect will not change basically.Furthermore, the harder the interburden rock stratum between coal seams is, the less the plastic deformation and stress reduction of the lower protected coal units will be, and therefore the worse the protective effect will become. Finally, the stress reduction would be larger if the differences of the neighboring rock stratum are large in coal and rock mass, and both the longitudinal squeezing caused by imbalanced vertical stress and the transverse shearing caused by imbalanced horizontal stress are the important precipitating factors that lead to the eccentric instability of coal and rock units under the gob and then result in the plastic deformation and failure.

Bearing characteristics of filling body and supporting intensity of working face during coal pillar mined with paste backfill

GUO Weijia，JIANG Ning，WANG Hailong，CHEN Shaojie

2016, 33(4):  585-591. 

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Taking paste backfill working face 2351 for an example in Daizhuang coal mine, bearing characteristics of filling body and supporting intensity of working face during coal pillar mined with paste backfill have been studied. Based on the analysis of the motion characteristics of roof in coal pillar mined with paste backfill, the mechanical model of roof stability during coal pillar mined with paste backfill has been established, and the mechanics related formula of supporting intensity has also been concluded. Research has shown that: the strength of paste is high, elastic modulus and Poisson’s ratio are lower, the discreteness of mechanical properties is obvious, and that the paste has typical plasticity hardening features; The overlying strata of paste backfill working face has breaks, bed separation and bending zone rather than caving zone; The distribution of vertical stress above the filling body is not a horizontal line nor saddle-shaped as the strip coal pillar but presents wave distribution. The stress above the filling body is not mutational but tends to be increasing and stable at last. The hydraulic supports used in paste backfill face 2351 can control the roof subsidence effectively, but the efficiency of hydraulic supports is lower, which should be optimized properly.

Study on mechanical model of aquifuge beam supported by filling strip in the water preserved mining

HUANG Qingxiang，LAI Jinqi

2016, 33(4):  592-596. 

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According to the typical conditions of special water preserved mining area, the aquiclude stability with two equispaced filling strips has been analyzed through the physical simulation experiment and theoretical analysis. The elastic foundation beam model of aquiclude supported by the strip filling has been set up, and the compression displacement of the filling strip and the deflection formula of the aquiclude beam have also been put forward. Furthermore, the position and depth of the downward crack have been determined. Based on the aquiclude stability criterion, the calculation method of reasonable filling strip width and interval width has been determined. The result has shown that the reasonable strip interval increases with the increase of the mining height, the thickness and strength of the beam. The depth of the downward crack mainly develops with the increase of the strip compression displacement.

The design of critical filling ratio in close distance coal seams by upward backfill mining technology

HUANG Peng，LI Baiyi，XIAO Meng，CHEN Zhiwei，PEI Yulong

2016, 33(4):  597-603. 

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Analyzing mechanics of overburden movement between traditional collapse upward mining and backfill upward mining, this paper has found that it would fail to mine in a certain close distance coal seams because upper coal and rock structures are damaged when applying traditional collapse upward mining. But backfill upward mining can reduce the damage level of the upper coal seam with a certain filling ratio. Using composite beam limit rupture and plastic zone development height discriminance to design filling ratio, the condition of backfill upward mining feasibility has been proposed, and then the backfill upward mining critical filling ratio has been obtained. Results have shown that according to geological conditions of Papaya mine , when upper coal seam (9^# coal) does not break, the critical filling ration of coal mining in sub-coal seam (10^# coal) is 82.1%, when plastic zone development height of sub-coal seam (10^# coal ) reaches upper coal seam (9^# coal), the critical filling ratio of the former is 70.4%, which means when implementing backfill mining 9^# coal in Papaya mine, critical filling ratio for backfill mining is 82.1%.

Spatiotemporal coupling law of mining pressure, strata movement and fracture field distribution in deep stope

WANG Xinfeng，GAO Mingzhong，LI Longqin

2016, 33(4):  604-610. 

