Abstract
During excavation of NATM tunnel in soft clays, stability problems could arise when the tunnel is too shallow, necessitating special treatment for the surrounding ground such as freezing and jet grouting. However, there is generally no specific rules to help the engineers to determine the critical depth of “shallow” tunnel, which is the dividing line between stably and unstable NATM tunnel design. When using factor of safety λ to quantify the stability of unlined shallow tunnel, λ can be considered as a function of tunnel depth D, height H, width W, cohesion, friction angle, and weight of the medium C, ϕ, and γ. In this paper, aiming at finding the relationship among D, W, C, ϕ, γ and λ of unlined shallow tunnel, we conduct parameters studies with an efficient numerical method for upper bound limit analysis: Discontinuity Layout Optimization. The results show it is possible to obtain λ only as a function of tunnel height and width when the other parameters are known, paving the way for quickly evaluate the stability of shallow tunnels for the researchers.
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