Fracture coalescence has been investigated on a variety of model rock specimens. The specimens contain two preexisting fractures which are either open or closed, and which are arranged at different distances and angles. Uniaxial and biaxial loading are applied. A biaxial machine was especially designed and built for this investigation. During the loading process crack initiation, propagation, and coalescence are observed. In the uniaxial tests, wing cracks and secondary cracks appear at the same time; wing cracks from open flaws initiate at 45 with respect to the loading direction, while secondary cracks initiate parallel to the flaw. In biaxial tests, wing cracks occur in the center of preexisting fractures or not at all. This research, as well as previous work at M.I.T confirm that secondary cracks which are often shear cracks play a very important role in crack propagation and coalescence.
Fractures and cracks are present in rock masses and intact rock, respectively. Their response and interaction under an applied stress field govern the behavior of intact rock and rock masses. Numerous experimental (e.g.: Hoek and Bieniawski, 1984; Horii and Nemat-Nasser, 1986; Lin and Logan, 1991; Reyes and Einstein, 1991; Chen et al., 1992; Dyskin and Germanovich, 1995; Shen et al., 1995), and analytical (e.g.: Segall and Pollard, 1980; Shen and Stephansson, 1992) efforts have been devoted to the understanding of crack initiation, and the propagation of pre-existing cracks. The present investigation is a continuation of research conducted at M.I.T. in the past years (Reyes, 1991; Takeuchi, 1991; Shen et al., 1995) to investigate fracture coalescence. This earlier research focused on the response of two preexisting fractures or cracks (the term flaw will be used from now on to describe preexisting fractures or cracks) in prismatic model rock specimens under uniaxial compression. Figure l(a) shows the geometry of the specimens used, and figure l(b) a detail of the flaws and the ligament, i.e. the intact material between the flaws. In these earlier experiments flaw lengths were usually the same, and different geometries were obtained by changing the flaw angle and the ligament angle. Also two series of experiments were conducted with open and closed flaws. Figure 2 shows a typical crack pattern up to the coalescence load obtained in one of the previous test series.
Model rock specimens of 152.4 ÷ 76.2 ÷ 30.0 mm. were prepared using, as in most previous tests, a mixture, of hydrocal B-Il (gypsum), celite, and water at mass ratios of: 700: 8: 280. Celite and water were put into a bowl and mixed for two minutes; then, the hydrocal was added and mixed for another four minutes. Afterwards, the mixture was poured into a mold and vibrated on a vibrating table for two minutes. After an hour, the specimen was taken out of the mold and air-dried for approximately one day. The two 152.4 ÷ 76.2 mm. faces were polished on a rotary grinding machine until a smooth surface was obtained.