REP18762

• Permit No: M1988112
• IBM Index Class Name: Report
• Doc Date: 7/19/1992
• Doc Name: TAILINGS DAM STABILITY AS RELATED TO LIQUIFACATION POTENTIAL WITHIN THE TAILS UNDER SEISMIC LOADING
• From: DMG
• To: HARRY POSEY
• Media Type: D
• Archive: No

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STATE
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1313 Sherman St., Room 215
Denver, CO 80203
Phone:[phone] (~
FA%:(3031832-8106 .~fl )l ..'1.
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DATE: November 19, 1/992
TO: Harry Posey /7~
FROM: Allen Sorenson
•
OF COLORADO
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Roy Rnmer
Governor
Michael B. Long
Division Doeaor
RE: Tailings Dam Stability as Related to Liquifaction Potential within the
Tails Under Seismic Loading, Battle Mountain Gold, San Luie Project,
File No. M-88-112
A study conducted by the State of Washington, Dam Safety Section, raises some
doubt ae to the stability of tailing impoundments constructed using upstream
raises subjected to seismic loading. My review of the San Luie Project file
indicates that potential instability related to liquifaction during an
earthquake was addressed by the Division during the adequacy review (see
attachments). The Division's review determined that if the tails that will
make up a portion of the foundation for the dam raises desiccate and density as
projected by the operator, that liquifaction will not occur during the design
earthquake.
In order to determine that projected desiccation and densification of the tails
is occurring, and that grain size distribution of the tails that make up the
dam raise footprint is as projected, the operator agreed to monitor the
tailings during operations. Apparently, the operator recently conducted testa
of ~t~ailing density using the sand cone method and a California tube sampler,
but ae far ae I know, pneumatic piezometere to measure phreatic s rface within
the tails have not been installed. This should be clarified duri~q the next
inspection of the mine. ~
L
The onus is on the operator to show that the tailings will form a stable
foundation for a dam raise prior to construction, otherwise center-line or
downstream raises should be utilized to increase impoundment capacity. One
item that was not addressed by the initial adequacy review is thelpo~sibility
of amplified ground motions within the tails. Ae the Washington tlam Safety
Section report indicates, data from the 1985 Hichoacan Earthquake shows that
certain combinations of soil column height and soft soil consistency can
greatly amplify the base rock motions in the overburden. if this affect was to
occur at the San Luis Project, the ground motions used in the stability
analysis, which were based on an appropriate design earthquake, would. not
accurately reflect site conditions and would result in an inflated factor of
eafety.~~ The operator should be required to analyze the possibility of
amplified ground motions within the taile.~i