Numerical analysis for umbrella arch method in shallow large scale excavation in weak rock

Abstract

Tunnelling in weak and shallow ground is common in urban areas and hence controlling ground subsidence is important. Large scale excavation using NATM and supported by auxiliary methods are widely used in urban areas. One of the popular auxiliary supports is the Umbrella Arch method. The Umbrella Arch method is used extensively in large-scale excavations in Italy and Japan. Despite its popularity, there is no simple procedure to model the Umbrella Arch method numerically. This paper compares the ground behaviour predicted by two simple and practical numerical solutions to simulate the reinforcing effect of the Umbrella Arch and investigates the behaviour of the ground when this pre-support method is used in tunnelling in weak and shallow ground. The first method is based on the principles of improvement of geotechnical material properties. An equivalent strength of the rock material is derived by the process of weighted averages. However, this method provides only crude approximation of the Umbrella Arch method. In the second method, the steel pipes are modelled individually. Beam elements are used to represent the steel pipes. Grout and rock material properties are combined using weighted averages process and an equivalent rock mass strength is derived. Using the Hoek-Brown criterion, corresponding rock mass parameters are used. These parameters only apply to a strip of rock material around the tunnel crown. Two-dimensional finite difference code, FLAC (Itasca, 1990), was used for the numerical simulations. The analyses conducted explicitly focused on surface settlement, crown displacement, and vertical and horizontal tunnel closures. The results of the analyses show that different approximations produce significantly different values of surface settlement and crown displacement. In order to produce an accurate 2-D approximation for the Umbrella Arch, the 2-D numerical solutions should be calibrated with field case histories and 3-D numerical analyses, which take into account of the 3-D deformation at tunnel face. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.