Interstate Technical Group on Abandoned Underground Mines
Fourth Biennial Abandoned Underground Mine Workshop
Abstract: Planning a Highway over Mined Ground
A Case History from the Tri-State Lead and Zinc District
Timothy E. Newton, MoDOT
An intensive investigation was made to plan the location of a proposed segment of U.S. Highway 249 in Southwest Missouri. Both the City of Joplin and the Environmental Protection Agency encouraged the Missouri Department of Transportation (MoDOT) to minimize the disruption of presently productive ground and to concentrate the roadway corridor through chaotic and derelict ground represented by evidence of former lead-zinc mining in the Tri-State District. The project area sits on the Oronogo Superfund Site due to the presence of lead, zinc, and cadmium that exist in the milled rock and tailings.
The Tri-State district of Missouri, Kansas, and Oklahoma was one of the foremost mining areas of the world, producing 50 percent of the zinc and 10 percent of the lead consumed in the United States from 1850 through 1950. The major ore minerals mined were sphalerite (zinc sulfide) and galena (lead sulfide). Extensive mining has produced large underground voids and the associated hazards of open shafts, subsidence pits, and tailing piles.
A combination of historic research, field reconnaissance, aerial photograph interpretation, strategic drill holes, and engineering geophysical surveys were utilized to conduct the investigation. The long-term objectives of the study are to minimize both the potential for contractor variable site condition claims and the potential for long-term subsidence-related problems. Many unmapped features were discovered, documented, and surveyed during the field investigation.
Despite these efforts, several mine features could not be avoided and their threat to the future highway required mitigation. Most features exist as small pits, 10 feet diameter and 10 feet deep. One cannot determine if the feature is a shallow prospect terminating at bedrock or a deep shaft. MoDOT decided to address these "pits" that fell under or near the highway footprint with dynamic compaction. Dynamic compaction is a ground improvement method where the application of energy is used to densify deposits by repeatedly raising and dropping a heavy tamper. The features will be repeatedly filled and tamped until they are brought up to roadway grade.
Other features that require mitigation are open shafts, which reach a depth of 180 feet. It was decided to use a polyurethane foam plug followed by two lifts of reinforced concrete. The thickness of the plug is the width of the shaft, typically 5 feet. The concrete will be placed in two separate 5 foot lifts above the plug. This technique requires a downhole camera to locate good bedrock to place the seal. The concept is that the polyurethane foam will bond with the bedrock and support the first lift of concrete. The concrete should support the load of the overburden.
Construction on this project began August 2001 and several sites have required dynamic compaction. Unanticipated problems include the discovery of numerous 6 to 8 inch diameter cased mine vents that required cutting at grade and filling with grout. The construction currently being performed will be discussed in this presentation.