A case study on Tunneling using NATM

Abstract

The new Austrian tunneling method (NATM) is widely applied in design and construction of underground engineering projects. The NATM is also known as sequential excavation method (SEM). The NATM is a construction method, which is very adaptive according to changing subsoil conditions and changing shapes of cross-sections. NATM/SEM enhances the self-supporting capacity of the rock or soil by mobilizing the strength of the surrounding ground. It is today by far the most resistant tunnel support system in earthquake endangered zones. Experience from past projects has helped to identify several critical success factors for safe and cost effective NATM/SEM tunneling. According to actual site conditions the design of NATM is with a certain degree of flexibility for adjustments. Unit price contracts to support flexibility, adaptability and balance risk sharing - a proven way to a cost effective and state-of-the-art product. Skilled contractors are familiar with the principles of NATM/SEM and the utilization of the ground support measures. Basic idea behind NATM is that immediate rocks are self-supporting and this method is economical. Here the cycle time achieved is 12 hrs. for class I, II and III but for the rock classes IV and V the cycle time achieved normally is 16–18 hrs. The NATM is most appropriate support system for tunneling in soft ground i.e. rock class IV and below. The installation of reinforced lattice girder with wire mesh, shotcrete and rock bolting provides a uniform load bearing structure in soft rocks.