Nong Zhang

A Fiber Bragg Grating-Based Monitoring System for Roof Safety Control in Underground Coal Mining.

Monitoring of roof activity is a primary measure adopted in the prevention of roof collapse accidents and functions to optimize and support the design of roadways in underground coalmines. However, traditional monitoring measures, such as using mechanical extensometers or electronic gauges, either require arduous underground labor or cannot function properly in the harsh underground environment. Therefore, in this paper, in order to break through this technological barrier, a novel monitoring system for roof safety control in underground coal mining, using fiber Bragg grating (FBG) material as a perceived element and transmission medium, has been developed. Compared with traditional monitoring equipment, the developed, novel monitoring system has the advantages of providing accurate, reliable, and continuous online monitoring of roof activities in underground coal mining. This is expected to further enable the prevention of catastrophic roof collapse accidents. The system has been successfully implemented at a deep hazardous roadway in Zhuji Coal Mine, China. Monitoring results from the study site have demonstrated the advantages of FBG-based sensors over traditional monitoring approaches. The dynamic impacts of progressive face advance on roof displacement and stress have been accurately captured by the novel roadway roof activity and safety monitoring system, which provided essential references for roadway support and design of the mine.

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.

Laboratory Testing of Silica Sol Grout in Coal Measure Mudstones

The effectiveness of silica sol grout on mudstones is reported in this paper. Using X-ray diffraction (XRD), the study investigates how the silica sol grout modifies mudstone mineralogy. Micropore sizes and mechanical properties of the mudstone before and after grouting with four different materials were determined with a surface area/porosity analyser and by uniaxial compression. Tests show that, after grouting, up to 50% of the mesopore volumes can be filled with grout, the dominant pore diameter decreases from 100 nm to 10 nm, and the sealing capacity is increased. Uniaxial compression tests of silica sol grouted samples shows that their elastic modulus is 21%–38% and their uniaxial compressive strength is 16%–54% of the non-grouted samples. Peak strain, however, is greater by 150%–270%. After grouting, the sample failure mode changes from brittle to ductile. This paper provides an experimental test of anti-seepage and strengthening properties of silica sol.

Simulation of the Load Evolution of an Anchoring System under a Blasting Impulse Load Using FLAC3D

A limitation in research on bolt anchoring is the unknown relationship between dynamic perturbation and mechanical characteristics. This paper divides dynamic impulse loads into engineering loads and blasting loads and then employs numerical calculation software FLAC3D to analyze the stability of an anchoring system perturbed by an impulse load. The evolution of the dynamic response of the axial force/shear stress in the anchoring system is thus obtained. It is revealed that the corners and middle of the anchoring system are strongly affected by the dynamic load, and the dynamic response of shear stress is distinctly stronger than that of the axial force in the anchoring system. Additionally, the perturbation of the impulse load reduces stress in the anchored rock mass and induces repeated tension and loosening of the rods in the anchoring system, thus reducing the stability of the anchoring system. The oscillation amplitude of the axial force in the anchored segment is mitigated far more than that in the free segment, demonstrating that extended/full-length anchoring is extremely stable and surpasses simple anchors with free ends.

Control Technology and Application for Surrounding Rock Deformation in T-Junction Area of Gob-Side Entry Retaining

On the bases of theoretic analysis, numerical simulation, and project practice, stress distribution of surrounding rock and movement characteristics of roof strata in T-junction area of gob-side entry were analyzed. Besides, control technology in T-junction area was proposed after indicating deformation characteristics of different stages in gob-side entry. Results show that there is an inclined block in T-junction area after fracture of main roof and the period when the block is tending to stability is a critical stage of pressure appearance for gob-side entry. Further, three stress zones: in-situ stress zone, abutment pressure zone and low stress zone appear in both strike and dip of mining panel and enhanced support of gob-side entry before excavation should keep away from the abutment pressure zone, which also indicates that deep bolting support can exert the bearing capacity of deep rock mass. Additionally, the whole procedure of surrounding rock deformation can be divided into four stages and deformation in gob-side entry is larger than that of entry excavation, which means that efficient support design should be taken into account. Ultimately, district control technologies for surrounding rock of gob-side entry in back zone, filling zone, and front zone are proposed and it showed a good adaption in project practice of coalmine.

Practice on No Pillar Simultaneous Extraction of Coal and Gas in Deep Mine

Based on four typical mining condition, such as implementation gob-side entry retaining under sandstone roof, long length and long time gob-side entry retaining, implementation gob-side entry retaining under mudstone roof and implementation gob-side entry retaining at depth of more than 900 m, this paper analysis the roadway maintenance-control in no pillar simultaneous extraction of coal and gas engineering, research on the roadway maintenance-control technology during exploitation and before mining, further study typical convergence characteristics of gob-side entry retaining after mining, drew in application of no pillar simultaneous extraction of coal and gas in deep mine, proposed evaluation about the project current existing problems and development trends.

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.

Experimental study of axial stress distribution and transfer along the bolt rods in an underground coal mine

Based on the data of experimental tests, underground pullout tests, and real-time monitoring of active anchor-bolt stress in the Dingji Coal Mine in the Huainan mining area, China, the axial stress distribution along the bolt rods in a coal mine roadway was obtained. The results show that the axial stress in bolts increases initially then decreases along the anchoring direction under a perfect anchoring state. As the pre-tension is increased, the stress variation displays nonlinear behavior. The external section of the anchoring bolt rod plays a primary role in restricting the deformation of the surrounding rock before failure occurs. The results also indicate that the bending moment in the bolt rod has a significant effect on the axial stress distribution and transfer, and the bearing and supporting effects of the bolt increase as the pre-tension increases to a certain extent. The axial stress and its transfer features in a working bolt along the embedded direction are nonuniformly distributed and fluctuate.

Experimental Investigation of Sandstone under Cyclic Loading: Damage Assessment Using Ultrasonic Wave Velocities and Changes in Elastic Modulus

This laboratory study investigated the damage evolution of sandstone specimens under two types of cyclic loading by monitoring and analyzing changes in the elastic moduli and the ultrasonic velocities during loading. During low-level cyclic loading, the stiffness degradation method was unable to describe the damage accumulations but the ultrasonic velocity measurements clearly reflected the damage development. A crack density parameter is introduced in order to interpret the changes in the tangential modulus and the ultrasonic velocities. The results show the following. (1) Low-level cyclic loading enhanced the anisotropy of the cracks. This results from the compression of intergranular clay minerals and fatigue failure. (2) Irreversible damage accumulations during cyclic loading with an increasing upper stress limit are the consequence of brittle failure in the sandstone’s microstructure.