2014-02-13_REVISION - M1977493 (2)

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Review Assistance Memorandum — Mayflower Tailings Storage Facility (aka 5 Dam) February 13, 2014 Page 2 of 6 2. Page 1 -2: The last paragraph describes the deposition berm constructed of compacted tailing sand when tailing deposition was resumed in 2012. It is assumed the projected raise of the impoundment will continue to use similar deposition berms, but this is not discussed in the report or shown on the seepage and stability analyses sections. Section 2 - Previous Analyses 3. The discussion of previous investigations and analyses was thorough, and the review of previous work provided a reasonable basis for the current analyses. Section 3 - 2012 Field Investigation 4. The most recent (2012) field investigation appears to have been thoroughly planned and executed to provide essential information on the existing project geometry and to provide additional information about the tails and foundation for use in the current analyses. Section 4 - Laboratory Testing 5. The laboratory testing program adequately identified and characterized the foundation and dam materials. The engineering properties selected for the analyses are supported by the test results, which are documented in the report appendices. Section 5 - Subsurface and Material Characterization The field investigation data (CPT soundings and SPT blowcount data) were used with the results of the laboratory tests to identify the engineering properties of the foundation and dam materials. 6. Section 5.1.2 - The conclusion that the tails are generally more dilative than contractive is adequately supported. The tails are not expected to experience widespread loss of strength due to earthquake- induced liquefaction from the Operating Basis Earthquake (OBE) event. However, the OBE event has a return period of only 475 years, which, while longer than the life of the mine, is much shorter than the time the closed impoundment must be managed. (See discussion of Section 7.1 below.) Section 5.1.3 - The shear strength of the slimes was estimated based on historic material characterization, previous testing, and experience at similar tailing dams. The slimes are estimated to have a measureable cohesiveness (500 psf), so they are not likely liquefiable. However, a substantial deposit of the slimes will underlie the final dam at a significant depth. Neither the potential for liquefaction nor the potential for loss of strength due to elevated pore pressures in this material was expressly evaluated and discussed in the report. 8. Section 5.1.4 - The description of the sludge layer location is somewhat confusing. The layer is estimated to be up to 3' thick at the beach at the upstream end of the current pond (1200' from the dam), increasing to about 30' thick at the "back" of the pond. Figure C -1 (Appendix C) shows the "back" of the pond to be farther upstream than the beach upstream of the dam, so the sludge layer does not appear to underlie the projected final dam. Section 6 - Seepage Analyses 9. Section 6.3 - The calibration of the seepage model to existing conditions is appropriate. However, the description of the calibrated model as using a phreatic surface 1 foot below the latest reading from Piezometer TH2006 -5 -1 (upstream of the blanket drain) but 8 feet above the highest reading for Piezometer P -4 (downstream of TH2006 -5 -1, and which appears to penetrate the blanket drain) is confusing. Is the highest reading in P -4 the latest reading? Using a higher phreatic surface would be conservative, but the description appears inconsistent. Piezometer locations are not shown on the model results.