scholarly journals Numerical Simulation on the Evolution of Mining-Induced Fracture Network in a Coal Seam and Its Overburden under the Top Coal Caving Method

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
Guangdi Deng ◽  
Heping Xie ◽  
Mingzhong Gao ◽  
Cong Li ◽  
Zhiqiang He
Keyword(s):  
Coal Seam ◽  
3D Modeling ◽  
Hard Rock ◽  
Fractal Theory ◽  
Soft Rock ◽  
Fracture Network ◽  
Hard Roof ◽  
Discrete Fracture ◽  
Mining Face ◽  
Overburden Strata

The evolution of the fracture network induced by mining has an important influence on the mechanical behavior of the rock and the safety of the mine. In this study, a new 3D modeling approach based on discrete fracture network (DFN) and fractal theory was developed and applied to the Tashan coal mine. The model results of the evolution of fracture in the coal seam are consistent with field observations. The evolution law of fracture network in the overburden strata was studied, and the results show that the fractal dimension of the fractures in the lower strata increased linearly and stabilized quickly within 50 m behind the mining face. In the higher strata, most of the fractures were generated behind the mining face and continued to develop farther than 100 m. More fractures were generated in the lower strata than in the higher strata, and the fractures more easily developed and expanded in the soft rock than in the hard rock. The evolution of the fractures of the main thick hard roof in the lower strata had a great impact on the generation of fractures in the higher strata.

scholarly journals Calculation Method of Overburden Damage Height Based on Fracture Mechanics Analysis of Soft and Hard Rock Layers

Geofluids ◽  
2019 ◽  
Vol 2019 ◽  
pp. 1-15 ◽  
Author(s):  
Liang Chen ◽  
Shaowu Fan ◽  
Can Zhao ◽  
Lang Zhang ◽  
Zhiheng Cheng
Keyword(s):  
Fracture Mechanics ◽  
Coal Seam ◽  
Calculation Method ◽  
Hard Rock ◽  
Soft Rock ◽  
Mining Area ◽  
Overburden Rock ◽  
Rock Interaction ◽  
Mechanics Analysis ◽  
Seam Mining

Under the geological condition of soft and hard rock interaction stratum, the overburden damage height can provide a quantitative support for the design of the locations of gas drainage boreholes in the roof mining fracture zone and the determination of the hydraulic fracture zone in coal seam mining. The interbedded structure of overlying mud rock and sandstone in the Lu’an mining area in Shanxi is a typical soft and hard rock interaction stratum. In view of the lack of soft rock fracture mechanics analysis and the improper calculation of the damage height of overburden rock caused by constant rock residual bulking coefficient to be used regularly in the analysis, in this paper, we constructed a fracture model of soft and hard strata by giving a quantitative classification criterion of soft and hard rocks and introducing a fracture failure criterion of soft rock strata and the space constraint condition of broken-expansion rock formation. Aiming at improving the calculation precision of overburden damage height, we presented a calculation method based on fracture mechanics analysis of soft and hard strata, which could delineate the extent of intact rock in overlying strata from bottom to top to determine the damage height of overburden rock. This research took Yuwu coal mine in Lu’an mining area as an example. Results showed that (1) by the calculation method, the overburden damage height of the N1102 fully mechanized caving face in Yuwu coal mine was 51.44 m, which was less than the value obtained by an actual borehole TV method as well as the numerical simulation result of 53.46 m, with a calculation accuracy about 96.22%, which is quite high for both. The calculation accuracy of the proposed method was higher than that of the three conventional theoretical methods, and it effectively solved the limitation of the fracture analysis method without the inclusion of the soft rock layer in design and the distortion problem due to the residual bulking coefficient to be improperly used in simulation. (2) There was no noticeable fractures in the broken soft rock zone, and the whole fractures were mainly low-angle rupture; the fractures in hard rock layer had obvious ruptures and multiangle cracks, and the average fracture width of soft rock was 2.8 mm smaller than that of hard rock. The fracture modes of soft rock and hard rock were mainly tensile failure and tensile shear failure, which verified the correctness of the fracture mechanics model of soft and hard rock layers constructed in this paper. (3) It is noticed that the tensile strength of rock in this method needs to be obtained through rock mechanics experiment on overlying strata in the study area, and our proposed method was applicable to the mining conditions of near horizontal coal seam. The calculation accuracy of this method meets the engineering error requirements and can be applied to the prediction of overburden damage height in near horizontal coal seam mining.

