REP17951

• Permit No: M1988112
• IBM Index Class Name: Report
• Doc Date: 7/30/1993
• Doc Name: FAX COVER
• From: BATTLE MOUNTAIN GOLD CO
• To: LARRY OEHLER
• Media Type: D
• Archive: No

BATTLE MTh. GOLD SL
Y
s
i` Ms. Alana Scott
July 28, 1993
Page 2
I1~19-672-3363 JUL ~93 1243 No .008 P.03
STEEPEN ROBERTSON ~1ND KIRSTEN
Liquefaction and Stabilsy An e~$
The cone penetrometer (CPT) soundings were located within the central portion of the
raise foundation as this area has the highest potential for containing saturated tailings and
tailings slime due to the deposition methods practiced at the mine. Other areas closer
to the periphery of the existing impoundment contain a greater percentage~of tailings sand
and overall denser better-drained tailings as determined through the surficial density test
work and hand auger borings discussed in Section 2.1 of the report.
The numerical factor of safety (FOS) computed throtigh any analytical procedure is
directly related to the conservatism which is employed in selecting input parameters. For
the same stability problem, incorporating non-conservative parameters Will result in a
higher factor of safety than conservative parameters but the actual stability will .be
identical.
As discussed below, the acceptance of a FOS of 1.1 is mitigated by several compounding
conservative assumptions. A FOS of 1.1 in this case conforms to gen4rally accepted
values. As discussed in the Permit Amendment adequacy response No. 215, the stability
analysis approach for the San Luis Project has been to adopt wnservative input
parameters.and the lower calcuhttod factors of safety which rrsult. Codservatism has
been imparted in the stability analyses for the dam raise through the following:
The embankment fill leas been modeled with a friction angle of 3tT degrees and
no cohesion. For the. 'dirty' sand and gravel used for construction of the main
embankment and proposed for construction of the raise, the fribtian angle is
probably several degrees higher and the material should exhibit cohesion on the
order of several hundred pounds per sgttue foot (psf).
The inhomogeneous and anisotropic nantre of the tailings has beeniconsidered in
the analyses by modeling all tailings as the weaker tailings slimes. A sample of
the slimes was tested for undrained shear strength in a triaxial shear test. As
discussed on page 3 of the report, the test resulu indicate a friction angle of 31
degrees and 358 psf cohesion at 5 percent strain and a friction angle of 32
degrees with 48ti psf cohesion at 10 percent strain. 'fie tailings were modeled
with a friction angle of 30 degrees and cohesion of 350 psf.
The embankment fill to Y1.DPE liner interface friction angle was gtodeled as 20 ~
degrces. When tested in direct shear, sand and gravel typically exhibit a friction
angle of 25 degrees or higher relative to sliding along VLDPB lindrs.
The tailings from elev 8552 to 8555 were indicated by the CPT work to be
saturated or «early so. For all dynamic loading, this layer has been assumed to