Tunnelling in changing geology

Abstract

Permanent technological improvements in mechanized tunnelling enabled new tunnelling projects, which were not realizable only a few years before. Considered to be the greatest challenges are projects consisting of mixed geologies, of high-strength abrasive hard rock, as well as unstable loose rock with clogging tendencies and high groundwater pressure. This requires a regular and professional maintenance concept for all tunnelling components participating directly and indirectly as well as an efficient construction operation for the support of the tunnel boring machine. Projects crossing soil, weathered and massive rock are challenging because of totally different machinery requirements between EPB and rock drilling. On one hand the tunnel boring machine should support the tunnel face to avoid settlements using a cutterhead which allows soil to enter the excavation chamber easily. On the other hand, the hard and massive rock requires a close cutter disc spacing and strong cutter head structure. Problems were encountered especially in mixed ground conditions when soil and rock run through the tunnel face. The result is extremely high wear on cutting tools as well as on the cutterhead structure itself and finally long downtimes for tool changes and cutterhead repair. New wear phenomena on the cutter discs were encountered and technical solutions developed. The design of a TBM in such conditions is a compromise between both excavation methods. To improve job site performance, all geological parameters and machine behavior have to be permanently analyzed. Possible measures like certain design features, conditioning of soil, special cutter and driving parameters will be pictured. Paris, Singapore, Porto and St. Gotthard/Switzerland are only some examples, where - through intensive technical support from the side of the TBM supplier - mechanized tunnelling is managed in extremely difficult geological and hydrological conditions. A sophisticated road tunnel is currently built west of Paris, which will complete the freeway circuit A86. Designed as a two-story road tunnel it is approximately 10 km long with a bore diameter of 11.57 m. Geological conditions of this area consist of heterogeneous soils, which cover the entire spectrum of cohesive and rolling soil to hard rock. The machine concept provides a combination of Mixshield and EPB-Shield that can host both conveyor systems at the same time - screw conveyor as well as feeder and pressure lines for the liquid operation. Singapore is currently replacing the existing sewage system (DTSS) up to 50 m deep, to comply with the requirements of a world metropolis in the 21st century. It is located in very difficult heterogeneous ground conditions: granite of up to 190 MPa, slightly to strongly disturbed clay, silt and sandstones as well as conglomerates, Kallang formation, consisting of soft clays and loose sands, old alluvium, additionally the hydro-static water pressure is of up to 4 bar. The Joint Ventures ordered Herrenknecht EPB-Shields equipped with cutterheads specially for hard granite formations. A 70 km extensive metro system is currently constructed in Porto, partially running underneath the city center of Porto. The tunnel boring machines have to face changes between very hard granite and loose rock as well as difficult hydrological conditions. In Switzerland four Herrenknecht tunnel boring machines are tackling the Gotthard from the north and the south for a distance of 75 km. After its completion in 2014, the Gotthard Base Tunnel with its 2 · 57 km will be the longest traffic tunnel of the world. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.