Project Description

The West Fork Relief Main is a 7,110-foot long, 8-foot diameter sewer tunnel through both soft ground and soft rock. Alluvial clays and terrace sands and gravels, clay shale, and limestone are the host materials for the tunnel. A related project, the 4th Street Deep Relief Sewer, consists of about 2,350 feet of 24-inch diameter sewer, about 60 percent of which was installed by trenchless construction methods. The West Fork Tunnel was constructed below the main downtown area of Fort Worth.

Scope of Services

LACHEL & Associates, Inc. (LFA), working on a team headed by Carter-Burgess, provided complete geotechnical services for the project including specification and supervision of the field and laboratory programs, interpretation of the results, development of subsurface engineering profiles, design of geotechnical instrumentation, and development of the Geotechnical Design Summary Report (GDSR). LFA also designed the initial and final support systems based on an evaluation of the ground response to the construction methods being considered and provided tunnel design plans and specifications. During the design, LFA evaluated the effects of swelling behavior and likely loading increases from swelling in the expansive clay shale and developed measures to mitigate the impacts of the swelling.

Initial support consisted of steel ribs with timber lagging. Contract documents allowed either precast reinforced concrete pipe or cast-in-place concrete as the final lining system. In addition to the geotechnical and design services, LFA provided cost and constructibility studies and consulting services through the tunnel construction period.

Before completion of the project, the City of Fort Worth requested a geotechnical investigation and design for about 3,000 feet of 36- to 42-inch finished diameter sewer along Commerce Street, to connect with the West Fork Relief Main by means of a deep shaft.

Client

City of Fort Worth, Texas