The Colsman Tunnel, located in Greenwood Village and Centennial, CO, is part of the Big Dry Creek Interceptor sanitary sewer system and is owned and operated by the Southgate Sanitation District (the district). Flow from the district’s entire waste water collection system (approximately 80,000 residents) is conveyed through the tunnel. Average flows of approximately 10 million of gallons per day are constant and cannot be turned off or diverted.
The tunnel was constructed around 1977 using hand tunneling and road-header equipment. The 2,320-m (7,614-ft) long, mushroom-shaped tunnel is approximately 175 cm (69 in.) wide, 190 cm (75 in.) tall and varies in cross section and shape along its length (Fig. 1). The tunnel was constructed with a slight downward slope of 0.36 percent to the west. At its deepest point, the top of the tunnel is about 27 m (90 ft) below the existing ground surface. Access to the tunnel is provided by a portal structure on the east side of the tunnel (east portal) and a buried outlet structure on the west side of the tunnel (west portal). Four ventilation shafts (designated Vent Shaft #1 through Vent Shaft #4) used during construction are spaced approximately 487 m (1,600 ft) apart along Orchard Road (Fig. 2). Access into the tunnel is complicated by the tunnel’s depth and its location beneath Orchard Road, a heavily traveled arterial roadway adjacent to a busy shopping center and residential areas.