SAK Tunnel Boring – Austin, TX
The I-35 Capital Express Central Project represents one of the most complex infrastructure projects in Texas' history. The SAK Tunnel Project will add vital underground drainage to mitigate flooding in downtown Austin and support the reconstruction of I-35 through the city's urban core. At the center of the effort is a dual-tunnel system extending more than six miles beneath the busiest streets in Texas, supported by a high-capacity pump station designed to move approximately 101,000 gallons of water per minute into the Colorado River.
Austin’s I-35 corridor is a major north-south corridor running through Texas and has long served as both an essential transportation backbone and a persistent chokepoint for congestion. With average daily traffic exceeding 200,000 vehicles, I-35 is not only a vital vein to the US infrastructure web, but absolutely critical for the state of Texas. Decades of underinvestment and unsystematic improvements left Interstate Thirty-Five extremely susceptible to stormwater surges, particularly in low-lying sections of downtown, posing a hazard not only to the entire Austin population but also to critical shipments being delivered into the city. Historic flooding events are now on the rise, disrupting traffic flow, damaging infrastructure, and continuing to endanger the Downtown population.
The Texas Department of Transportation (TxDOT) recognized that any long-term reconstruction of I-35 required a modern foundational drainage solution. Without it, lowering and expanding the highway through downtown would create a second Lake Travis, rendering Interstate 35 impracticable and unsafe. This realization led to the initiation of one of the most elaborate stormwater tunnel projects in Texas: a deep-bore system capable of capturing and redirecting enormous volumes of stormwater beneath the urban center.
The tunnel project is not only about flood control; it is part of Austin's comprehensive mobility strategy. To address the massive population increase over the past decade, TxDOT must first address the stormwater constraint. TxDOT and its partners are implementing the necessary conditions to rebuild I-35 with added capacity, safety enhancements, and multimodal improvements. The SAK Tunnel Project is the cornerstone of the infrastructure expansion, enabling above-ground transformation.
Shortfalls of Current Infrastructure
The I-35 pipeline, which runs through downtown Austin, has consistently faced challenges related to stormwater management. The combination of the steepest watersheds in the nation and vastly urbanized land cover, paired with limited coverage for stormwater mitigation, created recurring and ever-increasing flooding risks that standard upgrades could not address. For decades, incremental improvements, such as widening culverts and adding more storm drains, seemed to work to an extent; however, these proved inadequate for a corridor that is a vital vein for the state of Texas and the city of Austin.
As planning advanced for the Capital Express Central project, the eight-mile reconstruction of I-35 in Austin, Texas. It became clear that the existing drainage infrastructure would not support TxDOT’s vision for a lowered and expanded freeway.
Expanded difficulties:
Lowered highway sections would be prone to stormwater accumulation if not supported by a robust subsurface drainage system. Without a proper Stormwater Pollution Prevention Plans “SWPPP” plan in place, including a proper stabilized construction entrance like the FODS Trackout Control System, these sections could function as unintended detention basins, or become an area of major construction site tracking on the roadway, placing motorists at risk and threatening the integrity of the investment and the whole city of Austin.
Furthermore, limitations within Austin have already complicated the problem. The downtown area contains a dense network of utilities, limited right-of-way, and increasingly heavily developed properties owned by major US Fortune 500 Companies, such as Apple, Amazon, and Meta. Expanding surface drainage or constructing open channels was not feasible in such a setting. Any viable solution needed to be deep below ground, allowing for massive volume intake, and operating independently of the surface above.
TxDOT, therefore, faced a dual challenge:
1. Hydraulic capacity and Intensity: Develop a system capable of conveying huge stormwater volumes generated by concentrated rainfall events, more than sufficient to protect depressed freeway sections and surrounding neighborhoods.
2. Urban Sprawl: Design and build this system without disrupting surface mobility, utilities, major private and public investments, and or ongoing urban development, all while maintaining safety and compliance with state and federal stormwater regulations.
The solution was decided on a consolidated tunnel system paired with a high-capacity pump station. This subsurface approach provided both the ability to move stormwater at scale and the operational flexibility necessary to support the broader reconstruction of I-35, starting with drainage, then working on top of the subgrade, and expanding the lanes of Interstate-35.
