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Abstract Based on the experimental results of intact red sandstone under conventional triaxial compression，a set of microscopic parameters in particle flow code(PFC) reflecting the macroscopic mechanical behavior of intact red sandstone were obtained by analyzing the sensitivity of microscopic mechanical parameters in PFC. The particle flow simulation was carried out for the red sandstone containing two pre-existing fissures under different confining pressures. The influences of the confining pressure and the ligament angle on the strength failure characteristics of the red sandstone containing two pre-existing fissures were analyzed on the basis of the simulated results and the microscopic mechanical mechanism of crack coalescence of the red sandstone containing two pre-existing fissures was revealed. The parameters of peak strength of red sandstone containing two pre-existing fissures are smaller compared with that of the intact red sandstone，and the extent of reduction is related to the ligament angle β. The cohesion and the internal friction angle varied both nonlinearly with the ligament angle ?. When ? were 0°and 30°，the ultimate failure modes of the red sandstone containing two pre-existing fissures were similar to each other，no coalescence was observed between fissures ① and ②. When ? are 60° and 90°，the ultimate failure modes were similar，one crack coalescence was observed between fissures ① and ②. If ? was 120°，two crack coalescences were observed between fissures ① and ② at lower confining pressure，but only one crack coalescence occurred at higher confining pressure. When stress increased to a certain value，the bonds among grains began to break. The newborn micro-cracks initiate，propagate and coalesce to form the macro-cracks which results in the unstable failure of rock specimens. The increase of confining pressure improves the contact and bond force among grains at the microscopic scale，which leads to the increase of strength at the macroscopic scale. Existence of high confining pressure limits the propagating speed of microscopic cracks.
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Received: 22 August 2013
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2014, Vol. 33 
