DIFFERENT TUNNELLING METHODS

Technical principles behind the choice of different tunnelling methods

Technical principles behind the choice of different tunnelling methods

Introduction

This paper provides an in-depth illustration of the shallow tunnelling method (STM) used for tunnelling in shallowly buried soft ground. Limited arching effect and limited ground strength mobilization are the two mechanical characteristics of the STM. The stability of the cutting face and the dry tunnelling condition are the two preconditions that should be satisfied for the STM. Some “necessary” auxiliary methods mainly served to guarantee these two preconditions are highlighted. Five principles, namely proper auxiliary methods, sequential excavation with short advance length, rigid support with quick installation, short ring closure time and systematic deformation monitoring, which are required to follow when using the STM are summarized.

The state-of-art of the STM is classified into five different construction approaches according to tunnelling sequences, which are adopted in the construction of the nine subway stations in Beijing. The tunnelling procedures, support measures and settlement characteristics associated with excavation are demonstrated. Statistical analyses of the settlement data of 342 ground surface monitoring points above these nine stations are performed to illustrate the ground deformation characteristics of the STM. Numerical simulations are also employed to study the ground deformation characteristics of different construction approaches under the same geological conditions. This paper systematically demonstrates the applicability of STM in theory and practice. It is helpful in updating the database of the world tunnel projects and serving as a practical reference for future similar projects.

Discussion

The New Austrian Tunnelling Method (NATM) has become very popular for tunnel construction all over the world due to its technical feasibility, safety and economic competitiveness. This method is based on the principles established by Rabcewicz, 1964 and Rabcewicz, 1965 for the use of shotcrete as a support system in tunnel construction, together with observational method to determine whether the support system is sufficient. The use of NATM in soft ground was largely pioneered by Professor Müller for the Frankfurt Metro construction in 1968 and was later further developed by London Underground in the early 1990s.

Although the NATM has achieved remarkable successes (Tauern tunnel, Arlberg tunnel, Inntal tunnel, metro Rankfurt, Schweikheim tunnel, Tarbela caverns) all over the world (Kolymbas, 2005), as many of the NATM's recommendations were already in use, there exists some confusions and conflicts (Karakus and Fowell, 2004).

One debate of the NATM is whether it is suitable for soft ground construction (Brown, 1981). It is widely accepted that one of the major principles of the NATM is the deliberate mobilization of the strength of the ground around a tunnel to the maximum possible extent by allowing a controlled ground deformation (Brown, 1981, Sauer, 1988, Will, 1989 and Health and Safety Executive, 1996). Institution of Civil Engineers (ICE) claimed that any soft ground application of NATM is associated with the following principal measures: (1) Excavation stages must be sufficiently short, both in terms of dimensions and duration. (2) Completion of primary support—in particular, closure of the sprayed concrete “ring” must not be ...