A Wirth TBM reamer (pictured) is expected to start boring this month on the “Basel” tube of the twin tube Uetliberg tunnel project in Switzerland. Previously bored with a 5m diameter Wirth hard rock pilot TBM through a 2.7km molasse section, the reamer will now extend the tunnel diameter to the final 160m2 cross-section by undercutting, a principle whereby the cutting rollers work against the rock’s tensile strength, which is considerably lower than the compression strength. The head consists of a two-part boring head-base, and six boring arms. The boring head rotates on the inner kelly, which is braced and bearing-mounted in the pilot tunnel, and in the already enlarged tunnel cross-section. While the Basel tube is being extended, the second pilot tunnel will be driven for the “Chur” tube. The same procedure will then be used to extend it. The total length of the Uetliberg tunnel is 4.4km, with the soft ground sections already completed (T&TI November 2002, p24).
