REP29179

• Permit No: M1988112
• IBM Index Class Name: Report
• Doc Date: 7/1/1991
• Doc Name: SAN LUIS PROJECT TAILINGS IMPOUNDMENT CONSTRUCTION REPORT
• Media Type: D
• Archive: No

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4-in corrugated, perforated HDPE pipe with a filter fabric sock was installed in the invert of the
drain, During the fmal grading operations the drill casing was regrouted and sealed.
2.4 Lower Impoundment Foundation
The lower impoundment foundation excavation can be divided into three separate zones. These
zones would consist of the upper north abutment, the alluvial deposits in the lower reaches and the
south abutment.
The upper north abutment consisted of the Sante Fe gravel formation, i.e., very dense angular sands
and gravels with some cementation. This formation was generally encountered from approximately
the 8,540-ft bench upwazd. The contractor experienced some difficulty excavating t9»s material due
to the light cementation and high density. Much of this area had to be ripped before it could be
borrowed. This material was used for Type 1 material (not next to the liner) in the main
embankment fill and as Type 4 material in the main embattlortent and sputter embankment.
Early in the project it was observed that it would be extremely difficult to perform the earthworks
for the phase 3 expansion between the main embanlmtent and sputter embankment at a later date,
due to the steepness of the natural terrain. Some of the natural slopes in this area were standing
steeper than 2:1. A decision was made by BMG to redesign the slopes in this area too
accommodate the phase 3 earthwork construction in conjtmction with the phase 1 work. BMG
signed a change order for this work with TT'IR the earthworks contractor. The slopes were
redesigned so that the slopes below the 8,590-ft elevation were 5:1 or less, and the slopes between
the 8,590-ft elevation and the 8,620-ft elevation were 3:1. The design change called for placing
approximately 50,000 CY of structural fill material in the four major drainages along the north slope
of the lower impoundment. In total, the redesigned slopes generated approximately 140,000 CY
of material, leaving an excess of approximately 90,000 CY which was placed in the main
embattlrntent.
The south abutment slopes had shallow deposits of silts located on the east side of the ridges.
These silts were used for liner foundation material (T~rpe 3) and were borrowed and placed directly
on the fill At deeper depths the material appeared to be part of the Sante Fe formation with a
higher silt content than encountered on the north abutment. The majority of the material borrowed
from the south slopes was placed in the random fill (Type 4) of the main embartlottent, due to the
higher content of silt.
The original design called for skirting azound the large knob located on the south abutment slope.
However, after analyzing the borrow layout for the entire impoundment it was determined that the
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