2013-12-03_INSPECTION - M1986015

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PERMIT #: M- 1986 -015 INSPECTOR'S INITIALS: TC 1 INSPECTION DATE: November 15, 2013 from the agricultural ditch crossing 25th Lane pond a foot or more due to the access constructed on the north side of the adjacent pit. She also pointed out the stormwater drainage ditch TMOP has begun constructing to mitigate the flooding problem. Summary of Complainant's concerns: Ms. Cooper is concerns are: i. A large collapse that would damage her house and /or her property, ii. Falling into a crack or hole on her property, iii. Subsidence from a collapse damaging her utilities (e.g., ruptured gas line). Ms. Winters' primary concern is potential damage to her leach field. The large Phase 2 overburden/topsoil stockpile west of the Cooper property was observed to have multiple erosion rills and one larger erosion gully on the east side of the pile (see Photo 13). Ms. Cooper indicated a lot of dust blows off the stockpile during windy periods. The stockpile has clearly not been stabilized to protect against wind and water erosion in accordance with C.R.S. 34- 32.5- 116(4)(g). This is cited as a problem on page 1 of this report. Meeting Summary: I met with the seven aforementioned TMOP representatives to discuss my observations; relay the complainants' concerns; discuss potential causes of, and potential solutions for the observed problem. Mr. Sanchez provided me with a map showing well locations the mine has monitored over the past several years and water level data for most of these wells (see Attachment 1, Figures 1 through 5). He also stated the groundwater hydraulic gradient slopes downward to the east, primarily parallel to the Arkansas River. TMOP identified the portion of the mine operation adjacent to the properties on 25th Lane as Phase 2. They explained Phase 2 was undisturbed, with the exception of 4 acres related to the nearby Target Center construction project. The remaining excavation of the Phase 2 area began in 2007. In 2009, the northeast part of Phase 2 (near the MW6 label on Figure 1) was dewatered outside the slurry wall. It is understood by the DRMS that the slurry wall was completed in 2009, based on "As- Constructed' drawings noted under the "Records" section of this report. I explained to the TMOP representatives that the DRMS believes the local soils are likely predisposed to a phenomenon known as collapsible soils. This theory is based on conversations I have had with TC Wait, a part- time specialist with the DRMS and formerly with the Colorado Geological Survey (CGS). Ms. Wait previously investigated collapsible soils for the nearby Pueblo Airport. Mr. Sanchez briefly reviewed my copy of the "Collapsible Soils in Colorado" report (CGS, 2008). The DRMS believes the cracks and holes observed near 25th Lane are the result of dispersion and piping processes that physically removes sediment grains in the subsoil. TMOP offered to help mitigate the cracks and holes observed on the 25th Lane properties by using a skid steer, or similar equipment to partially excavate the cracks, backfill excavated cracks and holes with material provided by TMOP and compact the replaced soil by driving over it with the skid steer. TMOP discussed needing some kind of agreement with the property owners to proceed with the mitigation efforts. I explained that given the unknown size and geometry of the subsurface voids and the narrow nature of the surface cracks, the compacted surface backfill may not achieve the intended purpose, but that it was worth trying. TMOP offered to investigate undisturbed areas near the Phase 7 portion on the east end of the mine operation where they have been dewatering for over a year. If similar cracks and/or holes are found there, TMOP will excavate these Page 3 of 13