Failure Mechanism of Gob-Side Roadway under Overlying Coal Pillar Multiseam Mining

Roadway deformation and rock burst are the two key challenges faced by the safe operation of coal mines. Aiming at the issue of large deformation of the gob-side roadway under coal pillars in multiseam mining, this study has considered the case of the 8308 panel of Xinzhouyao coal mine in China. Based upon a combination of theoretical analysis, numerical simulations, and engineering practices, the mechanical model of “stress and deformation quantitative calculation of gob-side roadway under overlying coal pillars” was established in this study. The analytical solutions of the vertical stress distribution and the plastic zone of the gob-side roadway under overlying coal pillars were obtained. Finally, the accuracy of the mechanical model was verified using numerical simulations. The results showed that the coal pillar, upright above the gob-side roadway, and the cantilever roof around the gob-side roadway were the main factors leading to stress concentration and deformation around the gob-side roadway. For the particular cases considered in this study, the peak stress of the gob-side roadway could reach 1.8 times of the self-weight stress of overlying strata. The rates of the contribution of the gob-side roadway’s overlying pillar and the cantilever roof around the gob-side roadway to peak stress were 78.3% and 16%, respectively. The obtained results have an essential reference significance for stress calculations and rock burst prevention design of gob-side roadway under overlying coal pillars in multiseam mining.

Cause and Solution to Roadway Deformation in Vietnam Underground Coal Mines

The deformation and support method of roadways have always been important issues in safemining and production. Vinacomin's statistics show that, by 2021, there will be 64.19 km of roadwaysthat need to be repaired (accounting for 25% of the total new roadways). Thus, the problem of maintainingroadway stability is facing difficulties in underground coal mines in Vietnam. To find out the causes ofroadway failures, a case study at roadways of the Khe Cham I and Khe Cham III coal mines, Vietnam, ispresented in this paper. Based on the results of a detailed field survey, the deformation characteristics ofroadways and the failure mode of support structures were investigated. The results show that the roadwaydeformation is severe and the main support cannot control surrounding rock mass. Also, the destructionof support structure is frequent on reused roadways, affecting production efficiency and work safety.Therefore, to reduce deformation and increase roadway stability, a new support method called “multistageanchor of rock bolt + cable bolt” has been developed and a new longwall mining system with criticalcoal pillar width has been proposed. The new findings of the research can provide references for scientificstudies, and apply them in Vietnam's underground coal mine practices.

Comprehensive Study on Surrounding Rock Failure Characteristics of Longwall Roadway and Control Techniques

Research on underground coal mines has primarily focused on the stability of roadways. Mining activities lead to significant damage to the surrounding rocks and also degrade the support to the roadways. Considering the 21309 roadway of the Huojitu coal mine as a case study, this work comprehensively analyzed the characteristics of the surrounding rock using three methods: theoretical calculations, FLAC3D numerical modeling, and field observations. The results indicate that, under the influence of secondary mining, the failure range and stress concentration degree of the surrounding rock are considerably higher than those under the influence of primary mining. In this case, the maximum damage range in the surrounding rock can reach 1.8 m, the maximum principal stress can reach 19.82 MPa, and the ratio of the maximum principal stress to the minimum principal stress can reach 1.95. According to the results, the previous support design for roadways was optimized and applied in the field. Field monitoring revealed that the roadway deformation was effectively controlled, and the optimized support design was safe and reliable. This study is expected to serve as a reference for support designs or optimization under similar geological conditions.

Deformation Failure and Support Test of Surrounding Rock in Deep Arched Roadway with Straight Wall

The deformation and failure of the uphill roadway on the 3rd horizontal track in the No. 6 Mine of Pingdingshan Coal Group was taken as the engineering background. The similar simulation material of the roadway surrounding rock with quartz sand as the aggregate, cement as the cementing agent, and gypsum powder as the regulator was selected. Through mechanical tests on 25 sets of specimens with different proportions, the best proportion of similar simulated materials for simulating the deformation and failure of the surrounding rock of the roadway was obtained. Later, a large-scale deep mine roadway simulation test system independently developed by the company was used to carry out the roadway deformation and failure test. First, load the test body to the set initial stress state, and then carry out the full-face excavation and unloading of the roadway; finally, load it in the vertical direction until the roadway wall is damaged. It can realize the actual effect of simulation of roadway deformation and failure under the path of “high stress + internal unloading + stress adjustment.” The results showed that after the deep roadway is excavated with preload and high stress, the surrounding rock deformation, failure, and instability of the roadway mainly experience 3 periods: the first period is the period of uniform deformation of the roadway surrounding rock, the second period is the development period of the roadway surrounding rock slab structure, and the third period is the period of instability of the roadway surrounding rock slab structure. Combined with the time period of deformation and failure of the surrounding rock of the roadway, the damage scope of the surrounding rock and the actual situation of the site engineering. A step-by-step combined roadway repair and support plan of “bolt mesh + shotcrete + full-face hollow grouting anchor cable” with hollow grouting anchor cable as the core was determined. The stability of the repaired roadway has been significantly improved, ensuring the long-term use of the roadway.

