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Rockburst poses a great threat to the construction safety of deep tunnels under high geostress conditions. Recently, numerous deeply buried tunnels have been constructed in China, especially in the Sichuan–Tibet railway area. In this study, two rockburst experiments (strainbursts and impact-induced rockbursts) were conducted under different initial geostresses using the strainburst experimental apparatus and impact-induced rockburst experimental apparatus equipped with an acoustic emission (AE) monitoring system. A video acquisition system was used to capture the rockburst process. The results demonstrated that the rockburst process is similar under different initial geostresses, including grains ejection, rock plates spalling/buckling, and fragments ejecting violently. Based on the result of the AE characteristics, the large- amplitude AE signals were concentrated in the lower frequency band and the rockburst intensity was increased gradually with increase in the initial geostress. The b -value and β t -value fluctuated slightly at first, subsequently increased steadily, and then decreased abruptly when rockburst occurred. Furthermore, the displacement and divergence fields on the specimen surface were calculated using digital image correlation (DIC) technology. The result shows that the zone of large deformation and high divergence is consistent with the rockburst zone. Moreover, 80%σ peak is a critical stress state where rocks begin experiencing slight damage. • Failure process of strainbursts and impact-induced rockbursts is reproduced in laboratory. • Frequency-amplitude characteristics of AE under different geostress are analyzed. • Evolution of b -value and β t -value during the experiments is investigated. • Deformation and divergence fields are calculated, and microcracks begin to initiate and propagate at 80% of the peak stress. [ABSTRACT FROM AUTHOR]

Spalling is a typical failure mode encountered on the sidewalls after the tunnel face excavation of deep D-shaped tunnels or caverns. It significantly affects the safe and efficient construction and long-term stability of tunnels. To study the process and failure characteristics of sidewall spalling in D-shaped tunnels, four different three-dimensional initial stress states were established to perform a series of true-triaxial tests on cube specimens with a D-shaped hole. During the tests, a wireless micro camera was used to monitor and record the failure process of the sidewall in real time. Based on the test results, the process and failure characteristics of sidewall spalling were summarised, and the influence of cross-section shape on tunnel failure characteristics and tunnel stability were discussed. The results indicate that both sidewalls perpendicular to the maximum principal stress are cut into approximately parallel rock slabs, demonstrating typical spalling features. Failure zones on both sidewalls are located primarily between the corner and spandrel, and their outlines are "V" shaped (i.e., V-shaped notch). Spalling slabs are thin in both wings, and thick in the middle; they are cut into multilayer thin slices that are not completely detached from one another. With the increase in failure depth, the shape of the cracks first evolved from a straight shape into an arced shape, and subsequently evolved into a "V" shape. When the maximum principal stress is perpendicular to the tunnel axis, tunnel excavation along the minimum principal stress is conducive to the prevention of spalling. Compared with the circular tunnels, the D-shaped tunnels can effectively reduce the severity of rock failure and transform the rockburst into a static spalling. In a relatively low stress environment, the D-shaped cross section can improve the stability of the tunnel, while it is not conducive to the stability of the tunnel in a high stress environment. The result of this study benefits the design and prevention of spalling and rockburst of hard rock tunnels. [ABSTRACT FROM AUTHOR]

• Rockburst on the red sandstone containing an elliptical hole is reproduced. • The impact disturbance is considered in the experiment. • The average ejection velocity field of rockburst is investigated. • The influence of axial ratio on rockburst in elliptical tunnel is explored. The effect of the ratio of long and short axes on the rockburst of an elliptical tunnel structure was investigated by performing a series of true triaxial tests on red sandstone cuboid samples containing seven types of elliptical holes with different long–short axial ratios, using a self-developed true triaxial compression equipment. The entire process of spalling was monitored and recorded using a miniature camera. According to the test results, the failure stress, geometrical characteristics of the rockburst notch, fragment characteristics, and ejection velocity field characteristics of the rockburst are summarized, and the influence of the axial ratios of the elliptical hole on the failure characteristics and stability of the tunnel are discussed in detail. The test results indicate that the rockburst process of elliptical tunnels with different axial ratios experiences four processes; namely, calm period, small particle ejection period, spalling, and rockburst. The shape of the failure area after the rockburst presents a V-shaped feature. With the increase of the axial ratio, the area, angle, and depth of the V-shaped area show a decreasing trend, the initial failure stress of the cavern and the rockburst failure stress exhibit a trend of first decreasing and then increasing; the tangential stress of the cavern shows an increasing trend; the mass percentage of micro, fine, and medium grain decreases; and the average ejection velocity first decreases and then increases. Under the same stress conditions, rockbursts are more likely to occur on circular sections. Under the same section conditions, the ellipse can bear larger loads when its major axis is parallel to the disturbance stress. These conclusions are helpful to the efficient design of the tunnel structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • The initial crack affects the location and intensity of the rockburst. • Three mechanisms dominate the effects of the pre-existing crack on rockburst. • Four different rockburst evolution modes are proposed for the tunnel. The structural planes adjacent to the tunnel have a significant effect on rockburst, which has become an important engineering issue. To investigate the influence of such a rigid structural plane on rockburst, four different types of block specimens containing a through circular hole are designed with a prefabricated unfilled crack. A series of biaxial compression tests are then performed to simulate the rockburst process, followed by a detailed analysis of the strength characteristics, the strain field, the rockburst evolution process, and the microscopic characteristics of fracture surfaces. A prefabricated unfilled crack causes a decrease of the initial damage stress and the overall strength of block specimen, the earlier occurrence of rockburst but a decrease of initial rockburst intensity. Compared with the exposed crack, the non-exposed crack facilitates the occurrence of the rockburst more. Three mechanisms are proposed to explain these effects, including (i) the local stress concentration that alters the location of initial damage and the following crack propagation pattern, further affecting the location and scope of rockburst; (ii) the weakening effect that promotes rockburst evolution and inhibits rockburst intensity; and (iii) the sliding-induced secondary rockburst that contributes with significant energy release once the pre-existing crack is compacted. In light of the experimental observations, four different rockburst evolution modes are summarized to help to understand the rockburst in practical tunnels, and some engineering measures are recommended for the controlling and prevention of rockburst. [ABSTRACT FROM AUTHOR]