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With three typical deep mine working faces of Huainan mining area as research background, the dynamic evolutionary characteristics and spatiotemporal coupling effect of mining pressure, strata movement and fracture distribution in deep stope have been studied by using the comprehensive methods composed of numerical simulation, similar simulation and field monitoring. Furthermore, the dynamic response mechanism of mining stress field, overlying strata displacement field and roof fracture field has also been investigated. The results have shown that mining stress is significantly affected by mining progress, and the range within 20 m before and after working face squaring is the significantly affected zone of stress, with the dynamic response of the transient evolution features between mining stress and progress. Strata movement has strong spatial-temporal view, the same layer of strata with the speed increasing vertical displacement into a "Z" shaped distribution. Roof fracture range of plastic zone and mining advance progress keep synchronous coordination, and expansion space changes from lower to upper, damage depth from outside to inside, and affecting time from short to long. Roof breaking has spatial and temporal characteristics of transient, piecewise extension and partition mobility, strata fracture field has experienced spatial and temporal evolution process of pressure relief and instability losing, extensional cracking, fracture shrinking and turning small, fitting and closing. Mining response mode of advancing time and overburden failure has been obtained by field monitoring, and research results have provided a theoretical reference for the prevention of deep rock dynamic disasters.

Rib spalling mechanism and control with fully mechanized longwall mining in large inclination “three-soft” thick coal seam under closed distance mined gob

YANG Ke，HE Xiang，LIU Shuai，LU Wei

2016, 33(4):  611-617. 

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Rib spalling in “three-soft” thick seam with fully-mechanized and large height mining is difficult to control, especially under closed mined gob, and is still a main safety hazard. Based on geological conditions of No.12124 longwall panel in Panbei coal mine and its strata behaviors during initially extracting process, characteristics and factors of coal spalling have been investigated through lab experimental analyzing, theoretical calculating, and computer simulating with FLAC^3D. Strata behaviors, such as modes of coal wall spalling, mining-induced stress evolution characteristics, deformation and breakage area, have been systematically analyzed and obtained during the mining advancement in No.12124 longwall panel and after No.12125 longwall panel was mined. Coal properties, abutment pressure superposition of multiple-seam mining and performance of hydraulic support are the key factors to result in rib spalling. And the rib spalling mechanism with multiple mining-induce stress superposition in closed distance seams under complex geological conditions has been revealed. Technical measures have been proposed and put into practice, such as to increase support resistance, infuse water into coal, and pave double mesh networks. Engineering practice have shown that three technological applications have achieved safety effectiveness and provided references for longwall panel layout, spalling control, and hydraulic support design in similar geological conditions.

Mining plan simulation and rock migration analysis in steep seam mining

HAN Guang，QI Qingjie，CUI Tiejun，WANG Laigui

2016, 33(4):  618-623. 

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In order to understand the influence of steep seam mining on the surrounding rock mass in complex structure, the mining process has been simulated based on PFC^3D of particle flow. The coal seams are in synclinal left with horizontal fracture, and coal seam dip angle is 87°. Simulation scheme has been divided into 11 cycles, and the simulated mining depth is 240 m. This paper has analyzed the damage characteristics of rock mass in the excavation process, and put forward some possible measures. Simulation results have shown that the right side of rock mass of 40 m wide mined-out area is the most stable area. The left side of the rock mass dumps to the left before bending to the right. The left side of the rock mass of 30 m wide mined-out area is stable; the right side of the rock mass damage is the most serious. The mass between two mined-out areas is left dumping, leading to obvious subsidence in this part of surface. Before 30 m wide coal mining, rock mass on the right side should be reinforced, or the level constraints be imposed on rock mass; and the right side of rock mass of 40 m wide may not be reinforced.

Mechanism study of floor water inrush around mining field based on micro-crack extension

GAO Yubing，LIU Shiqi，LYU Bin，LI Kunqi

2016, 33(4):  624-629. 

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Based on the mine ground pressure and strata control theory, spatial distribution characteristics of ground pressure and floor water pressure around mining field have been analyzed in this paper, and then the mechanical effects of both these pressures on the coal floor have been determined. By setting up mechanics model, the effects of ground pressure and water pressure on a set of tilted cracks have been studied from a microcosmic point of view. Based on the thin plate theory, the effects of water pressure on the effective water-resisting layer have also been studied from a macroscopic point of view. Finally, a theoretical formulae of limiting hydraulic pressure has been deduced. The results have shown that the increase of abutment pressure can lead to the decrease of permeability coefficient; the increase of water pressure can cause the attenuation of rock mass strength and the increase of fissure length. The essence of water inrush in mining field is the result of propagation of micro-cracks and fracture damage of the effective water-resisting layer caused by ground pressure and water pressure.

