Failure Mechanism and Control Technology of Thick and Soft Coal Fully Mechanized Caving Roadway under Double Gobs in Close Coal Seams

Abstract

The surrounding rock of the roadway under double gobs in the lower coal seams is partially damaged by the mining of the upper coal seam and the stress superimposition of the stepped coal pillars. What is worse, the upper layer of the roof is collapse gangue in double gobs, which makes the anchor cable unable to anchor the reliable bearing layer, so the anchoring performance is weakened. The actual drawing forces of the anchor bolt and anchor cable are only approximately 50 kN and 80 kN, respectively. The roadway develops cracks and large deformations with increasing difficulty in achieving safe ventilation. In view of the above problems, taking the close coal seam mining in the Zhengwen Coal Mine as the engineering background, a theoretical calculation is used to obtain the loading of the step coal pillars and the slip line field distribution of the floor depth. The numerical simulation monitors the stress superimposition of stepped coal pillars and the distribution of elastoplastic areas to effectively evaluate the layout of mining roadways. The numerical simulation also analyzes the effective prestress field distribution of the broken roof and grouting roof anchor cable. A laboratory test was used to monitor the strength of the grouting test block of the broken coal body. Then, we proposed that grouting anchor cable be used to strengthen the weak surface of the roof and block the roof cracks. From on-site measurement, the roadway was seen to be arranged in the lateral stress stabilization area of the stepped coal pillars, the combined support technology of the grouting anchor cable (bolt) + U type steel + a single prop was adopted, the roadway deformation was small, the gas influx was reduced, and the drawing force of the anchor bolt and the anchor cable was increased to approximately 160 kN and 350 kN, respectively. The overall design and control technology of the roadway can meet the site safety and efficient production requirements.

A Numerical Simulation of the Subsidence Reduction Effect of Different Grouting Schemes in Multi-Coal Seam Goafs

“…Liu et al [23] conducted grouting tests on the fractured surface of the caving rock mass to solve engineering problems such as rock mass caving in a single coal seam goaf and studied the tensile characteristics of the goaf rock mass after grouting reinforcement. Xie et al [24] applied numerical simulations to analyze the effective pre-stress field distribution of broken roof and grouting roof anchor cables in a multi-seam goaf. The strength of grouting stone bodies in a multi-coal seam goaf was monitored using experimental laboratory methods, and a treatment method of using grouting anchor cables to fix the weak surface of the roof plate and seal roof cracks was proposed.…”

Control Techniques for Gob-Side Entry Driving in an Extra-Thick Coal Seam with the Influence of Upper Residual Coal Pillar: A Case Study

“…[27]. The mining of two adjacent coal seams tends to interact with each other due to the close distance [28,29]. The mining of the upper coal seam generally renders many section pillars, and lower coal seam roadways are usually designed with larger section coal pillars (usually over 20 m) to avoid safety hazards.…”

Instability Control of Roadway Surrounding Rock in Close-Distance Coal Seam Groups under Repeated Mining

“…Downward mining is the top-down mining between coal seams, and it is generally used in close-distance coal seam groups. Due to the disturbance of upper coal seam mining, the roof of the roadway is destroyed before the lower coal seam is mining, resulting in changes in the roof structure [1][2][3] and stress environment of the lower coal seam mining area [4,5]. In addition, the roadway surrounding rock is disturbed and destroyed repeatedly while mining the lower coal seam, which can easily lead to the instability and serious destruction of the roadway surrounding rock [6,7].…”