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Roadside Mine <br />#00489 -0004 <br />06/19/14 <br />'j�,t Huddleston -Berry <br />iy F"y- cti7M�.11C <br />Initial Hazard Evaluation <br />It is important to note that HBET was responsible for soil compaction testing during placement <br />of the earthen plug. Our records indicate that native silty sand soils were used to build the plug <br />and that the plug was placed in well compacted lifts. HBET understands that the `core' of the <br />earthen plug is at least 25 feet thick. While the slope at the back of the plug is unknown, the face <br />of the plug lies at 2H:1 V. Therefore, the earthen plug is at least 25 feet thick at the top and at <br />least 39 feet thick (assuming a 7 ft. high opening) at the base. However, the back slope of the <br />plug is likely not vertical and as a result, the actual thickness of the plug at the base is likely <br />significantly higher. <br />As indicated above, the seepage at the south side of the portal was very clear. This suggests that <br />piping is not occurring in the earthen plug. In addition, no mass earth movement was observed <br />in the saturated soils in this area. In general, based upon our observations and experience with <br />earthen embankments, HBET does not believe that there is an imminent risk of catastrophic <br />failure of the earthen plug at the south opening. <br />With regard to the north opening, the seepage from this area was minimal. In addition, the face <br />of the earthen fill was dry and no evidence of mass earth movements was observed in this area. <br />Again, based upon our observations, HBET does not believe that there is an imminent risk of <br />catastrophic failure of the earthen plug at the north opening. <br />Conseauences of Failure <br />Due to the fact that the precise conditions within the mine and /or earthen fill are impossible to <br />determine, in addition to evaluating the risk of failure, HBET conducted an evaluation of the <br />consequences of failure. <br />Maps of the Roadside Mine indicate that a concrete plug was installed in each of the two slopes <br />behind the earthen plugs. In addition, although HBET does not have any detailed cross sections <br />along the slopes, spot elevations on the mine maps indicate that the slopes rise for a few hundred <br />feet and then fall down towards the mine works. <br />Based upon measurements taken from the mine maps, HBET estimated that each slope could <br />contain approximately 100,000 gallons of water between the earthen plug and concrete plug. <br />However, this volume assumes that the slopes are full and that the earthen plug is vertical on the <br />back side (very conservative assumption). <br />In addition, assuming that the high spot elevation on the mine maps represents the high point of <br />the slopes, HBET estimated that each slope could contain approximately 200,000 gallons of <br />water behind the earthen plug. However, this volume assumes that both slopes are full (again, <br />very conservative assumption). Any water beyond the inflection point would tend to flow down <br />towards the mine works. <br />Review of the site topography suggests that the majority of any discharge of water /debris from a <br />catastrophic failure of the earthen plug would primarily follow the existing road and flow down <br />the steep slopes west of the portal. The flow would lose a significant quantity of energy at the <br />base of the steep slopes and would tend to spread out in the lowlands where HBET understands a <br />pond previously existed. <br />XA2008 ALL PROJECTS100489 - J& Storer Associales 1nc100a89 -0004 L061914.doc 2 <br />