I've already studied the
NS Cresent Corridor. The
NS Hatfield Tunnels have motivated me to study the Heartland Corridor.
(The references for the following information are
Heartland and
Conference.)
During the 1980's, the Heartland Corridor route was cleared for 19' 6" tall tri-level auto carriers. Most of this work was done by lowering the tracks. In the 1980s, other railroads developed the deep well, articulated flat cars to carry double-stacked containers, which effectively doubled the efficiency of railroad intermodal transport. But the 20' 3" height of a double stack would not fit in the tunnels on the eastern railroads.
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Not only was the height of the load important, but so was the "squareness" of the load. The covers on autoracks have curved corners so that they will more easily fit under the arched roof that many tunnels have. But containers have square corners. So to run double-stack trains between the ports in Virginia and the Midwest markets, NS had to enlarge 28 tunnels with a cumulative length of 5.5 miles.
Since many of the tunnels had bridges across the Tug River at their ends and the bottom of the tunnels were already lowered in the 1980s, going down further in the 21st Century was not an option. So they had to go up an average of 1.5' to provide the desired 21' clearance. (About half of the 28 tunnels were constrained by bridges on their end and had to go up. [
BLET-2010]) (
Hemphill Tunnels illustrates why a realignment done in 1903 caused so many tunnels to have bridges at their ends.) They also had to remove other overhead obstructions including bridges, signal structures, and slide detector fences. The Heartland Corridor will reduce the travel time from Hampton Roads to Chicago from 3 to 2 days.
Because Ohio wanted an intermodal terminal in Columbus as part of the transportation hub they were building around their airport and because Virginia wanted their Atlantic ports to remain relevant after the Panama Canal was expanded, they helped pay for the improvements. And Federal tax money was also involved. Taxpayers paid about half of the $188m clearance improvement project.
Because of a high-priority intermodal train in each direction run for UPS, the contractors had a work window each workday of just 10 hours. This allowed them and NS to reinforce the ceiling, grind away the top, and replace track, ballast and ties in just 15- to 20-feet sections each day. Work began in 2007 on a Saturday-Wednesday schedule because coal traffic was heavier at the end of the week. After the global recession in 2008 cut the demand for coal, the contractors worked Mon-Fri and avoided the expense of weekend overtime pay.
Some tunnels, such as the
North Hatfield Tunnel, were dug through solid rock, so it was just a matter of grinding off more rock.
Other tunnels were almost big enough, and it was just a matter of grinding little "ears" into the lining. This was "minor notching" because the amount of the tunnel liner removed (4 to 6 inches) did not compromise the structural integrity of the liner. I could not find an NS example. This photo is of the BNSF Cascades Tunnel.
Other tunnels required deep notches. I'm assuming that this Hemphill Tunnel #2 is an example of deep notching. Below we see two excavators, each with a diamond-tipped Alpine roadheader attachment, grinding the two notches.
Deep notching removes enough liner material that the "shear stresses in the concrete at the notch exceed acceptable values and additional support measures are required." The liner is reinforced by bolting it to the rock "to
develop arching action within the rock surrounding the tunnel, thereby allowing a stress
transfer of the current tunnel lining loads from the concrete liner to the reinforced rock
arch as the lining is notched. The maximum depth of the notch has been set to maintain at
least 10” of intact concrete in the lining." [
Conference, pp17-19] I reproduce this figure because it not only illustrates the rock dowels used to bolt the liner to the rock, it shows the clearance templates for autoracks and double-stacks. I presume that the clearance templates are wider at the top than the bottom to account for some rocking back and forth as the cars roll through the tunnels. But it doesn't allow for a lot of rocking. So the track must be kept in really good condition inside tunnels.
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elevation.maplogs,
License: Creative Commons Attribution-Share-Alike (CC BY-SA),
via Bridge Hunter
[This is Big Sandy #1. It was notched so deeply that it was treated as a crown
removal in terms of roof bolts and shotcreating. [BLET-2009] |
If notching would leave less than 10" of the liner intact, then they had to remove the tunnel's crown. Crown removal is complicated and it takes multiple work windows to work a single section of track and the work needed depends upon the strength of the ground above the lining. "A section of lining will be removed along the length of the tunnel with any rock above the lining needed for clearance. Rock bolts and mesh would be installed immediately and in advance of the next train. That operation would be followed by additional excavation support measures including multiple layers of shotcrete reinforcement, lattice girders, spiling, etc., depending on the level of excavation support required for the actual ground conditions encountered." [
Conference, p20] Below left we see the crown being removed in Cooper Tunnel. The photo of the portal shows that it is hard to determine how the tunnel was enlarged by looking at the portal because it looks like just notches.
The fourth option is to remove the tunnel entirely, called daylighting. This option has the advantage that the bulk of the work, removing all of the rock and soil above the lining, can be done while trains are running. Only the removal of the lining itself would need to be restricted to the 10-hour work window per day. I don't know if NS daylighted any of its 28 tunnels. But
BNSF removing the Cajon Pass Tunnels is a good example of daylighting.
Multiple contractors from all over the country were used so that multiple tunnels could be enlarged during each 10-hour work window. The project took three years to complete, 2007-2010. The roof of the Cooper Tunnel had eight courses of brick. [
BLET-2010]
