Department responds to mine shaft opening in Greene County
On the evening of Jan. 29, 2013, a subsidence occurred at the location of the Big Ben Mine Shaft on the eastern side of Springfield.
The Land Reclamation Program received reports about the shaft subsidence the morning of Jan. 31, 2013. The program began collecting information in preparation of a site investigation conducted later that morning.
Maps obtained from the department's Division of Geology and Land Survey and georeferenced on aerial photos in Geographical Information Systems by the Greene County Resource Management Department show a shaft entrance in the vicinity of two homes at 1515 and 1519 South Devon Road. The homes were constructed approximately 20 years ago.
Reportedly, the shaft originally was a vertical opening extending 190 feet and extending into two horizontal shafts. It is believed the vertical shaft was filled in the 1960's prior to the area being developed as a residential neighborhood. The subsidence around the vertical shaft created a crater approximately 31 feet long, 14 feet wide and 15 feet deep between the two houses. What appeared to be mine related concrete foundations and unconsolidated fill were present in the crater.
Both landowners, signed consent forms to allow the Land Reclamation Program to fill the crater created by the subsidence. The homeowners have contracted with an engineering firm for consulting and design.
The first priority was to ensure the foundations of both homes were stable and eliminate potential hazards to crews working in the area. The home located at 1519 South Devon was particularly vulnerable with the subsidence causing 60 to 70 percent of the footing under one side of the house to fall into the crater. If not stabilized, it was believed the home would collapse into the crater in the near future. The consulting engineer recommended constructing a cast in place concrete support beam that would span the length of the opening and would be used to reduce the pressure placed on the exposed foundation. The footing and foundation on the house located at 1515 South Devon appeared to be intact with some cracks appearing on the basement walls. An excavating company was contracted to place rock fill to stabilize the side slopes of the crater, install temporary supports for the foundations of both homes, and provide a suitable foundation for constructing the concrete cast in place support beam.
It is anticipated the cast in place concrete beam will be constructed within by mid-February 2013. After the homes are permanently stabilized, the Land Reclamation Program will work to characterize the site including conducting exploratory drilling to ascertain the extent of the shaft and excavating the crater to determine the depth and the extent of the mine shaft subsidence. The Land Reclamation Program anticipates finding a layer of bedrock that can be used as a load bearing surface for constructing a steel-reinforced concrete plug and backfilling the hole with suitable fill material.
On Feb. 28, 2013, a bid was awarded to Dewitt & Associates Inc. of Springfield in the amount of $46,336 to install a concrete support beam and micro-piers that will be used to stabilize the two houses at the site and allow AML staff or contractors to safely explore and repair the mine shaft collapse.
On March 7, 2013, a preconstruction meeting was held at the site by Palmerton & Parrish to discuss specific construction details, logistics and timeframes for this project with Dewitt & Associates and the homeowners. It was determined construction would most likely start on March 11, 2013, and could take as much as two or more weeks to complete. After the installation of the support beam and micro-piers is complete, the AML unit will be able to safely investigate the site without risking further damage to the structures attributed to additional soil collapse into the shaft. After the investigation is complete, the AML program will develop a plan to plug the shaft and backfill the hole.
On March 27, 2013, staff verified the installation of the support beam and micro-piers have been completed and both homes are stable at this time. The department staff are currently excavating the shaft opening and have encountered loose earthen soil that continues to break away from the opening. Staff will continue its investigation to determine the proper plan of action to stabilize and close the shaft opening.
On March 29, 2013, the contractor excavated a large chunk of concrete from the hole which appeared to have been covering the mine shaft opening. The removal revealed what appeared to be the mine shaft opening based on the color and contrast of the material compared to the native red clays surrounding the hole. Loose material was removed and material at the bottom compacted.. Plastic sleeves were installed on the micro piles to separate it from the concrete. The micro piles will allow the concrete to move around them in the event of future settlement. Some 3/4 inch plywood was applied along the exposed footing of the north of the house to ensure the concrete would not bond or overlap it and insure any future settlement of the concrete plug would not translate to the home's footing and micro piles . Approximately 77 cubic yards of concrete was pumped into the hole with No. 8 Rebar being placed throughout. The concrete was poured up and slightly over the very bottom of the structural beam installed on the home next door.
The following is a list of items need to be finished at the site:
- The void under the neighbor's house needs to be pressure grouted
- Soil and rock needs to be placed over the plug to within 2 foot of final grade.
- The remaining stockpiled soil and rock located in the backyard needs to be removed.
- Topsoil needs to be placed over shaft area.
- Remove all ruts and rocks.
- Seeding mulching.
- Set the monument for the shaft.