Study on soil arching effect caused by shield docking disintegration in soft soil strata Ⅱ.: theoretical calculation

Conducting theoretical research on the instability of the unsupported face and variations in overlying earth pressure induced by shield disassembly in soft soil strata is essential to reveal the intrinsic mechanisms of the soil arching effect under complex conditions, thereby providing theoretical guidance for shield docking and disassembly construction. Taking the soft soil stratum where a shield tunnel in Shanghai is located as the background, this study analyzes the soilarching effect in the ground under two working conditions: without reinforcement and with reinforcement in thedocking section. Based on the formation characteristics of the soil arch slip surface in the docking section,corresponding soil arch mechanical models are established. Furthermore, combined with the distribution of the trajectory of major principal stress, a calculation formula for theoverburden pressure on the tunnel within the docking region is derived. The rationality and applicability of theestablished model are verified by comparing the theoretically corrected solution with numerical simulation results. The findings reveal that, without grouting reinforcement, the soil arch slip surface extends obliquely upward from the tunnel to the ground surface, forming a basin-shaped failure surface in both transverse and longitudinal sections, with a wider range in the transverse direction. After grouting, the transverse slip surface exhibits either a “double-basin” or “basin-over-cone” pattern, while the longitudinal slip surface forms a conical shape within a limited range above the tunnel. Additionally, vertical stress decreases within the slip surface and increases outside it. Moreover, the difference between the average vertical stress and soil self-weight increases progressively with depth. However, in non-grouted conditions, the vertical stress curve remains smooth, whereas grouting reinforcement causes a distinct inflection point in the curve. Finally, without grouting reinforcement, the soil primarily resists external disturbances induced by shield tunneling by mobilizing the friction angle along slip planes. After grouting reinforcement, the strengthened soil undergoes stress redistribution, resulting in a significant reduction of the soil arching effect within the mass.​