Rock burst mechanism analysis on deep irregular island face

ZHU Guang’an，DOU Linming，LIU Yang，SU Zhenguo，LI Hui

2016, 33(4):  630-635. 

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To study the influence of deep irregular island face mining on rock burst, based on the spatial structure theory, the characteristics of overburden structure above island face 3112 have been analyzed in this paper, and the effect of large scale fracturing on the coal and rock has also been analyzed. Also the dynamic information monitored by microseismic monitoring system during roadway excavation has been added for the dynamic evaluation of rock burst risk at the working face. Meanwhile, through theoretical analysis and numerical simulation, the mechanism of rock burst induced by static and dynamic combined load in island pillar coal face has been analyzed. Based on energy conservation law, the release and conversion of the energy at the moment of the key strata fracture have been studied. Subsequently, the influence law of different mining scale effect on island pillar burst has been studied with the method of numerical simulation. The greater the mining scale is，the higher the risk of rock burst will be. Finally, the technologies in controlling rock burst have been developed for island face based on the mechanism. The engineering application has presented that strong tremors can be reduced by releasing pressure through large diameter drilling and break-tip blast in deep hole.

Theory and application of deep hole floor-break blasting in floor rock burst coal mine

ZHAO Shankun，LI Liyun，WU Baoyang，LIU Jun，OUYANG Zhenhua

2016, 33(4):  636-642. 

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The small deflection theory of elastic thin plate has been used to analyze the roadway floor stress condition and the main influence factors of floor rock burst have been figured out. Theory analysis and numerical simulation have been used to study the rock burst prevention mechanism and optimization scheme about deep hole floor-break blasting and the control effect have been tested in practice. It has been found that the mining horizontal stress is the key reason for floor heave and the layer thickness of floor, and that mechanical properties and the supporting intensity of roadway surrounding rock are the main influence factors of floor heave. If the floor strata is dense thick mudstone, the rock burst will happen for the elastic deformation energy released in the process of floor layer broken. The essence of deep hole floor-break blasting is the combination of borehole pressure relief and blasting vibration method for rock burst prevention. When the position of blasting holes in the middle of the floor and roadway centre line, not only the high elastic deformation energy accumulated in the floor can be released and the ability of continuously transfer stress/ energy be destroyed, but also the high stress in the roadway side can be released, which has made the prevention effect more apparent.

Research on mechanism for floor heave control in the roadway by base-angel-bolt and its type selection test

YANG Jun，SHI Haiyang，QI Gan

2016, 33(4):  643-648. 

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The geological and mechanical conditions of roadways become more complicated compared with those in the shallow part with the mining depth increasing. Consequently, the nonlinear mechanical phenomena especially the problem of floor heave in the roadway occur frequently. In order to explore the most effective type of cable fixed in floor, at first, the mechanism of floor heave controlled by the base-angle-bolt has been analyzed in two ways including cutting off plastic slip line effectively and decomposing pressure according to the mechanical analytical model of the bolt controlling roadway floor heave. Then six types of base-angel-bolt with different materials and specifications have been designed to carry out bending test. According to the optimized analysis of the results, the most appropriate type of I-3 has been determined and successfully applied in the engineering practice of Kongzhuang -785 mine in Datun. And the displacement monitoring curve has indicated that an obvious effectiveness has been obtained by applying the type of I-3 base-angle-bolt.

Mechanics performance testing and research of truss support in the soft rock roadway

ZHANG Guofeng，XIE Yi，XU Liying

2016, 33(4):  649-654. 

