Minimizing the use of concrete in tunnels and caverns: comparing NATM and NMT

For many decades, a tunnelling method has been in use which effectively minimizes the use of concrete, which should be one of the goals in our CO2-producing planet. We call the method NMT (Norwegian Method of Tunnelling) and emphasize its ‘single-shell’ characteristics, to distinguish it clearly from double-shell NATM (the so-called New Austrian Tunnelling Method), which is recommended to have (ASG, NATM: the Austrian practice of conventional tunnelling 2010): shotcrete, mesh, lattice girders, rock bolts (if in-rock), drainage fleece, membrane, and the final load bearing and often steel-reinforced concrete lining, including the invert when in poor rock conditions. This tunnelling method is inevitably several times more expensive, uses many times the volume of concrete, takes longer to build, and requires at least a ten times larger labour force than single-shell NMT. The single-shell tunnels for road or rail or hydropower or water transfer, or for large caverns for storage of oil or food, or for hydropower machine and transformer halls, can be made stable by judicious application of a well-used (>2000 case record based) the so-called Q-system of rock mass quality estimation. The latter encompasses a rock mass quality scale from 0.001 (equivalent to a serious fault zone, where we also may need a local concrete lining) to 1000 (equivalent to massive unjointed rock) where careful blasting will remove the need even for shotcrete. In general, rock masses where we need tunnels or caverns will lie closer to ‘mid-range’ (i.e. closer to Q = 1 which is described as ‘poor quality’). Here we would need combinations of corrosion-protected rock bolts and high quality fibre-reinforced shotcrete, with stainless steel or polypropylene fibres. We may also need systematic high-pressure pre-injection of micro-cement and micro-silica, which may add 20% to the (low) starting cost of the NMT excavation. Written as B + S(fr) in short-hand, NMT has rock bolt c/c spacing in metres and shotcrete thickness in centimetres, as specified by the range of Q values and excavation dimensions. The details are also affected by the planned use. For instance, at our record-breaking Olympic cavern of 60 m span (for housing 5400 spectators or later concert goers), B = 2.5 m c/c + S(fr) 10 cm were (and remain 25 years later) the stabilizing and permanent measures of support and reinforcement. Deformation monitoring and distinct element (jointed rock) numerical verification showed 7–8 mm of maximum deformation in the arch. The moderate Q value range of quality of 2–30 (poor/fair/good) and RQD = 60–90 indicated a well-jointed gneiss, which had only moderate UCS = 90 MPa compressive strength.