FRENCH TUNNELS HELP FORM INTER-CONTINENTAL LINK

When completed in 1981/82 the French highway a42, and a proposed spar, the F42, will complete an inter-european highway network. The 140 km a42 is being constructed across alpine terrain requiring the construction of two 500 M tunnels, the de La Crotte and St Germain, and also larger twin bores, the 1.4 km Vuache Tunnel and the 3.3 km Chamoise tunnel. The article describes the construction at Chamoise where a 9 sq M pilot tunnel is being driven through Jurassic geology where excavation is by drilling and blasting methods using three-boom jumbos. To overcome problems of rock decomposition after blasting, the new Austrian tunnelling method of applying shotcrete, mesh and resin rockbolts, followed by more shotcreting, was found necessary. Although no further lining will be applied to the pilot bore, the two 90 sq M main bores, when excavated will also have a 300 mm in situ concrete lining. It is planned to connect the pilot tunnel to the main bores by vehicular and pedestrian passages. (TRRL)