2008-09-16_PERMIT FILE - C1982056A (4)

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which is 6 feet less than the original design, however, the construction process was essentially the same as previously approved under TR97-30, as modified by MROl-175. Design calculations for the depth of fill required to distribute loading on the road are based on an estimate of soil shear strength of 5.21 psi in the low areas adjacent to the creek and 10.4 psi in the other areas. Discussions with an engineer with experience constructing roads in this azea indicate that typical sheaz strengths for this type of soil range from 13 psi to 20 psi, so the values used for design calculations are very conservative. Northwest Colorado Consultants developed a computer model of the road sections and determined factors of safety for deep failwe of the subgrade materials in the range of 2.0 and for shallow failwes in the range of 1.7; with°1-a~sideslopes°on the embanlanents. Based on this design information, a ' design incorporating geo-fabric and geo-grid materials to distribute loads over the soft soil base was developed. Tensaz design and stability analysis are provided in Exhibit 48. SEDCAD+ modeling runs were completed for the 10-yeaz, 24-how storm over the contributing drainage basin. The model assumes that the peak flow from Pond A will be occurring concurrent with the 10-yeaz, 24-hour event over the basin. A peak flow of 41.86 cfs was determined at the crossing and a minimum pipe diameter of 30 inches was determined to be adequate to handle the peak flow. Data was reviewed for USGS stream gauging station 09243800 (station 800), located approximately 2,500 feet upstream of the crossing. An instantaneous peak flow of 65 cfs was recorded on May 5, 1996. The recording period for station 800 is 1976 to 1996. An additional culvert run was performed with SEDCAD+ for the instantaneous peak flow of 65 cfs. Headwater depth for this run was based on the design roadway elevation of 6.5 feet above the invert of the culvert. A 36-inch culvert will safely pass the highest recorded instantaneous peak flow with the additional headwater depth provided by the roadway embankment. In order to assure additional capacity for potential blockage, a 70-foot, 42-inch CMP, or up to a 48- inch CMP, was installed at the crossing. Slope protection is provided on both the upstream and downstream sides of the crossing using rock rip-rap. Drainage from the road surface flows along the ditch line provided by the safety berm. Rock filters or other sediment control measwes are constructed at locations where road drainage flows into the creek as necessary. Drainage from the Batch Weigh/Loadout area is currently directed along the railroad tracks to Foidel Creek. The road on the west side of Foidel Creek is located in a previously distwbed old roadbed, therefore topsoil was not . available for salvage. The road on the east side of Foidel Creek is constructed along the toe of Pond C, which is also in a previously distwbed azea. On the east side of Foidel Creek, limited quantities of topsoil are available for use in final reclamation. The topsoil was windrowed to the side or placed in a stockpile adjacent to the road. Minor grading was done in the road corridor to provide a satisfactory base far subsequent road construction. A structwal geo-grid material was placed above geo-fabric to distribute loads over the soft soils more uniformly and reduce the requrred thickness of fill material required which, in tum, reduces the overall footprint of the roadway. Openings in the geo-grid structwe are approximately 1 inch by 1 inch. The fill material locks into these openings and increase the structural stability of the sub-grade. Safety berms were constructed on either side of the roadway. The berms are constructed of spoils materials from the mine site. This spoils material has been used extensively on the mine site in the past as fill material and is not acid-producing. During construction, temporary erosion control measwes were implemented as necessary. Long-term erosion protection is provided by seeding outslopes with an appropriate seed mix and use of selectively placed rock check dams. Mulching, fiber mats, topsoil, or other appropriate medium for the establishment of plant growth may be applied, as necessary, to asswe adequate vegetative establishment. Affected wetlands areas will be revegetated with vegetation native to the azea as detemuned in project wetlands delineations. The total azea of wetlands disturbed by this project is approximately 0.01 acres. Upon reclamation of this roadway, TCC will seed a wetlands area of at least 0.10 acres to satisfy the U.S. Fish and Wildlife's request fora 2:1 mitigation of wetlands area disturbed by the crossing. 3 Access Roads 3A. Fish Creek Borehole DewaterinQ Site Providing access to the Fish Creek Borehole Site will involve upgrading 6500 feet of an existing road and construction of an additiona12500 feet of road. The location of the area and access road is shown on Map 26, Fish Creek Borehole Area and Access Road. The existing road was designed so that construction, maintenance, and postmining condition of the road will control erosion, siltation, and pollution of water and prevent damage to public TR06-55 2.05-72 07/26/06