Grietas de friccion

To cite this paper: Int. J. Rock Mech. & Mm. Sci. 34:3-4, paper No. 335.

Copyright © 1997 Elsevier Science Ltd

Copyright © 1997 Elsevier Science Ltd Int. J. Rock Mech. & Mm. Sci. Vol. 34, No. 3-4, 1997 ISSN 0148-9062 To cite this paper: Int. J. RockMech. &Min. ScL 34:3-4, Paper No. 335

THE C O A L E S C E N C E OF F R I C T I O N A L C R A C K S A N D THE THE S H E A R Z O N E F O R M AT I O N IN B R I T T L E S O L I D S UNDER COMPRESSIVE STRESSES
R . H . C. W o n g ; K . T. C h a u

Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong
ABSTRACT

This paper investigates the mechanism of crack coalescence and peak strength of rock-like materials containing parallel frictional cracks under uniaxial compression loading. Thespecimens used in this study are the sandstone-like material proposed by Wong, Chau 1997 and are containing different number of parallel frictional cracks ranging from two to forty-two parallel cracks. Various values of inclination of preexisting cracks ~, bridge angle [3 (inclination between the inner tips of the two preexisting cracks) and the frictional coefficient ~t are used for parametricstudies. Two "rules of failure" for the specimens containing three cracks are proposed, and it is being tested with our results. In particular, we found that only crack coalescence between two cracks will occurs (not three) and the pair of cracks with a lower value of peak strength (as predicted by the results of Wong, Chau 1997) will dominate the process of coalescence; and that the mixed andtensile modes of coalescence are the preferential modes and the preferred orientation for crack coalescence is 75 °. In contrast to the observation by Horii, Nemat-Nasser 1985 for specimens made of resin, shear zone can also be formed in our specimens containing 18 and 42 cracks under uniaxial compression. In addition, we found that the peak strength of the specimens does not depend on the initialcrack density but depends on the actual number of cracks involved in the coalescence. This result provides a plausible explanation for our previous observation (Wong et al. 1996) that the peak strength in Yuen Long marbles does not decrease with the increase of the initial crack density as a threshold value is exceeded.
Copyright @ 1997 E l s e v i e r S c i e n c e Ltd

KEYWORDS Preexisting Crack• Crack Coalescence • Bridge Angle • Coalescence Angle • Crack Density • S a n d s t o n e - l i k e M a t e r i a l • F r i c t i o n a l C r a c k s • S h e a r M o d e • M i x e d M o d e • Tensile M o d e INTRODUCTION

Cracks and holes can interact with a compressive stress field in a way that stress intensity factor at the tip of the preexisting crack causes it to nucleate new cracks from theexisting crack tip (such as wing cracks) and to grow from the preexisting cracks (Griffith 1921, 1924). If these cracks extend to the surfaces of the sample, or if they interact with each other so that they grow unstably, then a macroscopic failure may follow. A number of experimental studies and theoretical models were proposed to explain and predict the processes of crack growth, interactionand rock failure (e.g. Hoek, Bieniawski 1965; Tapponnier,

ISSN 0148-9062

To cite this paper: Int. J. Rock Mech. & Min. Sci. 34:3-4, paper No. 335.

Copyright © 1997 Elsevier Science Ltd

Brace 1976; Olsson, Peng 1976; Kranz 1979; Batzle et al. 1980; Wong 1982; Horii, Nemat-Nasser 1985; Ashby, Hallam 1986; Sammis, Ashby 1986; Kemeny, Cook 1987; and Kemeny 1991). Although the coalescence ofmicrocracks is commonly believed to be the main mechanism for the shear zone and fracture zone formation in rocks and, thus, is responsible for the failure of rocks, there is relatively few experimental studies to examine this kind of crack coalescence occurring in the rock bridge area between preexisting cracks, fractures or joints. To stimulate the pattern of crack coalescence in brittle...