Background
Prior to the opening of the Furka Base Tunnel, all railway traffic on the Furka–Oberalp line traversed the challenging terrain between Valais and Uri via the historic high-level route using the 1.8 km (1.1 mi) long Furka Summit Tunnel.[2] This section of the line was particularly challenging to operate, its closure being necessary during the winter months due to the risks associated with heavy snowfalls. These seasonal cycles of closures and reopenings required civil engineering works each time, as large sections of the railway's overhead electrical lines to be dismantled and reinstated as well.[3][4]
The replacement of the Furka Summit Tunnel with a base tunnel had long been identified as an attractive solution, but the prospects for constructing such a tunnel were complicated by multiple factors in both its design and construction.[3] Initial authorisation from the Federal Council for preparatory work was given in 1971.[2] During 1976, the Parliament of Switzerland passed a comprehensive bill for the base tunnel's construction, sponsored by Roger Bonvin; the total cost of its construction had been initially estimated to be 76 million francs.[citation needed] However, these estimates proved to be drastically inadequate as the project proceeded, with figures in excess of 300 million francs being reached prior to the base tunnel's completion. There was substantial political tensions surrounding these cost overruns, which have been said to have contributed to Bonvin's ill health toward the end of the project; Bonvin died just prior to its opening in 1982. A plaque commemorating Roger Bonvin is installed as the base tunnel's entrance in Oberwald.[citation needed]
Construction
During 1971, work commenced on the boring of the Bedretto tunnel, which functioned as an annex tunnel for the Furka Base Tunnel.[2] Excavated by conventional techniques, including both drilling and blasting, it was a relatively narrow tunnel through the bare rock. It was used during the Furka Base Tunnel's construction for the removal of excavating spoil as well as to convey supplies into the construction site.[2] The Bedretto tunnel was abandoned for a time following the completion of the Furka tunnel, being used neither for emergency access nor for maintenance purposes, in part due to three separate collapses. During the 2010s, it was restored to accommodate a new ventilation system for the Furka tunnel, intended to supply fresh air in the event of a major fire within the latter tunnel.[2]
The construction techniques used for the Furka Base Tunnel were relatively innovative for the era. One such technique involved the application of a recently developed polyurethane-based grouting to make the rock surrounding the tunnel portals impermeable. It was reportedly the first use of such material within a railway tunnel.[3] The permanent support for the tunnel consisted only of rock anchors and a shotcrete lining along the majority of its length. Areas of particularly high rock pressure were addressed via the excavation of an elliptical or circular profile and the application of steel ribs. Engineers were guided in the use of support measures by a series of geological studies, along with construction experience, to predict and monitor systematic deformation.[3]
In spite of the presence of multiple complex geological features along the bore, work typically proceeded at a steady pace. Typically 400 metres of the tunnel were lined each month, this rate being supported via five individual shotcrete machines that were operated simultaneously. By the end of its construction, roughly 70,000 m3 of dry aggregate and cement mixture had been used, while 115,000 rock bolts have been applied.[5]
The completed Furka Base Tunnel accommodates a single track throughout, its cross section varying between 26 and 42 m2 throughout its length.[3] This often narrow cross section has been attributed as one reason for the tunnel's construction requiring ten years, as it hindered the movement of bulky construction equipment.[3]