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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]

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]

Pressure arch utilises the self-stabilizing capacity of surrounding rock and is benefit to the roof stability of underground excavations. The analysis of it provides a new stability assessment tool for underground engineering. In this paper, a series of numerical simulations were carried out to investigate the formation and properties of pressure arch and the self-stabilisation mechanism of surrounding rock. It is found that stress redistribution results in the formation of the pressure arch. The inner and outer boundary of arching area are determined as the principal stress trajectories through the inflection points of the principle stress-depth curves on the centre line of caverns. Furthermore, parametric analysis illustrates that in situ stress ratio and excavation size have more significant effect on the inner and outer boundaries and the arch thickness than roof shape and overburden depth. Finally, it can be observed that the various trends of the inner boundary height with different influence factors are same with the displacement of the crown, and it could be used as an indicator for stability assessment. Based on the observed phenomena, a design scheme is proposed to determine the maximum size of the underground openings cross-section without support in agreement with the geomechanical conditions. [ABSTRACT FROM AUTHOR]

To study the toughening effect and the anchoring mechanism of rock bolts, fractured rock specimens with different crack angles and different crack lengths are made of cement mortar. The prefabricated specimens are applied with different anchoring modes for reinforcement, containing no-anchoring, full-length bonded anchoring and pre-stressed anchoring. The uniaxial compression tests of fractured rock specimens under different anchoring modes are carried out, and then the mechanical properties and the failure modes of fractured rock specimens before and after anchoring are analysed. The 'Cavity Damage Effect' is defined and the test results show that the peak strength of the fractured specimens are roughly proportional to the crack inclination angle due to the 'Cavity Damage Effect', the crack is equivalent to a 'cavity damage' inside the prefabricated specimen if the opening of the crack is larger. The crack length can significantly affect the strength and deformation characteristics of no-anchoring specimens, while has little influence on the anchoring specimens. The full-length bonded anchoring and the pre-stressed anchoring can both significantly increase the mechanical parameters (the peak strength, residual strength and elastic modulus) of the specimens, enhance the resistance to deformation of fractured rock mass, and inhibit the expansion and coalescence of cracks. [ABSTRACT FROM AUTHOR]

Taking the tunnel face effect and time-dependent properties of soft surrounding rock into consideration, the spatial and temporal effects of squeezing deformation in the shield region are studied, and their determining influences on the interaction of contact-squeezing-friction between surrounding rock and shield is analysed. Then the mechanism of TBM shield jamming is revealed as follows: firstly, the radial convergence in the shield area increases with the increasing distance to the tunnel face and the growing stoppage downtime gradually; secondly, the convergence may exceed the overcut gap at a certain distance from the face where the surrounding rock starts to contact and squeeze the shield; as a result, frictional resistance is produced between the surrounding rock and shield; if the thrust supplied by the TBM propulsion system is insufficient to overcome the frictional forces when TBM advances forward, then the shield of TBM will become entrapped. Subsequently, based on the Convergence–Confinement method and the previously established constitutive model for squeezing soft rock, the calculation methods for the interaction curves (Longitudinal Deformation Profile, Ground Response Curve and Support Characteristic Curve) between soft ground and shield are put forward. Then the forecasting model for shield jamming inoculation process, the trigger condition and criterion for shield jamming are established. These forecasting models are illustrated and verified by a case study finally. [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]

Freedman's inequality is a supermartingale counterpart to Bennett's inequality. This result shows that the tail probabilities of a supermartingale is controlled by the quadratic characteristic and a uniform upper bound for the supermartingale difference sequence. Replacing the quadratic characteristic by, Dzhaparidze and van Zanten [On Bernstein-type inequalities for martingales. Stoch Process Appl. 2001;93:109–117] have extended Freedman's inequality to martingales with unbounded differences. In this paper, we prove thatcan be refined to. Moreover, we also establish two inequalities of type Dzhaparidze and van Zanten. These results extend Sason's inequality [Tightened exponential bounds for discrete-time conditionally symmetric martingales with bounded jumps. Statist Probab Lett. 2013;83:1928–1936] to martingales with possibly unbounded differences and establish the connection between Sason's inequality and De la Peña's inequality [A general class of exponential inequalities for martingales and ratios. Ann Probab. 1999;27(1):537–564]. An application to self-normalized deviations is given. [ABSTRACT FROM AUTHOR]

To resolve the image deconvolution problem, the total variation (TV) minimization approach has been proved to be very efficient. However, we observe that this approach has an over-minimizing TV effect in the sense that it gives a solution whose TV is usually smaller than that of the original image. This effect is due to the pre-pondering role of the TV in the corresponding minimization problem and prevents from finding the exact solution of the deconvolution problem when such a solution exists. We propose a modified version of the gradient descent algorithm, which leads to an exact solution of the deconvolution problem if it exists and to a satisfactory approximative solution if there is no exact one. The idea consists in introducing a control on the contribution of the TV in the classical gradient descent algorithm. The new algorithm has the advantage that the restored image has the TV closer to that of the original image, compared to the classical gradient descent approach. Numerical results show that our method is competitive compared to some recent ones. [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]