Numerical determination of mechanical properties of rock and of rock masses

Abstract

INTRODUCTION MANY experts, including Stini, Oberti, Mtiller, Jahns, Kastner and Haupt, have pointed out, not only that the mechanical properties of a rock mass differ from the mechanical properties of a rock specimen, but also that there is a relationship between them which depends on the jointing. Various investigations have also been carried out in an endeavour to determine a so-called rock modulus. Efforts in this direction are, of course, very expensive and results are not easy to achieve. It is well known that the rock and rock pressure conditions in Kiruna are good. No major difficulties arise during the development work in rock or ore and roof supports in the stopes are almost unheard of. The hanging wall and the foot wall of the massive magnetite orebody consists of quartz or syenite porphyry. These rocks are quite homogeneous in outer appearance, yet there are considerable differences in the strength of the rock mass of the various mine sections. These 'anomalies' in the strength of the rock mass compelled us in Kiruna to study in greater detail the mechanical properties of the rock mass, and not only to test the rock in the form of rock specimens. The object of these investigations was the estaolishment of numerical relations between the strength of the rock and of the rock mass. The strength of the rock or more accurately the strength of the rock sample depends inter alia on the mineral composition and on the compactness of the rock. Quite a low strength of the rock mass may be found to exist when the jointing is poor in spite of the fact that the strength of the rock is fairly high. The visible and invisible fissures are responsible for the separation of rock particles of varying size during development work, and in the worst cases the strength of the rock mass is found to be nil.