Initial Expansion and Evolution
The SAK Tunnel Project was conceived as the foundational drainage element of the I-35 Capital Express Central program. Designed through a conglomerate of BGE and Schnabel Engineering and executed under SAK, its scope is defined by two large-diameter stormwater tunnels running beneath downtown Austin, along with a high-capacity pump station at the south end near the Colorado River. Together, these elements form a system that will intercept, transport, and discharge stormwater on a scale that has not been previously attempted in the region.
Phase 1: Design
The earliest design iterations proposed three smaller tunnels branching beneath the corridor, each intended to capture runoff from separate drainage basins. While this approach offered flexibility, it also introduced significant constructability challenges. With three separate alignments, engineers faced conflicts with existing retaining walls, staging constraints in densely built areas, and the need for additional shafts to access and maintain each tunnel bore. These factors increased project risk, extended the anticipated schedule, and complicated future operations and maintenance.
SAK Tunnel Drainage Pipe
The Final Design
During the value engineering phase, the combined team of AECOM, HNTB, and BGE opted to consolidate the concept into two larger tunnels mined primarily through rock. Each tunnel features a finished inside diameter of 22 feet, amplifying the total volume transported by 10 times while reducing the number of required shafts.
· Tunnel A: Extends approximately 3.5 miles beneath I-35, intercepting stormwater at multiple drop shafts along the highway corridor.
· Tunnel B: Runs just over 3 miles beneath Cesar Chavez Street, providing parallel conveyance capacity and redundancy for downstream flow management.
In total, the system delivers more than six miles of new underground drainage infrastructure, strategically aligned to minimize surface disruption while maximizing efficiency.
Drop Shafts and Collection Points
Key to the system are multiple drop shafts, otherwise known as adit connections, which are being constructed at intervals along the CapEx drainage tunnels. These shafts act as vertical catch basins using gravity to capture stormwater from surface inlets and deliver it into the new main tunnels below. In addition to their stormwater mitigation function, the shafts provide long-term and easily accessible entry points for inspection and maintenance, ensuring the system remains serviceable for decades.
US 183 Pump Station
At the southern terminus, a new pump station dubbed US183 or the Capital Express Central Drainage Pump Station is in the beating heart of the system. Designed with a peak capacity of approximately 101,000 gallons per minute, the station ensures continuous flow through the tunnels during storm events. The station discharges directly into the Colorado River, providing a reliable outlet even when river levels rise.
The decision to centralize pumping capacity was critical to make the project worthwhile. Rather than relying on gravity alone, which would have been ineffective given the depressed lanes, the pump station provides a positive pull, allowing engineers to control stormwater removal. Its design allows for staged pumping, redundancy across multiple pump units, and adaptability to upgrade for future rainfall changes.
Evolution Through Design
By transitioning from three smaller bores to two larger tunnels with centralized pumping, the project team achieved several outcomes:
· Reduced risk by minimizing surface conflicts and shaft construction.
· Enhanced hydraulic present and future reliability with higher single-tunnel capacity.
· Simplified maintenance through fewer alignments and access points, allowing for increased coordination between SAK, constructing the tunnel, and Webber, LLC, which is building the US183 Pump Station
· Improved constructability by concentrating work zones and reducing traffic congestion and project staging conflicts.
This coordinated and streamlined design made the project a key component of long-term infrastructure capable of supporting Austin’s continued growth and I-35’s ability to handle Austin's ever-increasing supply chains.
Desert Soil and Subsurface
Constructing two 22-foot-diameter stormwater tunnels beneath Austin, Texas, presented challenges that went well beyond the design. The project alignment passes through some of the busiest and most congested parts of the city, where above-ground space is limited, utilities are dense, and traffic volumes exceed 200,000 vehicles per day. Each phase of the project required careful planning along each Adit, while simultaneously taking into account geotechnical conditions, as well as abiding by local, state, and federal stormwater regulations.
Urban Constructability Challenges
A central challenge was the limited availability of staging areas and shaft sites. Tunnel access points had to be located at intervals along the six-mile alignment, necessitating large-diameter shafts to convey stormwater and facilitate long-term maintenance continuously. However, these shafts had to be sited in areas with minimal surface disruption, avoiding direct conflicts with roadways, businesses, and neighborhoods.
Traffic management added further complexity. Work zones along I-35 and Cesar Chavez Street required coordination with TxDOT’s mobility office to minimize lane closures and maintain safety for motorists while keeping traffic flow manageable. Gradual phases were implemented to maintain traffic flow while still providing crews with the necessary space for excavation, remediation, and equipment delivery.
