DWR_3391215

• Document Date: 6/17/1981
• Document Type - Dam Safety: Report
• Division: 3
• Dam ID: 200224
• Subject: HOMESTAKE TAILING POND, LOWER DAM - LETTER REPORT, PHASE I REMEDIAL MEASURES
• DWR Send/Recipient: DSB
• Outside Send/Recipient: DAPPOLONIA CONSULTING ENGINEERS, INC.

L
Mr. Thomas M. Robertson 2 June 17 1981
Since the dam became operational in 1974, seepage has been observed in
the downstream toe area at increasing rates. Seeps were first observed
in 1978, estimated by Homestake personnel to flow at a rate of 10 to 15
gallons per minute (gpm) with some seasonal variation. In 1980, flow
rates increased to an estimated 45 to 50 gpm. In the early spring of
this year (1981), seepage rates were estimated to have increased to more
than 100 gpm.
The observed large increase in seepage rates is a concern for two reasons:
• The effect that elevated water pressures and
seepage velocities within and beneath the dam
may have on dam stability.
• The effect that the tailings water seepage may
have on groundwater quality in the area and in
downstream areas.
The information presented herein represents the results of a Phase I
investigation on the dam. The efforts of this investigation were aimed
toward developing remedial measures to reduce and/or control the seepage
and resolve the above concerns.
2.0 PHASE I ANALYSIS
The key to solving the seepage problem and developing remedial measures
is to define the path(s) and pressures of seepage from the tailings pond
to the downstream toe area. Phase I work involved the analysis of the
existing data in conjunction with additional information gathered by
Homestake personnel in an effort to define the path(s) and pressures of
seepage.
e
2.1 SITE GEOLOGIC CONDITIONS
Geologic information about the dam site was reviewed in order to identify,
if possible, zones of pervious material which could be transmitting the
observed downstream seepage. The information reviewed included:
' • As—constructed plans for dam construction,
prepared by Davis Engineering Service, Inc.,
dated September 19, 1974. Included were boring
{; logs by Lincoln—Devore Testing Laboratory.
• Report by Lincoln—Devore about boring program
4t for dam design in 1974.
• Soil and rock mapping of the eastern half of
the dam cut—off trench, prepared by Homestake
personnel in 1975.
• Boring logs prepared by Dames and Moore, as part
of a dam stability evaluation, in 1975.