Numerical investigation of the scale effects of rock bridges

TL;DR

This study numerically investigates how the scale and dispersion of rock bridges affect the mechanical response and failure processes of rock masses, revealing scale-dependent behaviors and proposing a threshold for characterizing rock bridges.

Contribution

It introduces a numerical approach to analyze the scale effects of rock bridges while maintaining joint persistence, filling a gap in understanding rock mass responses.

Findings

Shear resistance decreases with increased dispersion of rock bridges.

Stress concentration areas diminish as dispersion increases.

Wing crack propagation is insensitive to scale effects.

Abstract

The concept of joint persistence has been widely used to study the mechanics and failure processes of rock masses benefitting from the simplicity of statistical linear weighing of the discontinuity. Nevertheless, this term neglects the scale effects of rock bridges, meaning that the same joint persistence may refer to different numbers and spacings of rock bridges, leading to erroneous equivalent rock mass responses. To fill in this gap, an intact rock bridge was dispersed as multi rock bridges while maintaining a constant joint persistence, subjected to direct shear by conducting numerical simulations employing Universal Distinct Element Code (UDEC). In this way, scale effects of rock bridges were investigated from the perspective of load-displacement curves, stress and displacement fields, crack propagations and AE characterizations. Results revealed that the shear resistance and the area and value of stress-concentration decreased with increasing dispersion. Furthermore, uneven distribution of displacement fields in an arc manner moving and degrading away from the load was first observed, indicating the sequential failure of multi rock bridges. It was also found that the propagation of wing cracks was insensitive to scale, while the asperity of macro shear fracture mainly formed by secondary cracks decreased with increasing dispersion. In addition, increasing dispersion of rock bridges would overlap the failure precursors identified by intense AE activities. Based on the abovementioned results, we evaluated existing methods to characterize the joint persistence, and a threshold was observed to possibly define a rock bridge.