East Bank–West Bank Interceptor Connection Tunnels

A modern metropolis in north Texas, the City of Dallas needed to upgrade its wastewater system to better serve its population. The city’s Water Utilities Department embarked upon a $27.6 million project to replace aging infrastructure and provide a reliable solution for conveying wastewater from its East Bank interceptor to the West Bank interceptor and on to the central wastewater treatment plant in South Dallas.

The scope of the project involved the construction of two 40-foot-diameter access shafts and parallel 3,500-foot-long, 72-inch and 84-inch tunneled pipelines beneath the Trinity River, which were excavated by a rock tunnel boring machine. The tunnels and shafts are located in the Eagle Ford Shale, a formation with expansive properties and time-dependent creep behavior. During design, Schnabel specified the geotechnical investigation and laboratory testing requirements for soil and rock characterization, and documented the engineering properties and projected ground response of the materials to tunnel and shaft excavation.

Due to the large shaft diameters and parallel tunnel intersection geometry, we used numerical modeling to evaluate both the redistribution of stresses around the openings and the behavior of the shale pillar between the two tunnels at varying separation distances. Additional analyses were necessary to evaluate the stability of the running tunnels at the low point of the project, where only 1.5 tunnel diameters of rock were located above the tunnel crowns and below the river. We also recommended an instrumentation program to evaluate loads on the initial tunnel support, monitor tunneling performance, and detect potentially problematic deformations caused by construction operations. During construction, we provided management services including shaft and tunnel inspection, review of related contractor submittals, and general support of project administration.