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The roadway with soft rock has such problems as high ground pressure, strong rheology, and large deformation in deep coal mining in China, so the large supporting force is needed to stabilize the surrounding rock. The author has put forward a new technology with truss support, and the truss is made of eight 12^# double I-Beams. Each beam connects another beam with 390 mm×800 mm×30 mm welded steel plate and bolted connection. In laboratory the truss mechanical property about ultimate load and deformation has been achieved by the monitoring system with different combination with horizontal load and vertical load that is stepwise added. The results have shown that under the uniform pressure ultimate bearing capacity of the truss is 1 500 t, and the ultimate deformation is 12.18 mm; when the ratio of the vertical load to the horizontal load is 0.2, ultimate load of the truss is 900 t, and the elastic deformation limit is 26.3 mm; and the failure is the connection slide and the diameter shrinkage damage. Compared with the traditional U-shaped steel, the double truss support counterforce has been increased to 10 times. In the soft rock roadway, the author has put forward the first support with constant-resistance large-deformation anchor coupling support and reserved deformation; the support can release the deformation energy. After forming the anchor-surrounding rock coupling support circle, the strong truss force support has been used in the second support; this way can make full use of mechanical property. The truss structure may have an extensive application in fields of the deep roadway with the soft rock, high ground pressure and strong rheology in future.

Study on concrete strain of inclined shaft lining in deep alluvium during freezing sinking period

WANG Qianxing，YANG Weihao，WANG Yansen

2016, 33(4):  655-661. 

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For the purpose of studying the safety of inclined shaft lining in the period of freezing sinking, variations of concrete strain and steel stress of shaft lining have been carried out based on in-situ measurement of main inclined shaft in Hami Large South Lake mine. The structure safety of shaft lining has been discussed by taking ultimate concrete tensile and compressive strain into account, besides which, several optimized suggestions have been put forward. The result has shown that variations of concrete strain could include four periods: disordered period, accelerated increasing period, slow increasing period and stable period, and the valid concrete strain should be taken into account after disordered stage. The design of inclined shaft lining should mainly be controlled by concrete tensile strain. During the construction of main inclined shaft of Hami Large South Lake mine, the maximum hoop tensile strain of floor has been up to 1 280 με, which is considerably greater than the ultimate tensile strain value, and the maximum hoop tensile strain of circular section has exceeded 200 με with the shaft lining facing the conditions of fracturing.

Study on instability mechanism and thickness optimization of temporary ore wall based on cusp catastrophe model

WANG Yiming，XU Heng，WU Aixiang，AI Chunming，WU Peng

2016, 33(4):  662-667. 

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Aiming at the instability of temporary ore wall system, a main problem of non-pillar continuous mining, this paper has applied cusp catastrophe theory to analyze the instability mechanism of ore wall structure on the basis of the study of stability mechanical model of temporary ore wall structure. The cusp catastrophe equation of ore wall buckling instability has been obtained, as well as the necessary and sufficient condition of ore wall unstable failure. On this basis, the calculation method and the effect factors for limit thickness of ore wall unstable failure have been deduced and three control measures taken to decrease the thickness and increase the stability of ore wall have also been proposed. Combined with an example of Fenghuangshan copper mine in Anhui, the theoretical calculating value of its ore wall thickness is supposed to be bigger than 5.061 m, while the suitable value in production is 5.5 m, which has verified the theoretical value in industry. The industrial results have shown that the application of the cusp catastrophe model in the stability analysis and structure design of ore wall system is both reasonable and feasible.

Dynamic caustics test on the inclined cracks in roadway surrounding rock subjected to static-dynamic load

GUO Dongming，YAN Pengyang，YANG Renshu，YUAN Baosen，ZHOU Baowei

2016, 33(4):  668-675. 

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The transmitted caustics experimental system has been used to conduct the dynamic caustics research about the propagation law of cracks and the poly methyl methacrylate (PMMA) plates have been employed to make model specimens. The results have shown that: 1) All the b endpoints of the prefabricating cracks propagate under the action of the stress wave and generally extend along the horizontal direction with a phenomenon of warping seen from the propagation path; 2) As the dip angle θchanges, the extension displacement of b endpoint has the rule of variation between increasing and decreasing, and the extensiondisplacement curve is generally symmetrical about the linear θ=0° from -45° to 45°, but it has an apparent difference from -75° to -45° and 45° to 75°; 3) When θ=-45°, θ=0° and θ=60° theextensiondisplacement curve reaches the peak values of 22, 31 and 36 mm respectively, and reaches the low values of 9 mm and 11 mm when θ=±30°; 4) The change curve of stress intensity factor K_Ⅰ^d and the change curve of energy release rate G have a similar trend, which are both at a peak value at first, then the repeated oscillation change with peak value for several times; 5) The values of stress intensity factor and energy release rate are closely related to energy, which means the displacement of corresponding crack is larger when the values both have a relatively higher level, otherwise it is small relatively.

