Investigation of acting load and mechanical characteristics of shield tunnel lining in upper-soft and lower-hard stratum

When shield tunnels traverse the upper-soft and lower-hard stratum, the pronounced geological differences lead to uneven stress distribution on the segmental lining, thereby intensifying both construction challenges and safety risks. Investigating the loading conditions and mechanical characteristics of tunnel linings in such strata is imperative. Therefore, field measurements were conducted at two shield tunnel construction sites along Nanchang Metro Line 1, specifically within a silty clay-gravel layer and a homogeneous sand layer. The acting load and internal forces of the segmental lining were monitored over an extended period. By analyzing the monitoring data, the variation patterns of the loads acting on the segmental lining in the upper-soft and lower-hard stratum during the construction stage were summarized. Furthermore, a comparative analysis was carried out between the measured loading conditions and the theoretical analytical solutions. Subsequently, a refined numerical simulation incorporating bolt joints and bolt preload on the segment was performed to further explore the mechanical behavior of the segmental lining, with a comparison to the measured internal force data. The results indicate that synchronous grouting at the shield tail significantly affects the earth pressure in the upper soft soil layer, with the maximum earth pressure induced by synchronous grouting being approximately 1.9 times the final stable value. The vertical earth pressure in the upper part of the segmental lining exhibits a characteristic pattern of being “large in the middle and small at both ends”, with the measured maximum value after stabilization corresponding to approximately 72% of the theoretical value predicted by Terzaghi’s theory. Notably, a sudden change in lateral earth pressure is observed at the stratum interface. The bending moment and axial force at the invert of the tunnel segment are comparatively smaller than those at the vault. Additionally, the development of internal forces within the segment is fairly constrained in the lower hard stratum.