REDEFINING UNDERGROUND ENGINEERING: INTECSA AND THE SR99 MEGAPROJECT

How Intecsa's participation redefined the game in one of Seattle's most ambitious tunnel projects

ALASKAN WAY TUNNEL SR99, SEATTLE (USA)

The SR 99 Tunnel in Seattle is the largest-diameter tunnel ever built in the world. It runs beneath the city’s historic center and posed enormous technical challenges due to its complex geotechnical conditions and coastal location well below sea level. It remains one of the most ambitious urbans tunneling projects in U.S. history—defined not only by engineering audacity but also by a fundamental rethinking of risk, cost, and structural response.

The project was led on-site and remotely by Pablo Fraile, Head of Structural Engineering, and Stefano Murgia, Head of Tunnels and Geotechnics. Together, they coordinated a multidisciplinary team that combined European technical depth with U.S. execution standards. Their leadership was instrumental in aligning Intecsa’s approach with the complexity of the project, ensuring that technical excellence went hand-in-hand with strategic clarity.

Intecsa’s Tunnels and Structures departments became actively involved in the project from its earliest stages. The company worked closely with HNTB to prepare the technical documentation for Seattle Tunnel Partners (STP), a joint venture between Dragados-USA and Tutor Perini. 

“From the start, we knew this required more than technical precision—it demanded total strategic alignment with the city’s constraints,” explains Pablo Fraile.

After STP awarded the contact in 2010, Intecsa was invited to participate in the design phase as a subcontractor. A coordinator was stationed in Seattle throughout, supported by a team of specialists during peak phases. Intecsa’s role was not simply for support, but also it assumed technically demanding high-impact role from the start.

One of the most impactful contributions came during the tender phase. The firm detailed settlement assessment drastically reduced the number of buildings requiring mitigation—from over 40, as initially projected, to just 4. This reduction was not only technically robust but also budget-transformative, ultimately playing a decisive role in STP’s successful bid. The technical proposal submitted by HNTB received significantly higher scores than competing teams, tipping the scales in favor of the consortium.

“Our goal wasn’t just to reduce costs,” explains Pablo Fraile, “It was to bring our full technical expertise to the table—to anticipate risks, optimize every structural decision, and make every structural choice count.”

Throughout the design phase, Intecsa applied advanced modeling strategies that allowed for cost-effective, safe, and precise designs. The firm was tasked with analyzing settlement effects caused by the tunnel boring machine (TBM) on a wide range of structures, including approximately 40 buildings—many of them historic—as well as the Alaskan Way Viaduct and tunnels such as BNSF and Pike Street Adit.

In the design phase, Intecsa maintained the same high standards. While simplified empirical models were used for minor cases, many critical buildings required fully developed 3D FLAC models, including structural modeling, allowing precise analysis of settlement impacts and the effectiveness of proposed mitigation strategies. This rigorous approach helped optimize designs, balance costs, and ensure structural safety.

An outstanding example of this was the approach to reinforcing the Alaskan Way Viaduct. Initial elastic analyses had forecast dangerously high demand/capacity ratios, implying major structural work and traffic shutdowns. Instead, Intecsa implemented a nonlinear pushover analysis, simulating 38 plastic hinges within the viaduct. This method confirmed the structure’s stability and led to a highly efficient reinforcement solution—avoiding major disruptions and significantly balanced costs.

“We weren’t just facing a tunnel. We were facing a city’s history, its geology, and its limits. This project demanded everything we had—technically and creatively,” says Stefano Murgia, who describes SR99 as one of the most technically demanding of Intecsa's tunnel department.

Conclusion: Engineering Value Beyond Borders

Intecsa delivered significant value—both technically and financially—by contributing to the optimization of one of the most complex urban tunneling projects in the U.S. Its role in the SR 99 Tunnel stands as proof of what can be achieved when engineering rigor, transparent communication, and advanced structural modeling converge to meet challenging conditions. The project highlights how effective collaboration between international and U.S. teams can elevate outcomes through shared expertise and a unified focus on performance.