Abstract
The urban shallow tunnelling process in silty soil is easy to cause large displacement of surface and tunnel. Obviously, if the strata and the tunnel face are not treated by reasonable reinforcement method, instability and collapse phenomenon will be encountered during the tunnel excavation. There are a series of studies on construction methods of shallow tunnels, but these methods have limitations in silty soil. In this study, a comprehensive construction plan of the urban shallow tunnel in silty soil was proposed and applied to a case study in Fuzhou, Fujian Province in South China. The in situ monitoring tests and numerical simulation were employed to address displacement characteristics of surface and tunnel. Results indicated that the urban shallow tunnelling process could achieve good effect by dewatering of silty soil, reinforcing surface by vertical jet grouting piles, and advanced small pipes and circumferential grouting in the tunnel face; surface settlement during dewatering process accounted for about 30% of total surface settlement in silty soil; the excavation of the top heading, the middle, and lower benches had great effect on displacement of surface and tunnel for three-bench seven-step excavation method in silty soil; surface settlement troughs in silty soil were deeper and wider; lock-feet bolts had good effect on restricting horizontal convergence; and ratio of total crown settlement and total horizontal convergence was in range of 1.43∼1.59 when b/h was 0.88 in silty soil. The construction plan proposed in this paper is helpful for further study of shallow tunnel tunnelling process in silty soil.
Highlights
In recent years, there are more and more urban shallow tunnels in silty soil
Many problems need to be considered in construction process of the urban shallow tunnels in silty soil, for example, treatment of groundwater, selection of reinforcement schemes, and controlling of surface settlement [3]
According to the previous literature, the explorations on shallow tunnels have concentrated on studying stability of the tunnel face and characteristics of surface settlement and rock mass pressure in soft soil and sand by shield method; there are several researches about shallow tunnelling method (STM) of subway station in soft soil; there are a few researches about characteristics of silty soil [28]
Summary
There are more and more urban shallow tunnels in silty soil. Silty soil has characteristics of high compression, high sensitivity, high creep, high water content, low strength, and so on [1, 2]. According to the previous literature, the explorations on shallow tunnels have concentrated on studying stability of the tunnel face and characteristics of surface settlement and rock mass pressure in soft soil and sand by shield method; there are several researches about STM of subway station in soft soil; there are a few researches about characteristics of silty soil [28]. Most of existing studies are constructed by shield method, and few studies have focused on displacement characteristics of shallow tunnelling process in silty soil by STM. This paper retrospects the case study of STM in silty soil of the Hudonglu tunnel in Fuzhou, Fujian Province in South China, compares the in situ monitoring results and numerical results, analyzes displacement characteristics, and summarizes construction method and supporting measures for urban shallow tunnelling process in silty soil
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