REP22489

• Permit No: M1988112
• IBM Index Class Name: Report
• Doc Date: 6/10/1994
• Doc Name: 3RD PART TECHNICAL OVERSIGHT REPORT FOR THE SAN LUIS PROJECT TAILINGS DISPOSAL AREA PHASE II RAISE I
• Media Type: D
• Archive: No

' Third Perty Oversight 5 Weter, Weste & Lend, Inc.
Sen Luis Mine June 9, 1994
' The tailings disposal facility is a valley impoundment with an embankmerrt constructed
across the valley and tailings contained in the lined portion of the valley above the
embankment. The design intent of the tailings disposal facility was 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 to be deposited in relatively thin lifts (approximately 1-foot
thick). After discharge lines are moved, newly deposited tailings are allowed to alesiccate and
densify by evaporation and by drainage into underlying tailings. This type of managed
discharge system functions adequately when a sufficiently large tailings surface area in
' combination with a relatively dry climate allow drying of the thin lifts prior tco subsequent
tailings discharge.
The facility was designed for phased construction. After the initial phase of
embankment construction, subsequent lifts were to be 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 sections 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 minirmize the zone
of saturation (and driving head for flow) above the finer system, as well as provide a
protective layer for the liner system.
' The composite liner system (which extends beneath the free-draining embankment)
consists of a 40-mil thick very low density polyethylene (VLDPE) synthetic liner underlain by
' alow-permeability compacted silt soil zone.
2.3 GEOTECHNICAL fSSUES ASSOCIATED WfTH THE RAISE 1 EXPANSION
The tailings disposal facility was approved for Phase I construction, with the Phase II
design approved in concept. The detailed design of the Phase II, Raise I Expansion was
' submitted 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 become unstable or experience
excessive settlement if constructed over saturated tailings. As a saturated zome of tailings
was discovered beneath the embankment during field investigations (SRK, 1993a), the
potential for unacceptable deformation of the Raise I Embankment did exist IDMG, 1993a1.
These concerns were alleviated after BMRI installed DMG-mandated piezometers through the
' existing embankment (BMRI, 1993f), and it was verified that the phreatic surface was no
higher than 10 feet above the Phase I Embankment foundation/liner interface (BAJIRI, 1993f).
DMG also required that BMRI initiate a monitoring program to observe the quanrtity and rate
' of tailings consolidation which was occurring as construction of the Raise I ffmbankment
continued (DMG, 1993c1.