Study on instability fracture evolution of deep thick coal seams based on discrete element simulation

FANG Gang，YANG Shengqi，SUN Jianzhong，CHENG Jianlong

2016, 33(4):  676-683. 

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To study the instability fracture evolution process of roadway surrounding rock in deep thick coal seam, taking some coal mine as the engineering background, based on the discrete element, this paper has used the random distribution of triangle model unit blocks to analyze the failure process during roadway excavation process. According to the displacement and stress evolution laws of instability fracture process of surrounding rock in deep thick coal seam, the horizontal stress release has been found to be more intense in two side coal-walls than that in roof and floor of the roadway, while the vertical stress is the opposite because the main vertical stress release part is in the middle of the roadway. Numerical simulation results have shown two mains instability fracture evolution characters of surrounding rock with thick coal seam roadway. Firstly, there exists "spire" catastrophic collapse in roadway roof, and the middle of roadway is key position of surrounding rock instability. Secondly, roof and floor of the roadway delaminate seriously. The failure mechanisms corresponding with the test results have also been analyzed including factors such as the soft and broken features of roof and floor coal petrography, poor stability, the radial stress relatively intense release in roof and floor of roadway, poorbearing capacity in roof and floor of roadway with rectangular cross section, all of which have caused roof and floor of surrounding rock delamination deformation. Based on the failure mechanism of deep buried roadway with thick coal seam, a supporting method consisting of anchor cables has also been given, which can be applied in roadway support, and simulation results have shown that the supporting method can effectively control deformation of thick coal seam roadway.

Large deformation control technology for expansive and weak-cemented soft rock roadways in Shajihai coal mine

HAO Yuxi，WANG Jiong，YUAN Yue，WANG Xiaolei，ZHU Guolong，HE Manchao

2016, 33(4):  684-691. 

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The roadways excavated in expansive and weak-cemented soft rocks in Shajihai coal mine have caused serious large deformation problems such as roof failing, side collapse and floor heave in the field. In this paper the X-ray diffraction, SEM scanning, mechanical tests and water absorption tests have been performed to study the characteristics of this soft rock and mechanism of deformation and failure of roadway based on the actual site situation. Research on deformation mechanisms have shown that the main reasons for roadway failure is that the surrounding rocks are with features of weak-cemented and strong swelling ability. When exposed to severe excavation disturbance and low strength support, the surrounding rock cannot match the deformation of the support system by traditional anchor-bolt-mesh support, and the capacity of surrounding rock is wasted in roadway supporting system. Based on the above, the support capacity of surrounding rock has been employed for energy absorbing and ground pressure decreasing, and water isolation for reinforcement has also been performed. Based on the theory of absorbing energy support using Constant Resistance and Large Deformation (CRLD) bolts and anchors, a new support scheme has been put forward, which is the CRLD bolt + mesh + sprayed concrete layer + CRLD anchor + bottom anchor pipe. Field monitoring results have shown that the large deformation of weak-cemented soft rock has been better controlled than before by the application of new support scheme.

Deformation mechanism of narrow coal pillar in the fully-mechanized gob-side entry with incompletely stable overlying strata

ZHANG Hongwei，WAN Zhijun，ZHANG Yuan，MA Zhaoyang，ZHANG Jian，LIU Sifei，GE Lingjian

2016, 33(4):  692-698. 

