수직구 경사도와 불연속면 경사에 따른 안정성 분석 = Stability analysis of circular shaft based on inclination and discontinuity dip

다국어 초록 (Multilingual Abstract)

The stability of circular vertical shafts, while benefiting from stress arching, is highly susceptible to shear-driven kinematic failure in rock masses containing predominant discontinuities. Conventional continuum methods, such as FEM, inherently fail to accurately predict these structurally-controlled movements due to their inability to explicitly model large-strain block interaction and separation. This study overcomes this limitation by employing the Three-Dimensional Distinct Element Method (3DEC) to quantitatively assess stability, focusing on the combined effects of shaft inclination (θs), representing construction tolerance, and discontinuity dip angle (θd), representing geological structure. A systematic parametric analysis was conducted, varying θs (0° to 15°), θd (30° to 60°), and joint shear stiffness (Ks from 0.5 to 50.0 MPa/mm). The results demonstrated that the maximum horizontal displacement in the discontinuous model was approximately 44% higher than in the intact rock model, providing clear quantitative evidence that stability is governed predominantly by the structural properties of the joints rather than the rock matrix strength. Critically, the peak displacement(1.50 mm) was identified not at the maximum analyzed tolerance, but at a minor shaft inclination of θs = 5° (θd = 60° for Against Dip), establishing the novel finding of a Critical Geometric Misalignment Condition within standard construction limits. Displacement sensitivity reached its peak when the angular difference between the shaft axis and the discontinuity plane approached 45°, which is attributed to the geometric characteristics of stress redistribution and the reduction of the arching effect in circular openings. Furthermore, the Ks analysis confirmed a Stiffness Saturation phenomenon, indicating that stability control shifts from elastic stiffness to the joint’s residual shear strength above a certain threshold. Rock bolt analysis verified an asymmetric increase in axial forces under misalignment. These findings strongly suggest that Differential Support Systems (e.g., increased rockbolt length to 6.0 m and 20% increase in shotcrete thickness) must be deployed in kinematically vulnerable shaft sections identified by the established 5° misalignment criterion.