ABSTRACT:

The results of rock mass characterization typically are used to define the rock mass properties via empirical relations.The most commonly used empirical relation to define rock mass strength is the Hoek-Browncriterion. One limitation of this approach is that it often involves the use rock-mass rating techniques that ignore scale effects. Other limitations of the Hoek- Brown criterion also are discussed. The Synthetic Rock Mass(SRM) approach to jointed rock-mass characterization provides an alternative means of establishing a strength envelope that is not reliant on either Mohr Coulomb or Hoek-Brown criteria. This approach, based on two well-established methods, Bonded Particle Modeling in PFC3Dand Discrete Fracture Network (DFN) simulation, employs a sliding joint model that allows large rock volumes containing thousands of pre-existing joints to be ‘tested’ by applying stress or strain paths. These simulations, together with stochastic continuum simulations, demonstrate that rock masses have the ability to find failure paths of least resistance. The larger the rock mass being considered, the greater the number of potential failure paths. This observation leads directly to an understanding of scale effects and explains why larger rock masses have lower overall strength compared to smaller rock masses.

1 INTRODUCTION

Numbers derived from rock mass characterization are used to analyze the behavior of structures built in or on rock (i.e., the fundamental work of geotechnical engineers). Examples include slope stability studies, surface settlement/subsidence assessments, analysis of the behavior of underground openings, foundation studies, etc. These numbers typically come from geology or closely related fields (such as hydrogeology) and involve quantities that can be measured directly (usually on a small scale), such as rock density, intact rock strength, water pressure, discontinuity orientation, persistence, spacing and strength. The measured quantities are used to estimate other numbers required for larger-scale analytic, kinematic or numerical analysis. The procedure presently involves the use of empirical relations to estimate rock mass modulus and shear strength through one of several rock-mass rating techniques. Discontinuity behavior also usually is derived via empirical relations. Because many analyses, particularly for large structures, require the estimation of rock mass strength, this paper focuses on the current and future methods of estimating rock mass strength. First, the various rock-mass rating techniques are listed, followed by definitions of the methods used to estimate rock-mass shear strengths. The most widely used method based on the Hoek-Brown criterion is discussed in detail. Inherent deficiencies in all empirically derived rockmass strengths have led to the development of a Synthetic RockMass(SRM), and two applications of the SRMapproach in a mining environment are presented. Finally, some interesting results for stochastic analyses are used to elicit understanding about the behavior of rock masses and rock mass strength.

2 ROCK-MASS RATING TECHNIQUES

Systems of rock-mass rating techniques were developed for use in Civil and Mining Engineering in response to the need for ways to ‘rank’ a specific rock mass, based, in large part, upon the joints and their weakening effect on rock.

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