Mill Creek Interceptor – Contract 3

Completed in 2006 by Mole Constructors, The Mill Creek Interceptor – Contract 3 consisted of the excavation of 14,775 linear feet of 23 feet 8-inch diameter tunnel for a sanitary sewer through shale and 470 linear feet of 10-foot diameter tunnel through shale including shaft excavations to depths of 320 vertical feet. Tunnel excavation was performed using a Robbins Tunnel Boring Machine (TBM) for the 23 feet 8-inch diameter tunnel and via a Scotty TBM or Alpine Roadheader for the 10-foot tunnel. The initial tunnel support was steel ribs and wood lagging for larger tunnel and rock dowels for the smaller tunnel. The final lining is a 20-foot diameter reinforced, cast-in-place concrete. The shafts were supported via liner plates within soft ground, and rock dowels, welded wire fabric and shotcrete within rock. Tangential approach flow, baffle-type flow and standard drop structures are to be constructed within the shafts. The excavation of the tunnel was delayed during construction for approximately 9 months due to the unexpected infiltration of methane gas. The project was terminated for conveyance by the Owner after the concrete lining was completed, but prior to final completion.