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May 29, 2009 <br />Page 18 <br />Stresses on the sliding plane are limited to the lower portion of the gulch, suggesting <br />instability is only local when failure occurs. <br />• Conservatively modeling the fill as a weathered shale still results in a stable embankment. <br />Based on the modeling of the Horse Gulch fill, AAI believes the proposed fill in its <br />current configuration provides long-term stability and represents no safety hazard. <br />4.0 UNDERDRAIN DESIGN AND CONSTRUCTION RECOMMENDATIONS <br />The key to an effective underdrain design is to maintain drainage during construction and <br />for the life of the fill. Proper drainage can minimize the potential inflow in the life of the fill. <br />Therefore during the early phases of construction, the drain must be functional in order to accept <br />storm water. Silt fencing and or straw bales or other means of runoff control may be required to <br />mitigate fines infiltration into the core during construction. <br />Infiltration of the underdrain core with fines can cause the drain to fail which can result in <br />water rising into the spoils, destabilizing the fill. This phenomenon can be caused by either <br />under-sizing the drain or by having an inadequate filter system to prevent migration of the fines <br />into the drain core. The fines infiltration problem is most severe when the overburden is soil or <br />slakable rock. This is simply due to the amount of fines in the overburden. However, during the <br />early phase of drain construction, the drain is susceptible to fines from the overburden that are no <br />longer contained by vegetation. The regulations are written to mitigate the fines generated by the <br />overburden by requiring a 16-ft-thick drain core when the overburden is predominantly shale. <br />4.1 Underdrain Materials <br />The source rock for the underdrain in Horse Gulch is the series of sandstone layers that <br />form K-Ridge. Figure 21 shows the material in the tension escarpment in the G-Pit slide near <br />K-Ridge. Figure 22 shows the underdrain material that has been stockpiled on K-Ridge. This is <br />a very durable sandstone suitable for the underdrain core. <br />4.2 Filter System <br />Filter systems can be broken into two types: natural and synthetic. Natural refers to use <br />of on-site rock of a suitable gradation to prohibit migration of fines into the underdrain. <br />Synthetic systems use geo-textile fabrics to accomplish this. A major factor in determining the <br />suitability of either alternate is the transportation mechanism of fines. There is a tremendous <br />difference in mitigating the effects of gravity and mitigating those of groundwater. Based on the <br />hydrologic conditions at the site, this filter system will not have to contend with active <br />groundwater. <br />Rock filters can be quite effective in certain situations, such as sandstone overburden and <br />limited surface water infiltration. Observing the actual spoils from the G-Dip Pit stripping, AAI <br />believes that the materials for the Horse Gulch fill are suitable for constructing the underdrain <br />utilizing the natural segregation at the toe of the spoils pile to create a rock filter. Figure 23 <br />shows the natural segregation taking place from materials that are essentially the same as <br />anticipated in the K-Pit spoils. <br />Agapito Associates, Inc.