Tunnel Construction in the Mancos Shale Formation on U.S. Bureau of Public Roads Project 1-F Mesa Verde National Park, Colorado
Robert A. Bohman, 1958. "Tunnel Construction in the Mancos Shale Formation on U.S. Bureau of Public Roads Project 1-F Mesa Verde National Park, Colorado", Engineering Geology Case Histories Number 2, Parker D. Trask
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The decision to construct a 1476-foot highway tunnel between Morfield and Prater canyons in Mesa Verde National Park, Colorado, was motivated by vl) the high cost of maintenance to keep the only highway into the Park open? because of sliding and ravelling in sections where the road traverses practically vertical shale cliffs, (2) the possibility that some of these sections would be entirely lost despite the maintenance measures applied, and (3) the imminent danger to the traffic using the road. The tunnel itself is part of a relocated section of highway which bypasses the above critical areas.The original design of the tunnel was preceded by location, seismic, and geological investigations and was based upon the findings of these investigations.Three general types of material were excavated during construction: (1) colluvium (which consists of sandstone blocks interspersed in haphazard fashion in a sandy clay matrix; these materials are well-compacted, weathered products of the Mancos shale and Mesa Verde sandstone formations; variable thicknesses were excavated at the portal sites);(2) weathered and weakened Mancos shale; and(3) dense, bluish to gray, unweathered shale, sandy shale, and lensing sandstone units of the Mancos shale. The formations in the area dip gently (about 210 feet per mile) to the south.
The tunnel cross section for excavation indicates a dimension of 25 feet 2 inches on center line from subgrade to the tunnel back (roof) and is a semicircular section with a 15-foot finished radius above the spring line. At the spring line,