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Mechanical and fracture characteristics of single tunnel under the induced effect of a key joint

Wang Fei, Zhang Peng, Li Kaihui, Wang Cong, Cui Pengfei

Archives of Civil and Mechanical Engineering

2023

Vol. 23, no. 3

art. no. e206, 2023

The existence of joint at key position will change the stress distribution law around the tunnel, thereby changing the strength and stability of the tunnel. Therefore, a series of uniaxial compression tests were performed on the single-holed samples containing a joint to explore the effect of key joint parameters, including joint inclination (0°, 30°, 45°, 60°, and 90°) and length (20, 30, 40, and 50 mm), on the mechanical and fracture characteristics of the tunnel. Meantime, the DIC equipment and box dimension method were used to analyze the surface strain field of the sample and quantitatively characterize the surface crack of the failed sample, respectively. The experimental results show the uniaxial compressive strength of samples reaches the maximum and minimum values at joint inclination angles of 0° and 60°, respectively, and the strength of samples is negatively correlated with the joint length. Interestingly, the existence of some key joints can help to improve the strength of the single-holed sample. The strain bands and cracks mainly develop around the joint and the tunnel, which is well explained by the stress distribution results calculated by COMSOL software. The numerical results show that with the increase of joint inclination, the tensile stress at the upper endpoint of circular hole is increasing, and the compressive stress at left and right endpoints of the hole slightly increases. Analyzing the relative positional relationship between the coalescence path and the tunnel, the failure modes of the sample with different joint inclinations are classified into three types: center-symmetric failure (0° and 90°), through-joint failure (30° and 45°) and axisymmetric failure (60°). Additionally, the box fractal dimension of the surface crack of failed sample at the peak stress is closely related to the failure mode of the sample, and the box fractal dimension of samples with the same failure mode is positively correlated with their peak stress.

Shear rupture behaviors of intact and granulated Wombeyan marble with the flat-jointed model

Li Kaihui, Han Dongha, Fan Xiang, Yang Yi, Wang Fei

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 1

art. no. e51, 2022

Many large hydropower projects have been constructed in the highly interlocked and non-persistently jointed rock mass (e.g., the columnar jointed basalt). Regarding the granulated Wombeyan marble as an analog of such rock mass, a series of direct shear test simulations under constant normal loading (CNL) and constant normal stiffness (CNS) boundary conditions were performed to better understand its shear rupture behavior using the flat-jointed model based on the discrete element method. The results show that the shear process under the CNS condition can be divided into four stages: linear-elastic stage, yielding stage, shear wear stage and shear sliding stage. The strain-strengthening in the shear wear stage is evident due to the feedback normal stress provided by the cap induced by shear dilation. As the initial applied normal stress increases, the peak shear strength point of specimens under CNL approaches the yield point of specimens under CNS. Additionally, the shear rupture process under CNS is strain controlled in granulated specimens but stress controlled in intact specimens. Correspondingly, the shear rupture zone creation in granulated specimens is not but that in intact specimens is dependent on the initial applied normal stress. Finally, it is demonstrated that the CNS condition can improve the stability of rock slopes or underground excavations embedded in such rock mass and its improvement effect increases with the normal stiffness.