ABSTRACT:

Determining mechanical properties of joint rock mass under compression is a significant problem of rock engineering caused by the non-linear quality of joint media deformation up to failure. Results of experimental research demonstrate that non-linear diagrams of fragment deformation of joint rock masses s1 = f(e1) in many cases can be divided at three sections. It is proposed for determining the characteristics of joint rocks deformability to replace non-linear diagram of deformation by three linear ones. This paper presents the method of developing such a deformation diagram s1 = f(e1 of the model consisting of blocks under compression and using equations for calculating secant modulus values of deformation within every section, as well as values of stresses at the beginning of each section. Equations were obtained on the basis of numerical calculations (FEM) and experiment planning methods. This paper also presents comparison of results of numerical and physical experiments.

1. INTRODUCTION

When evaluating the interaction of engineering structures with rock mass, we are faced with the problem of determining its elasticity effective characteristics. The existence of scale factor, caused by structural heterogeneity of rock mass, limits the use of experimental methods of investigation, especially in cases where large areas of rock mass are involved (for example, high dams, large span excavations of underground hydro power stations, etc.). In such cases, provided that the scale of rock mass under investigation may be considered representative, the use of analytical methods for determining rock deformationproperties appears to be applicable. Representative volume is considered to be the volume which further increase does not materially affect or change its mechanical characteristics. Joint rock mass is also considered a structurally nonhomogeneous medium, especially when rock is dissected by joint systems of various order. At present, analytical methods based on the application of crack density tensor are used to define effective deformation characteristics of such medium (Merzliakov, Vlasov, 1993). Crack density tensor (CDT) takes into account hollowness induced by cracks, their orientation as well as the volume of rock under consideration, cut out from the rock mass. It is obtained by the following expression: (mathematical equation available in full paper) where nq - normal to crack mid-surface; bq - vector of crack opening at the same point; Vq - single crack volume; V - average volume of the considered area. The equality of CDT values at all points of the considered area (provided that the volume under consideration is representative) means that the material within the volume may be considered quasihomogeneous and quasi-continuous. In many cases, it makes it possible to represent with great reliability joint rock mass and its separate fragments as equivalent elastic-linear medium in engineering calculations. At the same time, numerous investigations show that as a rule, deformation of joint rocks is characterized by essential non-linearity. Three deformation sections are obvious at the deformation diagrams s1 = f(e1 for a fragment of gypsum sand blocks under biaxial compression test (Shiriaev et al., 1976) (Fig.1). Non-linearity of the first section is conditioned by deformations of inter-block joints, their closeness and the shear of blocks along joints.

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