Laserfiche WebLink
To: Susan Burgmaier, DRMS August 31, 2011 <br />From: Jill Carlson, CGS <br />Subject: Landslide repairs at Seneca II West <br />I visited Seneca II West with you, Sandy Brown, Brian Len of NWCC, and representatives of the mine <br />operator, Peabody Energy /Seneca Coal Company, on July 6 and 7, 2011. The purpose of the field <br />meeting was to view and discuss a number of new and recurring landslides within and adjacent to <br />reclaimed areas at the Seneca II West Mine, and to determine what actions, if any, should be taken to <br />repair any slides and unstable areas within the mine permit boundary. Table 1 summarizes the areas <br />discussed. <br />The Seneca II West Mine is underlain by Residuum on the Mesaverde Group (sand, silt, and clay soils), <br />derived from and grading down to weathered, interbedded shale, sandstone and coal beds of the <br />underlying Williams Fork Formation. The layers that compose the Williams Fork Formation were <br />deposited during the late Cretaceous in a fluctuating shoreline environment. The problematic shales and <br />clay soils associated with landslides, unstable slopes, and mud flows in this area were deposited as low - <br />energy marine sediments. <br />Spring of 2011 was a very active season for landslides in Routt County. Infiltration from an unusually <br />large snowpack and heavy precipitation caused soil and weathered rock mass strengths to decrease, and <br />caused increased instability. Numerous new and reactivated landslides, soil slumps and mudflows were <br />observed during routine inspections by DRMS beginning in May 2011. <br />It is important to note that the Seneca II West mine permit boundary contains portions of four landslides <br />mapped by Colton (Colton, R.B. et al, 1975, Preliminary map of landslide deposits, Craig 1° x 2° <br />quadrangle, Colorado: U.S. Geological Survey, Miscellaneous Field Studies Map MF -700, scale <br />1:250000), many unstable slopes, potentially unstable slopes, mud flows, and debris flows /debris fans <br />identified during geologic hazard mapping of Routt County by Allen Miller in approximately 1977, and <br />landslides identified by Madole during surficial geologic mapping. These mapped features predate the <br />mine. Landslides and unstable slopes are a relatively common feature of the natural landscape in the area <br />within and surrounding this mine. <br />During our site visit, we evaluated all of the landslides that you identified within the context of the <br />following questions: <br />1) Would further movement threaten structures such as roads, ponds, stock tanks and drainages that are <br />to remain as permanent features after final bond release? <br />2) Is there evidence of excessive surface or subsurface water, contributing to instability, that can be <br />corrected with a reasonable level of effort? <br />3) Would repairs, such as surface regrading, create more disturbance and surface compaction, thereby by <br />reducing the potential effectiveness of post- repair revegetation efforts, than is justified by the current <br />(temporary) unsightliness of any bare landslide surfaces? <br />Please note that the repairs and stabilization measures discussed below have not been analyzed to <br />determine the degree of stability (factor of safety) that will be achieved. Improving surface and <br />subsurface drainage should help reduce, but will not eliminate, the risk of continued or future movement <br />in the event of another high snowfall or high precipitation year. If the state, mine operator or property <br />