Analysis of unbalanced forces on large shield cutterhead in karst composite strata

Large-diameter shield tunnelling in karst strata presents challenges in quantifying unbalanced loads and attitude control during cave–cutterhead interaction. This study develops a 3D explicit finite-element dynamic model of large-diameter cutterhead–rock interaction, validated through cutting torque evolution and breakage morphology comparisons under homogeneous conditions. Parametric simulations examine how circular cave eccentricity and diameter affect unbalanced forces and overturning moments, analyzed through thrust-resultant point migration on the tunnel face. Results indicate that small eccentricity or diameter reduces cutter–rock contact length and resultant forces compared to homogeneous conditions. As parameters increase, resultant forces change minimally while overturning moments increase significantly due to thrust-point shift toward intact rock. Concrete backfilling effectiveness depends on stiffness matching with surrounding rock—soft fillings offer limited benefit, while overly stiff materials re-concentrate thrust and reduce mitigation. This study establishes the relationship between cave geometry and load response, emphasizing moment control priority in karst tunnelling for main-bearing protection, cutterhead design, and treatment strategies.