Breccias are composed of broken rock fragments (often called clasts) cemented together by a fine-grained matrix that can be similar to or different from the composition of the fragments. Breccias are among the most difficult rocks to sample and test because clast sizes are often larger than typical core sizes, making useful laboratory testing impossible. Numerical modeling using a Synthetic Rock Mass (SRM) approach is one way to estimate mechanical and hydraulic properties at scales of engineering interest (e.g., open pit slopes). The SRM approach was initially developed to estimate properties for jointed rock masses, but in this paper, the approach is modified to obtain properties for breccias. The key parameters in estimating properties are the clast/matrix ratio, the distribution of clast sizes, and the relative strengths of clasts and matrix. Numerical models are used here to derive rock mass properties for breccias of varying clast/matrix ratios. An important finding of this work is that clast-dominated breccias may have higher friction angles at low confinement when compared to either of the constituents. This finding is in line with laboratory testing of thermally brecciated marble, which shows higher friction at low confinement when compared to intact marble. We also use a 3DEC model to obtain permeability and then compare results to similar measurements for thermally brecciated marble.
Synthetic Rock Mass (SRM) models were originally developed to understand and quantify the behavior of jointed rock masses by reproducing the combined effects of intact fracture and discontinuity movement. The main inputs are intact rock properties, joint properties, and a Discrete Fracture Network (DFN). SRM models are not new. For example, Carvalho et al. (2002) used twodimensional UDEC (Itasca, 2014) models of diorite to estimate rock mass strengths for slope stability studies. Worthy of note is the observation that the failure mechanism in the triaxial samples was mainly a consequence of tensile failure of the intact rock bridges in the rock mass. Clark (2006) used FLAC (Itasca, 2005) with ubiquitous joints to construct an SRM model. The orientation of the ubiquitous joints was sampled from the actual distribution of joint orientations. The SRM model exhibited anisotropy, scale effects and reasonably reproduced empirical strengths.
