Technical Parameters of Rock and Rock Masses
In the introductory lecture of this “Course on rock mechanics”, the fundamental differences between the rock properties and the rock mass properties were mentioned. In this lecture, we shall examine more closely, first of all, those engineering properties
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* All figures quoted in the text are at the end of the lecture. L. Müller (ed.), Rock Mechanics © Springer-Verlag Wien 1972
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L. Muller
In the introductory lecture of this "Course on rock mechanics", the fundamental differences between the rock properties and the rock mass properties were mentioned. In this lecture, we shall examine more closely, first of all, those engineering properties of the rock masses which require to be determined in our engineering structures. It need not be emphasized that the engineering structure and the geological ground form together a physical system in which the rock or the ground is usually the weaker part, and consequently determines the overall safety of such a system. Only by a correct appraisal of the properties of the rock masses the design of an engineering structure, e.g. tunnels, slopes in open pit mines, will meet the safety requirements and will be economically dimensioned. Even today, though the critical principles have been known for the last two decades, considerations are often based on the properties of the rock elements instead on the properties of the rock masses. In this respect it is typical that in many rock mechanics laboratories a major part of research and testing activities are still devoted to the rock material and far too little to the mechanical behaviour of the complex rock masses. How big this difference between the properties of the rock material and the geological rock body formed by this rock material can be, was very clearly illustrated by HALL (1963) in the case of the slope of a Swedish open pit mine (Fig. 1). The back calculation of the steep granite slopes gave a frictional angle of tg P = 0,8 and a cohesion intercept of only 3,0 kp/ cm 2 while the granite rock itself had a compressive strength, as determined from a test cube, of about 2500 kp/cm 2 and a correspondingly high shear strength. A rock mass, i.e. a rock formation. is, in general, made up of a large number of elements. Such a material is much more complex than most other materials engineers have to deal with. It is more polymorphic, more inhomogeneous, i.e. changing from place to place. As mentioned before, the material and its mechanical behaviour is characterized and basically influenced by the two basic properties, its imll1ense anisotropy and discontinuity. Anisotropic with respect to its mechanical behaviour is the rock material itself which constitutes the rock mass whose strength and deformation behaviour depend on the direction. This anisotropy of the rock material is of a dimension - up to about 1:5 which may be neglected or, by considering it as orthotropic. (or plagiotropic respectively) be treated analytically. Much greater anisotropic behaviour 1Il regard to mechanical properties and particularly to the deformation and strength is caused by the numerous discontinuities such as cleavage and bedding planes, joints and geological
Technical Parameters of Rock and Rock Masses
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faults which cut through the rock. They may be summed up here under the general term of sep
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