Numerical investigation on the effect of cutterhead shapes on tunnel face stability

The stability of tunnel faces under the stationary condition of an Earth Pressure Balance (EPB) machine is critical in tunnel construction. While extensive research has focused on the operational stability of tunnel faces, a significant gap remains in understanding the influence of cutterhead geometry on face stability under stationary phases. This study employs three-dimensional (3D) Finite Element (FE) analysis to investigate tunnel face stability in clay and sandy soils, emphasizing the effect of cutterhead opening area ratio (COA), cutterhead shape, and tunnel cover depth (C/D) on stability under stationary conditions. Three distinct cutterhead shapes, exhibiting varying COAs (35 %, 40 %, and 45 %), were analysed across a range of cover depths from 0.5D to 4.0D (D represents the tunnel diameter). The results indicate that larger COAs (45 %) significantly increase soil displacement and instability risks, particularly in clay soils, with critical displacements occurring after reductions of up to 40 % in support pressure. In contrast, sandy soils demonstrated enhanced stability even with larger COAs. Furthermore, the study revealed a significant influence of cutterhead design on soil displacement and support pressure. Cutterhead shape 3, characterized by symmetrical openings and a large central panel, exhibited superior performance, minimizing soil displacement and requiring up to 20 % less support pressure compared to other cutterhead shapes investigated in this study. The cover depth in the three shapes was found to influence stability, with deeper tunnels (C/D = 4.0D) at various COAs experiencing greater displacement and requiring higher support pressures, especially in clay soils. Stress distribution analysis revealed that increased COA and larger cover depths contribute to higher horizontal stress, which exacerbates face instability. Additionally, clay soils exhibited a higher propensity for instability compared to sandy soils, particularly under conditions of larger COAs and deeper cover depths. This research provides a novel approach to optimizing EPB machine performance by considering face stability in the cutterhead opening areas. The findings offer valuable insights for tunnel boring machine (TBM) design and operational planning in various ground conditions.