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.