Experimental Study of Prevention and Control of Rock Burst in Steeply Inclined Coal Seams by Mining Sequence and Filling

The control and prevention of rock burst in a steeply inclined coal seam are essential. In order to figure out the effects of filling and mining sequence on rock burst in the steeply inclined coal seam, B3+6 and B1+2 coal seams in Wudong coal mine are chosen as the research objects, and an in-house experiment system of similarity simulation is established in this study. Combined with numerical simulation, the characteristics of collapse, stress distribution, and displacement variations can be measured, which provide useful information to study the effects of the filling body and mining sequence on rock burst. Experimental results show that the key reason for rock burst in a steeply inclined coal seam is the stress concentration of the rock pillar between B3+6 and B1+2 coal seams instead of the stress-lever-effect of a deeper rock pillar. The filling body can support the middle rock pillar, share the geological structure stress in the horizontal and vertical direction, eliminate the stress concentration zone largely, and prevent the occurrence of rock burst. When multiple working faces are working, the opposite side of the coal seam should be mined first to release the energy in the rock in advance, thus preventing the rock burst effectively. The research results provide fundamental information for better understanding the reason for rock burst and preventing rock burst in the steeply inclined coal seam.

Mechanism of Rock Bursts Induced by the Synthetic Action of “Roof Bending and Rock Pillar Prying” in Subvertical Extra-Thick Coal Seams

When studying the rock burst mechanism in subvertical extra-thick coal seams in the Wudong coal mine in Xinjiang, China, most studies focus on rock pillars, while the effect of the roof on rock bursts is usually ignored. In this paper, a rock burst mechanism in subvertical extra-thick coal seams under the control of a “roof-rock pillar” is proposed. A theoretical analysis is first performed to explain the effect of roof-rock pillar combinations on rock bursts in coal seams. Numerical modeling and microseismic analysis are implemented to further study the mechanism of rock burst. The main conclusions are as follows: 1) During the mining of the B3+6 coal seam, an obvious microseismic concentration phenomenon is found in both the roof and rock pillar of B3+6. The rock bursts exhibited obvious directionality, and its main failure characteristics are floor heave and sidewall heave, but there will also be some failures such as shoulder socket subsidence in some parts. 2) The stress transfer caused by rock pillar prying is the main reason for the large difference in rock burst occurrence near the vertical and extra thick adjacent coal seams under the same mining depth. 3) Under the same cantilever length, the elastic deformation energy of the roof is much greater than that of the rock pillar, which makes it easier to produce high-energy microseismic events. With an increasing mining depth, the roof will become the dominant factor controlling the occurrence of rock bursts. 4) The high-energy event produced by the rock mass fracture near the coal rock interface easily induces rock bursts, while the high-energy event produced by the fracture at the far end of the rock mass is less likely to induce rock burst. 5) Roof deformation extrusion and rock pillar prying provide high static stress conditions for the occurrence of rock bursts in the B3+6 coal seam. The superposition of the dynamic disturbance caused by roof and rock pillar failure and the high static stress of the coal seam is the main cause of rock burst in the B3+6 coal seam.

Influence of Parallel-Joint Position on Mechanical Behavior and Acoustic Emission Characteristics of Rock Pillar

In this paper, the parallel-joint rock column model is established based on PFC software, and the effects of different joint positions on the mechanical properties, acoustic emission characteristics, and damage evolution characteristics of rock columns are analyzed. In the PFC models, the intact rock is simulated by parallel-bond model, and the joints are simulated by flat-joint contact model. The research result shows that on the whole, when the joint is outside the rock pillar, the UCS of the rock pillar is higher than that of the joint inside the rock pillar, while the elastic modulus is less than that of rock pillar with joint inside. The evolution characteristics of acoustic emission of rock pillars at different joint positions are basically the same. However, the maximum value of acoustic emission events and corresponding deformation of rock pillars at different joint positions are different. The damage of jointed rock mass can generally show three stages: no damage, slow damage increase, and sharp damage increase.

