Laserfiche WebLink
rating 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 16, 2011 site visit <br />we observed that VWP-05 and VWP-06 had been buried by gob and could not be located for <br />reading. <br />Pore pressures measured to date are presented on the attached figure. Pore pressures have <br />remained relatively steady for piezometers VWP-02 and -03. Pore pressures at VWP-07 and <br />VWP-08 increased slightly between the December 2010 and March 2011 readings. This is most <br />likely due to the placement of new gob material near the instruments at those sites. Readings <br />at VWP-08 have generally 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. All measured pore pressures are well below the <br />pore pressures at failure for similar confining pressures for the CU triaxial shear tests that have <br />been conducted as part of the on-going stability evaluation for this refuse bank. All other VWP <br />readings showed slight to significant decreases in pore pressures since the December 2010 <br />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 />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(abuckhornaeo.com. <br />Sincerely, <br />BUCKHORN GEOTECH, INC. <br />Daniel C. Quigley, P.E. <br />Bowie Gob Pile 2 Instrumentation Monitoring 1st Qtr 2011.doc <br />Project #05-126-GEO <br />Page 2 of 3