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In order to understand the deformation mechanism of the narrow coal pillar in gob-side entry employing TCC under unstable overlying strata, borehole inspection camera has been utilized to observe the boreholes in the narrow coal pillar. Evaluation method of broken level based on the basic measuring scale has been proposed to undertake the statistical analysis of borehole fracture and the characteristic of fracture distribution has also been investigated. The results have shown that: 1) Transverse fractures and beddings are the two main fracture forms of the pillar; abnormal fracture zone is mainly located at both sides of the pillar; besides, there exist many transverse fractures with certain space intervals and small fractures in the middle of the coal pillar. 2) According to different broken levels, the pillar can be divided into three zones, viz., fracture zone A, stable zone B and fracture zone C; fracture zone A is near the roadway while fracture zone C is near the gob. Because of the asymmetry of the geometric distribution of these three zones, the stable zone is closer to the fracture zone C. 3) The key to control the deformation of the coal pillar is to control the rotating and slippage of the key block, and its coupling support control method has been proposed.

Study on surrounding rock deformation control of finishing cut in backfilling mining face

LI Meng，ZHANG Jixiong，JU Feng，LIU Shiwei，CAO Yuanwei

2016, 33(4):  699-705. 

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To solve support dismantling problems caused by big cross-section finishing cut and to control strata movement in solid backfilling mining face，this paper has adopted the theoretical analysis and FLAC^3D numerical simulation method to study laws of surrounding rock deformation and failure mechanism of finishing cut. Then the comprehensive support technology of anchor (rope) mesh reinforcement, temporary support of “#” type wooden cogs filled with grouting and permanent support of finishing cut grouting have been put forward. In addition，the supporting parameters of concrete pier consisting of wooden cogs has been optimized. The results have shown that when the reasonable spacing of concrete pier is 2 000 mm, the pier bearing capacity can satisfy the requirement of temporary support. The technology has been applied in Yangzhuang coal mine of Huaibei Coal Mining Group, and the accumulated sink value about 80 m above the working face has been only 60 mm, which has indicated that surrounding rock deformation control of finishing cut is well controlled.

Study on bearing and failure laws of roadside support influenced by end restraint

LI Guodong，CAO Shugang，YANG Hongyun

2016, 33(4):  706-712. 

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In order to acquire the regularity of deformation and destruction of the support in gob-side entry retaining, FLAC^3D has been used to build the model of roadside support. By using trial method, the reduction function relationship between cohesion of the model and interior angel and plastic sheer strain has been obtained, which helped to ensure that the stress-strain relationship gained from numerical model and filling materials in the physical compression test is consistent with the fracture law. Based on such parameters, the deformation and destruction characteristics under different end constraints and inclination angles have been analyzed. The main conclusions are as follows. Without end restraint, the stress distribution within support before peak is approximately uniform with unlimited lateral deformation, and the dip angle does not have obvious impact on the peak load before plasticity. With end constraints, the peak stress of support increases a little and lateral inhomogeneous deformation occurs. The destruction forms of the support differ according to different angles: when the angle is between 0° and 10°, multiple shear failure occurs; when it is between 10° and 25°, the double crack shear failure is produced; when it is greater than 25°, the top-bottom boundary friction shear failure presents. Additionally, due to the slope of coal seam, with end restraints, the stress concentration of sharp angle is higher, so it should be reinforced during supporting.

Analysis of influencing factors of interaction between double-shield TBM and surrounding rock in squeezing ground

CHENG Jianlong，YANG Shengqi，LI Xuehua，PAN Yucong

2016, 33(4):  713-720. 

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In order to study the interaction between double shield TBM and the surrounding rock affected by the LDP relax ratios, overcut, conicity of the shield, length of shield and lateral pressure coefficient, a completely three-dimensional TBM model usingFLAC^3D has been adopted to investigate the features of the surrounding rock deformation, the contact force and the frictional resistance act on the shield based on the above influence factors considering the non-uniform gap between shield and surrounding rock. Moreover, the LDP curve relax ratios has also been defined in this paper. The simulation result has indicated that the LDP curve release rate and the resistance act on the shield of TBM have been affected significantly when the stress release rate exceeds a specific value, therefore a sensitivity analysis should be conducted under the specific engineering geological conditions. The nonuniform gaps between the shield and ground will gradually close from invert to crown and subsequently contact the shield surface and the most significant contact areas occur at the end of the front shield and rear shield. Both the friction force acting on the front shield and rear shield and the total friction force will increase linearly with the increasing overcut and appropriate increasing of overcut can effectively reduce the shield resistance; the total friction force acting on the shield will increase linearly with the increasing of the length of shield as well; with the increasing shield conicity, the friction force acting on cutterhead and front shield and the total friction force will increase slowly while the force acting on rear shield will decrease obviously. The frictional resistance acting on the front and rear shield is remarkable when the lateral pressure coefficient is more than 1.2. The above achievements have important reference significance for further understanding the interaction between TBM and surrounding rock and for forecasting TBM entrapment subsequently.

