The mechanics and physics of fracture are briefly reviewed, with special attention to their applicability to problems of rock fracture and fragmentation. The desirability of analyzing energy changes associated with fracture directly, rather than following the more restrictive procedures of linearly elastic fracture mechanics, is stressed. Application of the direct approach to hydraulic fracturing, rock bursts and coal bumps in mining, laboratory investigations of rock fracture, blasting and drilling are mentioned. The report concludes with some recommendations for future research.
INTRODUCTION
Fractures, or essentially linear discontinuities, are pervasive in rock masses and can be observed at any scale, be it from high altitude aerial photography or through a microscope. They represent a deformational response to forces imposed on the rock at some time in its geological history. Active tectonism or engineering activities may change the current equilibrium sufficiently to reactivate movements along the existing fractures and may also generate new fractures. Consequences include such effects as devastating earthquakes, rock bursts in mines, failure of slopes or dam abutments. New fractures are also generated intentionally in such processes as hydraulic fracturing, drilling, blasting, comminution and grinding. Understanding of the mechanics of rock fracture is thus of broad general importance.
The mechanics of fracture initiation and propagation have been most intensively studied in relation to fabricated materials, usually metals. As in other branches of rock mechanics, the theoretical basis of rock fracture draws heavily on the research and approaches followed for these materials. Although this work has been of considerable value in explaining rock fracture, there remains a tendency, also evident to varying degrees in other branches of rock mechanics, to adopt uncritically the approaches and procedures of the studies on fabricated materials. Some of the questions of most significance in the problems of fabricated materials may be of relatively little import in problems of rock engineering. It is important if maximum benefit is to be gained in rock mechanics from developments in other fields to recognise these differences. Some of the more important differences are the following:
Forces in geological situations are generally compressive, either gravitational or tectonic in origin, and may vary in magnitude and orientation. Non-planar extension of fractures is not uncommon.
Outer "boundaries" in the earth are essentially infinite so that forces may always be redistributed away from a fracture, which can then stabilize after some extension. Stable fracturing is a common occurrence in mining and tunnelling.
Fractures abound in rock, formed at various times over many millions of years. Amelioration of the potentially damaging effects of continued movement along fractures is important, as in earthquake control on one scale and in rock-bolt reinforcement of jointed rock on another. Generation of new fractures is a frequent goal, as in hydraulic fracturing, blasting, and comminution. Prevention of fracture, the main interest in fracture mechanics applied to fabricated materials, is of smaller concern in rock mechanics.
The ultimate tensile resistance of most rocks is very small compared to the compressive resistance, so that tensile fractures are frequently observed in rocks.
