To address the issue, CSX hired Skanska-Fay JV as its progressive design-build contractor via a $242-million contract to increase the clearance from top-of-rail to 21 ft. The JV worked with lead designer Hatch and local firm WRA to advance the design. Members of the project team say the flexibility that the progressive design-build model provides has been essential for optimizing their work and meeting the owner’s needs.

“This job is pretty ideal for progressive design-build because there’s all these different options, says Mike Goetz, project manager at Skanska. “It’s not like we have one thing to build, and there’s only one way to do it. There’s many different ways to do this and achieve CSX’s goal.”

They considered options such as notching the crown in some areas and notching the invert or both in others. But as they took core samples of the brick to check if there would be enough courses of brick left after notching, they found too much variation, says Tom Costabile, Skanska’s construction manager on the project. 

“You could take a core right here, and then 10 ft away you’d have a different thickness, so it was hard to have schedule certainty there,” he says. “There’s a lot of risk. So we proposed doing a full invert replacement, which gives the owner a nice uniform invert as well.”

Plans also evolved based on planned outages. The team scrapped an early plan to work during weekend and overnight tunnel closures, which would have involved repeatedly mobilizing and demobilizing to complete work in late 2027. Ultimately, CSX agreed to a full shutdown of the tunnel starting in February and running through October, with substantial completion now expected this fall—“huge savings there,” Costabile says.

With so many improvements to the plan throughout design and construction, the team adapted “improve, adapt and overcome” as their slogan for the project. 

“We had a lot of time ahead of this project to do planning, and then once we actually got boots on the ground, a lot of that planning had to be modified as we found what worked and what didn’t work,” Costabile says. “We continue to adapt to this day. Almost every week we’re coming up with something to do better, so our cycle times continue to decrease and we’re going to continue improving probably until the last day of the project. It’s worked out very well.”

1895 Tunnel

Map courtesy Skanska

The Howard Street Tunnel is about 8,700 ft long, running under downtown Baltimore from Camden Yards in the south to the former Mt. Royal train station in the Bolton Hill neighborhood in the north. It was originally completed for the Baltimore and Ohio Railroad in 1895. Most of the Howard Street Tunnel is mined, but it also has a cut-and-cover section and a concrete box section added in the 1980s to accommodate construction of a highway. 

The project also covers track work extending about 700 ft on the south side and 2,300 ft on the north side, including the 265-ft-long Mt. Royal Tunnel. 

Early work included collecting LiDAR scans, utility preparation and establishing drainage volumes, because the project’s scope includes improving drainage in the tunnel. It’s a free-draining tunnel, designed to allow water to leak in, but the team needed to ensure the water coming into the tunnel has somewhere to go so it doesn’t end up in the track bed and break down the ballast. About 550 gal of water comes through the tunnel per minute. They ended up with a pair of drains running along the walls and two more trough drains below the tracks. The water discharges at Camden Street.

New Invert

Once complete, the new tunnel invert will consist of 1,188 precast invert slab (PCIS) segments. Each PCIS segment is 6 ft long, with a flat bottom and two recessed trough drains, and wings that come up at an angle, measuring about 18-19 ft across from wing to wing, depending on the width of the tunnel at a given location. The team operates in cycles, installing three segments at a time. Improvements to their means and methods—”improve, adapt and overcome”—have allowed the team to increase their rate from two cycles per day to three, Goetz says. 

At the start of each cycle, the team uses a vacuum de-watering system on the soft soil found in most of the tunnel, typically out 150 ft ahead and 50 ft back. Then they demolish the existing invert, excavating 21 ft ahead. 

Photo by James Leggate/ENR

The project’s restrictions and unusual conditions required specialized equipment, especially as the team found hi-rail equipment, dual-mode vehicles that can operate on or off tracks, “is in short supply, and it is also notoriously unreliable,” Costabile says. For demolition, they initially started with a typical hydraulic hammer, but found it relatively inefficient. They ended up using three remote-controlled Brokk demolition robots, including the two largest models the company has made. Costabile says they’re extremely efficient at demolishing brick masonry like that found inside the tunnel.

“They’re kind of like an endangered species of equipment,” Costabile says of the larger Brokk robots. “There are not a lot of them globally available.” 

