Abstract: The uniaxial compression experiments on the sandstone samples containing three pre-existing fissures (the sample scale of 80 mm×160 mm×30 mm, and fissures angles of ? = 30°, ? 1 =60°) and carried out by using rock mechanics servo-controlled testing system, to investigate the effect of ligament angle ? 2 on strength failure and crack coalescence behavior. Compared with intact sandstone sample, the stress-strain curves of sandstone sample containing three fissures show more stress drops, and their peak strengths also take on a reducing tendency, but the reducing extent is closely related to ligament angle ? 2. When ? 2 increases from 75° to 90°, the peak strength reduces to 77.82 MPa from 82.04 MPa; whereas when ? 2 increases from 90° to 120°, the peak strength does not have a distinct variance. The failure mode of intact sandstone sample is a typical axial splitting fracture; but the sandstone sample containing three fissures results from a lot of crack initiation and coalescence at the tips of pre-existing fissures, so as to lead to the ultimate unstable failure. By using photographic measuring technique, the effect of ? 2 on the crack coalescence behavior of sandstone sample containing three fissures is discussed. The results show that the coalescence in the specimen containing three fissures (? 2 =75°, 90° and 105°) occurs between fissure ① and ③, fissure ② and ③, but no coalescence is observed between fissure ① and ②; but in the specimen containing three fissures (? 2 =120°), the coalescence occurs between fissure ① and ③, fissure ① and ②, but no coalescence is observed between fissure ② and ③. Finally, the relationship between microscopic deformation behavior of sandstone samples containing three fissures and crack coalescence process is constructed.

Key words: rock mechanics, three fissures, brittle sandstone, strength behavior, crack coalescence