Abstract
To reveal the mechanical mechanisms and energy release characteristics underlying progressive failure of columnar jointed basalts (CJBs) with various model boundaries and confining pressures, by combining the meso-damage mechanics, statistical strength theory, and continuum mechanics, inhomogeneous CJB models with different dip angles to the column axis are constructed. In the cases of plane stress, plane strain, and between plane stress and plane strain, the gradual fracture processes of CJBs are simulated under different confining pressures and the acoustic emission (AE) rules are obtained. The results show that: 1) in the case of plane stress, the fracture process of CJBs along direction I orthogonal to the column axis: at the initial stage of loading, the vertical joints and the transverse joints in the CJB specimen are damaged. Then, more columns in the upper middle part are cracked; 2) in the case between plane stress and plane strain, the fracture process of CJBs along the direction parallel to the column axis: at the initial stage of loading, the columnar joints are damaged. Then, the area of the damaged and broken columns at the top of the specimen increases and the crushing degree intensifies; 3) for the case between plane stress and plane strain, the AE energy accumulation before the peak stress is higher than the plane strain state along the direction orthogonal to the column axis. Meanwhile, along the direction parallel to the column axis, this value becomes larger when changing from the state between plane stress and plane strain to the plane strain state. These achievements will certainly improve our understanding of the fracture mechanism and energy evolution of CJBs and provide valuable insights into the instability precursor of CJBs.
Highlights
A columnar jointed structure is a kind of tensile fracturedominated structure with regular or irregular columns, which is common in volcanic lava
From the perspective of confining pressure, in the case between plane stress and plane strain, the increase of the compressive strength (CS) was relatively small with the growth of confining pressure; in the case of plane strain, the CS increased greatly with the growth of confining pressure
3.1.2 In the Direction Parallel to the Column Axis: Strength and Deformation Characteristics of columnar jointed basalts (CJBs) With Different Model Boundaries and Confining Pressures It can be seen from Figure 5E that, in the direction parallel to the column axis and with a confining pressure of 0 MPa, the CS of the three model boundary conditions changed in a U-shaped trend with the increase of the column dip angle, in which the CS reached the lowest value at β 30° and changed slowly at the angle range β 60°–90°
Summary
A columnar jointed structure is a kind of tensile fracturedominated structure with regular or irregular columns, which is common in volcanic lava. It is mostly found in basalt, and sometimes in intermediate acid lava, fused tuff, subvolcanic rock, and basic dike. Columnar jointed rock masses (CJRMs) usually present with the forms of quadrilateral prisms, pentagonal prisms, hexagonal prisms, etc. In Southwest China, many large-scale hydropower stations have been built in the areas filled with columnar jointed basalts (CJBs), such as the Baihetan Hydropower Station, Wudongde Hydropower Station, Jin’anqiao Hydropower Station in Lijiang, Yunnan, and the Xiluodu Hydropower Station in the canyon section of Jinsha River. With CJRMs being a special type of rock mass, their mechanical properties can be obtained from in situ tests, laboratory tests, numerical simulation, and some other methods (Min and Jing, 2003; Eshraghi and Zare, 2015; Liu and Shao, 2017; Yu et al, 2017; Yu et al, 2018)
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