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• An integrated study of physical and numerical modelling is conducted. • Unloading rates affect both crack patterns and depths. • Low-speed unloading produces closed arc-shaped cracks on both sides of the tunnel. • Crack depth caused by low-speed unloading is larger than high-speed unloading. Excavation of tunnel is the unloading process of surrounding rock, and the unloading rate is one of the important factors affecting the stability of underground tunnel. Through the combination of large-scale physical model test and numerical simulation, combined with the water bag filling technology to simulate high-speed unloading excavation, the influence of unloading rate and unloading magnitude on the stability of surrounding rock of underground tunnel is quantitatively analysed according to the crack depth, displacement and stress concentration of surrounding rock. The results show that low-speed unloading produces closed arc-shaped cracks on both sides of the tunnel, while high-speed unloading produces closed circular cracks around the tunnel surrounding rock. Tunnels excavated with low-speed form wedge-shaped cracks and flaky destruction. The cracks distribute on the left and right sides of the tunnel, and form closed arcs. Tunnels excavated with high-speed form wedge-shaped closed cracks, which are circularly distributed around the tunnel. The crack depth of tunnel surrounding rock caused by low-speed unloading is larger than that caused by high-speed unloading, and the local stress concentration of low-speed unloading is also more obvious in the sidewall of the tunnel. However, the overall stress of the surrounding rock of the high-speed unloading is more significant, which means it accumulates much energy, and dynamic disasters may occur. High-speed unloading is more suitable for projects in low ground stress or soft rock mass stratum, which may cause less damage to the surrounding rock. In contrast, low-speed unloading can reduce the possibility of dynamic disaster for projects in high ground stress or hard, brittle rock mass. [ABSTRACT FROM AUTHOR]

• 32 loading levels in 3 physical models with different σ max direction are conducted. • The smaller the angle of σ max direction and tunnel axial, the tunnel is more stable. • If σ max direction is perpendicular to the tunnel axial, spalling is occurred. • If σ max direction is parallel, ring-like failure is occurred when σ max is large. • With increase of σ max , the effect of its direction increases. The major principal stress direction of the tunnel is one of the most important factors affecting the stability of an underground tunnel. This paper is devoted to the quantitative analysis of the effect of the major principal stress direction respect to the tunnel long axis on the stability of an underground tunnel. A total of thirty-two loading levels with three physical models combined with the numerical simulation were conducted. The results indicated that, with the decrease of the angle between the major principal stress direction and the long axis of the tunnel, the tunnel becomes more stable. For the load at which cracks begin to occur in the tunnel, when the major principal stress direction is parallel to the tunnel axis, there are no cracks in the tunnel until the load reaches approximately twice as much that of perpendicular. When the major principal stress direction is perpendicular to the tunnel axis, the failure involves spalling. The cracks initially occur at the corner of the tunnel, and then propagate along the sidewall in the form of a parabola. When the major principal stress direction is parallel to the tunnel axis, the failure involves ring-like damages. Cracks appear at the top and bottom of the tunnel, and connect with the cracks on the sidewall. With the increase of the principal stress, the effect of the major principal stress direction increases. [ABSTRACT FROM AUTHOR]

• 74 specimens with seven bedding angles and five confining pressures were tested. • The lowest strength occurs at bedding angle of 45°–60° and greatest below 15°. • The fracture angle of slate presents a U-shaped curve. • The lowest modulus occurs at bedding angle of 45°–60°, and greatest at 75°–90°. • Even if confining pressure is small, the modulus is significantly increased. The physical properties of slate in buildings are affected by its bedding angle and confining pressure. A total of 74 slate specimens with seven bedding angles and five levels of confining pressure were tested in uniaxial and triaxial compression tests. The effects of bedding angle and confining pressure on the mechanical behaviour of the brittle slate were quantified. The test results showed that, with the increase of confining pressure, the compressive strength of rock increased significantly. The variation in compressive strength of slate under confining pressure could be described by the parabolic Mohr-Coulomb strength criterion. The strength of slate is the lowest when the bedding angle is 45°–60°, and greatest when it is below 15°. When unconfined, the fracture angle of slate is 65°–90°. With increased confining pressure, the fracture angle of slate tends to 45° from above. As the bedding angle increases from 0° to 90°, the fracture angle of slate presents a U-shaped curve. When the bedding angle is 75°–90°, the deformation modulus of slate is greatest, and is minimised when the bedding angle is 45°–60°. Even if the confining pressure is low, the deformation modulus of slate is significantly increased compared to that in unconfined conditions. It proves that shotcrete treatment of the rock surrounding a tunnel will greatly improve the supporting effect thereon. [ABSTRACT FROM AUTHOR]

In order to study the influence of bedding angle and loading rate on the initiation fracture toughness (IFT) of coal, notched semi-circular bending (NSCB) specimens were made by using a split Hopkinson pressure bar (SHPB) system. The dynamic IFT of NSCB coal specimens with various bedding angles and crack lengths under different loading rates were analyzed and discussed. A high-speed high-resolution digital camera was used to record the processes of cracking initiation and propagation in coal specimens. The experimental results indicate that the effects of bedding plane on the cracking pattern decreases with the increase of both loading rate and crack length in the specimens. The impact velocity dominates the evolution of the dynamic IFT of coal, and the anisotropy caused by bedding planes in the specimens plays a more critical role in the evolution of dynamic IFT than the crack length. The effects of bedding angles on the dynamic IFT decrease with the increase of impact velocity. The growth behavior of dynamic IFT of coal distinctly changes after the loading rate exceeds a critical value. In addition, a model of rate dependency was established to describe the dynamic IFT of coal. [ABSTRACT FROM AUTHOR]