3D Particle-Based DEM Investigation into the Shear Behaviour of Incipient Rock Joints with Various Geometries of Rock Bridges

Abstract

A 3D particle-based DEM model was established taking into account the geometries of rock bridges. The model was used to investigate the shear behaviour of incipient rock joints. Fifty-seven direct shear tests were conducted under constant normal load (CNL) boundary conditions using the established model, in which rock bridges with nineteen different geometries and incipient joints with various areal persistence (between 0.2 and 0.96) were involved. Our results show that, for the cases having a single rock bridge, cracks often initiated around the edges of the rock bridges and coalesced first in the middle of the rock bridge areas. While for other cases containing multiple rock bridges, cracks initially appeared at the connection points (located in the middle of the joint planes) of the rock bridges and then propagated to the edges. High crack initiation stresses were measured, which were often more than 60% of the shear strength of the tested incipient rock joints. Sudden failures of the rock bridges subjected to shearing were observed, accompanying dramatic increases in the number of cracks. Another important conclusion derived from this research is that both joint areal persistence and rock bridge geometry played significant roles in the shear failure of the simulated Horton Formation Siltstone joints. The present study has shown that shear strength increased gradually when joint areal persistence was decreased. Interestingly, different shear strength values were measured for rock joints with the same areal persistence (e.g. K=0.5). Shear velocity was also found to have a significant influence on the shear characteristics of the Horton Formation Siltstone joints. A higher shear strength was measured when the shearing velocity was increased from 0.01 to 1 m/s.

Optimized Analysis Method for Evaluating the Shear Strength Parameters of Rock Joint Surfaces

“…Without distinguishing the difference between specimens, the shear strength parameters obtained by directly fitting the data can neither faithfully reflect the effect of normal stress on the shear strength nor can faithfully reflect the shear strength of a group of specimens even if the value of R 2 is high. Nevertheless, shear test under varying normal stresses provides a considerable amount of useful information that should be fully analyzed and applied [14,15]. As such, developing a suitable analysis method to obtain shear strength parameters that can better re ect the shear characteristics of the rock mass joint surface is of paramount importance.…”

“…Moreover, regardless of whether shear test is conducted with natural joint specimens collected on-site [3][4][5][6] or artificially prepared, joint specimens [7][8][9][10][11][12][13] have been widely studied, and it is impossible to ensure that all specimens have the same morphological feature on the joint surface. For example, by the study of Shang et al [14], even for some rock joints with the same areal persistence, the measured shear strength is still different. At the same time, Ji et al [15] found it is different to remove the effect of surface irregularity from the test results.…”

Geological discontinuity persistence: Implications and quantification

“…Many investigations have been undertaken to measure the shear strength of discontinuities, mostly focusing on mechanical discontinuities with zero true cohesion (Barton, 1976). For non-filled and non-persistent rock joints, shear strength is however controlled by four components including fundamental shear strength of rock bridges (Shang and Zhao, 2017), internal friction in solid bridges (after rock bridges are mobilized), friction from the persistent joint segments (Lajtai, 1969b) as well as geometry and location of bridges (Ghazvinian et al, 2007). An equivalent shear strength calculation method was developed based on the Mohr-Coulomb failure criterion, in which strength contributions from rock bridges and persistent joint areas are linearly combined (Lajtai, 1969a;Hudson and Harrison, 2000) as expressed by the following equation:…”

“…Rock bridges significantly increase the shear strength of individual incipient rock discontinuities (Shang and Zhao. 2017), especially under constant normal stiffness boundary conditions (Shang et al, 2018).…”

“…Zhang et al, 2006). In numerical analysis, non-persistent rock joints containing rock bridges with different geometrical parameters are readily analysed (Shang and Zhao, 2017); the brittle failure of rock bridges often lead to a dramatic drop in shear strength ( (Rao et al, 2006) and therefore mode of fracture initiation and propagation.…”