Colsman Tunnel Sanitary Sewer Rehabilitation

CLIENT Southgate Water & Sanitation District

Location Centennial, CO

Shannon & Wilson provided geotechnical engineering and tunneling services for this 1.5-mile long tunnel rehabilitation project as part of a design-build team. 

The approximately 6-foot high by 5-foot wide horseshoe-shaped tunnel was constructed in the 1970s and has reached the end of its design life.  The sanitary sewer tunnel was rehabilitated by slip lining it with a 48-inch diameter HDPE pipe under live flow conditions.  The HDPE pipe was installed using a horizontal directional drill (HDD) rig to pull the pipe string through the tunnel, followed by backfilling with low density cellular concrete.  To accommodate the 1.1-million-pound pullback forces of the HDD rig, Shannon & Wilson designed sheet piles driven into bedrock to support the HDD anchor block.

Other Shannon & Wilson services included designing a soil nail retaining wall and micropile foundation for a pre-fabricated bridge in a tightly-constrained section of the project adjacent to a narrow steam crossing.  Construction access was limited, and the project required more specialized geotechnical engineering to provide the best solutions to the owner.  We also provided pavement design for a new large vehicle turnaround area and repairs to existing pavement sections.  Shannon & Wilson worked closely with the project team to develop the best engineering approaches given the geologic conditions, site constraints, permitting requirements, and budgetary and schedule limitations.  

Shannon & Wilson also provided construction materials testing services for soil, concrete, asphalt, and low density cellular concrete, and provided construction observation for services for drilling interim grout injection points into the existing tunnel, low density cellular concrete placement, and micropile installation.

water Water Supply / Wastewater
Pavement Laboratory Geotechnical Engineering Tunneling / Underground


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