| Thawing Auriferous Gravel 1
| in Alaska with Cold Water §
The use of cold wate#—that is,
water at the normal summer tempera
ture of ponds, ditches and creeks—
in thawing perpetually frozen muck
and gravel is a recent development
that promises to be of value to the
placer miners of Alaska. The idea
is perhaps not entirely new, but its
application to mining was not success
fully demonstrated until the season of
1918. The method is far from being
established, and much experimental
work remains to be done before its
real value and limitations are known.
It promises to be an economic means
of thawing ground for dredging and
open-cut mining. The available data
on the subject as given below Ib ab
stracted f rom a report now in course
of publication by the United States
geological survey, department of the
interior.
Experiments with cold water, hot
water and steam were made during
the season of 1917 under the direction
of John Miles for the Alaska Mines
Corporation, on its ground near Nome,
for the purpose of determining the
most efficient and economical means
of thawing the deep Nome gravels for
dredging. Superheated steam at a tem
perature of 1,000 degrees Fah., steam
at boiler temperature, hot water at
150 to 180 degrees Fah., and water
at stream temperature of 52 degrees
Fah. were used in the experiments.
In the experiments in which steam
and hot water were used three holes
were drilled to bedrock at the points
of fin equilateral triangle twelve feet
apart, and at a point intermediate be
tween them a test hole was drilled,
which was cased and kept open to de
termine when the ground had thawed
to the center. Shafts were sunk in
the thawed ground, and drifts were :
.. limita nf the thawed area.
in the tests in which hot water and
steam at boiler temperature were used
the thawed areas tended to come
downward, leaving horsts of unthawed
ground on bedrock. Superheated |
steam at 1,000 degrees Fah. thawed a 1
mass of gravel 00 feet deep, 32 feet
in diameter at the bottom and 27 feet
in diameter at the top. The thawed
area was greatest on bedrock and
where a clay stratum was encountered
the hole did not pinch.
Two holes were thawed by the cold
water method, both single holes with
out the test holes used in the forego
ing experiments. One hole was 44314
feet, and the other 00 feet deep. Both
were six-inch holes drilled to bedrock
with an Empire drill. A two-inch pipe
was inserted to bedrock, and a small
gas engine developing not more than
30 pounds’ pressure was used to force
water down the pipe. The water was
allowed to run from the bottom of the
pipe and flow over the surface, the
circulating water thawing the ice out
of the muck and gravel. As no test
holes were used, it was not possible
to tell how rapidly the thawing ptog
ressed until the ground began to
slough in at the surface. This required
from five to six days. The shrinkage
in the thawed ground was about one
third, which represents the volume of
ice present in the muck and giavel.
The temperature of the water issuing
from the hole was not taken. A shaft
sunk in the thawed ground showed
the holes to have passed through a
stratum of clay. The area thawed was
in cross-section like an hour glass,
having a width of twenty-two feet on
bedrock, pitching to six feet at the
clay stratum, and widening to eighteen
feet at the surface. It is to be noted
that the thawing was most effective
where it was most desired, on bed
rock.
Sufficient work was not done nor
were sufficient quantitative data col
lected to warrant any extensive con
clusions, but the experiments are irt
teresting in that they demonstrate
the ability of cold water to thaw
frozen ground, and to thaw effectually
on bedrock. Otherwise stated, they
demonstrate the efficiency of a heat
unit which is near the freezing point
and available by natural processes.
The moderate efficiency of hot water
and boiler steam and the high effi
ciency of superheated steam are
equally shown. Any preference for
thawing by cold water rather than by
superheated steam must, of course,
rest on the ground of economy.
Success with Cold Water
During the season of 1918 the cold
water method just outlined was used
with some important modifications and
under different conditions in several
operations.
Pierce and Johnson, in the Candle
district, drilled a six-inch hole in 24
feet of muck (no gravel), and intro-,
duced water by a 334-inch pipe under
a head of 15 feet (6 pounds pressure).
The water entered the ground at 42
degrees Fah., and left at 32 to 34
degrees Fah. The time of thaw was
seven days, and the diameter of the
thawed area eight feet. Steam had
been tried under the conditions pre
vailing here and was not satisfactory.
Water did the work well.
The same company used cold water
in thawing creek gravels preparatory
to dredging. The ground thawed was
12 or 13 feet deep and consisted of
four to five feet of clay and muck,
four to five feet of gravel and three
I
feet of soft blue -clay bedrock. A bat
tery of forty three-fourths-inch points
was used, the points spaced in squares
five feet apart. It was found by trial
that five-foot spacing required forty
eight hours to thaw, and that the thaw
was perfect, whereas 10-foot spacing
required 96 hours to thaw, and horsts
of unthawed material were left on
bedrock. The water entered the ground
at 42 deg. Fah. and came from the
thaw at 38 deg. to 39 deg. Fah. Water
under a head of 24 feet (10.4 pounds
pressure) was used. The thawing
took place first and was widest on
bedrock. Shrinkage of as much as 1
three feet was observed, and was con
fined almost entirely to the muck. It
is estimated that 60 miners’ inches
of water was used to run the 40 points
continuously. Three men were em
ployed, two on the day and one on the <
night shift. A 50 horse-power engine
was used to pump water to a reservoir
24 feet above creek level, and was run
three hours in each eight. In this op
eration holes were not drilled. The
points were set with water and re- <
quired only an occasional twist with
a Stilson to force them to bedrock as
thawing progressed.
Problems to Be Solved
Some of the questions still to be an- ^
swered that will probably influence
the successful working of the process
are: What quantity of water and
what water pressure will give best re
sults—that is, thorough thaw in the
least time, in deep or shallow gravels,
in the presence of sand, clay, muck,
etc.? Is a heat unit just above freez
ing as efficient as one 5 degrees
above freezing? If so,- the least possi
ble pressure to insure circulation
should be used; if not, the pressure <
should be increased to insure removal
(Continued on Page Three)
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