ABSTRACT
As part of a study of the origin of rockbursts and sheet structure, in which fractures are parallel or subparallel to directions of maximum compression, I have conducted a series of uniaxial compression experiments using specimens of Chelmsford granite. Through-going fractures that were parallel (axial) or nearly parallel (subaxial) to the direction of applied stress were produced in the experiments. The size of the specimens was 12.7 x 12.7 x 5.1 cm. Fracturing was induced by a flaw that had been cut through the 5.1 cm dimension of each specimen. Samples were loaded parallel to one of their long dimensions and normal to the plane or axis of the flaw. Thus, the experiments were run under conditions of plane stress. Three types of flaws were used in the experiments: a. notches in edges of specimens, b. slits that did not intersect the edges of specimens, and c. circular holes with radial slots cut in their peripheries. Fracturing first occurred in areas along the boundary of a flaw where compressive stress was greatest and consisted of localized crushing of the rock. With a further increase of the applied stress, axial or subaxial zones consisting of enlarged microcracks emanated from the areas of maximum compression at the margins of the flaw. Further increase of the applied stress produced a sudden, through-going fracture that propagated axially or subaxially away from the flaw and coincided with one of the zones of enlarged microcracks that had developed earlier. These fractures formed at applied stresses that were about 50 to 60 percent of the uniaxial compressive strengths of cylinders and prisms of Chelmsford granite that contain no flaw. Some of the sample geometries used in the experiments are similar to the geometries of large blocks in quarries of Chelmsford granite. These blocks are subjected to in situ loads that are approximately uniaxial and have been the sites of destructive rockbursts. Similarities between rockbursts and fracturing of specimens containing manufacture flaws suggest that the same fundamental mechanism is responsible for fracturing in both cases.
INTRODUCTION
The experiments described herein are part of a larger study of the origins of sheet structure and rockbursts in Chelmsford granite in northeastern Massachusetts. Rockbursting and the formation of sheet structure are processes that are currently taking place and can be observed especially well in granite quarries. In the quarry of the H. E. Fletcher Company rockbursts occur frequently, and often originate in areas where regional stresses have been concentrated by quarrying activity. Regional compressive stresses measured in quarries in Chelmsford granite are commonly about 300 bars, and in areas of stress concentration stresses greater than 500 bars have been measured (Hooker and Johnson, 1969; Holzhausen, 1976). In Fletcher quarry large blocks of granite are developed along each of four quarry walls. The blocks, typically about 25 meters high, are produced by cutting "channels "into the vertical walls and contain two or more sheets of granite separated by natural, subhorizontal sheet fractures. Rockbursts commonly originate at the ends of the channels separating adjacent blocks (Fig. 1) and consist of the sudden, unstable propagation of one or more steeply dipping fractures, often accompanied by a loud report.
