Causality Mechanism of Coal Bump Occurred in Longwall Roadways

Coal bump occurring in longwall roadways accounts for more than 87% of total coal bump events. However, there is no practical mechanical model that can explain the causality and process of coal bump, resulting in that the prediction and prevention of rockburst heavily depend on engineering experience or lessons. Considering the mechanical characteristics of surrounding rock, the suspended roof behind working face and stress state of the rib coal, a seesaw structure model related to incentive of the coal bump and a mechanical model related to instability of rib coal block are established in this paper based on theoretical analysis and numerical calculation, which is capable to clarify the causality mechanism, process, key factors and critical state of coal bump. The hard suspended roof behind the working face produces periodic abutment pressure on the coal around the working face. The result of numerical calculation shows that, under the condition of high compressive stiffness of hard coal around the working face, the roof in front of the working face rebounds upward. The phenomena of roof rebound suggests that the overlying strata of the working face form a seesaw structure. In the area of roof rebound, the normal stress on the roof-coal interface is reduced. Stress analysis of a rib coal block has been conducted. Result shows that, the tectonic stress perpendicular to the rib can overcome the fractional resistance pushing the rib coal into the roadway once the normal stress on the roof-coal interface de-stresses to a certain level. Accordingly, a critical state equation of coal bump has been established. It can be concluded that, de-stressing of the roof-seam interface caused by roof rebound triggers coal bump. The tectonic stress is its force source, and the tensile strength of coal and the shear strength of the coal seam with roof and floor are bump resistances. This study clarifies the causality mechanism and process of coal bump occurred in longwall roadway that provides a theoretical basis for prediction and prevention technology.

Prediction and Evaluation of Coal Mine Coal Bump Based on Improved Deep Neural Network

Coal bump prediction is one of the key problems in deep coal mining engineering. To predict coal bump disaster accurately and reliably, we propose a depth neural network (DNN) prediction model based on the dropout method and improved Adam algorithm. The coal bump accident examples were counted in order to analyze the influencing factors, characteristics, and causes of this type of accidents. Finally, four indexes of maximum tangential stress of surrounding rock, uniaxial compressive strength of rock, uniaxial tensile strength of rock, and elastic energy of rock are selected to form the prediction index system of coal bump. Based on the research results of rock burst, 305 groups of rock burst engineering case data are collected as the sample data of coal bump prediction, and then, the prediction model based on a dropout and improved Adam-based deep neural network (DA-DNN) is established by using deep learning technology. The DA-DNN model avoids the problem of determining the index weight, is completely data-driven, reduces the influence of human factors, and can realize the learning of complex and subtle deep relationships in incomplete, imprecise, and noisy limited data sets. A coal mine in Shanxi Province is used to predict coal bump with the improved depth learning method. The prediction results verify the effectiveness and correctness of the DA-DNN coal bump prediction model. Finally, it is proved that the model can effectively provide a scientific basis for coal bump prediction of similar projects.

Application of Distributed Optical Fiber Technology for Coal Bump Prevention

The 8939 working face in Xinzhouyao coal mine is a high coal bump proneness panel. For coal bump prevention, rib holes are drilled for pressure relief purpose. The deformational behaviour of the pressure relief borehole is studied using distributed optical fiber sensing technology. The strain of the surrounding coal and the pressure relief range were measured from 0 hrs to 402 hrs after hole drilling. Based on the analysis of pressure relief procedure, combining with borehole observation, the crack development, limited equilibrium, collapse, and compaction stages of the borehole were estimated as 0∼72 h, 72∼190 h, 190∼402 h, and greater than 402 h, respectively. Consequently, the hole drilling is modified to 110 m ahead of the working face to achieve better pressure relief effect. Microseismic monitoring shows that, after hole drilling optimisation, the high-energy microseismic events and average energy of microseismic events are reduced significantly.

Mechanism of Coal Bump among Mine Group under the Control of Large Geological Body: A Case Study of Yima Mining Area, China

Coal bump often occurs in coal mining among many working faces in mine group under the control of large geological bodies. In order to study the coal bump mechanism between adjacent working faces under the conditions of large fault and huge thick overburden conglomerate, this paper regards Yima mining area as a practical engineering background and theoretically analyzes the mechanical behavior of overlying rock in the spatial structure. Then, the deep-ground and whole-space measurement is carried out in the 13230 working face of Gengcun mine and 21121 working face of Qianqiu mine. The results show that the basic structural unit in Yima mining area is composed of two goafs, middle coal pillar, and overlying conglomerate. Under the condition of nonsynchronous mining in adjacent working faces, there is a comovement effect similar to lever’s “prying” phenomenon in thick conglomerate beam—the conglomerate strata above larger goaf side induce an overall uplift movement of the corresponding strata above smaller goaf side, and uplift length of the conglomerate strata is related to the mining length, coal pillar width, caving angle, and coal-conglomerate distance. The results of surface subsidence, microseism, and stress in the two working faces verify the conglomerate’s phenomenon of comovement effect and disturbance range and further explain the role of active movement of F16 fault and overall causes of huge thick conglomerate on the coal bump. The vertical stress of the 13230 face is relatively low at the beginning, and high horizontal stress by fault activation causes typical bump accident with the horizontal sliding of coal body. With the increasing development of 13230 face, the intensity and frequency of coal bump in horizontal direction decrease obviously, but with high proportion in vertical direction. The results provide a theoretical basis for the study on the mechanism of coal bump between two adjacent working faces under the conditions of huge thick conglomerate and large thrust fault.

Stability Control Mechanism of High-Stress Roadway Surrounding Rock by Roof Fracturing and Rock Mass Filling

Large deformation of roadway and coal bump failures have always been the focus in deep underground engineering. By considering the Lu’an mining district in China, the failure mode and stability improvement process of high-stress roadways were analysed with the field tests and numerical simulations. The field test results showed that a great amount of deformation and serious damage occurred in surrounding rocks during panel retreat due to the suspended roof. A novel approach employing roof fracturing and collapsed rock filling effect was adopted to maintain the roadway stability. A numerical model was established with the Universal Distinct Element Code (UDEC) to research the fracturing characteristics between the roadway and gob roofs and the stress change in the surrounding rock. The modelling results demonstrated that, without fracturing roof, the peak vertical stress of the coal pillar was 18.3 MPa and the peak vertical stress of the virgin coal rib was 15.6 MPa. The roadway was in a state of high stress. With fracturing roof, the peak vertical stress of coal pillar was 9.3 MPa and the peak vertical stress of virgin coal rib was 13.4 MPa. The fractured rock mass in the gob expanded in volume and provided supporting resistance to the overlying strata, which relieved stress concentrations in the coal pillar. Field measurement results indicated that the roadway large deformation was successfully resolved during excavation and panel retreat after implementing the novel approach, providing useful references for the application of this novel approach in similar coal mines.