At the heart of New York City’s ever-evolving transit landscape, STV’s geotechnical and tunneling engineering team is playing a pivotal role in a groundbreaking project set to transform the way passengers move through Midtown Manhattan.
As part of a dynamic design-build team led by Skanska, STV served as the designer- and engineer-of-record for a new passenger tunnel connecting the No. 7 Subway Line’s 42nd Street Station to the iconic Grand Central Terminal. The project was introduced to the public this week.
This project represents more than just the addition of a transit link – it embodies a leap forward in urban infrastructure, seamlessly integrating cutting-edge engineering with the demands of one of the world’s busiest transit hubs. Our approach prioritizes not just functionality but also passenger experience so that the new tunnel enhances convenience, safety, and operational efficiency.
Navigating a Complex Urban Landscape
Located beneath the bustling intersection of Manhattan’s East 42nd Street and Lexington Avenue, this project is designed to improve circulation and accommodate the city’s growing transit demands. By significantly increasing ingress and egress capacity, the passageway will alleviate congestion at a crucial juncture. However, constructing a tunnel in a city as dense as New York presents unique challenges – balancing extensive rock excavation and tunneling within a highly complex urban environment of heavy traffic, existing businesses, and historic structures.
Through cutting-edge technology and meticulous planning, our team is transforming these challenges into opportunities for innovation. Utilizing state-of-the-art 3D Finite Element Analysis (FEA), we have designed a tunnel that prioritizes stability and public safety under shallow rock cover while minimizing ground settlement and construction impacts on existing utilities, including sensitive steam pipelines.
A Delicate Balance: Engineering in a High-Stakes Environment
Positioned beneath the intersection of 42nd Street and Lexington Avenue – adjacent to Grand Central Terminal and the Chrysler Building – the tunnel navigates a subterranean maze of underground utilities and infrastructure. The geological conditions add another layer of complexity, with only 6 to 10 feet of rock cover above the tunnel’s 17-foot-high crown, resting beneath a 30 to 34 feet deep soil layer. While excavation methods focus on stabilizing the rock mass, reinforcements must also secure the overlying soil to mitigate settlement risks and safeguard vital surrounding infrastructure.
This project is emblemized by how the team is carefully threading a passage between existing structures while establishing a direct and efficient connection between the Flushing Line 42nd Street Station and Grand Central Terminal’s subway station.
Innovative Techniques for Urban Tunneling
To execute this delicate excavation, STV is employing the Sequential Excavation Method (SEM), a highly adaptable approach that allows for controlled excavation in challenging ground conditions. This method, coupled with specialized equipment such as Brokk-mounted drills and hydraulic machinery, enables us to perform precise drilling, breaking, and excavation.
To maintain stability throughout the excavation process, the tunnel is divided into sections: a top heading and bench configuration. This phased approach facilitates the installation of initial ground support without requiring lifts or scaffolds, enhancing both safety and efficiency.
Our advanced support systems include:
- Rock dowels and lattice girders for structural integrity
- Fiber-reinforced shotcrete for immediate reinforcement
- Rock spiles for pre-supporting the ground ahead of excavation
- Real-time geotechnical and structural monitoring to ensure safety and minimize impact on adjacent infrastructure
Once excavation is complete, a smoothing layer of unreinforced shotcrete is applied, preparing the tunnel for a comprehensive waterproofing system. The final structural work includes the installation of a reinforced concrete permanent liner to form the finished tunnel lining. The project will culminate in architectural finishes and system installations that will bring the passageway to life.
STV’s structural engineers, part of the Tunneling and Geotech Centers of Excellence, developed an innovative design solution to protect the public and minimize construction impacts on MTA’s services during construction. A temporary shield was designed and installed in the station cavern. All tunneling activities, including breaching a more than century-old unreinforced structure, were conducted under live traffic conditions without causing any interruptions.
Additionally, as part of this project, the entire fire alarm system of the Grand Central Terminal complex was redesigned and upgraded.
Beyond Construction: A Commitment to Excellence
Our role in this transformative project extends far beyond construction. We have conducted extensive geotechnical and geophysical exploration, compiled a Geotechnical Baseline Report for Construction (GBR-C), and developed a Construction Impact Assessment Report (CIAR) to guide decision-making and mitigate risks.
Every aspect of this project has been carefully designed with safety, efficiency, and sustainability in mind. Our approach balances cost-effective engineering solutions with long-term durability, ensuring that this passageway not only serves the present needs of New York’s transit system but also stands resilient for generations to come.
With the successful completion of this ambitious endeavor, we celebrate our commitment to engineering excellence, continuous innovation, and enhancing the fabric of New York City’s transit infrastructure. This tunnel is more than just a passageway – it is a testament to the power of engineering to shape the future of urban mobility.
