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Floor heave is a disaster that is frequently encountered in tunnel engineering. Existing explanations of floor heave mechanisms have many limitations, and the most important of which is that the stress concentration, release and transfer phenomena and the influences of the in-situ stress lateral coefficient and tensile strength are ignored. Therefore, the combined finite-discrete element method (FDEM) is used to study the failure process of floor rock masses and propose a new mechanical mechanism of floor heave that can consider the influences of the in-situ stress lateral coefficient and tensile strength. The type I fracture energy corresponding to different tensile strengths is calibrated using a direct tensile simulation test. Then, the floor heave mechanism is investigated under different lateral coefficients and tensile strengths, and five different floor heave modes are proposed under different in-situ stress lateral coefficients and tensile strengths. The floor heave modes controlled by the various in-situ stress lateral coefficients and tensile strengths are different, but they can all be explained by the maximum concentrated tangential stress. [ABSTRACT FROM AUTHOR]

To investigate the effects of the oval hole on the strength and the failure mechanisms of rock materials, a series of uniaxial and biaxial compression numerical simulations were carried out using the combined finite-discrete element method (FDEM). Based on the numerical simulation results, the effects of dip angle (β), major–minor ratio (ω) and position of the oval hole on rock strength characteristics crack initiation and development process and final failure modes were analysed in detail. The numerical simulation results indicate that: (1) Under uniaxial compression, the ultimate failure modes of rock containing oval holes can be mainly classified into single inclined plane shear failure, X-shaped tensile-shear composite failure and Y-shaped tensile-shear composite failure. Under biaxial compression, the ultimate failure mode is single-inclined plane shear failure dominated by shear cracks. Whether it is uniaxial compression or biaxial compression, the crack initiation position of the main fracture surface is near the end of the major axis of the oval hole. (2) Under uniaxial compression, the axial stress–strain curve of rock containing the oval hole is smooth before and after the peak, but under biaxial compression, there is obvious fluctuation in the post-peak stage due to crack initiation and propagation. In both uniaxial and biaxial compression, the pre-peak elastic stage of the axial stress–strain curve of the rock containing the oval hole is significantly shortened and the peak stress is significantly reduced compared with the intact rock, which indicates that the bearing capacity of the rock is seriously reduced by the existence of oval hole. (3) When the position and major–minor axis ratio of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength increase with the increase of the dip angle of the oval hole. When the position and dip angle of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength decrease with the increase of the major–minor axis ratio of the oval hole. (4) At different dip angles, the brittleness index Bi of rock containing oval holes decreases with the increase of confining pressure, and the brittleness degree shows a gradual weakening trend. At the same time, when the dip angle increases, the decreasing amplitude of brittleness index Bi decreases with the increase of confining pressure. Three types of failure modes of rock with elliptical hole are systematically summarised, and the failure mechanisms of elliptical hole rock are revealed. The influence of different positions, major–minor axis ratio, and dip angle of rock containing oval holes on uniaxial compression and biaxial compression are studied. The relationship between brittleness index and failure mode of rock containing the oval hole is investigated. [ABSTRACT FROM AUTHOR]

The synergistic effects of inherent minerals and specific organic structures in coal on steam gasification were comprehensively investigated herein. The steam gasification of lignite, anthracite, and their corresponding demineralized samples were tested by a laboratory fixed-bed micro reactor system. Lignite showed much higher gasification reactivity than anthracite, which led to a significant reduction of gasification temperature. Moreover, it was proved that minerals could promote the release rate of H2 but decrease the release rate of CO, which resulted in high H2/CO of the syngas for lignite, but the effects of minerals on the steam gasification of anthracite were not so obvious. These gasification behaviors could be attributed to the existence of inherent minerals in lignite, which indicated that inherent minerals had a remarkable catalytic effect on steam gasification, but limited impact on that of anthracite. The inherent minerals and organic structures of coal were characterized by scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Raman). The results showed that interactions of inherent minerals with oxygen-containing functional groups such as carboxyl functional groups and the more amorphous carbon structures in lignite played a key role in the catalytic effect during steam gasification. [ABSTRACT FROM AUTHOR]

The degradation of rock mechanical behavior after cyclic wetting and drying could affect the stability of rock slope, embankment, and tunnel significantly. A series of tests were first conducted on sandstone collected from a reservoir bank slope. The results showed that the mechanical parameters (including tensile strength, uniaxial compressive strength, elastic modulus, and internal friction angle) decrease as the cyclic number of wetting and drying increases. Then, a damage variable, which can consider the combined influence of cyclic wetting-drying and loading, was first proposed based on test results. Further, an improved damage model, which considers the effect of cyclic wetting-drying on internal friction angle and the nonlinear deformation characteristics in the fissure closure stage, was established with statistical damage mechanics. The determination method for the parameters in the proposed damage model was also introduced. Finally, the effectiveness of proposed model was verified by comparing with experimental data. The verification indicates that the results from the improved model agree well with test results, and the improved model can enhance the prediction accuracy over the original model. [ABSTRACT FROM AUTHOR]

The study of coal oxidation and weight gain at low temperatures is helpful in understanding the spontaneous combustion of coal and its mechanisms. In this study, we comprehensively investigated the low-temperature oxidation properties and the oxygen adsorption weight gain kinetics of Chinese Shenhua subbituminous coal (SH). Using thermogravimetric experiments with various particle sizes, heating rates, and coal ranks, the oxygen adsorption mass gain stages of coal oxidation at low temperatures were investigated. According to the results of these experiments, as the particle size of SH coal decreased, the dry cracking temperature (T1) decreased, the ignition temperature (T2) changed slightly, the weight gain increased, and the spontaneous combustion tendency increased. The three-dimensional diffusion model (Zhuralev-Lesokin-Tempelman) (D5) is suitable for SH coal samples with different particle sizes. A higher heating rate is not conducive to the formation of carbon-oxygen complexes. The D5 model can also be used to fit the kinetics of the SH coal samples at 5 K/min, 10 K/min, and 30 K/min. Unlike the SH coal samples, the Shengli lignite (SL) and Tavan Tolgoi anthracite (TT) samples fit the three-dimensional diffusion model (Jander) (D3). Based on the fitting results of the Achar differential method and the Coats-Redfern integral method, the activation energy E of the low-temperature oxidation weight gain of the coal samples decreases with decreasing coal particle size and with increasing oxidation rate and coal rank. [ABSTRACT FROM AUTHOR]