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Third Party Oversight 13 Weter, Waste & Lend, Inc. <br />San Luis Mine June 9, 1994 <br />As shown in the tables above, the major change to the Type 1 material specifications <br />was to allow for a significant increase in the percent passing the #40 and #200 (fines) <br />screens. With increasing fines content, permeability of the embankment could potentially be <br />' lower than that which was intended in the original design and lower than that of the original <br />(Phase II embankment. A lower permeability within the embankment could result in a <br />decrease in the rate of tailings fluid drainage into the embankment, and a t:orresponding <br />' increase in the phreatic surface within the tailings <br />In addition to the change to the gradation specification, a larger maximum particle size <br />' was defined in the revised specifications. The potential for piping of tailings into and through <br />the embankment existed if significant quantities of oversized materials were placed in the <br />embankment concurrently with low compactive effort. <br />4.2 INITIAL CONSTRUCTION <br />' During the initial site visit by WWL and DMG on October 28, 19913, significant <br />quantities of material larger than the revised specification for maximum size (6 inches) were <br />observed on the top surface and upstream face of the embankment. Materials up to 30 <br />inches in diameter were located along the upstream face of the embankment. It was not <br />possible to determine the actual quantity (percentage) of oversized materials being placed as <br />' embankment fill was not being hauled and placed during the inspection. <br />4.3 TECHNICAL ISSUES <br />The key technical issues associated with Raise I Embankment materiials are shear <br />strength (for acceptable static and seismic stability), permeability (for drainage of tailings <br />fluids), and filter compatibility (for protection against piping of tailings into the embankment). <br />The well-graded material proposed for the Raise I Embankment in the original specifications <br />(SRK, 1993c) would provide acceptable shear strength and protection against piping, if <br />properly compacted. <br />The remaining issues concerning embankment material are permeability and drainage. <br />' Although materials used for the embankment were not within the specifications stated in the <br />original design report ISRK, 1993a1, acceptable drainage can be verified by the installation and <br />monitoring of piezometers as shown in Figure 1. Five piezometers installed at the crest of the <br />' embankment and screened at or near the base of the fill, and five piezometers irnstalled at the <br />toe of the upstream face of the embankment would allow the phreatic surface within the <br />embankment and within the tailings near the embankment to be monitored (if present). Water <br />levels within the piezometers should be monitored on a quarterly basis. <br />' 4.4 FINAL CONSTRUCTION <br />Although some oversized materials were incorporated into the embankment, equipment <br />' operators provided good compactive effort and attempted during material placement to either <br />break down or move the oversized materials to the upstream and downstream embankment <br />faces. <br /> <br />