A framework for inclusion of faults in coal seam gas risk assessments

2021 ◽  
Vol 61 (2) ◽  
pp. 695
Author(s):  
Titus A. Murray ◽  
William L. Power
Keyword(s):  
Coal Seam ◽  
Fracture Network ◽  
Coal Seams ◽  
Coal Seam Gas ◽  
Near Surface ◽  
Discrete Fracture ◽  
Flow Through ◽  
Flow Pathways ◽  
Pressure Changes ◽  
Assessment Context

Assessments for coal seam gas (CSG) projects may be required to consider the potential hydrological impacts of faults on surface and near-surface groundwater assets. This study presents three distinct end-member geological scenarios and outlines methods for characterising fault-related groundwater flow within a risk assessment context. Scenario 1: a regional aquitard isolates the water assets from the coal seams. There is little risk of leakage across the aquitard because there are no faults, or because the faults have maximum displacements less than the thickness of the aquitard. Scenario 2: a region-wide aquitard is not present, and the seams and the groundwater assets are located within the same groundwater system. In this scenario, CSG development may cause pressure changes to propagate parallel to the strike and dip of the fault in the fault damage zones. Scenario 3: regional aquitard(s) are present, but larger displacement faults breach the aquitards, allowing for possible combinations of across-fault connections between the different aquifers, and between aquifers and the coal seams. In this scenario, potential flow pathways between the groundwater and the CSG field need to be characterised using Allan Maps (fault plane profiles). It is essential to compare calculated flow rates of any new or potentially new flow pathways with the predevelopment flow regime. It is also important to recognise that flow estimates are best made using a Darcy’s law treatment for flow across fault zones and within the aquifers, and a Snow’s law treatment (discrete fracture network) for flow through fractures in fault damage zones.

Evolution of a mining induced fracture network in the overburden strata of an inclined coal seam

2012 ◽  
Vol 22 (6) ◽  
pp. 779-783 ◽  
Author(s):  
Xiujun Wei ◽  
Mingzhong Gao ◽  
Youchang Lv ◽  
Xiangchao Shi ◽  
Hailian Gao ◽  
...  
Keyword(s):  
Coal Seam ◽  
Fracture Network ◽  
Overburden Strata

scholarly journals Analysis of gas migration patterns in fractured coal rocks under actual mining conditions

2017 ◽  
Vol 21 (suppl. 1) ◽  
pp. 275-284
Author(s):  
Mingzhong Gao ◽  
Ting Ai ◽  
Zhiqiang Qiu ◽  
Zetian Zhang ◽  
Jing Xie
Keyword(s):  
Coal Seam ◽  
Computer Aided Design ◽  
Gas Flow ◽  
Test Site ◽  
Fracture Network ◽  
Gas Migration ◽  
Gas Seepage ◽  
Mining Conditions ◽  
Coal Rock ◽  
Mining Face

Fracture fields in coal rocks are the main channels for gas seepage, migration, and extraction. The development, evolution, and spatial distribution of fractures in coal rocks directly affect the permeability of the coal rock as well as gas migration and flow. In this work, the Ji-15-14120 mining face at the No. 8 Coal Mine of Pingdingshan Tian?an Coal Mining Co. Ltd., Pingdingshan, China, was selected as the test site to develop a full-parameter fracture observation instrument and a dynamic fracture observation technique. The acquired video information of fractures in the walls of the boreholes was vectorized and converted to planarly expanded images on a computer-aided design platform. Based on the relative spatial distances between the openings of the boreholes, simultaneous planar images of isolated fractures in the walls of the boreholes along the mining direction were obtained from the boreholes located at various distances from the mining face. Using this information, a 3-D fracture network under mining conditions was established. The gas migration pattern was calculated using a COMSOL computation platform. The results showed that between 10 hours and 1 day the fracture network controlled the gas-flow, rather than the coal seam itself. After one day, the migration of gas was completely controlled by the fractures. The presence of fractures in the overlying rock enables the gas in coal seam to migrate more easily to the surrounding rocks or extraction tunnels situated relatively far away from the coal rock. These conclusions provide an important theoretical basis for gas extraction.

scholarly journals Cooperative Control Mechanism of Long Flexible Bolts and Blasting Pressure Relief in Hard Roof Roadways of Extra-Thick Coal Seams: A Case Study

2021 ◽  
Vol 11 (9) ◽  
pp. 4125
Author(s):  
Zhe Xiang ◽  
Nong Zhang ◽  
Zhengzheng Xie ◽  
Feng Guo ◽  
Chenghao Zhang
Keyword(s):  
Coal Seam ◽  
Cooperative Control ◽  
Field Tests ◽  
Surrounding Rock ◽  
Deep Hole ◽  
Coal Seams ◽  
Thick Coal Seam ◽  
Structure Damage ◽  
Hard Roof

The higher strength of a hard roof leads to higher coal pressure during coal mining, especially under extra-thick coal seam conditions. This study addresses the hard roof control problem for extra-thick coal seams using the air return roadway 4106 (AR 4106) of the Wenjiapo Coal Mine as a case study. A new surrounding rock control strategy is proposed, which mainly includes 44 m deep-hole pre-splitting blasting for stress releasing and flexible 4-m-long bolt for roof supporting. Based on the new support scheme, field tests were performed. The results show that roadway support failure in traditional scenarios is caused by insufficient bolt length and extensive rotary subsidence of the long cantilever beam of the hard roof. In the new proposed scheme, flexible 4-m-long bolts are shown to effectively restrain the initial expansion deformation of the top coal. The deflection of the rock beam anchored by the roof foundation are improved. Deep-hole pre-splitting blasting effectively reduces the cantilever distance of the “block B” of the voussoir beam structure. The stress environment of the roadway surrounding rock is optimized and anchorage structure damage is inhibited. The results provide insights regarding the safe control of roadway roofs under extra-thick coal seam conditions.

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