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San Luis Project - Phase 11 Raise 1 Expansion Conawction States Report ' densify. As the tailings densify, the permeability is reduced and results in a low permeability barrier to downwazd seepage migration from subsequent tailings deposition. Critical to the performance of this deposition technique is the rate at which the tailings layers aze deposited. ' The rate of deposition must be slow enough to allow for the evaporation and desiccation to occur. During the initial stages of tailings discharge into the facility, the geometric configuration of the ' disposal area did not allow for a slow enough rate of tailings deposition for full desiccation to occur. Therefore the embankment was designed using the fully drained embankment dam concept to provide a pathway for interstitial water to escape from these tailings in addition to ' the drainage blanket overlying the liner. In addition, during the startup of the facility prior to the development of a beach of tailings against the embankment, the potential existed for free water (from deposition and storm events) to impound against the fill. The design therefore initially incorporated conventional water retaining dam design for which a zoned, filter compatible structure was requ1red. 1 With the tailings beach against the fill and a free water pond remote from the embaNcment, the Phase I requirements for water retention no longer exist. The use of a zoned fill was therefore not considered necessary for the Phase II raises. ' As the quantity of tailings in the impoundment increased, the rate of deposition, or rate of rise, decreased and allowed for the full thin-lift, subaerial deposition processes occur. Thus, the ' ability of the embankment to allow drainage from the tailings to occur became less critical as the tailings level in the facility increased. Prior to the need for the embankment raise, very little ' or diminimus amounts of the slurry water aze actually drained through the embankment. The water which does drain into the embankment comes only from the tailings immediately adjacent ' to the embankment. Conswction of the main, Phase I, embankment consisted of a zoned earthfill incorporating Type 2 material, similar to the material currently utilized to construct the raise, in the upstream shell of the embankment. This material was also required as a filter to prevent piping of tailings into the embankment fill. Field work conducted prior to construction of the raise has demonstrated a mazked lack of saturated horizons within the tailings deposited adjacent to the Phase I embankment. In fact, the tailings in this area aze less saturated than was anticipated during the ' October, 1993 16 SRK Project No. 14725