REP48449

• Permit No: M1988112
• IBM Index Class Name: Report
• Doc Date: 1/31/1994
• Doc Name: THIRD PARTY TECHNICAL OVERSIGHT REPORT FOR THE SAN LUIS PROJECT TAILINGS DISPOSAL AREA PHASE II RAIS
• Media Type: D
• Archive: No

Third Party Oversight 5 Water, Waste & Land, Inc.
San Luis Mine January 10, 1993
' The design intent of the tailings disposal facility and in particular the tailings impoundment was i
that it was to perform as a managed discharge or thin lift tailings disposal facility. A thin lift
disposal operation discharges tailings from different locations around the perimeter of the disposal
' facility. The discharge locations are rotated on a schedule that allows the tailings tc be deposited
in relatively thin lifts (approximately i-foot thick). After discharge lines are moved, newly
deposited tailings are allowed to desiccate and density by evaporation and by drainage into S9"`8`:~tr
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9
underlying tailings. This type of discharge system works with a sufficiently large tailings surface P°'
area and dry climate to allow drying of the thin lifts prior to subsequent tailings discharge.
The facility has been designed far phased construction. After the initial phase of
' embankment construction, subsequent lifts are constructed in an upstream manner, with each
embankment raise constructed over previously deposited tailings. The embankment was designed
' as afree-draining or "leaky" embankment, constructed with sands and gravels to expedite drainage
of tailings near the embankment. The design cross section is shown on Figure 1.
' In addition to the free-draining embankment, a drainage layer was designed above the liner
system to promote drainage of tailings fluids and convey this drainage through the toe of the
' embankment (Figure 11. This drainage layer was also designed to minimize the zone; of saturation
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liner system.
The liner system (which extends beneath the free-draining embankment) consists of a
-tompesite-synthetic liner underlain by aloes-permeability soil zone. The synthetic li~ier is a 40-mil
/
1 thick very low density polyethylene (VLDPE), and the low-permeability zone is a compacted silt.
' 2.3 GEOTECHNICAL ISSUES WITH RAISE I EXPANSION
The tailings disposal facility was approved for Phase I construction, with! the Phase II
design approved in concept. The detailed design of the Raise I Expansion was subrcitted for DMG
review in July 1993 (SRK, 1993a).
DMG's original concern with the Raise I Expansion was with potential instability of the
Raise I Embankment as it was to be constructed over recently deposited tailings near the original
embankment. The Raise I Embankment could became instable if saturated zones of tailings were
located beneath the future Raise I Embankment. As a saturated zone of tailings e>:isted beneath
the embankment (SRK, 1993a1, the potential for unacceptable deformation of the Raise I
Embankment due to liquefaction of tailings did exist (DMG, 1993a). These concerns were
alleviated after BMRI installed DMG mandated piezometers through the existing embankment
(BMRI, 1993f). It was verified that the phreatic surface was no higher than 10 feet above the
Phase I Embankment foundation/liner interface (BMRI, 1993f). DMG also required that BMRI