Symmetry and Asymmetry of EPB Shield Posture in Urban Metro Tunneling: Tail Void Indices, TBM Parameter Correlates, and Twin-Tunnel Comparison

The Gaussian settlement-trough model assumes that ground settlement occurs because tunneling is symmetrical about the tunnel centerline. However, the shield causing the settlement operates in an asymmetric posture during earth pressure balance (EPB) tunneling. This study characterizes the postural asymmetry of the shield in EPB tunneling. Tail void (TV) clearance data for 1580 rings from published data on Guangzhou Metro Line 9, China, were analyzed. Two types of asymmetry indices—the horizontal asymmetry index (HAI) and the vertical asymmetry index (VAI)—were proposed. The two indices indicated systematic deviations from symmetry, with high statistical significance (all p-values < 0.001). The systematic horizontal deviation was to the left, with a pooled mean value of −0.055. In contrast, the stronger systematic vertical deviation was reduced crown clearance, with a pooled mean value of −0.097. Validation against independent shield axis records showed HAI tracked horizontal deviation (p < 0.001), but VAI did not track vertical attitude. The results indicated that postural asymmetry in EPB tunneling was a systematic, parameter-associated phenomenon, with implications for predicting ground settlement, annular grouting strategy, and risk-based monitoring design. However, the absolute values of the two indices were specific to this project and were not transferable without recalibration.