Ground surface movement of Shallow-Buried Large-Sectional tunnel under Full-Ring Pipe-Jacking roof and ground freezing

Abstract

The Gongbei Tunnel is an important part of the Hong Kong–Zhuhai–Macao Bridge, with a super-large section of 336.9 m2 and an extremely shallow cover section 5 m from the tunnel crown. The Gongbei Tunnel mainly passes through a weak muddy soil layer with a high water level. In addition, Gongbei Port (connecting Zhuhai city and Macao) is immediately above the tunnel. Thus, the environmental concerns and political sensitivity regarding this area necessitate very strict control of ground surface movements. Therefore, a full ring of a pipe-jacking roof (with 36 pipes) was used to control the ground movements caused by tunnel excavation. In addition, a ground-freezing method was adopted to prevent water inflow during the tunnel excavation. The tunnel excavation was divided into 14 sections and was conducted step-by-step to control the excavation-induced ground surface settlement. Generally, the ground surface movement appeared as an uplift, owing to the ground freezing. The most significant ground surface heave was greater than 500 mm, and the settlement at the tunnel crown was less than 20 mm, indicating that the new construction method protected the tunnel effectively during the excavation but had a significant effect on the surrounding environment. To better understand the coupled effects of ground freezing and tunnel excavation on the ground response, the ground surface movement induced by ground freezing was predicted using the stochastic medium theory. Consequently, the ground surface settlement induced by tunnel excavation was obtained by subtracting the calculated ground surface heave caused by the ground freezing from the observed ground surface movement. The calculated freezing-induced ground surface movement was consistent with the measurements. The calculated results show that the distribution shape of the ground surface movement changes from a concave type to a spandrel type when the frozen wall exceeds the tunnel cover depth. Finally, three key parameters influencing the ground surface movement were discussed: the development velocity of the frozen wall, the frost heave ratio, and the primary influence angle. A parametric study shows that the frost heave ratio has the most significant effect on the ground surface movement and should be addressed more carefully via freezing temperature control in the ground freezing process.