NMT-NATM hybrid method for sustainability and risk reduction

Summary

Abstract

In 1992 a group of authors from Norwegian companies proposed the name NMT (the Norwegian Method of Tunnelling) as a distinctive alternative to the well-known NATM of Austria. In a nut-shell one can call NMT ‘single-shell’ while NATM can be called ‘double-shell’. NMT is most appropriate for tunnel and cavern construction in medium to hard jointed rock. Support and reinforcement are guided by Q-system application. NATM with its final cast concrete lining can be considered most appropriate for lower quality, weathered and faulted rock. Temporary support and reinforcement methods: shotcrete with mesh or fibre reinforcement, bolting and lattice girders (LG) are typically associated with NATM, prior to the drainage fleece, membrane and cast concrete. In the case of NMT the permanent bolting and shotcrete are deliberately chosen to be of high quality and corrosion protected and are both applied up to the tunnel face, with the final layer of shotcrete perhaps making a total thickness of 10cm also applied only a round or two behind the face. In case of poor rock conditions such as faulted rock, a more robust alternative to LG is used in NMT, called RRS (rib-reinforced shotcrete arches). These consist of systematically bolted arches of reinforcing bars typically 6 x 16mm diameter, welded to cross-bars and sprayed into the form of a thick local arch that may be repeated at 1.5 to 3m intervals. NMT may also have a final concrete lining if the rock mass quality is very poor such as Q < 0.01. In such cases cost and time involved will have approached that of NATM. Systematic high-pressure pre-injection is a frequent first stage of NMT. This radically reduces risk and actually also reduces support and reinforcement needs. In this article a tentative NATM-NMT-NATM hybrid approach is suggested for a ‘typical hard rock tunnel through a weathered hillside’ as so frequently encountered for motorways in tropically weathered regions. Both ends of the tunnel would be NATM while a longer central NMT construction would save cost and time overall. Two case records will be described in which, due to unaccounted conditions, the NATM portions were over-loaded and LG folded due to high vertical loads. Risk reduction measures will be suggested. Due to overbreak effects on concrete volumes in NATM, greater lengths of NMT means greater sustainability as overbreak is not filled in the case of NMT.