CapEx Drainage Tunnel Sets Stage for Historic Highway Project
The I-35 Capital Express (CapEx) Central project in Austin, Texas, under development by the Texas Department of Transportation (TxDOT), involves improvements to 8 miles of this vital corridor serving the state’s capital. The project will add new high-occupancy vehicle lanes; improve frontage roads, ramps and interchanges; remove elevated portions of the highway; and lower the alignment through the downtown area.
When completed, the $4.5 billion program will not only improve vehicular transportation, but also facilitate pedestrian and bicycle traffic while enhancing east-west connections across the highway. Additionally, the plan provides the option of capping the sunken highway in areas to enhance connectivity and create greenspace.
In order for the project to be realized, however, the infrastructure must be designed and built to support the finished facility. One of the first steps is the construction of the CapEx Drainage Tunnel underneath the highway. To accommodate a project of this size, the drainage tunnels are designed to be 22-ft internal diameter and stretch approximately 6.5 miles, one leg of 3 miles and one leg of 3.5 miles.
Planning for the CapEx Central Project dates back to the 1980s and has included input from the community that has helped shape the current design. The project runs along the I-35 corridor from approximately the intersections with US 290 in both the north and the south.
The tunnel is needed to collect drainage from I-35, as well as flows that currently cross the highway which will be impacted by lowering the roadway. The I-35 drainage tunnel will run north-south under the highway for about 3 miles, where it will flow into a tunnel running east-west under Cesar Chavez Street. The tunnel under Cesar Chavez will run about 3.5 miles, terminating at the US 183 interchange.
With the environmental decision finalized, TxDOT and its design team are fast-tracking the schedule to get the roadway renovations completed and open to traffic in one decade by 2033 or sooner. “We want to minimize how long Austin is under construction – it’s our state capital and where the legislature meets, it’s home to many businesses and residents, and adjacent to the University of Texas,” said Heather Ashley-Nguyen, transportation and planning and development director for TxDOT’s Austin District. “We have a very aggressive schedule and are doing anything we can to deliver this project to Austinites as quickly as possible.”
As mentioned, the drainage tunnel serving the lowered roadway is a critical element in allowing the overall project to proceed to construction. BGE began its design process in January 2023 with 30 percent in June, 60 percent expected by the end of September and final design at the end of the year. Meanwhile, TxDOT and its team is conducting informational workshops and one on one meetings with contractors and is planning to bid the project in July 2024, with construction scheduled for 2024-2027.
The team is planning to deliver the project via design-bid-build with an A+B (cost and schedule) component. Under that model, qualified teams will submit a schedule component in addition to cost, with credit given to proposals with shorter completion times. Qualifications include financial capacity as well as experience and key personnel considerations. TxDOT is proposing an Alternative Technical Concept approach to help incorporate sealed input from contractors within the design-build-build delivery approach.
A gravity-drained tunnel was the preliminary design, but due to the geology in the area, final design lowered the tunnel within the Austin chalk layer to mitigate risks. Lowering the alignment helps mitigate potential utility conflicts and settlement impacts along Cesar Chavez Street, which is home to businesses and residents.
The Austin chalk provides a good medium for bored tunneling, and offers the potential for a quicker and less expensive construction compared to building a shallower tunnel with the potential for mixed-face conditions.
“The Austin chalk is a weak limestone with relatively low abrassivity in which you can achieve good penetration rates,” said James Parkes, tunnel design lead with Schnabel Engineering. “There have been a lot of projects in the area completed locally in the same formation that have been very favorable, so we want to keep the profile within that stratum as much as possible.”
Challenges include the possibly of encountering some of the Eagle Ford shale formation in some areas, as well as the crossing of known and unknown faults.
In keeping with the aggressive schedule, TxDOT and its team is requiring the use of two TBMs to mine the I-35 and Cesar Chavez tunnels concurrently.
“The schedule is a critical component of this project, so to accomplish that we are looking at using two TBMs,” Dan Goodin of BGE, design team lead. “The other thing that we have done is positioned the launch shaft locations in TxDOT right of way so they are ready on Day 1 – as soon as the contractor gets notice to proceed, they can start sinking the shaft.”
TBMs are being prescribed as shielded rock TBMs installing segmental liners. The segmental lining is considered favorable for the working conditions under the water table, as well as the possibly of encountering potentially swelling Eagle Ford formation and crossing fault zones. Additionally, installing a one-pass lining is anticipated to help shorten the construction duration. The TBM may include probing and grouting capabilities.
The CapEx Drainage Tunnel is a unique project for TxDOT. Typically, utility construction for the agency includes shallower jack-and-bores or microtunnels for culvert installation vs. deep, large diameter TBM tunnels.
“This is a one-of-kind project for TxDOT, not only in the size and depth of the tunnels, but also the speed of the design and construction,” said Joe Goessling, hydraulics engineer for TxDOT’s Austin District. “While a tunnel of this diameter in this geology may not be new, the fact that we are doing this in such a short amount of time makes it unique. In addition, the drainage tunnel is in advance of a much larger project that everyone in the city is looking forward to getting completed.”
CapEx Drainage Tunnel Project At-A-Glance
- Construction Schedule: 2024-27
- Length: 6.5 miles
- Depth: 70-200 ft
- Diameter: 22 ft ID
- Owner: TxDOT
- Consultants: BGE, Schnabel Engineering, Balcones Geotechnical, HDR
- Anticipated Construction Method: Rock TBM with precast segmental liner
Jim Rush is editor of TBM: Tunnel Business Magazine.