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Third Party Oversight 13 Water, Waste & land, Inc. ' Sen Luis Mine June 9, 1994 As shown in the tables above, the major change to the Type 1 material ecifications was to allow for a significant increase in the percent passing the #40 and~#200 (fines) screens. With increasing fines content, permeability of the embankment could potentially be ' lower than that which was intended in the original design and lower than that df the original (Phase I) embankment. A lower permeability within the embankment could result in a decrease in the rate of tailings fluid drainage into the embankment, and a corresponding increase in the phreatic surface within the tailings. ' In addition to the change to the gradation specification, a larger maximum particle size was defined in the revised specifications. The potential for piping of tailings into and through the embankment existed if significant quantities of oversized materials were placed in the embankment concurrently with tow compactive effort. 4.2 INITIAL CONSTRUCTION ' During the initial site visit by WWL and DMG on October 28, 1993, significant quantities of material larger than the revised specification for maximum size 16 ilrlchesl were observed on the tap surface and upstream face of the embankment. Materials up to 30 inches in diameter were located along the upstream face of the embankment. It was not possible to determine the actual quantity (percentage) of oversized materials being placed as embankment fill was not being hauled and placed during the inspection. 1 4.3 TECHNICAL ISSUES The key technical issues associated with Raise I Embankment materialls are shear strength (for acceptable static and seismic stability), permeability Ifor drainage of tailings fluidsl, and filter compatibility (for protection against piping of tailings into the enbankment). The well-graded material proposed for the Raise I Embankment in the original slpecifications (SRK, 1993c) would provide acceptable shear strength and protection against piping, if ' properly compacted. The remaining issues concerning embankment material are permeability ahd drainage. ' Although materials used for the embankment were not within the specifications stated in the original design report (SRK, 1993a1, acceptable drainage can be verified by the installation and monitoring of piezometers as shown in Figure 1. Five piezometers installed at the crest of the embankment and screened at or near the base of the fill, and five piezometers installed at the ' toe of the upstream face of the embankment would allow the phreatic surface within the embankment and within the tailings near the embankment to be monitored (if present). Water levels within the piezometers should be monitored on a quarterly basis. 1 4.4 FINAL CONSTRUCTION ' Although some oversized materials were incorporated into the embankment, equipment operators provided good compactive effort and attempted during material placement to either ' break down or move the oversized materials to the upstream and downstream embankment faces.