Archived: TRB Mid-Year Meeting Symposium On The Effects of Abandoned Underground Mines on Transportation Facilities
Planning a Highway over Mined Ground:
A Case History from the Tri-State Lead and Zinc District
Allen W. Hatheway+, Timothy E. Newton^, Neil L. Anderson*
+Department of Geological Engineering, University of Missouri-Rolla, Rolla, Missouri
^Missouri Department of Transportation, Jefferson City, Missouri
*Department of Geology and Geophysics, University of Missouri-Rolla, Rolla, Missouri
Presented by Tim Newton, Geotechnical Liaison, Missouri Department of Transportation
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 the chaotic and derelict ground represented by evidence of former lead-zinc mining. The area is part of the Oronogo Superfund site due to the presence of lead, zinc and cadmium that exists in the milled rock and tailings covering the ground. The post mining conditions of this ground are "geotechnical uncertainties", as they have the potential for causing loss of ground support by methods of slow settlement or rapid collapse. The risks are compounded, for had there not been bad ground geologic conditions, there would not have been the associated mineralization.
This represents some of the most geotechnically difficult construction ground imaginable. Lead and zinc ore in the Joplin area was preferentially deposited along pre-existing near-vertical joints or faults, along the margins of the Pennsylvanian sinkholes, and as sheet ground deposits within the Mississippian host rocks at depths on the order of 60 m. The sinkhole and near-surface joint/fault zone ores are shallower and were recovered first (1850-1900) using either interconnected shafts or open-pit mining techniques. The sheet ground deposits are deeper and were mined later (1900-1950) using room-and-pillar methods. Typically, these mines were abandoned without mitigation and were often later robbed of ground support, thereby now constituting potential highway and/or construction hazards.
From the perspective of a highway engineer, there are three principal types of abandoned-mine hazards in the study area. These are associated with the shallow mines that employed interconnected shafts, the shallow open-pit mines, and the deeper room-and-pillar mines that employed access and ventilation shafts.
1. Shallow mines that employed interconnected shafts that were not properly in-filled when abandoned represent highway hazards for two reasons. First, open shafts could collapse under the new loads, resulting in incremental to catastrophic surface subsidence. Additionally, the open works of these abandoned shafts could provide vertical conduits for contaminant-bearing runoff and could also bring about the progressive "washout" of fine-grained sediment beneath the highway sub-base. (As noted previously, shallow mining activities were focused about the margins of Pennsylvanian sinkholes.)
2. The abandoned and in-filled open pit mines represent sites of potential gradual to catastrophic subsidence for two reasons. First, their non-engineered in-filling material is under-compacted and could consolidate and settle when loaded. Second, improperly abandoned shafts may also extend outward from the open-pit mine into the adjacent strata, and cause additional gradual to catastrophic surface subsidence.
3. Improperly-abandoned access and ventilation shafts to the deeper room-and-pillar mines pose subsidence hazards because of the presence of void space and under compacted fill. They also could provide vertical conduits for surface runoff and facilitate the progressive "washout" of fine-grained sediment beneath the highway. The deeper room and pillar mines themselves are not considered to be a significant risk due to the likelihood that natural upward-failing back (roof) strata would choke-up by bulking and therefore would prevent significant collapse-related surface subsidence.
A combination of historic research, field reconnaissance, aerial photograph interpretation, strategic drill holes, and engineering geophysical surveys were utilized to conduct the investigation. The fieldwork was initiated by navigating to known locations of mines and shafts using a GPS receiver. Missouri Division of Geology and Land Survey had studied their mine map archives and calculated coordinates of these features, which they stored in a database and referenced to by township, range, and section. MoDOT also digitized the available mine maps and plotted them on the highway plans. Many unmapped features were discovered, documented, and surveyed during the field investigation.
