Study on Vertical Displacement Law of Existing Pipe Gallery Caused by Tunnel Underpass

Shield tunneling beneath existing utility tunnels may induce vertical settlement and deformation of the structure. To reduce adverse effects, bottom grouting and settlement reduction piles are commonly adopted for reinforcement. In this study, the vertical displacement behavior of an existing utility tunnel induced by double line shield tunneling is investigated before and after reinforcement. The reinforcement mechanisms of the grouted layer and settlement reduction piles are analyzed. The strengthening effect of the grouting zone on foundation soil and the influence of settlement reduction piles on additional stress transfer are considered. Based on the Mindlin solution for additional stress caused by tunnel excavation and the energy variational method, a theoretical calculation formula for the vertical displacement of the utility tunnel under reinforced conditions is derived. An engineering case is used to compare the theoretical results with field monitoring data and numerical simulation results. Parametric analyses are further conducted to study the effects of grouting length L g and foundation resistance enhancement coefficient k m on tunnel settlement. The results show that the theoretical predictions agree well with measured and numerical results. Grouting and settlement reduction piles can effectively reduce vertical settlement of the utility tunnel. Increasing L g enhances settlement control, but the improvement rate gradually decreases. The coefficient k m increases with the number of settlement reduction piles, and a larger k m leads to smaller settlement. In engineering practice, the combined use of a grouted layer and settlement reduction piles provides better settlement control.