French hill road tunnels: A pragmatic rock support approach

Abstract

The French Hill project involved the design and excavation of 3.5km of tunnels in an urban environment, including intersections and passages between the tunnels. The geology encountered in the tunnel face ranged from fill material to chalk and carbonate rock with karstic voids of variable size and shape. The tender design, which followed the empirical guidelines of the Q-System, was optimized for the construction phase. The final design, described in this paper adopted a more pragmatic approach, adopting either systematic bolting or shotcrete shell support classes, depending on the expected rock mass quality. The applicability of the various support options was evaluated using both continuum and discontinuum computer models. The utilization of the rock mass capacity using this optimized design led to a successful excavation of the tunnels. In order to meet the tight construction schedule, up to 12 excavation faces were mined simultaneously.