Xigui Zheng

Strengthening borehole configuration from the retaining roadway for greenhouse gas reduction: a case study.

A monitoring trial was carried out to investigate the effect of boreholes configuration on the stability and gas production rate. These boreholes were drilled from the retaining roadway at longwall mining panel 1111(1) of the Zhuji Coalmine, in China. A borehole camera exploration device and multiple gas parameter measuring device were adopted to monitor the stability and gas production rate. Research results show that boreholes 1~8 with low intensity and thin casing thickness were broken at the depth of 5~10 m along the casing and with a distance of 2~14 m behind the coal face, while boreholes 9~11 with a special thick-walled high-strength oil casing did not fracture during the whole extraction period. The gas extraction volume is closely related to the boreholes stability. After the stability of boreholes 9~11 being improved, the average gas flow rate increased dramatically 16-fold from 0.13 to 2.21 m3/min, and the maximum gas flow rate reached 4.9 m3/min. Strengthening boreholes configuration is demonstrated to be a good option to improve gas extraction effect. These findings can make a significant contribution to the reduction of greenhouse gas emissions from the coal mining industry.

Strength Restoration of Cracked Sandstone and Coal under a Uniaxial Compression Test and Correlated Damage Source Location Based on Acoustic Emissions

Underground rock masses have shown a general trend of natural balance over billions of years of ground movement. Nonetheless, man-made underground constructions disturb this balance and cause rock stability failure. Fractured rock masses are frequently encountered in underground constructions, and this study aims to restore the strength of rock masses that have experienced considerable fracturing under uniaxial compression. Coal and sandstone from a deep-buried coal mine were chosen as experimental subjects; they were crushed by uniaxial compression and then carefully restored by a chemical adhesive called MEYCO 364 with an innovative self-made device. Finally, the restored specimens were crushed once again by uniaxial compression. Axial stress, axial strain, circumferential strain, and volumetric strain data for the entire process were fully captured and are discussed here. An acoustic emission (AE) testing system was adopted to cooperate with the uniaxial compression system to provide better definitions for crack closure thresholds, crack initiation thresholds, crack damage thresholds, and three-dimensional damage source locations in intact and restored specimens. Several remarkable findings were obtained. The restoration effects of coal are considerably better than those of sandstone because the strength recovery coefficient of the former is 1.20, whereas that of the latter is 0.33, which indicates that MEYCO 364 is particularly valid for fractured rocks whose initial intact peak stress is less than that of MEYCO 364. Secondary cracked traces of restored sandstone almost follow the cracked traces of the initial intact sandstone, and the final failure is mainly caused by decoupling between the adhesive and the rock mass. However, cracked traces of restored coal only partially follow the traces of intact coal, with the final failure of the restored coal being caused by both bonding interface decoupling and self-breakage in coal. Three-dimensional damage source locations manifest such that AE events are highly correlated with a strength recovery coefficient; the AE events show a decreasing tendency when the coefficient is larger than 1, and vice versa. This study provides a feasible scheme for the reinforcement of fractured rock masses in underground constructions and reveals an internal mechanism of the crushing process for restored rock masses, which has certain instructive significance.

Implementation of a Pretensioned, Fully Bonded Bolting System and Its Failure Mechanism Based on Acoustic Emission: A Laboratorial and Field Study

Currently, bolt-support technology is commonly applied throughout the world during mining activities, civil engineering, and hydraulic projects. The theory of a pretensioned and fully bonded bolting system has long been proposed for use as tunnel supports; however, this technology is difficult to implement in coal mining, particularly for roadways that require immediate support. In this study, a modified cement with water-to-cement ratio of 0.25 was used to work in tandem with a traditional resin cartridge to create a pretensioned, fully bonded bolting system. The fast-setting epoxy resin is positioned at the bottom of the borehole for pretension, and the cement is positioned along the rest of the borehole for full-length bonding along the bolt. Based on a series of pullout tests on the proposed bolting system with different pretension forces, this study shows that a pretensioned, fully bonded bolting system is more durable and stable than the end-encapsulated resin bolting system that is currently utilized in most of China’s coal mines. Additionally, an acoustic emission detection test was simultaneously conducted to further describe the inner fracture mechanisms of the bolting system under different pretension forces ranging from 50 to 140 kN, with an increment of 30 kN. The results of this test indicate that a higher pretension force can decrease the damage events in the bolting system. Additionally, the failure form of a pretensioned, fully bonded bolting system is more likely to be dominated by the fracture of the bolt rod in most circumstances. Finally, this combined bolting technology is successfully implemented in a coal mine, and the monitored results confirm that the proposed bolting measure is effective and reliable.