By Kevin Wilcox
Liberty University utilizes a novel placement method to install large concrete tunnels beneath working railroad lines.
Thick reaction walls, foreground, are used to pull the tunnels under the railroad tracks, approximately 7 in. at a time. When complete, the tunnels will open up access to the campus. Courtesy of Liberty University, Inc.
December 10, 2013—Liberty University has reached the halfway point in a project aimed at replacing an at-grade railroad crossing with two large concrete tunnels, eliminating a traffic choke point at the edge of the growing campus. The project employs a novel approach by which the tunnels are being jacked into place beneath one of the busiest sections of rail line in the southeastern United States.
"Any at-grade crossing is dangerous both for vehicle traffic as well as pedestrians," says Brad Barber, the design coordinator in Liberty University's Planning and Construction Department. The crossing has become a growing concern because the campus is growing at the same time train traffic is increasing.
"We have a considerable amount of freight that comes through here," Barber says. "Recent tunnels completed through the mountains now bring a considerable amount of the traffic coming out of Chicago and the Midwest down to Atlanta." Amtrak trains also utilize the lines.
The university considered a temporary bypass bridge created by using precast-concrete box culverts and such standard tunneling methods as driving two 20 ft diameter steel tubes beneath the track, according to Maggie Cossman, P.E., M.ASCE, the corporate engineer for the university. But this method had drawbacks. "As you can imagine, with the circular shape of the steel tubes, we lose a lot of space because we fill the bottom in to make a wide enough travel path, and we only end up with two lanes, when we really need four lanes," Cossman says. "We were looking for other methods so we could get the four lanes through economically."
Because the tunnels will be placed beneath working rail lines of the Norfolk Southern Railway, the university had to have the company's approval on any tunneling project. The university hired the Syracuse office of Brierley Associates, an engineering firm specializing in tunnels, to help resolve the design challenge. The solution that the team devised was to cast two heavily reinforced concrete boxes on-site and jack them into place beneath the rail line.
The heavily reinforced concrete tunnel boxes were cast on-site and weigh approximately 4.2 million lb apiece. The project will not disrupt rail traffic on the busy line. Courtesy of Liberty University, Inc.
"The jacked box tunneling method has been utilized around the world," Barber notes. "It is a proven method. However, our method of jacking the tunnel boxes is different in that the boxes are pulled instead of pushed. This provides for the boxes to be more accurately placed, and keeps the soil confined tightly between the tunnel box on one side and the reaction wall on the other side."
The reaction walls are 12 ft wide, 4 ft thick, and 22 ft tall. They are heavily braced by large steel H piles that are grouted into bedrock and H beams that are placed diagonally. Six large jacks, each with a 1,680,000 lb pulling capacity, are mounted to the reaction wall to pull the tunnel boxes into position via a network of cables that are housed in predrilled, high-density polyethylene pipe sleeves.
The interior dimensions of each of the tunnel boxes are 16 ft high, 28 ft wide, and 130 ft long. The bottom slab and walls are 2 ft thick; the top slab is 2.5 ft thick. Each box weighs approximately 4.2 million lbs. The tunnels were cast with a roof slab that is longer than the floor slab and diagonal walls. This creates an overhang that provides a safety measure during excavation.
The edges of the boxes are fitted with a cutting shield made of 1 in. thick steel plate. The face of the cutting shield is set at a 25 degree angle. The cutting plate acts as a wedge to cut through the soil and protects the concrete box from damage. The tunnel walls are also fitted with ports through which a bentonite slurry is injected during the pulling operation to reduce friction.
"The jacks pull the box about 7 or 8 inches per pull," Barber says. "You can pull as often as every 15 minutes, but it's all based on how fast your excavators can dig out that amount of soil. We are dealing with a railroad hill that has been in place for more than 100 years."
The team placed the first box this fall and during the 15-day process found steel channels, railroad ties, and large boulders in the excavation area. The team is required by the railroad to take a break in construction until after the busy holiday season. The second tunnel will be placed early in 2014, Barber says.
"Once you get started pulling the boxes, it's a 24-hour-a-day operation until it's done," Barber says. "The railroad does not allow us to stop. When you start pulling one of the boxes, you have to go until it is pulled into place."
The boxes are 30 ft longer than is currently necessary to accommodate the possible addition of a third set of tracks at the site. Once the second box is in place, the reaction wall will be demolished and the tunnel ends will be finished with retaining walls.
The tunnel installation will greatly improve traffic flow into and out of campus, where a new library, science hall, 1,400-space parking deck, school of music with a 1,600-seat auditorium, medical school, and 1,200-student dormitory are under construction, as well as a new campus and student center.
"It's been exciting because it is different," Barber notes of the tunnel construction. "With everything going on, it's been a whirlwind."