Brown. Empirical Strength Criterion For Rock Masses

SEPTEMBER 1980 15715 : .r,,tv,^b- GT9 JOURNAL OF THE GEOTECHNICAL ENGINEERING DIVISION tsv ■ • j-5 .ban; ' ri> i EMPIRICAL STRENGTH CRITERION FOR ROCK MASSES I By Evert Hoek1 and Edwin T. Brown,2 M. ASCE INTRODUCTION The design of an excavation in rock requires an assessment of the likely response of the rock mass to a set of induced stresses. In order to predict this response, a knowledge is required of the complete stress-strain behavior and strength characteristics of the rock mass and of the influence of time on these properties. This paper is concerned with only part of this total response, the peak strength developed under a given set of stresses; the influence of time is not considered. In many cases such as slopes (30) or shallow tunnels (29), instability may be associated with the structural features in the rock mass, and the shear strengths of these discontinuities will be required for use in design calculations. In some deep underground excavations, stability may depend on the relationship between the induced stresses and the strength of the intact rock (27). The processes of drilling and blasting and excavation by tunnelling machinery are also strongly influenced by the strength of the intact rock material (41,47). There is a further class of rock engineering problem in which the overall stability of a deep surface cut or the components of a system of underground excavations will be determined by the mass behavior of the rock mass surrounding the excavation. In some cases, the rock mass may be heavily jointed so that, on the scale of the problem, it can be regarded as an isotropic assembly of interlocking angular particles. The transition from intact rock to heavily jointed rock mass with increasing sample size in a hypothetical rock mass surrounding an underground excavation is shown in Fig. 1. Which model will apply in a given case will depend on the size of the excavation relative to the discontinuity Note.—Discussion open until February 1, 1981. To extend the closing date one month, a written request must be filed with the Manager of Technical and Professional Publications, ASCE. This paper is part of the copyrighted Journal of the Geotechnical Engineering Division; Proceedings of the American Society of Civil Engineers, Vol. 106, No. GT9, September, 1980. Manuscript was submitted for review for possible publication on October 25, 1979. 1 Principal, Golder Associates, Vancouver, Canada. 2 Prof, of Rock Mechanics, Dept. of Mineral Resources Engrg., Imperial Coll. of Sci. & Tech., London, England. «nil i 1013 1014 GT9 SEPTEMBER 1980 spacing, the boundary conditions, the imposed stress level, and the orientations and strengths of discontinuities. A significant proportion of past rock mechanics re