Abstract: The authors present a case study to investigate the damage and destabilization mechanisms of broken and soft surrounding rocks and provide a reasonable support scheme design for an inclined shaft bored using a tunnel boring machine(TBM). This study is conducted on the Kekegai subinclined shaft bored by a TBM in Yulin, China. A discreteelementsimulation software, UDEC, is employed to analyze the stability of the surrounding rocks as well as the support effects under four conditions: no support, anchornetwork shotcrete support, anchornetwork shotcrete cable support, and anchornetwork shotcrete cable + steel belt support. Based on the numerical results, a supportoptimization scheme is proposed and applied in practice. Accordingly, the following results are verified: (1) The rock destabilization in the TBM tunnel demonstrates ovalshaped plastic damage, which is dominated by shear plasticity and controlled by tensile plasticity, requiring the timely adoption of support means. (2) The anchor rod, anchor cable, shotcrete, and steel belt support can effectively limit the deformation and damage to the surrounding rocks. Therefore, the anchornetwork shotcrete cable + steel belt support exhibits the best support effect compared with all the other support types, with a reduction of 50.9% and 99.85% of the roof displacement and the tensile plastic area of the surrounding rocks, respectively, compared with those of the system without any support. The anchornetwork shotcrete cable scheme is finally used as the support solution, and a Wtype steel belt is added to strengthen the support in the roof area, which displays a falling risk. The adopted support scheme can effectively control the deformation of shallow surrounding rocks and plastic expansion of deep surrounding rocks, thereby achieving precontrol of sudden falls.

Key words: shaft, open tunnel boring machine, discrete element analysis, failure mechanism, surroundingrock damage zone, support optimization