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REV91728
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REV91728
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Entry Properties
Last modified
8/25/2016 3:13:14 AM
Creation date
11/21/2007 11:14:20 PM
Metadata
Fields
Template:
DRMS Permit Index
Permit No
C1996083
IBM Index Class Name
Revision
Doc Date
5/18/2006
Doc Name
Adequacy Review & Attachments
From
DMG
To
Bowie Resources, LLC
Type & Sequence
PR10
Media Type
D
Archive
No
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<br />ass umptions listed above, the resulting tan ~, of 22 degrees could be overestimated The research presented <br />in support of the results is only one of many studies relating residual strength to physical properties of clay <br />type soils. Other studies show lesser values for residual shear strength are possible for soils with similar <br />physical characteristics to the slide zone material. Another axes the report does not address are the <br />additional displacements that have occurred since the original slide took place. Subsequent movement since <br />the initial slide displacement would contribute to further strain softening along the existing failure plane, <br />Therefore, the existinglandslide geometry should also be modeled to determine residual shear strength along <br />the failure plane. If a lesser residual sheaz strength results, the pseudo-static factors of safety and the <br />dynamic deformations should be reevaluated. <br />The total computed cumulative deformation of the landslide from seismic loading was reported to <br />be 1 I inches, but the report does not address a possible progressive failure mechanism the landslide would <br />likely incur or additional strain softening along the failure zone with each increment of movement. Eleven <br />inches of strain would appear sufficient to cause rupture along the entire length of the failure surface. <br />Should complete displacement of the landslide occur when the reservoir is full, water displaced by the soil <br />mass could overtop the dams. If the dams survive overtopping should it occur, resulting damage may be <br />limited to decreased reservoir volume and possible blocked intake strucrure for the outlet works. The <br />report provides some assurance the extent of the landslide does not infringe on the footprint of the main <br />embankment. Boring BD-104 (maximum depth 23.5, drilled in the upstream shell at the right end of the <br />main dam, revealed no indication of a potential shear zone nor other weakness in the foundation or the <br />embankment. Based on the subsurface investigations, movement of the landslide should not trigger <br />displacement or deformation in the main dam. <br />In summary, the report addresses the prunary issues pertaining to potential impacts on the integrity <br />of the embankment dams from mininginduced seismic loadings. Borings drilled through the embankments <br />and into the foundation revealed no apparent zones of weakness or zones susceptible to liquefaction. For <br />the seismic loads analyzed, the embankments would not appear to experience significant deformation. <br />However, the landslide warrants further analysis and evaluation of the potential consequences. The <br />parametric model used to determine shear strength for the slide zone did not evaluate probable variability <br />of all parameters. Variability of data such as geometry of the failure surface, strength of the other materials, <br />and level of the phreatic water surface in the slide have a dtrect bearing on the results. Therefore, additional <br />analysis of the landslide is recommend to evaluate the full range of plausible shear strengths and conditions. <br />In addition, because of uncertainties in the seismic loads evaluated, the requested change in monitoring <br />requirements was not substantiated. Collection of background data is even more imperative to corroborate <br />study assumptions, especially crucial since the revised plan shows miningwould now occur within a 1 /4 mile <br />radius of the dams. From a dam safety standpoint, the recommended frequency of three times each pear <br />for measurement of piezometers is not sufficient to establish baseline performance levels. Initially, <br />piezometers should be measured at least weekly for the first two years in order to establish a historic <br />performance record along with the reservoir level at the time of the measurement. Measurements would <br />be especially critical in the Spring and throughout the irrigation season when storage of water is most likely <br />at maximum levels. Monitoring should also include periodic surveys of movement monuments. <br />Formulation of a comprehensive monitoring plan as described in my previous letter of November 7, 2001 <br />would be prudent. Monitoring results associated with the dams must be evaluated by a registered <br />professional engineer and a report submitted to the State Engineer annually. <br />I hope this provides sufficient information to prepare your Division's adequary response. Please <br />call me if you have questions or need further clarification. <br />Sincerely, ~ ~~~ <br />es G. Norfleet, P.E. <br />Dam Safety Engineer <br />cc: Frank Kugel, Acting Division Engineer <br />Jack Byers & Doug Boyer, Dam Safety Branch <br />
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