Study on efficiency promotion of dragline with single-operation cycle

SUN Jiandong，ZHANG Ruixin，MA Xingen，ZHANG Shuai，WEN Xiaoke，WU Linlin

2016, 33(4):  721-727. 

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In order to improve the efficiency of dragline operation，statistical study has been made on time consuming of dragline single-operation cycle in Heidaigou surface mine. The results have shown that mining methods and geometry of soil bench are the main influence factors of digging efficiency and put forward combined-type digging and changed partial geometry shape of soil in order to improve digging efficiency. Time consuming of equipment rotation is the main factor that influence the efficiency of single operation cycle, so rotation angle ratio of acceleration and deceleration process should be 3/2 in rotating and 4/5 in returning step respectively. Time consuming of bucket ascension is the key factor which restricts the efficiency of coal trench excavation. Reference table of dragline operation has been made according to the restrictive relation between bucket ascension and equipment rotation.

Comparative study on stability and mechanical structure of straight slope and horizontal concave slope

MA Li，LI Kemin，HAN Liu，SHU Jisen，MENG Qingwu

2016, 33(4):  728-733. 

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In order to study the internal mechanism causing the differences of stability between straight slopes and horizontal concave slopes, the mechanical models of simply supported beam for straight slope and two hinged arch structure for horizontal concave slope were established, basis parameters formulas of axial force, shearing force and bending moment were deduced, and the effect of entirety mechanical structure difference on stability were revealed. The effect of straight slope and horizontal concave slope on stability was studied within a three-dimensional analysis model. The results have shown that：1) The stability of horizontal concave slope is greater than that of straight slope, because the curved arch structure of horizontal concave slope can alleviate shear stress concentration distribution effectively and reduce slope failure risk; 2) End-slope stability can also be enhanced effectively if bottom width is lowered by increasing advancing speed of inner-dump working face; 3) Straight and horizontal concave slope stabilities decrease progressively in a linear gradient with slope angle increasing;4) And with slope height increasing, the rising amplitude of the stability coefficient of horizontal concave slope decreases gradually compared to that of straight slope.

Research on permeability characteristics of karst collapse column fillings in complete stress-strain process

ZHANG Boyang，BAI Haibo，ZHANG Kai

2016, 33(4):  734-740. 

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The floor water-inrush accident caused by karst collapse column is easy to break over the aquifer and coal seam. This kind of accident has become the serious danger of lower group coal mining in northern China. For the well-consolidated collapse column, the permeability of the fillings directly affects that of the overall collapse column, and the permeability of fillings constantly changes under the influence of mining. The study on seepage characteristic of collapse column fillings is conducted with MTS815.02 seepage test system in a complete stress-strain process and the variation of permeability under different strain states. Meanwhile the influence of initial moisture content and confining pressure on the fillings permeability is analyzed in this test. The results show that: 1) The representative stress-strain-seepage relationship of fillings is divided into pressure elastic segment, damage and creep stage, and the permeability presents the change rule of decrease-increase-decrease in complete stress-strain process; 2) The peak value of permeability decreases with the increasing confining pressure, and the ratio of the permeability in the complete stress-strain process has a exponential function relation with the confining pressure difference in complete stress-strain process; 3) The initial permeability and porosity of fillings decreases with the increase of initial moisture content, and the ratio of permeability and porosity have a power function relationship. The difference between peak and initial permeability decreases with the increasing initial moisture content in complete stress-strain process.

Effects of normal stresses on crack propagation and coalescence mechanisms of rocks upon shear

CHENG Lichao，XU Jiang，FENG Dan，LIU Yixin

2016, 33(4):  741-746. 

