Colsman Tunnel Rehabilitation Project (Progressive Design-Build)
Centennial, Colo.

Owner: Southgate Water & Sanitation Districts
Lead Design Firm: Dewberry
General Contractor: Garney Cos. 
Engineer: Burns & McDonnell
Geotechnical Engineer: Shannon & Wilson Inc. 
Subcontractors: Big R Bridge; Cematrix Inc.; Global Underground Corp.; HEI Civil; ISCO; Marine Diving Solutions; SewerVUE; Sunbelt Rentals; Tweisz; Weston Landscaping; Xtreme Drilling

Concerned about the reliability of the Colsman Tunnel, the Southgate Water & Sanitation District hired a design-build team to keep it from collapsing. Because the 7,600-ft-long tunnel is the only connector in a wastewater collection system that supports 80,000 residents, the team proposed to slip-line the existing tunnel with HDPE pipe to maintain continuous sewage flow, thus eliminating the cost of an expensive new tunnel or a temporary bypass during rehab of the old one. 

This slip-lining presented a challenge, however, as the tunnel was only accessible at its east and west portals. Four ventilation shafts, 24 in. in diameter and spaced approximately 1,600 ft apart, also limited accessibility to the tunnel. The team hired a drilling contractor to drill nine additional grout-injection shafts. The reduced length of grouting helped to keep the project on track.   

Sections of the HDPE pipe were pulled through the existing tunnel and connected with a fusion machine that pushed the welds together. Once the HDPE pipe was in place, the sewage flow was diverted from around the pipe and into the newly pulled pipe. This solution provided the district with a 75-plus-year structural design at a cost less than half of the original budget, saving approximately $15 million.

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