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Based on non-linear Hoek-Brown criterion, a new approximate solution of deformation and plastic zone radius of circular tunnel is deduced under non-uniform pressure. In the plastic region, three different, Young's modulus attenuation models are applied to solve the plastic zone deformation. The results show that the lateral pressure coefficient (LPC), dilatancy coefficient, buried depth and Young's modulus attenuation exert important effects on the surrounding rock state. Under the influence of LPC, the radius and displacement of plastic zone show non-uniform variation; then, the maximum value of the ground response curve is gradually shifted from the side to the roof. With the burial depth and dilatancy coefficient increase, the surface displacement presents the non-linear increase characteristic. Besides, the influence of Young's modulus on the plastic zone deformation is not only related to its attenuation model, but also closely related to the surface location and LPC of surrounding rock. Based on the above research, a non-linear dynamic support optimization method for non-uniform pressure circular tunnel is proposed. [ABSTRACT FROM AUTHOR]

The strength criterion is an extremely important basis for evaluating the stability of surrounding rock and optimizing the support pressure design. In this paper, nine different strength criterions are summarized and simplified based on the reasonable assumption. And then, a new unified criterion equation is established which includes all of the strength theories proposed by this paper. Meanwhile, a new unified closed-form solution for circular opening based on the newly proposed unified criterion equation is deduced with the non-associative flow rule under plane strain conditions. In the plastic zone, four different elastic strain assumptions are applied to solve the plastic zone deformation. The validity of the solution is also verified by comparing with the traditional solution. Finally, the influences of strength criterions, dilation coefficient, elastic strain form of plastic zone and rock mass damage on the mechanical response of surrounding rock are discussed in detail. The research result shows that Tresca(TR) and Von Mises (VM) criterions give a largest plastic zone radius, followed by Inscribe Drucker-Prager (IDP), Mohr-Coulomb (MC) and Middle Circumscribe Drucker-Prager (MDP) criterions and seem to underestimate the self-strength of rock mass. Circumscribe Drucker-Prager (CDP) criterion gives a smallest plastic zone radius and may be overestimated the self-strength of rock mass; Unified Strength Theory (UST0.5), Generalized SMP criterion (GSMP), Mogi- Coulomb (GMC) and Generalized Lade-Duncan (GLD) criterions which reasonably consider the effect of internal principal stresses give an intermediate range and can be strongly recommended for evaluating the mechanics and deformation behaviour of surrounding rock. As the dilation coefficient gradually increases, the dimensionless surface displacement presents the nonlinear increase characteristics; The deformation of plastic zone which are closely related to the strength criterions, are also greatly influenced by the elastic strain assumption in the plastic zone and rock mass damage degree. The assumption that the elastic strain satisfies Hook's law (Case 3) may be more reasonable compared to the continuous elastic strain (Case 1) and thick-walled cylinders (Case 2) assumptions. In addition, the Young's modulus power function damage model seems to give more reasonable solution for the deformation of plastic zone and is suggested to be a preferred method for solving plastic displacement. The research results can provide very important theoretical basis for evaluating the tunnel stability and support design reliability for different lithology rock mass in underground engineering. [ABSTRACT FROM AUTHOR]

The aim of this paper is to determine the influence of heating rate on the dynamic mechanical performance of coal measure rocks after heat treatment. Uniaxial compression tests were conducted using a split Hopkinson pressure bar system on coal measure rocks after heat treatment at 800°C at different heating rates. The results reveal that a high-temperature treatment changed the physical and mechanical properties of the sandstone. When the heating rate was increased, the natural and mechanical properties were reduced gradually. The failure mode of sandstone after heat treatment was tension destruction, and shear fracturing occurred locally only at high heating rates. Original cracks grew inside the sandstone after heat treatment, while the number of defects (including cracks and pores) increased gradually with an increased heating rate, which was the main reason for the change in the sandstone’s macromechanical properties. The energy dissipation during fracturing was correlated closely to the fracture characteristics and increased with increased heating rates. The results of this work can provide references for future engineering projects, such as underground coal gasification and coal-bed methane thermal exploitation. [ABSTRACT FROM AUTHOR]

In this paper, the dynamic loading tests of sandstone after heat treatment at 25-800 °C are carried out by using the split Hopkinson pressure bar test system. Combined with the theory of energy dissipation, the dynamic fracture and energy consumption characteristics of coal-series sandstone are systematically studied. The test results illustrate that the dynamic fracture and energy consumption characteristics of sandstone are mainly related to the changes of internal moisture and cementing materials. [ABSTRACT FROM AUTHOR]

It is of great significance, for economic, environmental and security reasons, to investigate the strength characteristic of underground cemented paste backfill (CPB). Consequently, an ultrasonic test, uniaxial and triaxial compression experiment, and acoustic emission (AE) monitoring were carried out on CPB, for which the particles satisfied Talbot gradation. The homogeneity of CPB specimens was evaluated by ultrasonic pulse velocity (UPV). The stress--strain behavior and AE characteristic of CPB specimens under different Talbot indices and confining pressures were investigated. The effects of the particle size distribution and the confining pressure on the peak strength of CPB were analyzed. The strength parameter model of CPB under the coupled influence of the particle size distribution and the confining pressure was constructed based on the Mohr--Coulomb strength criterion. The results show that the peak strength of CPB is positively linear with confining pressure, however, the relationship between its strength parameters and the Talbot index can be characterized by a quadratic polynomial function. This suggests that there is an optimal gradation of particles reflected in the maximum strength of CPB. [ABSTRACT FROM AUTHOR]