Lateral abutment pressure distribution and evolution in wide pillars under the first mining effect

The wide pillars are generally popular due to the high productivity and efficiency in Northwest China.The distribution of lateral abutment pressure in coal pillars is important for mining safety.To reveal the effect of the first mining on the lateral abutment pressure distribution and evolution in wide pillars,an in-situ experiment,theoretical analysis and numerical simulation were performed.First,the field monitoring of lateral abutment pressure was conducted from the perspective of time and space in the Chahasu Coal Mine,Huangling No.2 Coal Mine and Lingdong Coal Mine during the first mining.Based on the field monitoring stress,a theoretical model was proposed to reveal the lateral abutment pressure distribution.The methodology was demonstrated through a case study.Aiming at the distribution mechanism,a numerical experiment was conducted through the finite-discrete element method(FDEM).Last,field observations of borehole fractures were performed to further study the damage distribution.In addition,two types of lateral abutment pressure evolution with mining advance were discussed.Suggestions on the stress monitoring layout were proposed as well.The results could provide foundations for strata control and disaster prevention in wide pillars in underground coal mines.

Deformation characteristics and reinforcement technology for entry subjected to mining-induced stresses

The entry at Zhangcun coal mine in Lu'an coal mining area in Shanxi Province suffered from severe mining-induced stresses with the heading face driven oppositely to an adjacent working face.In this paper,the characteristics of deformation and failure of the entry were investigated in terms of the tempo-spatial relations between heading and working faces through field study and numerical modeling.The three-dimensional(3D) finite difference models were built to investigate stresses,displacements and damages in the surrounding rocks of the entry and the working face.The field study includes selection of reinforcing methods and materials,design parameters,and determination of cable prestress.The monitoring data of entry deformation and stress along the cables during every stage were presented.The state of the reinforced entry was evaluated based on the monitoring data.The results demonstrate that before the heading face of the entry crosses the adjacent working face,the influence of advanced abutment pressure caused by adjacent working face upon the entry is not significant.After they cross each other,however,the lateral abutment pressure will have an evident impact on the entry.The displacement rate of the entry will be greatly increased and reaches a certain value within a certain distance between the heading face and the working face.Then,it will increase again with the presence of secondary mining-induced pressure on the entry when the present working face advances.The fully-grouted cable with short length,high strength and high prestress is an effective way to reinforce the entry suffering from severe mining-induced stresses,which greatly reduces the displacement and failure possibility of the entry.Finally,the principles and recommendations for reinforcing design of entries suffering from severe mining-induced stresses were proposed according to field study,numerical modeling and experiences from other coal mines.Problems encountered in field study and suggestions for reinforcement were also discussed.

Support technologies for deep and complex roadways in underground coal mines:a review

Based on geological and mining characteristics,coal mine roadways under complex conditions were divided into five types,for each type the deformation and damage characteristics of rocks surrounding roadways were analyzed.The recent developments of roadway support technologies were introduced abroad,based on the experiences of supports for deep and complex roadways from Germany,the United States and Australia.The history and achievements of roadway support technologies in China were detailed,including rock bolting,steel supports,grouting reinforcement and combined supports.Four typical support and reinforcement case studies were analyzed,including a high stressed roadway 1,000 m below the surface,a roadway surrounded by severely weak and broken rocks,a chamber surrounded by weak and broken rocks,and a roadway with very soft and swelling rocks.Based on studies and practices in many years,rock bolting has become the mainstream roadway support form in China coal mines,and steel supports,grouting reinforcement and combined supports have also been applied at proper occasions,which have provided reliable technical measures for the safe and high effective construction and mining of underground coal mines.

Effect of pre-tensioned rock bolts on stress redistribution around a roadway—insight from numerical modeling

The importance of the pre-tensioned force of rock bolts has been recognized by more and more researchers. To investi- gate the effect of pre-tensioned rock bolts on stress redistribution around roadways, a numerical analysis was carried out using FLAC3D and a special post-process methodology, using surfer, is proposed to process the numerical simulation results. The results indicate that pre-tensioned rock bolts have a significant effect on stress redistribution around a roadway. In the roof, pre-tensioned rock bolts greatly increase vertical stress; as a result, the strength of the rock mass increased significantly which results in a greater capacity of bearing a large horizontal stress. The horizontal stress decreases in the upper section of the roof, indicating that pre-tensioned rock bolts significantly reduce the coefficient and the size of the region concentration of horizontal stress. At the lat- eral side, pre-tensioned rock bolts greatly increase the horizontal stress; therefore, the rock mass strength significantly increases which results also in a greater capacity of bearing a large vertical stress. The greater the size of pre-tensioned force, the larger the region of stress redistribution around a roadway is affected and the higher the size of the stress on the roadway surface the more the rock mass strength increases.

