This 377-foot long tunnel is an important connective element of the City and County of Denver’s Dakota Avenue Outfall stormwater improvement project. An open face 129.5-inch outside diameter TBM was utilized to complete this tunnel which is approximately 17 feet below five active rail lines travelled daily by BNSF , Union Pacific and Denver’s RTD- Light Rail passenger system.
Challenges the Brierley design team managed included shallow tunneling under active rail lines in a mixed-face condition consisting of alluvial sands over claystone and the sandstone of the Denver Formation. Due to the potential for running sands and the shallow cover separating the rails and tunnel crown, permeation grouting was selected as a means of ground improvement for the cohesionless sands.
The directly jacked reinforced concrete pipe was connected to other components of the Dakota Avenue Outfall project. The completed project provides needed stormwater conveyance improvements that are vital to future re-development of this underutilized urban locale.