2010-04-12_REPORT - C1996083 (2)

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Vibrating Wire Piezometers Five vibrating wire piezometers, designated VWP-01 through VWP-05, were installed in coal refuse at CWDA No. 2 in June 2005. The piezometers were grouted in drilled boreholes as described by BGI (2006). Piezometer locations are presented on the attached site plan. Since installation two of the vibrating wire piezometers have stopped working correctly (VWP-01 and VWP-04). Diagnostic tests indicate that shorts have developed, likely due to damaged cables. Three additional vibrating wire piezometers, designated VWP-06 through VWP-08, were installed at CWDA No. 2 in June 2009 to supplement the remaining piezometers. Completion details of these piezometers are described by BGI (2009). Pore pressures measured to date are presented on the attached figure. Pore pressures have remained relatively steady for piezometers VWP-02, -03, -05, and -06. Pore pressures at VWP- 07 and -08 have decreased significantly since the last readings in December 2009. This is a good indication that internal drainage is occurring and that waste material is dewatering due to consolidation and trending towards equilibrium. Additionally, some of the stockpiled material has been shifted from the top of the waste pile and contributed to the decrease in pore pressure from December 2009 to March 2010. The measured pore pressures are well below the pore pressures at failure for similar confining pressures for the CU triaxial shear tests that have been conducted as part of the on-going stability evaluation for this refuse bank. Pore pressures are anticipated to continue to decrease as drainage occurs. In any event, on- going monitoring of all piezometers is recommended to monitor pore pressure responses to the on-going refuse placement. Otherwise, the measured pore pressures are consistent with the revised stability evaluation for the coal refuse performed by BGI (2008). Closing On-going monitoring of inclinometers and piezometers at CWDA No. 2 provides an indication of the stability of the refuse as well as an indication of probable performance for other refuse piles at Bowie. For these reasons, we recommend continued monitoring of this instrumentation on a quarterly basis. We sincerely appreciate the opportunity to work with you on this project. If you have any questions regarding this letter or the instrumentation monitoring at CWDA No. 2, please don't hesitate to contact me at (970) 497-8852 or dan0tuckhorn eg o.com. Sincerely, tws A*?"__ ""'?°• c, 11 BUCKHORN GEOTECH, INC. 38334 -a . e- Daniel C. Quigley, P.E. ?fsl0(ypL Enclosures: Instrumentation Site Plan, Inclinometer Cumulative Displacements, Summary of Pore Pressure Monitoring cc: Jim Stover, J.E. Stover and Associates Bowie Gob Pile 2 Instrumentation Monitoring 1st Qtr 2010.doc Project #05-126-GEO Page 2 of 3