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Vibrating Wire Piezometers <br />Five vibrating wire piezometers, designated VWP -01 through VWP -05, were installed in coal <br />refuse at CWDA No. 2 in June 2005. The piezometers were grouted in drilled boreholes as <br />described by BGI (2006). Piezometer locations are presented on the attached site plan. Since <br />installation two of the vibrating wire piezometers have stopped working correctly (VWP -01 and <br />VWP -04). Diagnostic tests indicate that shorts have developed, likely due to damaged cables. <br />Three additional vibrating wire piezometers, designated VWP -06 through VWP -08, were <br />installed at CWDA No. 2 in June 2009 to supplement the remaining piezometers. Completion <br />details of these piezometers are described by BGI (2009). During the March 29, 2012 site visit <br />we were able to extract information at installations VWP -06 and 08. All other vibrating wire <br />piezometers have been damaged or buried beyond readability. Damaged or missing <br />piezometers will be repaired or replaced in the second quarter 2012 when site conditions are <br />acceptable and the spring runoff has subsided. <br />Pore pressures measured to date are presented on the attached figure. Pore pressures at VWP - <br />06 decreased slightly between the December 2011 and March 2012 readings, while the readings <br />at VWP -08 remained constant during that period. Those reading are consistent with consistent <br />loading from new gob material at those sites and the readings indicate generally stable <br />conditions at those piezometer sites. Readings at both piezometers have generally decreased <br />since they were installed. This is a good indication that internal drainage is occurring and that <br />waste material is dewatering due to consolidation and trending towards equilibrium. All <br />measured pore pressures are well below the pore pressures at failure for similar confining <br />pressures for the CU triaxial shear tests that have been conducted as part of the on -going <br />stability evaluation for this refuse bank. <br />Pore pressures are anticipated to continue to decrease as drainage occurs. In any event, on- <br />going monitoring of all piezometers is recommended to monitor pore pressure responses to the <br />on -going refuse placement. Otherwise, the measured pore pressures are consistent with the <br />revised stability evaluation for the coal refuse performed by BGI (2008). <br />Closing <br />On -going monitoring of inclinometers and piezometers at CWDA No. 2 provides an indication of <br />the stability of the refuse as well as an indication of probable performance for other refuse piles <br />at Bowie. For these reasons, we recommend continued monitoring of this instrumentation on a <br />quarterly basis. We sincerely appreciate the opportunity to work with you on this project. If <br />you have any questions regarding this letter or the instrumentation monitoring at CWDA No. 2, <br />please don't hesitate to contact me at (970) 497 -8852 or dan @buckhorngeo.com. <br />Bowie Gob Pile 2 Instrumentation Monitoring 1st Qtr 2012.doc <br />Project #05- 126 -GEO <br />Page 2 of 4 <br />