The Appalachian Mountain range is nearly 1,500 miles long stretching from the southeast corner of Canada to Alabama. The Cumberland Gap, a natural break in the range near the eastern borders of Kentucky and Tennessee, is now a paved highway known as US 25E. A section of US 25E that now passes through a tunnel underneath the mountain is called the Cumberland Gap Tunnel.
Cars and trucks have replaced Conestoga wagons, but the route, now a paved highway, still offered challenges to travelers. The steep grade and sharp turns of US 25E were the cause of many traffic accidents. So many, in fact, that the 2.3-mile stretch of highway earned the unfortunate nickname, “Massacre Mountain.”
To make the route safer, a section of US 25E was relocated under the mountain by constructing a tunnel. This also helped restore the appearance of the mountain to the way it looked in the late 1700s, in the days of Daniel Boone.
The Cumberland Gap Tunnel passes through a large limestone formation within the Cumberland Mountain. Limestone, made of the mineral calcite, is slightly soluble in water. When rainwater seeps through cracks and between layers of limestone, it dissolves some of the calcite it contacts. Bit by bit, the flowing water removes calcite and the cracks grow larger, eventually forming caves. The resulting cave system is a natural conduit for underground water flow.
Where the tunnel passed through a cavern, concrete was poured to form an artificial ceiling structure. This, of course, blocked the water flow. The cavern, which runs almost perpendicular to the tunnel, was kept in a nearly natural state by allowing the water flow to continue its normal course through a conduit five feet in diameter built under the roadway. There is also a passageway constructed above the tunnel to allow for air and water movement. Thus, the cave system continues to function as a natural drainage system.
Engineers designed an additional drainage system to collect percolating groundwater that hits the impervious tunnel walls. This system includes a manmade streambed parallel to the tunnel, running beneath the roadbed from end to end.
The streambed is made of limestone gravel, and water flows between and around these gravel particles. When groundwater percolates through limestone rock layers before reaching the gravel bed, that drainage water is already saturated with calcium carbonate (the chemical name for calcite) and does not dissolve calcite from the gravel. Without loss from dissolving, loss of gravel caused by abrasion and erosion occurs at a slow and predictable rate.
However, most of Cumberland Mountain is made of rock other than limestone, such as sandstone, shale, and siltstone. This means that much of the water entering the tunnel’s drainage system is not already saturated with calcium carbonate and does dissolve gravel material. Similar to the way the cave system developed long ago in the limestone formation within Cumberland Mountain, the artificial streambed is gradually washing away. During the first ten years of the tunnel’s existence, enough gravel matter was removed to cause the roadway above to sag and need repair. As of 2009, a long-term solution had not yet been determined.
To learn about the construction of a different kind of tunnel, one that went underwater, check out Building the Channel Tunnel.
For more information about the movement of plates in Earth's crust, check out Mountain Maker, Earth Shaker.