Richard Simon

A multiaxial stress criterion for short- and long-term strength of isotropic rock media

Natural and induced stress states around man-made openings in rock generally have different magnitudes along the principal axes. Accordingly, appropriate stability analyses should be based on expressions that take into account the full stress tensor. Based on the main features of compressive and tensile failure of isotropic media, the authors propose a general multiaxial criterion for describing the short-term failure (STF) strength as well as the damage initiation threshold (DIT) of rocks and rock masses. Time and size effects are explicitly included in the formulation. The influence of time is described by an extension of the subcritical crack growth theory in which the DIT is introduced. This damage initiation threshold represents the stress state below which there is virtually no crack growth, and hence corresponds to the long-term rock strength. Size effects, on the other hand, are treated using a generalized version of existing power-law expressions in which limiting values have been imposed for the relative size of representative elements. Application of the criterion to rock masses is done using simultaneously the size effect function for the case where no new types of defects are introduced, and a function of the RMR geomechanical classification to take into account the influence of new types of discontinuities such as joint sets. Sample applications of the criterion, using simplified calculations for rock structures, are shown and briefly discussed.

A damage initiation criterion for low porosity rocks

Abstract When low porosity rock samples are loaded beyond a certain threshold, microcracking occurs. This threshold can be seen as a yielding condition in an elasto-plastic model, or as a damage initiation criterion in an internal state variable constitutive model. In this paper, the authors present a simple multiaxial formulation used to describe this threshold in the conventional stress space. The criterion is applied to a hard rock (granite) and to a soft rock (rocksalt), and its significance is discussed.

Underground stope optimization with network flow method

A new algorithm to optimize stope design for the sublevel stoping mining method is described. The model is based on a cylindrical coordinate defined around the initial vertical raise. Geotechnical constraints on hanging wall and footwall slopes are translated as precedence relations between blocks in the cylindrical coordinate system. Two control parameters with clear engineering meaning are defined to further constrain the solution: (a) the maximum distance of a block from the raise and (b) the horizontal width required to bring the farthest block to the raise. The graph obtained is completed by the addition of a source and a sink node allowing to transform the optimization program to a problem of maximum flow over the graph. The (conditional) optimal stope corresponding to the current raise location and height is obtained. The best location and height for the raise are determined by global optimization. The performance of the algorithm is evaluated with three simple synthetic deposits and one real deposit. Comparison is made with the floating stope technique. The results show that the algorithm effectively meets the geotechnical constraints and control parameters, and produce realistic optimal stope for engineering use.

Modelling Shear and Normal Behaviour of Filled Rock Joints

The behaviour of filled rock joints depends on the properties of the joint and of the fill material. In this paper, the authors present a new model (called CSDS) that can represent the shear and normal behaviour of clean and filled rock joints. From a geometrical analysis of joint surface asperities and fill thickness, equations are established to represent the evolution of joint parameters using the properties of fill material. The rock joint shear constitutive model CSDS is modified to simulate the complete shear behaviour of filled rock joints. The validity of the proposed approach is then assessed through applications to laboratory test results.