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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). <br />Pore pressures measured to date are presented on the attached figure. Pore pressures have <br />remained relatively steady for piezometers VWP-02, -03, -05, and -06. Pore pressures at VWP- <br />07 and -08 have decreased since a peak was recorded in January 2010. This is a good <br />indication that internal drainage is occurring and that waste material is dewatering due to <br />consolidation and trending towards equilibrium. Additionally, some of the stockpiled material <br />has been shifted from the top of the waste pile and contributed to the decrease in pore <br />pressure from January 2010 to December 2010. The measured pore pressures are well below <br />the pore pressures at failure for similar confining pressures for the CU triaxial shear tests that <br />have been conducted as part of the on-going stability evaluation for this refuse bank. Note that <br />VWP-03 showed a slight increase in pore pressure between the March 2010 and June 2010 <br />readings which was interpreted as an indication of gob materials being temporarily stored in <br />this area during compaction efforts. This site has shown a trend in decreasing pore pressure <br />which is to be expected if either (a) internal drainage is working or (b) if temporarily stored gob <br />material has been moved since June 2010. All other VWP readings showed slight to significant <br />decreases in pore pressures since the September 2010 readings. <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 />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 danCa buckhorngeo.com. <br />Sincerely, <br />BUCKHORN GEOTECH, INC. <br />0 too, <br />'Q38334 <br />Daniel C. Quigley, P.E. ,? • <br />,V %12-.3,0_11D go 4 <br />% <br />Bowie Gob Pile 2 Instrumentation Monitoring 4th Qtr 2010.doc os•'••"? <br />Project #05-126-GEO 4NAL <br />Page 2 of 3