Entry Into An Abandoned Underground Mine - New York …



Entry Into An Abandoned Underground Mine

Precautions and Preparation - Protecting the Public

By George H. Davis, R.G., C.P.G.

Senior Geotechnical Specialist

Missouri Department of Transportation

Abstract

On December 1, 2005, a unique entry was made into the old Breckenridge mine adits on the property of the Fred Weber, Incorporated’s O’Fallon Quarry near St. Louis, Missouri. This property, where current open-pit operations mine concrete aggregate and fill, also abuts Missouri Department of Transportation right of way for Interstate 70, a major east-west thoroughfare. It had been alleged by a private individual that the abandoned underground on the property crossed underneath the Interstate. A team of six professional firefighters and one geologist entered the abandoned mine to either prove or disprove this allegation. The safety precautions used and awareness of the potential dangers present led to successful conclusion of this entry, with the reemergence of the team from the mine. The allegation was disproved, measurement of the size of the underground cavity was made, and the team was able to directly observe advanced stages of collapse in this limestone mine. This paper discusses the precautions taken to insure the team’s success. Safety was the primary concern during the course of this investigation for the entire team.

Mine location and geologic setting

Fred Weber, Incorporated is a major contractor in the St. Louis, Missouri area with multiple business divisions that engage in heavy construction, solid waste management, and production of crushed limestone. The O’Fallon open-pit quarry operation and rock crusher, where this investigation took place, is adjacent to Interstate 70 in the City of O’Fallon, St. Charles County, Missouri. There are two pits at the current operation, one which has been mined out and abandoned, The other pit has not yet been mined out, though this is likely to occur within the next 10 years. In ascending stratigraphic order, Mississippian Limestones of the Choteau Group, Fern Glen Limestone, Burlington Limestone, Keokuk Limestone, and Warsaw Limestone have been mined for aggregate and construction rip-rap (Smith and Beste, 2001). The location of the mine is shown in Figure 1.

The Choteau limestones at this location are about 70 feet in thickness, and are exposed near the base of the quarry. These limestones are gray to yellowish-brown, fine grained, thin- to medium-bedded with a wavy-bedded or nodular appearance that commonly contains calcite-filled geodes. Above the Choteau is the Fern Glen Formation, which is a yellowish-brown, fine-bedded to massive, sometimes cherty, fossiliferous

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Figure 1. Location of Weber O’Fallon Quarry. From USGS O’Fallon 7 ½’quadrangle map. Note close proximity of Interstate 70. Entry was made into adit in quarry face going south from quarry

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limestone which may also be dolomitic in part. The thickness of the Fern Glen is approximately 90 feet. The Burlington and Keokuk limestones are gray, fine-to coarse grained, medium bedded to massive limestones containing abundant bedded and nodular chert, sometimes comprising up to 70% of the formation. Beds of the Burlington-Keokuk form the floor of the adits that were entered. Finally, atop the Burlington-Keokuk is limestone of the Warsaw Formation. This limestone varies in quality, but was once mined at this location underground. These beds constitute the walls and roof of the mine. The Warsaw consists of a fine- to medium-grained, shaly and thin-bedded, sometimes cherty limestone that may also be dolomitic, and tends to split quite easily on its bedding planes. Above the Warsaw, which has an irregular surface contact, is soil up to 50 feet in thickness. The stratigraphy of the entered portion of the mine is shown as Figure 2.

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Reasons for Concern, and Legal Status

Chapter 444 of the Revised Statutes of the State of Missouri governs the rights and duties of mine owners in Missouri. One section of the law, Section 320, prohibits mining under public highways and railroads. Specifically, it reads:

“444.320. No person or persons, firm, or association of persons, company or

corporation shall, within this state, mine or excavate beneath the surface of any

public highway or railroad right-of-way, in such a manner as to cause the surface

of the ground over which such public highway or railroad track is constructed to

cave in.”

The mine location and proximity to MoDOT right-of-way became a possible cause for concern when a private citizen alleged to the City of O’Fallon board of Aldermen that the mines at one time had gone under Interstate-70. How could this be proven or disproven?

Surveying of the mine entrances in relation to the location of the Interstate proves little other than the distance of the entrances from the right-of-way. The location of most openings on the far quarry wall made them relatively inaccessible for direct visual inspection. Only one opening was accessible for entry and possible inspection. Drilling on state right-of-way and/or the lanes of Interstate 70 was deemed too dangerous to crews who would have to establish lane control in lanes with limited sight distance and a 65 mile per hour speed limit. In addition, work would have to be done at night since daily traffic volumes far exceeded the capacity of a single lane to carry during daylight hours. Geophysical survey adjacent to the lanes of I-70 was also considered to be too dangerous in the limited right-of-way. In addition, since access was through an active quarry, jurisdictional questions of inspection also arose.

Since the initial complaint was filed with the Missouri Department of Labor and Industrial Relations, Division of Labor Standards, their offices sought a legal opinion to clarify the law, and to determine the role and responsibilities of the state agencies involved. In September of 2005, this legal opinion was returned with the result that the Department of Labor and Industrial Relations, Division of Labor Standards, Mine and Cave Inspection Section was deemed the authority to inspect any mine suspected to be in violation of the law. The mine inspector and Assistant Director of that Division, Steve Dunn, used a ranging prism to look back into the mine 210 feet, still short of MoDOT right-of-way. This still did not satisfy all of the parties involved, since areas in the mine were still not visible to inspection in that determination.

The only option for visiting those areas of the mine that could not be seen, thus, was for man-entry into the abandoned mine. MoDOT was contacted by Randy Sanders, Deputy Chief of Operations of the City O’Fallon Fire Protection District, about entering the mine with a geologist, to aid them in determining the best way in which to make entry, and how to insure the maximum safety for all team members.