They then use crawler carriers to remove excavated material. The chassis has a rotating point, so the equipment can be driven in, rotate on an axis, get loaded with material and then drive straight out without having to back up or turn around. The crawlers also have low ground pressure because of the width of their tracks, so they don’t tear up the existing invert or create a muddy mess. 

The team needed an excavator with a lot of breakout force, but found most equipment was too large with boom knuckles that would have hit the side of the tunnel, and they would not have been able to rotate inside the tunnel. They found a Mecalac excavator with a unique multi-piece boom that allows it to work in unusually constrictive geometries, according to Costabile. It’s a piece of equipment he says is popular in Europe, but hard to find in the U.S. It features a sandwich coupler system, which allows the team to switch out the tilt rotator for a hydraulic hammer “without having a mechanic disconnecting hoses and everything,” further boosting efficiency, Costabile adds. 

Photo courtesy Skanska

After excavation, the team then compacts and prepares the subgrade with geotextile fabric and crushed stone, plus four leveling pads that will support the PCIS segments where they meet. 

To place the PCIS segments, a conventional crane or gantry system would not have worked in the tunnel, especially with the short work windows between trains under the original plan. So the team commissioned a pair of custom gantries, which Costabile calls the project’s “crown jewels.”

Kelley Engineered Equipment designed the gantries, which weigh about 225,000 lb. Each has a flat rail car with a 120-ft gantry rail above it, a 30-ft cantilever for moving the segments ahead of the car and a 15-ton electric hoist. They are designed to be “as simple as possible,” Costabile says, so that it’s easy to replace components as needed because it’s such a critical piece of equipment for the project.

The team ordered two with an early plan of having two headings during short work windows between trains, but with the full tunnel closure have opted to instead use one heading, with equipment moving in from the east portal, near the former train station, and excavated material going out via the west portal near Camden Yards. Abingdon, Va.-based Wolf Hills Fabricators built the gantries. 

Once three PCIS segments are in place, crews then grout underneath them through seven grout ports. Next, they place rebar and concrete for the extended wings in order to reestablish an inverted arch shape for the invert, which transfers the two footers’ loads. 

Rendering courtesy Skanska

Concrete Needs

With work proceeding 24/7, the team needs to be able to place concrete at any given point on any given day, which ruled out using a local ready-mix plant. But they also don’t have enough room for a batch plant, so the team obtained a Fiori concrete batching vehicle. The team also got super sacks full of bulk concrete material, with a custom mix designed with some additives to accelerate the reaction for the high early strength needed for the project. The CBV picks up a super sack, discharges it into its drum and then adds liquid and then provides concrete right inside the tunnel where it’s needed. 

“We need early strength in order to meet our schedule,” Goetz says. “We cannot start that next 21 ft until that concrete is cured to at least 1,000 PSI. We place the concrete, wait about an hour and we go right into the next cycle.”

Photo by James Leggate/ENR

To accelerate the process further, the team started using shoring posts—basically screw jacks—to bear upon the footer on one side and the segment on the other, allowing them to instantaneously transfer the load across and through the segments, so crews can advance to the next cycle right away. Once the gantry passes, the team brings in concrete via hi-rail truck and they place a more conventional mix. That change is part of what helped the team up their rate to complete a third cycle in a day. 

The final step of the cycle is using the gantry to fly a track panel down to the newly installed PCIS segments. Then the team advances the gantry and starts the next cycle. 

The track panels use timber crossties, rather than direct fixation or steel ties. That way it’s easier for CSX to replace a section of rail, should it ever become damaged, Costabile says. 

“They can cut out the section of track that’s damaged, put in track panels that they have staged throughout their network, and get it operational again,” he says. 

The tunnel’s closure to trains is scheduled to end in October, and substantial completion is scheduled for this year. So far, more than 400,000 labor hours have gone into the project, which is more than 80% complete. 

Derek Mihaly, construction director at CSX, says work is also advancing on the other remaining obstructions to double-stack service on the railroad’s core intermodal network. Around the end of last year, the railroad cleared several projects in Philadelphia. One site in Delaware has two bridges that need to be raised. And then in Baltimore, there are some smaller tunnels and three overhead bridge replacements that need to be addressed in addition to the soon-to-complete Howard Street Tunnel reconstruction.