Downtown Austin’s Subsurface
The downtown corridor contains a web of water, sewer, gas, and electrical utilities. Relocating or protecting these assets was crucial to the tunnel excavation process. In several cases, shafts had to be shifted from preferred locations to avoid critical subsurface utilities. Extensive mapping and coordination with the City of Austin were required to confirm alignments, resolve conflicts, and maintain service continuity during construction.
Geotechnical Considerations
Subsurface conditions in Austin vary from limestone and dolomite formations to silty soils near the river. At the same time, the tunnels were designed to run primarily through hard rock, transitioning into softer ground required essential careful risk assessment. Shafts were supported with temporary liners to protect workers and maintain stability until permanent structures could be installed. By aligning the tunnels to maximize rock cover, designers reduced settlement risks to adjacent buildings and infrastructure.
Environmental Permitting and Compliance
Beyond engineering and logistics, the project had to comply with a suite of environmental regulations. Because tunnel construction involves significant earth disturbance, activities were regulated under the National Pollutant Discharge Elimination System (NPDES) and its Texas equivalent, the Texas Pollutant Discharge Elimination System (TPDES), along with TXDOT oversight.
· Stormwater Pollution Prevention Plans (SWPPPs): The contractor was required to prepare and implement SWPPPs for each staging area and shaft location. These plans identified potential pollution sources such as sediment, fuels, and construction materials, as well as implementing BMPs to prevent discharges. Additionally, the team asked themselves, “What is the best stabilized construction entrance system we can deploy, leading to the inclusion of FODS Reusable Construction Entrance System.
· Erosion and Sediment Controls: Given the size of the shafts and remediation stockpiles, erosion controls were critical. Perimeter silt fencing, inlet protection, and stabilized access points were installed to prevent sediment from leaving the site.
· City of Austin Watershed Protection: Local ordinances required additional measures to protect downstream waterways, notably Lady Bird Lake and the Colorado River. Coordination with the City's Watershed Protection Department ensured that BMPs met or exceeded municipal standards.
Role of Trackout Control
One of the most visible compliance challenges on a heavy civil construction site like this is the risk of vehicles tracking mud and debris carried out by the construction activity. In a dense urban corridor, even small amounts of sediment can pose safety hazards for pedestrians and motorists, leading to regulatory violations. The project, therefore, prioritized using FODS, the best trackout control mats, which have been proven effective and are a low-maintenance trackout solution that functions across varied soil types and in all weather conditions.
With strict regulations and timelines in place, SAK chose to follow the BMP handbook and implement FODS Trackout Control Mats, which offered more reliable sediment control than traditional rock entrances. By addressing trackout proactively, the team reduced the risk of fines, minimized street sweeping frequency, and maintained good community relations throughout the prolonged construction period.
Key Stakeholders & Expertise
The success of the SAK Tunnel Project rests not only on engineering design but also on the collaboration of a multidisciplinary team of contractors, consultants, and public agencies. Each stakeholder brought a specialization in something necessary to complete this urban undertaking.
Owner and Program Oversight
The Texas Department of Transportation (TxDOT) serves as the project owner, providing overall oversight, funding, and integration with the larger I-35 Capital Express Central program (CapEx). TxDOT’s role extends beyond conventional construction management. Because the tunnel system is the cornerstone of the $4.5 billion infrastructure upgrade. TxDOT has treated it as an essential element, requiring constant attention to scheduling, budgeting, and risk management. Funding support from the Capital Area Metropolitan Planning Organization (CAMPO) reflects the regional priority placed on both flood protection and mobility.
Contractors
A joint venture between SAK Construction and J.F. Shea Construction is executing the tunneling and shaft excavation.
· SAK Construction, based in Missouri, is recognized nationally for its trenchless technology and tunnel expertise. The company’s experience in underground infrastructure projects positioned it as a strong partner for managing the complex logistics of deep-bore stormwater delivery.
· J.F. Shea Construction, headquartered in Walnut, California, is one of the most renowned civil contractors in the United States. With a history that includes landmark projects such as the Hoover Dam, the Golden Gate Bridge, and the San Francisco-Oakland Bay Bridge, Shea provided technical expertise and a historically proven record of delivering megaprojects under demanding conditions.
