Abstract
The growth of double cracks is the main factor leading to progressive rock failure under hydromechanical coupling. The initiation modes and interaction behaviors of double cracks were investigated by using laboratory tests, and the influences of water pressure were analyzed. The maximum energy release rate criterion was modified to determine the crack growth characteristics. A numerical model was established and then verified by the test results. Based on the simulation, the distribution of stress fields and key fracture parameters of double cracks was investigated. Then, initiation characteristics and interaction behaviors of parallel and nonparallel cracks were quantitatively analyzed. The results indicate that the increase in water pressure leads to the crack initiation being inclined to the original surfaces and the growth length along the crack fronts tending to be uniform; the small tensile stress zones are formed close to the crack tips, and significant compressive stress zones are formed at both sides of the crack surfaces; stress superposition and interaction occur when crack spacing is less than 2.5a; the interactive weakening effect is mainly present in the inner side (rock bridge zone) of cracks, while a certain degree of interactive enhancement effect exhibits in the outer sides; the cracks are much easier to initiate at the outer wing cracks when the spacing is less than the critical length (0.5a); and cracks with a dip angle of 45° are much easier to initiate at the endpoints of long axis. The research results provide certain theoretical guidance for the safety assessment of underground engineering.
Highlights
Under the action of uniaxial compression and water pressure, the wrapping wing cracks were generated at the long-axis ends of prefabricated cracks, with large deflection angles to the original crack planes. e growth of the outer and inner wing cracks showed a certain degree of asymmetry due to the crack interaction effect, which is mainly reflected in the fact that the lengths of outer wing cracks were slightly longer than those of the inner ones
E crack initiation angle is defined as the acute angle between the new and original crack surfaces. e spatial pattern of 3D crack growth is primarily controlled by the initiation angles and extension lengths at the endpoints of crack long and short axes. erefore, the initiation characteristics of cracks reflect the growth mode and interaction behavior between cracks to some extent
In order to accurately capture initiation characteristics of the double cracks, the crack initiation stress was measured through multiple pretests and test results indicated that the double cracks initiated when the axial compressive stress reached about 1/3 of the specimen peak strength. e ratio was much lower than the initiation stress threshold reported by the previous studies [24, 25], which is ranging from 42%∼ 53%σp. erefore, it is more accurate to observe the crack initiation characteristics when the axial pressure reaches about 1/3 of the peak strength of the specimen
Summary
Discontinuities such as joints and cracks that are widely present in rock masses have a significant impact on the stability and safety of underground structures [1, 2]. e unsteady growth and coalescence of joints and cracks are triggered due to the coupling effect of stress and seepage fields, especially in complex environments with high geostress and high groundwater pressure, leading to the rock failure and serious engineering accidents. erefore, the research on the growth and interaction mechanisms of cracks in rocks under hydromechanical coupling has received more and more attention in recent years.To date, the problem concerning the growth of a single 3D crack has been studied extensively [3,4,5]. e previous work has laid a foundation for the prediction of the failure of rock containing a single crack [6,7,8]. E unsteady growth and coalescence of joints and cracks are triggered due to the coupling effect of stress and seepage fields, especially in complex environments with high geostress and high groundwater pressure, leading to the rock failure and serious engineering accidents. Studies and many engineering practices indicate that the initiation and growth modes of multiple cracks are quite different from that of the single crack due to the interaction between cracks, which is the main reason to cause large-scale rock instability [9, 10]. The stress and seepage fields are significantly affected by the double cracks interaction behavior, which, in turn, Advances in Materials Science and Engineering changes the penetration of the rock bridge as well as the rock failure modes [15,16,17,18,19]
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