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Operations as detailed in the technical revision application for this project will be completed as the initial phase of <br />• construction activities. In the event construction is significantly delayed, the associated geotechnical investigation <br />drill sites will be evaluated and appropriate temporary sediment control measures will be implemented as <br />necessary to ensure off -site sediment transport is controlled. <br />Geotechnical Investigation - Sandstone Cliff Mitigation Measures (MR95 -117) <br />Twentymile Coal Company will conduct a geotechnical investigation of the proposed construction sites for mitigation <br />measures to be utilized to protect County Road 27 from hazards associated with rockfall potential beneath the <br />Twentymile Sandstone. Detailed designs and a technical revision application for construction of the specific <br />mitigation measures are presented as TR -19 (see Exhibit 7F). The geotechnical investigation for the proposed <br />measures entails the drilling of five (5) shallow holes. Four (4) ofthe proposed drilling locations are located north of <br />County Road 27 at locations within areas proposed for disturbance by construction of the sandstone cliff mitigation <br />measures. For purposes of the mitigation measures design study, County Road 27 in this vicinity was divided into <br />several zones (see Exhibit 7F). The drill sites located within proposed mitigation measure construction areas are <br />located in zones I, 3, 4, and 5. One additional proposed drill site is located south of County Road 27 in zone 2. This <br />site is intended to provide information for the construction a portion of rerouted County Road 27 and is also within an <br />area proposed for disturbance. <br />Drilling will be conducted with a self - leveling track - mounted portable drilling rig (CME 45 mounted on Bombardier <br />Snow Cat). Four inch (4 ") diameter continuous flight power augers will be utilized. Drill holes will be driven <br />approximately 10 to 15 feet with core samples obtained as necessary to evaluate construction requirements for the <br />proposed mitigation measure structures. No drilling fluids will be utilized in association with the rig and no structures <br />or preliminary site preparation work will be necessary to support the drilling effort. <br />• All five sites are located in areas of previously undisturbed native topsoil. Drilling is estimated to take approximately <br />one day although holes will be left open for two to three days to evaluate the potential presence of groundwater. No <br />topsoil salvaging operations are proposed in association with the drilling program as drilling will not result in any <br />adverse impacts to the topsoil resource. All drill sites are located in areas to be further disturbed during construction <br />the sandstone cliff mitigation measures. Topsoil will be salvaged as appropriate during the initial constriction phase <br />of the mitigation measure project. <br />The track mounted equipment utilized for the geotechnical investigation will produce minimal disturbance in <br />traveling over undisturbed ground and drilling for short durations. Particular care will be taken to access the sites <br />during a period when dry ground conditions are prevalent. To the extent possible, cuttings generated during the <br />drilling process will be returned to the drill holes prior to abandonment to minimize potential sediment transport from <br />each site. In the event an aquifer is intercepted during drilling at any site, the affected drill hole will be sealed using a <br />grout plug in the aquifer zone and then will be backfilled to the surface with cuttings. In the event formation water is <br />encountered during drilling and liberated on the surface, silt fences will be erected as secondary sediment control <br />measures to minimize the transport of sediment off site. <br />Construction of the mitigation measures is anticipated to commence during the summer of 1995. In the event <br />construction initiates on schedule, permanent sediment control measures and topsoil salvage operations as detailed in <br />the technical revision application for this project will be completed. In the event construction is significantly delayed. <br />the associated geotechnical investigation drill sites will be evaluated and appropriate temporary sediment control <br />measures will be implemented as necessary to ensure off-site sediment transport is controlled. <br />Water System <br />Water for the surface facilities and the mine is supplied by two water systems. The first source is TCC well No. 1 <br />which is a 407 foot deep well. Well water is pumped into a 10,000 gallon surge tank and a centrifugal pump delivers <br />MR 97 -154 2.05 -49 Revised 9 /9/97 <br />