Together, the SAK–Shea joint venture combined regional stormwater expertise with high-profile national civil experience to form the ideal partnership for the job.
Pump Station Contractor
The pump station is being constructed by Webber, LLC, a Texas-based contractor and member of the Ferrovial group. With its North American headquarters in Houston, Webber specializes in large-scale civil works and water infrastructure. The company’s integration with PLW Waterworks further strengthened its capacity to deliver a complex hydraulic facility with multiple high-capacity pump units, electrical systems, and discharge structures into the Colorado River.
Engineering and Design Consultants
· BGE, Inc., also headquartered in Houston, provided much of the design for the tunnels and pump station. BGE’s engineers developed the alignments, hydraulic models, and structural systems required to ensure both functionality and resilience under long-term and scalable service conditions.
· Schnabel Engineering, based in Glen Allen, Virginia, contributed expertise in feasibility review, shaft and tunnel support, and risk management. Their role ensured that geotechnical challenges, support systems, and contractor methodologies were evaluated in advance, reducing the likelihood of cost, accident, and schedule overruns.
Collective Expertise
The combination of TxDOT leadership, CAMPO funding, and contractors like SAK and Webber, as well as designers and engineers from BGE and Schnabel. Few projects require such a balance of skills, including design, geotechnical engineering, heavy civil contracting, and environmental compliance, all working in harmony. The SAK Tunnel Project demonstrates that no single firm or agency can deliver such infrastructure alone; instead, it requires the deliberate alignment of expertise across the public and private sectors.
Timeline & Budget
The drainage tunnel program officially broke ground in May 2025, with shaft excavation beginning at priority sites along I-35 and Cesar Chavez Street. The project is anticipated to span approximately four years, with final tunnel lining, pump station commissioning, and surface restoration scheduled for completion in 2029.
While TxDOT has not yet issued an official statement, the larger Capital Express Central program is budgeted at approximately $4.5 billion, making it one of the most significant transportation investments in Texas' history. Within this program, the stormwater tunnels and pump station are considered critical infrastructure; without them, the highway reconstruction cannot proceed.
The cost framework reflects not only excavation and construction but also risk management contingencies, extensive utility coordination, and environmental mitigation. TxDOT has prioritized this package early in the program schedule to mitigate risks associated with downstream phases of highway work.
The financial value of the tunnels extends far beyond the direct construction costs. By providing robust flood protection, the project safeguards billions in long-term highway investment. Without adequate drainage, any surface-level reconstruction would face unacceptable risk of flood damage, roadway closures, and emergency response costs. Thus, while difficult to isolate in a budget line item, the tunnel system is in many respects the most critical investment within the Capital Express Central program.
Construction BMPs: Trackout Control with FODS
In addition to its engineering complexity, the tunnel program demanded rigorous stormwater compliance. Among the most persistent environmental risks was the potential for trackout, where sediment, clay, and debris carried by truck tires or tracked equipment could be deposited on adjacent streets.
Limitations of Traditional Rock Entrances
Historically, projects of this scale relied on stabilized construction entrances composed of crushed aggregate. While effective under limited conditions, rock entrances posed three significant drawbacks for the SAK Tunnel Project:
1. Maintenance Intensity: Rock entrances degrade quickly under high traffic volumes, particularly when soils are saturated. Maintaining their effectiveness requires constant grading, stone replacement, and supplemental sweeping to ensure optimal performance.
2. Embedded Material: Heavy trucks often press aggregate into underlying soils, reducing the stone’s effectiveness and creating rutted, muddy approaches that worsen rather than prevent trackout.
3. Cost Profile: Continuous stone replacement drives up both material and labor costs. On a multi-year urban project with multiple shaft sites, the cumulative cost of aggregate could reach hundreds of thousands of dollars.
FODS Trackout Control Mats
To address these limitations, the project team deployed FODS Trackout Control Mats at staging areas and shaft access points. Constructed of high-density polyethylene, the mats feature raised pyramids designed to use the vehicle's tire pressure to dislodge sediment and debris from tires and tracks. Unlike rock, which deteriorates over time, FODS mats deliver consistent performance across all soil and weather conditions.
