Trench Engineered Systems Case Study

Largest Hydraulic Struts in America Speed Historic Tunnel Project

An engineered hydraulic shoring solution saves time and money for a major contractor building a tunnel in the Chesapeake Bay.

A historic expansion of the iconic Chesapeake Bay Bridge-Tunnel required digging a one-mile tunnel 134 feet below the surface using a tunnel boring machine. The logistics were challenging: The excavation sites for the machine’s launch and receiving pits were extremely small manmade islands in the bay, and the project timeline was tight.

United Rentals identified an opportunity to replace traditional weld-in-place shoring with hydraulic shoring. The engineered solution, which leveraged the largest hydraulic struts available in America, improved constructibility, enhanced safety and saved the contractor time and money. Just-in-time delivery solved for space constraints.

“Deploying high-capacity hydraulic struts gave the contractor a unique advantage, delivering performance that couldn’t have been met with conventional domestic shoring equipment.”

-Brett Sondergard, Region Project Development Manager, United Rentals

A major infrastructure contractor specializing in heavy civil construction took on a historic, $850 million undertaking: building an underwater tunnel to expand the capacity of the Chesapeake Bay Bridge-Tunnel. The iconic 17.6-mile bridge and tunnel complex, considered an engineering marvel, needed separate tunnels for north- and south-bound traffic to increase safety and accommodate a growth in traffic volume.

The new one-mile tunnel, which will sit 134 feet below the water surface, will be the first transportation tunnel in the Mid-Atlantic carved with a tunnel boring machine.

To launch the 6-million-pound machine, named Chessie, the contractor first had to construct a launch pit. The challenge: The construction site was a small island in the bay, only about 5.25 acres, and the project timeline was tight. Shoring a deep excavation in the face of extreme space limitation while keeping the project on schedule and budget required outside-the-box thinking.

Hydraulic shoring reduces installation time and complexity

From the earliest stages of planning, United Rentals collaborated closely with the project partners, helping them evaluate the initial shoring plan, which called for traditional weld-in-place steel shoring.

Welded-strut shoring would require welding at every level, which would take significant time and pose considerable safety risks. It would also require the contractor to obtain hot work permits and hire specialized labor during a welder shortage, potentially further slowing progress.

United Rentals quickly identified an alternative approach that would improve efficiency and overall system performance: a custom-engineered hydraulic shoring system capable of handling high loads. Modular in nature, the system would shrink installation and removal time and help solve constructibility challenges.

Massive, high-capacity struts support the largest loads

The hydraulic shoring plan called for 250-tonne, 375-tonne and 750-tonne heavy-duty struts to support the excavation. The 750-tonne strut is the largest in North America, currently available only from United Rentals. It is designed to handle massive loads for the deepest excavation projects. The tunnel project marked the first U.S. deployment of these high-capacity struts.

Limited storage and laydown space at the excavation site created a logistical hurdle. United Rentals rose to the challenge by assembling the struts offsite and delivering them on a will-call basis as they were needed and crews were ready for installation. Truck-to-excavation installation meant the struts never had to be stored on the site. Congestion and crane time were reduced in turn.

Meeting preloading and load monitoring requirements

United Rentals offered technical support for the installation of the system. To meet preloading requirements, United Rentals’ engineers created a detailed chart based on the shoring design plans that calculated the appropriate force to apply to each strut during initial use. Onsite advisors then stoked out each strut to its specified pressure.

For enhanced safety, United Rentals installed load pins on key struts. The pins monitored loads in real time, giving the project team early visibility into unexpected load changes.

Successful outcome led to second contract

United Rentals’ innovative shoring solution helped the contractor build the launch pit on time and on budget. As a result, the customer contracted United Rentals to provide a similar solution for the receiving pit, which was also completed on time and on budget.

“Deploying high-capacity hydraulic struts gave the contractor a unique advantage, delivering performance that couldn’t have been met with conventional domestic shoring equipment,” said Brett Sondergard, United Rentals’ regional project manager.

Chessie has since finished her work, breaking through the receiving pit in January 2025. The tunnel extension is on track to open in early 2028.

“The success of this project shows that contractors can count on United Rentals to execute what we commit to,” Sondergard said. “Our size, experience and innovative shoring products provide a competitive advantage on the largest excavation projects.”