Integration of Design and Risk Management in Large Soft-Ground NATM Tunnels

Large tunnel cross-sections in soft ground require a design methodology that can adequately deal with the complex stress state surrounding the underground opening and the associated excavation stages; this is especially the case at bifurcations where adjacent tunnels merge to create a transition zone of a larger cross-section. Vertical profile design constraints may further lead to shallow overburden conditions that add to the risks that have to be addressed as part of the tunnel design. This paper presents an advanced NATM design methodology that is integrated with design and construction risk management schemes to meet major risk-bearing aspects; namely, large tunnel cross-section (area of up to 470 m²), a shallow overburden (as low as 10.5 m), soft-ground conditions (cemented sands and gravels), and the location of the tunnel inside an urban area. Special emphasis was paid to controlling the settlement risks associated with tunneling via an oriented risk triggers and alarm plan.