To address the large deformation of the surrounding rock of deep gob-side entry retaining under high stress, lithological characteristics of the surrounding rock and failure model of support body and their evolutionary processes are analyzed through field investigation and theoretical analysis. Failure mechanisms of surrounding rock and the technology to control it are studied systematically. The results show that the causes of the large deformation of the surrounding rock are weak thick mudstones with softening property and water absorption behavior, as well as its fragmentation, dilatancy, and long-term creep during strong disturbance and highly centralized stress states. The cross-section shape of the roadway after deformation and failure of the surrounding rock is obviously asymmetric in both the horizontal and vertical directions. Since the original system supporting the surrounding rock is unable to completely bear the load, each part of the supporting system is destroyed one after the other. The failure sequences of the surrounding rock are as follows: (1) roadway roof fracture in the filling area, (2) filling body fracture under eccentric load, (3) rapid subsidence of the roadway roof, and (4) external crack drum and rib spalling at the solid coal side. Due to this failure sequence, the entire surrounding rock becomes unstable. A partitioned coupling support and a quaternity control technology to support the surrounding rock are proposed, in which the roof of the filling area plays a key role. The technology can improve the overall stability of gob-side entry retaining, prevent support structure instability caused by local failure of the surrounding rock, and ensure the safety and smoothness of roadways.
This study investigates water-sand bursting disasters associated with fractured rock that affect safe mining in the mining areas of Western China. A broken rock water-sand seepage rheological test device was developed, and rheological tests were conducted on multiple groups of broken rock samples with single-stage axial loading and different load levels. When the rheology of each group of broken rock samples was stable, water-sand mixed fluid was injected into the samples at a certain pressure gradient to conduct water-sand seepage tests on broken rock masses. It was found that when the porosity of a fractured rock mass is within a certain range, the water-sand mixed fluid does not completely pass through the fractured rock mass and some sand particles are filtered by the fractured rock sample. There is an exponential relationship between the sand breaking ability and the sand filtration ability of fractured rock and its initial porosity, and the permeability of fractured rock decreases by a certain extent after sand filtration. However, for different load levels, when the flow through a fractured rock mass tends to be stable, the final porosity of the fractured rock mass decreases exponentially with axial compression. Based on the classical Kelvin rheological model and the basic theory of fractional calculus, a new fractional rheological model has been proposed and the rheological parameters under different load levels were fitted to the model. The new fractional rheological model is better able to describe the rheological characteristics of broken mudstone.
It is one of the important safety problems in the process of mining shallow coal seams in western China that the rock mass affected by mining stress breaks and forms a penetrating fracture, leading to a sand burst in the working face. The self-developed test system is used to carry out the experimental study on the flow characteristics of Aeolian sand in fractures. The research work is focused on the influence of several parameters, such as the thickness of the Aeolian sand layer, the fracture opening, and the fracture dip angle on the velocity of sand particles in fractures. The results show the following: (1) The influence of fracture opening and fracture angle on sand burst rate is much greater than that of sand thickness. No matter what the fracture angle and fracture opening value are, the influence weight of sand thickness on sand burst rate is almost zero. (2) When other conditions are unchanged, with the increase of fracture dip angle, the sand burst rate increases significantly, and the relationship between the sand burst rate and the fracture dip angle is exponential. (3) The influence weight of fracture opening is the largest. With the increase of fracture opening, the sand burst rate increases logarithmically. Finally, according to the test results, the relation equation which can simultaneously describe the influence of fracture opening and fracture inclination on the rate of the sand burst is fitted. This study can provide a theoretical basis and scientific guidance for the prevention and control of coal mine sand inrush disasters caused by roof cracking in western coal mines.
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