Evolution of Characteristics of the Zone of Rock Loosening with Cross-Sectional Area

Because of the large deformation of surrounding rock mass and the different deformation characteristics of roadways with different cross-sectional areas, it is difficult to determine the means of support of roadways and the hole-sealing depth of extraction boreholes, which will cause serious roadway deformation and reduce the gas drainage rate. In order to solve these problems, this paper studies the evolution law of the zone of rock loosening around the roadway at four different cross-sectional areas (15 m2, 20 m2, 25 m2, 30 m2) by means of the acoustic field tests and numerical simulation. The results revealed three key points: first, the zone of rock loosening around the roadway is symmetrically distributed around the center of the roadway, and its shape is approximately that of a “butterfly.” Second, field tests results indicate that the rock loose zone of 1302 North floor mining roadway is in the range of 2.3–2.4 m on the side of the roadway and is 2.7–2.9 m on the central auxiliary transportation roadway. The simulation results show that the rock loose zone of 1302 North floor mining roadway is 2.5 m on the side of the roadway and is 3.0 m on the central auxiliary transportation roadway. The simulation results under four different section areas matched the field test results well, and the range of the surrounding rock loosening zone increases with the increased cross-sectional area. Third, the loose zone at the top corner and side of the roadway has a linear relationship with the cross-sectional area, and the loose zone at the bottom corner of the roadway does not change significantly with the cross-sectional area. These results have significance for determining the cross-sectional area of mine roadways in the same geological conditions, the sealing depth of the borehole, and the surrounding rock support.

Analysis and Monitoring Technology of Upper Seam Mining in Multiunderlayer Goaf

Based on the background of close coal seam mining in the Qianjiaying coal mine, Tangshan, China, the feasibility of the upper seam mining in complex underlaying goaf is analysed using the roof caving analysis and numerical method. The deformation of the mining seam and roadways is monitored and analysed by field measurement and 3D laser scanning. The deformation characteristics of #5 seam after mining 1378P, 2071P, 2072P, and 2091P working panels with a depth of 39–54 m below the #5 seam are analysed using roof caving analysis and numerical method. Results show that the maximum deformation of #5 seam in the superimposed area of the lower goafs reaches 2.5 m and the maximum deformation in the single coal goaf is 1.5 m. The maximum seam inclination is 1.9°. The subsidence of the floor of 1359P roadways is obtained by field measurement, and the result is consistent with numerical calculation. ZEB-HORIZON 3D laser scanner was used to measure and model the roadway deformation. Based on the analysis of multiple scanning data, the deformation of the 1359P roadways was obtained. Results show that the deformation of the surrounding rock of the roadway is not great, the maximum displacement of the roof fall is 30 mm, and the maximum rib convergence is 25 mm. It can be concluded that the #5 seam can be recovered in this complex underlying lower seams’ mining condition.

A Control Approach of the Roof in No-Pillar Roadway Formed by Roof Cutting and Pressure Releasing

Roof control is one of the eternal themes of mine pressure theory, and it is also a key step of roadway formed by roof cutting. Based on the analysis of abutment pressure distribution, the viewpoint of controlling roadway roof by roof cutting distance is put forward according to the failure limit of roadway roof, and the calculation method of roof cutting distance is given. Based on the Qiuji coal mine’s background, the numerical and field test study is conducted to verify the theoretical analysis, and the allowable variation range of roof cutting distance is obtained. The research shows that the roof cutting distance, abutment pressure, and roadway deformation are closely related. By controlling the roof cutting distance, the roadway roof can be placed in the low-stress area, the roadway deformation can be reduced, and the support cost can be saved. This study provides a theoretical basis for explaining the roadway’s abnormal mine pressure and controlling the roadway roof by roof cutting.

Study on the Lagging Support Mechanism of Anchor Cable in Coal Roadway Based on FLAC3D Modified Model

Aiming at the broken failure of anchor cable in the mining roadway roof during the mining process, the lagging support scheme of anchor cable is proposed. Based on the results of indoor anchor cable pull-out test, the Cable element in FLAC3D is modified to realize the extension breaking of anchor cable in the calculation process. Furthermore, the minimum principal stress and volume strain rate mutation point are used as the failure criteria of the anchor cable. Through the comparative analysis of five anchor cable lagging support schemes of 6208 transport tunnel in Wangzhuang Mine Coal, the results demonstrate that the lagging support reduces the initial support resistance of the supporting structure. With the increase of lagging time, the ability of anchor cable to adapt to deformation increases gradually. When the lagging time reaches the gentle area of roadway deformation, its ability to adapt to deformation remains stable. Finally, it was determined that the support should start at 10–15 m of the anchor cable lagging head of the 6208 transport tunnel. Industrial tests show that the lagging support scheme ensures that the anchor cable can withstand a certain deformation, and the support body has no broken failure, which effectively controls the large mining-induced deformation of surrounding rock.

Numerical Investigation on Time-Dependent Deformation in Roadway

The roadway deformation normally relates to time especially for underground coal roadway. The strength of soft coal is low, and therefore the deformation increases gradually under constant stress with time, which is called rheology deformation. In this study, based on a field case, the rock mass properties and deformation data of the roadway were obtained according to the field test. A 3D numerical model was then established, and the rheological deformation including horizontal and vertical deformation of the coal roadway was systematically analyzed. The results showed that the rheological deformation of horizontal sidewall accounts for almost 30% of the whole deformation, and the stable time for such roadway is around 60 days after excavation. The tendency of the roof deformation is similar to the sidewalls, and however, the floor deformation is different. Then the related suggestions for maintaining the stability of such roadway were proposed, which is useful in-field application.