Abstract: Considering the weakening effect of strength and stiffness of the shale layer, the two dimensional particle flow numerical models of shale specimens are established using the granular distinct element method. Based on the evolution law of the macroscopic mechanical parameters with the relative water absorption rate, which is obtained from the triaxial compression tests of shale specimens, the mesoscopic parameters of the numerical model under different water absorption time are calibrated, in order to study the unloading mechanical properties and failure characteristics of shale specimens under different water absorption time. The results show that the strength parameters under loading and unloading stress paths decrease with the increase of water absorption time. However, the cohesion is generally smaller in unloading condition compared with that of loading condition, while the internal friction angle presents an opposite law. Under different water absorption time, the rock samples mainly fail along or at a certain angle to the bedding plane. More fractures are found in the rock samples with the increase of water absorption time. Thus the failure modes of the samples, changing from single shear to double shear, are more easily controlled by the bedding planes. The development law on the number of microcracks in rock samples under different unloading stages is also different. The growth rate of tensile and shear cracks in the yield stage under unloading status is differentiated, showing that the growth rate of shear cracks gradually tends to be stable while that of tensile cracks continues to increase. When the stress residual point at the post-peak stage is reached, the growth rate of tensile cracks significantly decreases, and then basically tends to be stable.

Key words: water absorption time, shale, unloading, failure characteristics, particle flow