Planning for safety

An abandoned underground mine should, under normal circumstances, never be entered unless there is a legitimate reason for entry that will either save or preserve human life. In this case, two equally compelling reasons for entry were present:

1) It was not known whether or not the mine cavity crossed under I-70, nor its proximity to the road. Since there were areas of visible collapse in the mine that could be seen from the entrance, collapse under I-70 might be catastrophic.

2) It had been determined that there was no other option to locate the limits of the mine and its relationship to I-70.

Once it was determined that entry would be made, plans were made to insure the safety of the team, based on knowledge of the method of mining and on safety precautions from methods of cave exploration. Preparation was necessary to address anything that might be encountered underground. The team decided in a pre-entry safety meeting that if hazardous materials were encountered, they would exit the mine immediately. An equally valid concern was the presence of bats, since they are known to carry rabies. Areas that appeared to be actively collapsing were avoided. It was planned to determine the overall area of the mine by seeking out the limits of the mine. One additional concern for safety was the possible presence of toxic, asphyxiant, or explosive gases.

To determine the location of the mine entrance, a hand-held Global Positioning System (GPS) system was used, a Trimble Pro XL with a one-foot accuracy. Two survey coordinate points at the edge of Interstate 70 right-of-way outside the quarry were also established. With three points outside the mine, sufficient control was established such that an excellent estimate of the location of the mine could be established once underground. This allowed the team to spend a minimum amount of time underground.

A safety meeting was held prior to entry, where all team members were advised of the proper methods of procedure. Each team member carried three sources of light, a standard procedure in cave exploration. Hard hats and steel-toed boots were required. Two team members carried gas detection equipment, and 2-way Motorola radios were tested for line-of-sight transmission. The team members were instructed to stay away from all rib (wall) faces that had cracks (since this might be indicative of overstressed areas prone to collapse) and from any rocks that might be perched precariously on the roof of the mine. No team member would go anywhere alone at any time, and gas detector checks would be made periodically.

Entry into the mine

The mine was entered shortly after 2:30 p.m. Dec. 1. Floor conditions were treacherous, as large roof slabs had fallen in a large thirty-foot tall pile near our entry. (See Figure 3).We looked up and found that the roof was some 20 feet above the top of the pile, and that the roof appeared to be stable at that time. There were only one or two hanging slabs (referred to as ‘widowmakers’ for obvious reasons) that appeared to be precarious. We avoided the areas under those slabs as we proceeded inwards.

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Figure 3. Team members crossing the rubble pile after the initial entry. The rubble pile extended nearly a hundred feet inward into the mine.

Gas detection readings were taken at hundred-foot intervals, measuring particularly for methane, carbon monoxide, and sulfur dioxide and trioxide. An extremely low threshold limit alarm was set on the detector to insure that the team would be warned of any change in gas levels that could be harmful. The first traverse proceeded east in the mine, periodically checking roof and floor conditions to insure safety. One team member brought a video camera so that the condition of both the roof and the floor could be recorded for further reference as needed.

It was verified that the mine was in the Warsaw formation, and that the rock was a thin-bedded, fine-grained limestone with chert in both nodular and bedded form. Parts appeared to be dolomitic, and susceptible to freeze-thaw damage. One interesting phenomenon noted was the tendency for a decrease in the amount and abundance of roof spall further back in the mine. This was explained by two possible mechanisms:

1) Blast effects from the adjacent quarry caused the roof to spall in an already-weak limestone formation, the Warsaw. Blast effect decreased with distance from the quarry to a point where it no longer affected the roof of the mine approximately 150 feet in.

2) Freeze-thaw effects caused the roof and pillars to spall, decreasing in severity and frequency with distance from the mine adit entrances.

After examining spalled blocks on the floor of the mine, no conclusion could be reached as to which mechanism was responsible, though it is believed to be a combination of both. The morphology of the roof is similar to that of a classic “climbing cave” in which roof spall gradually decreases in diameter of effect as a cave “cones and domes” as the roof moves upward. Fred Weber, Inc. agreed to suspend blasting at the adjacent quarry during the mine exploration.

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Figure 4. East pit of Fred Weber, Inc. O’Fallon Quarry. Note location of horizontal shafts in side of quarry face. Photo taken facing southeast.

No standing water was found except in the back of the mine, which was 225 feet back from the entryway. There were three main galleries of shafts, all paralleling the quarry face and separated by pillars approximately 25 feet by 25 feet square. The main galleries extended nearly 2000 feet. The team managed to reach all four corners of the mine prior to exiting at 4:15 p.m. December 1st.

Conclusions

Entry into the former Breckenridge mine adits of the Fred Weber, Inc., O’Fallon quarry allowed a unique opportunity to view weathering and collapse features where geophysical methods and targeted drilling were not possible for safety reasons on the adjacent Interstate. Entry would not have been advisable if the roof were shale or other poorly consolidated rock, such as that found in a coal mine.

It is believed that blasting at the adjacent open-pit operation of Weber-O’Fallon Quarry in combination with periodic freeze-thaw near the entrance to the mine caused acceleration of a normal karst-like process of roof “dome-out” within the mine. From measurements taken of the cavity, the mine does not go underneath I-70, and thus the public is not at risk.

References

Smith, David C. and Beste, Robert, 2001. “Selected Industrial Mineral Producers Along Interstate 70 and Missouri Highway 94, St. Charles County, Missouri.”, Guidebook of the 48th annual Meeting of the Association of Missouri Geologists, St. Louis, Missouri, Sept. 28 and 29, 2001.

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