2017-01-30_PERMIT FILE - C1981041 (8)

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Roadside Mine #00489-0004 Huddimon-scrry- 05/09/16 The ERT survey was completed by Fugro in April 2016. The survey included data collection along seven profile lines. The referenced report completed by Fugro is attached. Conclusion As indicated in the referenced Fugro report, the ERT data clearly shows a vertical low resistivity zone in the subsurface. This low resistivity zone correlates extremely well with the location of the old air shaft. Based upon the results of the ERT survey, test pit observations, drilling information provided by others, and site geology, HBET believes that the old air shaft is the pathway for water migration into the mine workings. The ERT data does not show any other significant anomalies within the survey area. Therefore, HBET believes that the old air shaft is the only pathway within the survey area. As discussed previously, considering the depth of the mine workings, the site geology is simply not conducive to natural development of sinkhole pathways originating at the mine workings. Any additional pathways which could exist elsewhere on the site would also likely be man-made vertical features such as additional air shafts, improperly backfilled boreholes, etc. However, HBET is unaware of any such features within the project area. Recommendations Unfortunately, while the old air shaft is believed to be the preferential water pathway, the ERT survey data does not indicate precisely how open the shaft is. The Fugro report suggests that the shaft is backfilled and that the lower resistivity is indicative of a higher permeability zone rather than air filled or water filled voids. In addition, due to the resolution of the geophysical method, the precise pathways from the observed sinkhole openings at the ground surface to the shaft are not well delineated in the ERT data. As a result of the uncertainties regarding the sinkhole pathways and precise condition of the old air shaft, HBET recommends two alternative approaches to prevent future water migration into the mine from these features. The first approach would be to excavate the sinkhole surface openings and dig along the pathways until they intersect the shaft. The shaft itself should be exposed at this point in order to evaluate the amount of open area, etc. within the shaft. With the shaft exposed, HBET recommends that low slump concrete be placed to seal off the shaft at this elevation. The native soils excavated from the area should be screened to 6 -inch minus and placed in compacted lifts back up to original grade. This approach would likely be the least expensive in terms of the actual mitigation effort. However, excavation of this magnitude could create other problems with regard to reclamation due to the fact that the work is associated with a mine site regulated by State and Federal agencies. A lower disturbance alternative would be to conduct low mobility displacement (compaction) grouting within the anomaly area. HBET recommends compaction grouting to a depth of at least 25 feet within an approximately 100 feet radius of the air shaft. Primary grout holes should be installed on a grid pattern at a 10 to 20 feet spacing which will be established in coordination with an experienced grouting contractor. Secondary grout holes should be installed at half - spacing within the primary grid pattern. The compaction grouting program will fill any open voids in the subsurface and densify the surrounding soils to limit the potential for any future water migration into the shaft. X:\2008 ALL PROJECTS\00489 - JE Stover Associates Inc\00489-0004 Roadside Ivtine\200 - Geo\00489-0004 LR050916.doc TR -69 A14-14-3 09/16