Theoretical Analysis of the Variation of Hydraulic Pressure in Cracks Under Uniaxial Compression of Water-saturated Rock Pillar

In order to analyse the variation of hydraulic pressure in cracks of water-saturated rock pillar under uniaxial compression,taking the water-saturated rock pillar as the research object,in which the cracks are divided into two types: longitudinal crack and inclined crack, and the elastic-brittle plastic model is used to describe the mechanical behavior of rock. Assuming that the long axial direction of the crack is consistent with the axial direction of the rock pillar, the expression of tensile stress in the direction perpendicular to the long axial direction of the crack under axial compression is derived by using Maxwell model and Inglis formula. Simplifying the crack to flat elliptic, clinical hydraulic pressure in the case of tensile shear failure and compressive shear damage of the cracks are deduced, and the distribution of clinical hydraulic pressure in uniaxial compression cracks with different growth pattern is analysed. The results show that with the propagation of cracks, the clinical hydraulic pressure near the tip is approach to zero, and in case of hydraulic fracturing, the extension should exhibit the characteristic of discontinuity.

Determination of Water-Proof Coal (Rock) Pillar Height in Mining Coal Seam Group under Water-Bearing Rock Stratum

In order to determine the reasonable height of water-proof coal (rock) pillar when mining multiple coal seams under aquifer, this paper analyzes the expansion height of water-conducting fracture zone when coal seams mining. Considering the expansion law of water-conducting fracture zone in coal seams mining, two schemes of coal seams mining in upper and lower groups and one-time mining of all coal seams are designed for comparative analysis, and the height of water-proof coal (rock) pillar is determined based on the expansion height of water-conducting fracture zone. The results show that the height of water-proof coal (rock) pillar is calculated as 91.08 m when mining upper and lower groups and 105.46 m when mining all coal seams at the same time. According to UDEC numerical simulation results, the height of water-proof coal (rock) pillar is 56.08 m when mining upper and lower groups and 86.36 m when mining all coal seams at the same time. Comparing the results of theoretical calculation and numerical analysis, the maximum value is selected as the final result, and the reasonable water-proof coal (rock) pillar height is determined to be 105.46 m.

Rock Pillar Design Using a Masonry Equivalent Numerical Model

In underground mining, the design of rock pillars is of crucial importance as these are structures that allow safe mining by maintaining the stability of the surrounding excavations. Pillar design is often a complex task, as it involves estimating the loads at depths and the strength of the rock mass fabric, which depend on the intact strength of the rock and the shape of the pillar in terms of the aspect ratio (width/height). The design also depends on the number, persistence, orientation, and strength of the discontinuities with respect to the orientation and magnitude of the stresses present. Solutions to this engineering problem are based on one or more of the following approaches: empirical design methods, practical experience, and/or numerical modeling. Based on the similarities between masonry structures and rock mass characteristics, an equivalent approach is proposed as the one commonly used in masonry but applied to rock pillar design. Numerical models using different geometric configurations and state of stresses are carried out using a finite difference numerical approach with an adapted masonry model applied to rocks. The results show the capability of the numerical approach to replicate common types of pillar failure modes and stability thresholds as those observed in practice.

Experimental Study on the Temporal and Spatial Variation Law of Gas Flow in Different Rock Pillar Drainage Boreholes

In order to study the temporal and spatial variation of gas flow in different rock pillar extraction boreholes, gas pressure, gas content, gas emission from 100-meter coal holes and coal seam permeability coefficient were measured on site. The site inspected the gas flow in the boreholes at 15m, 7.5m, directly above, 7.5m, and 15m at the lower slab of different rock pillar floor roadways. It analyzed the change law of gas flow in boreholes of 710 floor lanes and 505 floor lanes, which provided a basis for the layout of gas drainage boreholes.