Westcliff Drainage Tunnel
A city of cowboys and culture, Fort Worth embarked on a multiphase capital improvement project recommended to reduce the flooding of homes and other structures in its watershed. The overall Westcliff Drainage Improvements Phase 2B project consisted of 1,500 linear feet of stormwater conveyance line. Schnabel was retained by the lead designer, Wade Trim Inc., to provide tunnel engineering design services.
The Westcliff drainage project included a 175-ft long trenchless crossing beneath Granbury Road, a busy four-lane road, and a Fort Worth Western Railroad (FWWR) line to facilitate the installation of the 90-inch diameter steel pipeline. Ground conditions at the crossing location consist of sediments, primarily clays, overlying weak limestone, resulting in a mixed-face condition for tunneling. With less than 9 feet of cover under the railroad, a 104-inch diameter tunnel was excavated by means of hand mining and the installation of two-flange steel liner plate for initial ground support.
We were responsible for providing input to the subsurface investigation, characterizing the subsurface conditions, evaluating applicable tunneling means and methods, performing tunnel settlement analyses, evaluating settlement mitigation measures, and evaluating locations and available space for jacking and receiving pits. We summarized the results in a geotechnical tunnel report and developed the tunneling specifications accordingly. We also helped, in coordination with FWWR, to ensure the analyses satisfied the railroad’s requirements for assessing settlements, and that appropriate mitigation measures were included in the contract requirements.
During construction, we provided review of contractor submittals related to tunneling, evaluated a change proposal from the contractor to switch the tunneling methodology to hand mining, performed periodic site visits during construction to evaluate tunnel and shaft construction and evaluate the ground conditions, and evaluated instrumentation readings for surface settlement submitted by the contractor. Ultimately, the tunnel was completed without any significant ground settlement or other issues.