Characteristics and Countermeasures of Tunnel Heave due to Large-Diameter Shield Tunneling Underneath

Structural deformation and damages may occur in existing tunnels because of new tunneling underneath. There is limited research on the interaction between overlying tunnels and large-diameter slurry shields in soft clay. This paper presents a case on the response of existing metro tunnels to large-diameter slurry shield underpassing in the muddy clay of Hangzhou, China. Through long-term displacement monitoring, displacement characteristics of existing tunnels during and after new tunnel construction were analyzed. Unusual tunnel heaves were observed after the passing of the shield tail because of the buoyancy effect. The existing tunnels heaved with the shape of an inverted V, which was well-described with the Gaussian curve. The location of maximum tunnel heave was affected by the skew angle between the new and existing tunnels. Because the overburden pressure decreased in the under-crossing area, increased maximum ground heaves were observed. The adopted countermeasures for controlling tunnel heave were introduced and analyzed. By controlling the uplift of the new tunnels, excessive tunnel heave can be prevented. Increasing the clearance between the new and existing tunnels and optimizing the grouting parameters can diminish tunnel heave effectively. Long-term tunnel settlement caused by grout and soil consolidation developed considerably after shield underpassing. Decreased tunnel settlement and increased differential settlement could be observed during the second shield undercrossing. Detailed monitoring data and analysis provided useful experience for similar projects. (C) 2019 American Society of Civil Engineers.