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Using goof as water storage space plays a vital role in the ecological environment and economic development of arid mining areas, while the rock strength and the stability of coal pillars in underground water reservoirs are closely related to creep process. In this work, triaxial creep-seepage tests were conducted for coal samples to develop new insights into the creep behavior and permeability evolution. The results showed that the creep deformation and permeability evolution of coal samples exhibit three stages, namely, the compaction hardening stage before the stress threshold, volumetric compaction stage, and volumetric dilatancy stage. The coal permeability decreases first and then increases with the creep strain and it is well correlated with the variation of volumetric strain. [ABSTRACT FROM AUTHOR]

As a long-term urban transportation infrastructure, the stability of metro construction is very important. With the continuous expansion of urban metro network, metro adjacent construction is more and more frequently encountered. In this paper, taking the double-line adjacent construction problems encountered during excavation of Beijing New Airport Line metro in sandy cobble stratum as an example, the surface settlement caused by excavation of single-line metro and double-line adjacent metro is studied by computed tomography scan, electron microscope scanning, laboratory mechanical tests, orthogonal inversion and numerical simulation and other methods. The results show that 1) the sandy cobble particle size and sandy cobble distribution of Beijing sandy cobble samples are relatively dispersed, and the sandy cobble particle size is from 3 to 20 cm, of which the largest proportion is the sandy cobble particle size from 5 to 8 cm. 2) The internal friction angle of Beijing sandy cobble samples with different moisture content is from 27° to 40°, which is much larger than that of other soil samples. Even at the post-peak stage, the shear stress of some sandy cobble samples does not gradually tend to be gentle, but continuously increases or has secondary peaks. 3) For the excavation of double-line adjacent metro, the maximum settlement difference and the impact range of excavation are proposed as indicators to investigate the degree of interaction of the adjacent metro. 4) The results of numerical orthogonal analysis show that the width of surface settlement zone increases significantly with the increase of tunnel diameter and spacing when excavating adjacent metro in sandy cobble stratum, but the rate of increase with tunnel diameter (from 6 m to 14 m) almost reaches 2 times of the rate of increase with the spacing (from 3 m to 15 m). 5) The maximum settlement value of sandy cobble stratum increases with the increase of tunnel diameter and decreases with the increase of spacing. The orthogonal linear analysis shows that the influence of tunnel diameter on the maximum settlement value is much greater than the spacing. The maximum settlement increases with the increase of tunnel diameter, which is about 3.6 times of that with the decrease of spacing. [ABSTRACT FROM AUTHOR]

A reliable prediction of the strata behavior and support capacity of longwall workings has always been a challenge for mining engineers. It is still difficult to quantitatively predict some important mining parameters, such as support capability and the interval of periodic caving of the major roof under given mining and strata conditions. The reason is that several factors controlling the strata behavior are difficult to determine, such as the bulk factor of the caving zone, strength factor of the major roof and load factor of the overlying rock strata. In this study, the back analysis method is employed to address this problem. This paper illustrates an integrated approach to predict the strata movement behavior and optimum capability of powered roof supports for safe mining. First, this approach uses the empirical method to qualitatively analyze the mining influence zone, then establishes a quantitative prediction model by theoretic analysis method, and solves the optimum values of the factors described in the model with the back analysis method. On the basis of the above research, the support capability and interval of periodic caving of the major roof under similar geological conditions may be calculated by using these suitable values of the factors. Finally, an illustration of the above process is presented in this paper to examine the reliability of this approach. [ABSTRACT FROM AUTHOR]

The extensive bedding and cleats have a great influence on the mechanical properties of coal. The previous research mainly focused on the bedding effect, and few studies took the influence of both bedding and cleats into consideration. The scanning electron microscopy (SEM), uniaxial compression experiments, and acoustic emission (AE) monitoring were conducted on one hard coal specimen with different bedding and cleat angles. The experimental results show that for the bedding dip of 0° and face and butt cleat dip of 90°, the properties of coal specimens were characterized by the highest peak strength, the drastic failure process, the post-peak brittle behavior, the long initial quiet stage, and the short violent booming stage of AE hits number. For the bedding and face cleat dip of 45°, and the butt cleat dip of 90°, the properties of coal specimens were characterized by the lowest peak strength, the slip failure mode, the step-like drops of the post-peak stress-strain curve, and the several peaks of AE hits number. On the other hand, for the bedding and butt cleat dip of 90°, and the face cleat dip of 0°, the properties of coal specimens were presented as tensile failure mode with plate cracks, the short non-linear stage and the slight step-like drops of stress-strain curve, the short initial quiet stage of AE hits number, and the large value of elastic modulus. Then, a theoretical model of coal containing bedding and cleats was established in this paper. The simulation results of peak strength and failure mode verified the proposed model. The simulation of coal strength shows that bedding had the decisive influence on the uniaxial compressive strength of coal, while the weakening effect of cleats was non-negligible. When bedding and cleats were taken into consideration together, the minimum peak strength was reduced and the bedding dip related to the minimum peak strength changed. [ABSTRACT FROM AUTHOR]