Influence of composite stratum surrounding rock zoning on tunnel stability and optimization of support

Conventional studies on tunnel stability in composite strata often neglect the variations in stratum dip angles and localized joint distributions, resulting in conservative and uneconomical support designs. This study investigated the influence of surrounding rock zoning and joint parameters on tunnel stability through model experiments and discrete element numerical simulations (3DEC), based on the Fengjie (FJ) tunnel project in Zhuyi Town, Chongqing. A targeted support optimization scheme was then proposed. The results show that composite strata exhibit poorer stability compared to homogeneous strata, with settlements extending toward weaker rock zones as the dip angle increases. During excavation, soil pressure decreases at the arch crown and invert, but increases significantly at the arch waist and sidewalls. Tunnel instability is aggravated by steeply inclined, closely spaced, or intersecting joint sets. Expanding joint fragmentation zones leads to a reduction in the factor of safety, while increasing stratum dip angles improve it. Fragmentation zones in the arch section have a greater destabilizing effect than those in the sidewalls or invert. An uneven support scheme (Ⅲa-type with 20 cm thickness for intact rock zones and Ⅳb-type with 25 cm thickness for fractured rock zones) reduces concrete usage in cross-sections a, b, and c by 20.5%, 10.0%, and 8.0%, respectively, while ensuring safety and enhancing cost-effectiveness. These findings provide practical guidance for optimizing tunnel support design and construction in composite strata.