Key benefits realized on the SAK Tunnel Project include:
· Reduced Maintenance Labor: Once installed, mats require minimal maintenance. Set aside 10 minutes from your regular cleaning schedule and use the FODS Shovel, Sweeper Truck, or contained water to clean the FODS Mats so they are prepared for the next day. Placing composite mats frees up site crews from constant aggregate management, allowing labor to be reallocated to other, more vital operations.
· Cost Savings: By avoiding recurring stone deliveries and disposal, the project will reduce material expenditures and waste. Over the multi-year duration, contractors such as SAK are saving money long term because FODS Trackout Control Mats are designed as an easily deployable solution. As the project progresses, SAK can move and relocate the mats through the phases of the project, drastically cutting installation, refresh, and remediation costs.
· Urban Compatibility: Mats could be installed on pavement and later removed without surface damage, a significant advantage in a dense downtown corridor where many shaft sites are located on existing streets or lots.
· Regulatory Compliance: With trackout effectively minimized, contractors maintained compliance with TxDOT’s SWPPP requirements and avoided potential fines or remediation costs.
Quantifying the Difference
A typical rock entrance for a high-traffic shaft site might consume 80–100 tons of stone for the initial, and an additional 10-15 tons per month during the wet season, delivered at a cost of $35–$50 per ton in the Austin market. Over the scheduled four years, this equates to hundreds of thousands of dollars in direct expenditures and waste exclusive to trucking, disposal, and maintenance labor.
By contrast, FODS mats represent a one-time investment that is extremely easy to deploy as well as to relocate, and provides a 10-year service life.
Results & Anticipated Benefits
Although construction remains ongoing, the benefits of integrated BMPs like FODS are already becoming clear.
Interim Construction Benefits
· Cleaner Streets: Austin's urban core can expect a reduction in trackout, compared to past projects, maintaining community goodwill and minimizing complaints from adjacent businesses.
· Regulatory Compliance: SWPPP inspections have reported fewer deficiencies at shaft sites equipped with FODS mats, reducing the administrative burden of corrective actions.
· Operational Efficiency: By minimizing trackout and site maintenance, contractors have been able to allocate more resources to excavation and tunneling operations, thereby keeping the project on schedule.
Long-Term Flood Protection
Once completed, the tunnel and pump station system will fundamentally change the flood profile of downtown Austin:
· Flood Reduction: Hydraulic models indicate significant reductions in surface flooding along I-35 during 10-, 50-, and 100-year storm events.
· Resilient Highway Design: Depressed sections of the reconstructed freeway will remain serviceable during storm events, avoiding costly closures and emergency detours.
· Neighborhood Protection: Adjacent communities, which have long been impacted by street flooding, will benefit from improved drainage and reduced risk of property damage.
Community and Economic Benefits
The tunnels also support Austin’s broader economic vitality. By ensuring reliable mobility along I-35, the project safeguards both local commuter traffic and interstate freight operations. The reduction in flooding risk enhances resilience for downtown businesses, state offices, and the University of Texas campus. In this way, the tunnels serve not just as hydraulic infrastructure but as a foundation for continued economic growth and urban redevelopment.
Engineering Resilience in the Heart of Austin
The SAK Tunnel Project illustrates the scale of infrastructure required to protect and modernize America's most congested highways. By combining two 22-foot-diameter stormwater tunnels, multiple drop shafts, and a 101,000-gallon-per-minute pump station, TxDOT and its partners are delivering one of the most ambitious drainage systems ever undertaken in the state of Texas.
The project demonstrates how engineering, constructability, and environmental compliance must converge in dense urban settings. From geotechnical alignment to traffic management and SWPPP adherence, the system reflects years of planning and collaboration among TxDOT, CAMPO, SAK Construction, J.F. Shea, Webber, BGE, and Schnabel Engineering.
Within this effort, FODS Trackout Control Mats have provided a small but critical innovation. By replacing rock entrances with a reusable, durable, and cost-effective alternative, the project has realized meaningful savings in labor, materials, and compliance risk. The mats’ contribution underscores how thoughtful selection of construction best management practices can improve outcomes on even the largest and most complex projects.
Ultimately, the tunnels are not simply an underground conduit for stormwater—they are the foundation upon which the new I-35 will be built. By addressing chronic flooding first, TxDOT is ensuring that the Capital Express Central program can proceed with confidence, delivering a safer, more reliable, and more resilient transportation corridor for Austin and the state of Texas.
