An Unprecedented Hydrodemolition Project
Published: 28/12, 2023
The Fort McHenry Tunnel is a unique structure that connects the harborside Locust Point and Canton areas of Baltimore. It stands out because it crosses under the Patapsco River just south of historic Fort McHenry. The four-tube tunnel is a vital link in Interstate 95 — the main North-South interstate highway on the East coast — that spans eight lanes and sees more than 100,000 vehicles pass through each day. With large structures like the Fort McHenry Tunnel comes considerable ongoing maintenance. A current project involves removing and replacing two 4-in (101.6mm) conduits buried in a mile-long wall in the ventilation plenum below the roadway. The conduits house 15,000V cable that powers the tunnel facility. The scope of the work also includes 26,000 ft3 (736m3) of concrete rehabilitation to the underside of the roadway slab and side walls supporting it.
Hydrodemolition was specified for the project due to health and safety requirements in the tunnel beneath the river. Dust from impact demolition — such as jackhammering — poses several serious risks. First, flammable material is prohibited and could cause flames to quickly spread in the event of a fire. Second, giant fans force air through the tunnel during traffic jams and emergency situations, such as a car fire. If there was a layer of dust when the fans kicked in, it would cause whiteout conditions with silica-laden dust. These situations could be disastrous for everyone in the tunnel. With hydrodemolition, there is no dust due to using water to remove the concrete.
The Maryland Transportation Authority set out to choose a contractor for what would prove to be a daunting task. The contractor would need to employ Hydrodemolition methods to remove a large amount of material in the ventilation plenum that was a cold, windy, dirty environment. Freyssinet, Inc. was awarded the project with a bid of nearly $30 million. Freyssinet’s portfolio highlights extraordinary projects across several continents. The Fort McHenry Tunnel job stands on its own, however. Not only is the worksite beneath the tunnel roadway in a cold, dirty, wet, environment, but being restricted to working at night throws more obstacles into the mix, such as limited access to the worksite and difficulty obtaining supplies not already on hand. Ceilings of the workspace only stretch around 6 ft (1.8m) tall at their highest point, making powerful but compact equipment a necessity. Campbell formed a fleet of Aquajet machines featuring three Aqua Cutter 410As to tackle the daunting project. With its compact size and electric motor, the 410A is the ideal Hydrodemolition robot for a limited-access, underground setting like the Fort McHenry Tunnel. The 410A’s hydraulic and articulated arm features two rotating joints and three extensions that allow the hood to tilt and remove concrete on the tunnel wall and overhead roadway deck. The 15-man crew uses the Aqua Cutters to remove portions of the wall 30 in (762mm) tall and approximately 12 in (305mm) deep for a total distance of 4 miles to access the cables and conduit. Each shift, they remove about 6 cubic yards (162 cubic feet) of concrete and replace the conduit and rebar each shift. The final step is to finish the repair with shotcrete. Two robots run during the shift while the third is rotated out for maintenance. To get the Aqua Cutters into the worksite, Freyssinet lifted them in with a crane through access hatches into the pump room at the bottom of the tunnel structure. They had to remove the hoods and cutting heads to navigate the narrow passages, stairways and doors with 1 in (25.4mm) of clearance. With the challenges getting equipment in and out of the tunnel, routine maintenance is performed inside the tunnel and can be completed during the shift without affecting production Crew members are typically spread more than 1,000 feet apart during a shift in the name of safety. The shape of the tunnel creates challenging angles on the work surface. It’s a semicircle that turns vertical at the walls with only two short flat spots on each end—like working in a pipe cut in half, creating an angle on which the robots travel. Freyssinet built a rail system to ensure the robots stay on track during operation.
The Key to the Operation
Freyssinet performs hydrodemolition in the tunnel below the roadway during 10-hour overnight shifts, six of which are dedicated to Hydrodemolition. The other four hours are used to set up, tear down and move the mobile operation. The road is closed during the shift and then reopens to traffic in the morning.
As a result, they must move any equipment positioned on the roadway, including a tool trailer, debris trailer, two high-pressure pumps towed by a semi, a 5,500-gal (20,819-liter) tanker truck, a concrete pump, a 500-gal (1,829-liter) water trailer, four generators, a flatbed truck loaded with concrete and their Aquajet EcoClear water treatment system. The EcoClear is a self-contained unit in a 20-foot Conex container that is mounted on a lowboy trailer. This setup allows the crew to pack up and drive off at the end of each shift. All hoses for the water and power run through 4-in (101.6mm) fresh air vents from the roadway down to the tunnel basement.
Hydrodemolition projects generally require methods of collecting and treating the water used in the process. Throughout the six-hour work shift, the 410As each push out 42 gallons (159 liters) of water per minute. Campbell purchased the EcoClear specifically for this task. The EcoClear features a compact footprint and user-friendly operation and treats wastewater on-site with minimal monitoring.
The Freyssinet crew made a surprising discovery about the EcoClear once they had their system up and running. Their water source stems from the tanker truck. Without recycling water, the crew would use about six tankers per shift, but they’re only using about one. This provides not only considerable time savings, but also cost savings in both water and fuel consumption. The EcoClear recycles the water five times during the work shift. Work on the Fort McHenry Tunnel began in April 2021 and has an expected completion date of June 2026.
