COMMON
QUESTIONS &
ANSWERS
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Volume 4, Number 1 (April 1999)
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Q: |
Have these recent 'wet' years
affected the Horseshoe Bend diversion? |
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A: |
Yes. When ARCO and Montana
Resources (MR) began diverting the Horseshoe Bend stream that
once flowed into the Pit from the northeast rim, they had expected
to treat about 2.6 million gallons of water per day. But so far,
they've been treating about 4 million gallons per day.
Recent years of wet weather may have also played a role in last
September's Pit wall sluff that sent about 3 million tons of
rock and dirt into the water. Rising groundwater saturated and
gradually weakened a section of the southeast wall, eventually
causing it to break away.
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Q: |
Have there been any more major
rock slides along the Pit walls? |
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A: |
No. But MR is taking steps
to stabilize the piles of waste rock that form sections of the
Pit walls. Two options are available: The first is to remove
material from the tops of the dumps (the crests) to relieve pressure,
and the second is to add material to the bottoms of the dumps
(the toes) to bolster their foundations.
MR is employing both strategies to minimize future problems.
Using a bulldozer, crews have shaved the crest of what's called
the "Bird Watch Dump" along the Pit's south wall. And
to shore up the underwater toe of another dump in the Pit's southeast
corner, they're pushing in material from the Bird Watch Dump
plus additional waste rock and dirt from the active Continental
Pit. In all, MR expects to move approximately 5 million tons
of rock and dirt into the Pit to stabilize this area.
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Q: |
How would an earthquake affect
the Pit and surrounding area? |
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A: |
This question was asked
because a series of faults (including the Continental Fault),
which are cracks in the earth's crust, run just east of the Pit.
The answer would depend of course on the magnitude and duration
of the quake. A major earthquake most likely would cause material
from the Pit's walls to cave in, boosting the water level. And
unfortunately, 'earthquake-proofing' the Pit is not possible.
However, MR is taking steps to ensure the stability of the Yankee
Doodle Tailings Dam, the east end of which sits directly above
the Continental Fault. In 1993, MR hired a San Francisco-based
engineering firm to evaluate the dam's seismic stability. The
investigators declared the dam "safe" and said it could
likely withstand an earthquake of Richter magnitude 6.5.
The consultants' report also provided specific recommendations
for maintaining this level of safety as the dam expanded. MR
officials said that they have been following the recommendations
and that a report documenting their actions is nearing final
draft stage.
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