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Using self-developed mesoscopic test equipment for coal and rock upon shear, taking sandstone as the research object, the research on rock shearing mesoscopic cracking propagation and coalescence mechanisms has been carried out under different conditions of normal stresses, and the sandstone shearing mesoscopic cracking propagation process and mesoscopic coalescence mechanisms have been analyzed, and the effects of saturated coefficient on mesoscopic cracking and crack morphology have been discussed. Results have shown that with the increase of normal stresses, the post-peak residual strength has an increasing trend, and the length of surface initiation crack has a decreasing trend. The time of crack initiation occurred before the peak since normal stresses have been applied. Rock bridges emerged in the process of middle crack formation and coalescence, constantly deflected in the process of the friction and slip, and finally lead off, forming local stripping. After the crack at the end of the specimen formed, because of the restriction of normal stresses and the stress concentration of the crack at the end, the secondary crack appeared at the end, which is connected with the original coalescence crack, then, the bifurcation formed, and caused the end partial peeling. Affected by normal stresses, many microcracks appeared between particles in the surrounding, which intensified the damage range in crack walls. With the increase of the normal stresses, the number of the macro discontinuous cracks appeared with the increasing trend during shearing, whose length was gradually decreasing, and both the number of secondary cracks and the range of crack development zone showed an increasing trend.

Non-Darcy seepage properties of mixture particles of crushed gangue under confined compression

MA Dan，BAI Haibo，CHEN Zhanqing，LI Shuncai，JIANG Bangyou，HUANG Weixing

2016, 33(4):  747-753. 

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In room-pillar and roadway gangue backfill mining, the coal pillar and gangue are interval, and crushed gangue are under confined compression, besides which, the compaction, porosity and seepage properties of crushed gangue play a significant role in the control of coal seam gas and groundwater flow. Based on the steady-state flow measurement and axial displacement control method, a self–designed testing system for water flow in crushed rocks has been used to monitor the non-Darcy seepage properties of mixture particles of crushed gangue under confined water pressure, and the experiment has obtained the relation between seepage properties (permeability k and non-Darcy coefficient β) and porosity. Testing results have indicated that: 1) The Re calculation and the pore pressure gradient and flow velocity curves have shown that the water seepage flow in crushed gangue is a non-Darcy flow. 2) The porosity of crushed gangue is strongly influenced by compaction (axial displacement) and particle mixture. The porosity decreases with the increase of axial displacement and decrease of bigger particle size, respectively. 3) During the compaction, some larger particles being crushed and broken is the main reason that gave birth to particle size of 0-2.5 mm, and small particles have been washed away because of the effect of water seepage. 4) In general, under confined compression, the permeability k decreases while the non-Darcy coefficient β increases. The fluctuation of seepage properties curves has shown that the seepage properties of crushed gangue are not only related to compaction levels, mixture sizes, particle crushing, style of arrangement, but also to the pore channel.

Research on loading rate effect of uniaxial compressive strength of coal

LI Yanwei，JIANG Yaodong，YANG Yingming，ZHANG Kexue，REN Zheng，LI Haitao，MA Zhenqian

2016, 33(4):  754-760. 

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In order to investigate how the loading rate affects the uniaxial compressive strength of coal, the mechanical properties testing of 4^-1 coal from Zhengli coal mine in Shanxi province has been carried out with the TAW-2000 electro-hydraulic servo rock mechanical experimental system. The correlation between peak strength, elastic modulus, axial strain and loading rate has been studied, and how the release and dissipation of elastic strain energy react with loading rate has also been discussed. The results of the study are as follows: 1) Unlike the brittle rocks, the peak strength of the coal samples will increase as the loading speed grows, but there is a critical value when the rate exceeds, the strength will decrease. 2) The damaging stress of the coal samples has negative correlation with the loading rate. 3) The quicker the loading rate becomes, the faster the axial stress grows. But when it exceeds 1.16×10^-3 mm/s, the increment of the load would be stable. And the faster the rate is, the earlier damaging stress could be observed, and also the faster the samples could be damaged. 4) Conversion rate of the strain energy dissipation is relatively low in the first phase of the uniaxial compression tests, and has a negative correlation with the loading rate. In the second phase, the dissipation strain energy first increases and then decreases as the loading rate increases. The maximum conversion rate of strain energy dissipation occurs at the peak point or the point where axial the stress falls abruptly after the peak point on stress-strain curve.