Two tunnels were constructed in the Martin Marietta Fort Calhoun Quarry at a 14.5 percent decline gradient from the portal face at the bottom of the cut and cover notch to the level of the underground mining complex within the Gilmore City Limestone. The two declines, each with a length of 3,136 ft., were primarily constructed with mechanical excavation by road header with drill and blast excavation within the limestones as necessary. The tunnels were constructed in sub-horizontal shales with limestone interlayers and coal seams with the final 80 ft. excavated in limestone at a near horizontal grade with a rectangular profile matching the mine development drift geometry.
Gall Zeidler Consultants evaluated the encountered ground conditions and the in-tunnel monitoring data to delineate rock mass behavior and identify the causes of the failures of the initial lining and provided recommendations for initial support rehabilitation and safe continuation of tunneling works.
Fort Calhoun, Nebraska
Quarry Decline Tunnels
3,140 ft (957 m) each
27 ft width x 25.5 ft height (8.3 x 7.8 m)
Shale with limestone interlayers and coal seams