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C. <br />• <br />50 <br />5.2.5 Pilot dewatering well cluster <br />Groundwater <br />As discussed in Section 1, one of the objectives of the project was to evaluate the <br />behavior of the various units of the Williams Fork Formation during long -term <br />dewatering. The location for the pilot dewatering test was chosen to evaluate <br />groundwater conditions in the proposed initial box cut, which will be located at the north <br />end of Phase 1 (Figure 1.2). The pumping well and piezometers were designed as an <br />integrated system and installed accordingly. <br />The pilot dewatering well cluster consisted of the pumping well (C- 04 -16B), six standpipe <br />piezometers (SP -1 though SP -6), and 18 VWPs installed in six borings. The location of <br />the pilot dewatering well cluster is shown in Figure 5.8. The piezometers were laid out <br />along lines corresponding to the major joint patterns reported for the area (Section 2). <br />Pumping well C- 04 -16B was designed to pump from the entire stratigraphic section <br />down to approximately 50 ft below the planned pit floor in the box cut. This will <br />eventually be the deepest part of the mine and represents the worst case dewatering <br />scenario. Three pilot borings for well C- 04 -16B were drilled using an 8.75 -inch bit and <br />air rotary to a depth of approximately 700 ft. Each boring was logged with a multi - <br />spectrum geophysical tool. Short term (air lift) tests were also performed to estimate <br />potential well yields. Although the results from the three pilot borings were very similar, <br />borehole C- 04 -16B was estimated to have the highest yield during airlifting and was <br />selected as the pumping well. The other two bore holes were completed as <br />piezometers. <br />The dewatering well was screened from 302 to 702 ft below land surface (bls), with <br />blank casing from 522 to 562 ft bls and 642 to 662 ft bls. The blank casing from 642 to <br />662 ft bls serves as a pump gallery. A well completion diagram and geophysical log are <br />provided in Appendix 5.D. The static water column of C- 04 -16B extends from <br />approximately 280 ft bls (just above the D coals) down to the H sandstone at a depth of <br />702 ft bls. <br />Eighteen piezometers were installed in six borings adjacent to the pilot dewatering well <br />(Figure 5.8) to monitor water levels in specific units of the Williams Fork Formation <br />during the long -term pilot dewatering test. In addition to the SP and VWP designations, <br />each location was also identified by using the C -04 -16 hole number and a letter (A and C <br />through M). <br />2572 -R2 Colowyo Coal Company <br />Water Management Consultants <br />