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In this section, we provide answers to the most common tunnelling related questions.
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In 1848, in Pribram (south of Prague) the use of fast setting mortar was used in place of heavy timber support in a coal mine at Wejwanow. At the same time, a Swiss engineer, Karl Ritter proposed curved tunnel sections with immediate closure of the invert - providing a closed ring in any squeezing ground. This was probably as a result of the 1818 patent of Brunel's circular soft ground tunnelling shield.
In 1907, shotcrete was effectively invented by Carl E Akeley. A taxidermist in Chicago - he developed a machine to spray mortar on the skeletons of dinosaurs to protect them against erosion. The first recorded application of the shotcrete method in the tunnelling industry was in the USA in the early 1920's.
The New Austrian Tunnelling Method (NATM) is often linked to a patent by Professor Ladislaus von Rabcewicz, who invented the duel lining support for tunnels (initial and final linings). This concept, however, had little to do with the application of the shotcrete but merely expressed the concept of allowing the rock to deform before the final lining is applied, reducing the load. The idea behind ensuring the deformation of the rock was based on the theoretical investigations by Engesser in 1881 and was applied by Schmid in 1926.
The original advocate of the use of shotcrete as immediate support in sqeezing ground instead of more traditional heavy timber or steel support was Anton Brunner, a mining engineer from Salzburg, Austria. The 'Shotcrete Method', as NATM was known at the time, gained worldwide recognition when it was applied under the consulting guidance of Professor L Muller and Professor L von Rabcewicz in the Schwaikheim Tunnel in 1964.
The first successful application of the method in soft ground conditions in an urban environment was in Frankfurt/Main in Germany in 1968
The flexibility of NATM approach as illustrated by Dr. Sauer and Partners' "NATM Tool Box" means that it can be adapted in all geotechnical conditions. Ground conditioning/improvements such as jet grouting, ground freezing, grouted pipe spiling or Barrel Vault Methods are also available to stabilize the face. The application of the Doorframe Slab Method, a semi cut-and-cover construction method also allows very shallow tunnel alignments. The Caisson Method, for example provides a safe and cost effective alternative to Immersed Tube tunnels.
For shorter tunnel sections (generally less than 2 km), tunnels with variable geometry and tunnels in mixed ground conditions, NATM provides more cost effective, flexible and safe tunnelling without the long mobilization process associated with TBM procurement. Compared to the Cut & Cover construction method, it minimizes the impact on the environment by avoiding surface disruption. Today, NATM works in any ground with minimal overburden and is therefore well suited for tunneling in urban environments.
Because of its flexibility and highly reliable construction schedule, NATM has become widely accepted by clients and contractors alike.
This is also demonstrated in the pie chart extract from 'Tunnel' magazine (edition 8/2003) which shows over 64% of transportation tunnel construction in Germany is now procured using this method.
Graph: Comparison of Tunneling Methods in Germany
A. Haack, Tunnel 8/2003
A detailed cost comparison between Cut & Cover and NATM construction methods has been carried out for a pedestrian tunnel at Dulles International Airport in Virginia (USA). This tunnel, which serves as an underground corridor for pedestrians between two of the airport terminals, is approximately 235m long. The NATM option has a cross sectional area of 85m² which is comparable to a typical subway station cross section. The crown of the tunnel is located 4.7m below the ground surface, and the bottom of the invert at 13.1m below ground surface. A significant portion of the tunnel’s crown is located in a mixed face (i.e. soil and rock interface). Also, several existing utility lines traverse the alignment of the tunnel, such as fuel, storm sewer and fibre optic lines. The construction time was estimated to be around 50 weeks.
Based on the preliminary engineering the Cut & Cover version was envisioned as an open-cut construction with typical support for excavation consisting of shotcrete with tie-backs, soldier pile and lagging. The cross section was 10.8m wide and 5.9m high with a minimal overburden.
The comparable construction costs (NATM was estimated to be 25% cheaper) and shorter construction time as well as the substantial advantage of low surface impact by the NATM alternative convinced the owner to proceed with the project as a mined tunnel.
Volume loss is usually expressed as a percentage of the excavated volume and can be reasonably represented by a Gaussian distribution. With NATM the early application of the primary lining and ring closure which restrain the "squeezing" effect of the soil and induce ground arching, results in volume losses being limited to less than 1%.
The presence of reinforcement and the possibility of "shadowing" (voids around the re-bar) and also aggressive ground conditions have led to concerns over the longevity and durability of the primary lining. This in turn has resulted in some clients insisting that the primary lining is ignored with the final design accommodated in a secondary lining which may include fibre reinforcement.