TxDOT I-35 CapEx Drainage Tunnel Project

Located in Austin, TX, the $4.5B Interstate 35 (I-35) Capital Express Central project aims to alleviate traffic congestion along the corridor between US 290 East and SH 71/Ben White Boulevard. Before these improvements can start, the essential infrastructure must be designed and constructed to support new above-ground facilities. The CapEx Drainage Tunnel project marks the initial phase of this infrastructure development. Schnabel is serving as a core member of tunnel project’s design team.

The Capital Express Central project includes the creation of new high-occupancy vehicle lanes; the enhancement of frontage roads, ramps, and interchanges; the removal of elevated highway sections; and the lowering of the roadway alignment in the downtown area. The construction of a 22-foot finished diameter drainage tunnel and its 9 large diameter maintenance and drop shafts are imperative for the collection of drainage water from I-35, including flows that currently intersect the highway, which will be impacted by the lowered roadway.

Given the tunnel’s considerable size, it will be executed in two segments spanning a total of 6.5 miles. One segment will run north-south beneath I-35 for three 3.5, ultimately feeding into a second tunnel that runs east-west. The latter tunnel will extend beneath Cesar Chavez for nearly 3 miles, concluding at the US 183 interchange.

The Texas Department of Transportation (TxDOT) has faced several constructability challenges, as the original tunnel alignment included obstructions, conflicts with future retaining walls, staging area concerns, and significant mixed-face tunneling. Consequently, the tunnel alignment was lowered into the Austin Chalk layer to mitigate potential risks. In line with TxDOT’s tight schedule, two tunnel-boring machines (TBMs) will operate simultaneously to excavate the tunnels. Segmental lining was chosen due to expedite the construction schedule and mitigate low rock cover instability concerns. The final design was completed in 2024, with construction scheduled to occur between 2025 and 2028.

Rendering: Courtesy of Texas Department of Transportation