The University of Missouri-Rolla (UMR) conducted a reflection seismic/ground-penetrating radar survey for MoDOT along segments of the proposed roadway corridor. A total of 14,600 lineal meters of shallow reflection seismic data, nine down-hole seismic calibration check shots, and 15,000 lineal meters of ground penetrating radar (GPR) data were acquired. The purpose of the geophysics was to delimit "unmapped" areas of probable historic mining activity. Seismic data were acquired to map Mississippian bedrock, locate and identify paleosinkholes and abandoned mine features, and to determine structural geologic trends in the study area. The GPR data were acquired to identify and locate abandoned mine access and ventilation shafts in areas that were overlain by surficial milled ore rock (chat). Pre-construction knowledge of these anthropogenic and natural features will assist in route selection and geotechnical site mitigation, and minimize both the potential for contractor variable site condition claims and the potential for long-term subsidence-related problems.
The drilling program was then based on the data described above. Anomalies were drilled to confirm the presence of voids and poor quality rock. Drill holes were also placed adjacent to the mapped features, shallow pits and open shafts. Route selection was based on the entire investigation and most of the potential hazards were avoided in the chosen corridor through the area. This investigation created a rational examination pathway for potential site development in previously mined areas that represent a standard of care essential to protect human safety and the financial investment of engineered facilities.
Several mine related features could not be avoided and their threat to the future highway had to be mitigated. Most exist as small pits, 3 meters diameter and 3 meters deep. One cannot determine if the feature is a shallow prospect terminating at bedrock or a deep shaft. It was not feasible to investigate these features due to safety and the difficulty of drilling through the rubble of a filled shaft. It was 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. MoDOT calculated the energy required to compact the material in a typical 1.5 x 1.5 meter shaft to the average depth of rock, which is 4.2 meters. It was decided to use a 9 Megagram tamper dropped from a height of 8 meters, typical of what the average crane on the jobsite can handle. A minimum of two passes of 12 drops will be required to meet the energy requirements. There will be five drop locations in an "X" pattern, with the shaft in the middle and the spacing 1.5 times the diameter of the tamper. The features will be repeatedly filled and tamped until they are brought up to roadway grade. The pit features that fall on right-of-way but not close to the roadway footprint will only be filled and compacted.
Other features that require mitigation are open shafts, which reach a depth of 60 meters. Several sealing techniques were reviewed, but they all required excavation to bedrock. MoDOT believed this to be extremely dangerous and difficult, especially where rock was deeper than 7 meters. The U.S. Bureau of Mines is both researching and using polyurethane foam plugs to seal shafts associated with large coal mines. 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 1.5 meters. The concrete will be placed in two separate 1.5 meter lifts. This technique requires a downhole camera to locate good rock to place the seal. The concept is that the polyurethane foam will bond with the rock and support the first lift of concrete. The 3 meters of concrete should support the load of the overburden. The remainder of the shaft will be filled with chat.
The EPA requested that MoDOT cleanup the area near the project by incorporating off right of way chat and tailings in highway embankment. EPA designated areas were delineated in the field and mapped with differential GPS. Ground penetrating radar was then used to estimate the depth of the mine waste, providing the third dimension for volume calculations. MoDOT will be reimbursed for the hauling of material and the grading of the excavations. Once excavated, the material will be placed in alternating 15 centimeter layers with Class A soil material for a 50/50 mixture in compacted embankments.
The project as of May 14, 2001, will undergo the bidding process this month and construction is scheduled for August 2001. Despite MoDOT's best efforts, it is certain that additional mine features will be discovered during excavation. MoDOT believes they have conducted a thorough investigation, but realizes that the mitigations will require adjustment to meet the actual site conditions. Job special provisions for dynamic compaction, polyurethane plug, downhole camera, and other mine related construction aspects are available upon request.
Sources:
- FHWA Demo Project 116 GROUND IMPROVEMENT TECHNICAL SUMMARIES Publication No. FHWA-SA-98-086
- DESIGN PROCEDURES FOR RIGID POLYURETHANE FOAM CLOSURES Contract J0219001 - US Bureau of Mines
- MoDOT Job Special Provisions J7U0436E