Abstract: When drilling in soft and weak rock, the drilling is difficult to form and collapses frequently. To solve this engineering problem, based on the assumption that the rock cohesion weakens linearly along the plastic zone, the elastoplastic analysis of surrounding rock of drilling was completed and the secondary stress, displacement and plastic zone expressions of distribution are obtained. Gained self-defined rock mass cohesion by using FISH language that embedded in FLAC3D software. The numerical simulation analysis of elastoplastic distribution of rock around drilling was done. Compared to the original ideal M-C standards, the contrast results indicate that the theoretical calculation is correct. Theory and numerical simulation results show that: The plastic zone radius formula (e.g. fixed Fenner solution) that based on ideal elasticplastic model is a special solution when cohesion co equal to cs. After considering the cohesion weakening and residual cohesion, the plastic zone radius is 1.4 times as big as fixed Fenner solution. The numerical simulation shows that the maximum tangential stress in surrounding rock of drilling present to approximately 2.9 times drilling radius from drilling center; the stress concentration factor is less than 2. When under the same in-situ stress conditions, the plastic zone radius expand obviously after considering the cohesion weakening. Through analysis it is gained that the best drilling reinforcement radius is 3 times of the drilling radius; these achievements were applied to a drilling engineering of a certain coal mine in China.

Key words: soft rock, drilling collapse, cohesion weakening, elastoplastic analysis, drilling reinforcement