Resist-decreasing effects of rock bolts on strength of rock mass around roadway——insight from numerical modeling

To investigate the resist-decreasing effects of rock bolts on the strength of the rock mass around a roadway,a comparative study has been carried out using the numerical analysis code FLAC3D.An unsupported and a rock bolt supported model have been built for comparison.Two types of rock mass strength,the uniaxial compressive strength(UCS) and the triaxial compressive strength(TCS) of rock mass have been obtained from each model,using a prepared Fish based on the Mohr-Coulomb criterion.The results indicate that when a roadway is excavated,both the UCS and TCS in a definite local rock mass around the roadway would inevitably decrease,no matter whether the roadway is supported or not.The major decreasing region did not settle in the middle of the roadway surface,but within a deeper horizon into the rock mass.The resist-decreasing effects of rock bolts both on the UCS and the TCS of rock mass around roadway are significant.

不同拉拔加载速率对全长锚固锚杆力学行为和声发射响应的影响

考虑到采矿扰动应力的影响,更好地理解不同拉拔加载速率下全长锚固锚杆的承载特性具有重要意义。本文采用一系列的实验室拉拔试验研究了不同拉拔加载速率对全长锚固锚杆力学性能和失效特性的影响。结果表明,锚固试件的力学性能表现出明显的加载速率依赖性和剪切增强特性。随着拉拔加载速率的增加,锚固试件的峰值拉拔载荷增大,而峰值拉拔载荷对应的位移和时间却减小,锚固试件的耐久性显著降低。同时,累积的声发射振铃计数、能量和事件数均随着拉拔加载速率的增加而减小。声发射事件的峰值频率呈现出明显的分区集中分布特征,振幅主要分布在50~80 dB的范围内。随着拉拔加载速率的增加,锚固界面的局部应变增大,且锚固界面的失效逐渐由局部剪切失效向整体剪切失效过渡。累积声发射振铃计数可用于评估拉拔过程中锚固界面的损伤特征,且分析结果与实验结果具有较好地一致性。该研究结果可为全长锚固锚杆的支护设计和性能监测提供参考。

Forecasting of destroyed height of overlying rock with the top coal caving based on ANN

Analyzed the rule of the Water Flowing Fractured (WFF) zone's development during the fully mechanized top coal caving.Six influence factors of WFF's height were selected,viz.mining thickness,base rock thickness,dip angle,uniaxial compressing strength of roof,mudstone proportion in overlying rock,and structure of overlying rock. The height-forecasting model of WFF was established based on the Artificial Neural Net-work techniques,and was applied in the first fully mechanized top coal caving face under sea in China.

Tests and analysis of mechanical behaviours of rock bolt componentsfor China＇s coal mine roadways

A series of laboratory tests were performed to study the mechanical behaviours of newly developed high strength rock bolt components, including rebar, thread, plate, and domed washer. The characteristics of deformation and damage of each component were presented. The stress distribution of plate and domed washer was investigated through fi nite element modelling. The numerical results show that the yield and tensile strengths of the developed high strength rebar are 33.6%e58.3% and 17.2%e28.7% greater than those of the conventional rebar, respectively. The increase in yield strength was higher than that in tensile strength, suggesting an increase in yield to tensile strength ratio and a decrease in elongation. It is well-known that the thread processing may not be of high precision and accuracy as expected, which is characterised as rough thread surface, non-identical tooth height, toe stripping, and cracks in the surface.Hardening during thread processing tends to increase the thread yield and tensile strengths. In this paper, the typical deformation process of arch-shaped plate is classi fi ed into fi ve stages. The tested plates exhibited distinct deformation characteristics and bearing capacities due to variations in shape, size,material and presence of washer. It was observed that uneven bottom surface, low bearing arch and large radius of the transitional arc connecting bearing arch and bottom surface were the major reasons accounting for low load-bearing capacity of plates. The performance of domed washer has a close relation with the shape, size, strength, and deformation compatibility with plate. Stress concentration was observed on the periphery of the contact surface between domed washer and plate, which is signi fi cantly in fl uenced by the strength of domed washer and is considered to be 20%e30% higher than that of plate.Finally, a case study in the Datong coal mining district was presented, and the support pattern and effect of the developed rock bolt were described.