DEPTH AND TIME OF FREEZING OF
A SILTY SOIL UNDER DECIDUOUS FOREST
NEAR FAIRBANKS, ALASKA
ENVIRONMENTAL PROTECTION AGENCY
WATER QUALITY OFFICE
NORTHWEST REGION
ALASKA WATER LABORATORY
College, Alaska
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DEPTH AND TIME OF FREEZING OF A SILTY SOIL
UNDER DECIDUOUS FOREST NEAR FAIRBANKS, ALASKA
by
Frederick B. Lotspeich
for the
WATER QUALITY OFFICE
ENVIRONMENTAL PROTECTION AGENCY
ALASKA WATER LABORATORY
COLLEGE, ALASKA
Working Paper No. 12
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A Working Paper presents results of investigations
which are to some extent limited or incomplete.
Therefore, conclusions or recommendations—expressed
or implied—are tentative.
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DEPTH AND TIME OF FREEZING OF A SILTY SOIL
UNDER DECIDUOUS FOREST NEAR FAIRBANKS, ALASKA
Depth and time of frost penetration are important design criteria
that control or limit underground domestic water supply and waste disposal
systems in any climate where winter temperatures cause soils to freeze.
In Alaska, especially the Interior, these factors become important
because of the extreme cold and the duration of below-freezing tempera-
tures. Many areas of Interior Alaska are free of permafrost, but still
experience deep seasonal freezing of soils. Near Fairbanks a range of
low hills, mantled with silty soils, is becoming important for home sites
and all these slopes with a southerly exposure are free of permafrost.
On such sites it is important to know the seasonal depth of freezing
because a frozen waste disposal system here poses a serious pollution
problem that may be a significant health hazard. This temperature station
was established because data -on soil freezing were not readily available.
METHOD
The site for this station is about eight miles northwest of Fairbanks
on a deep, silty soil under a birch and aspen forest where the trees were
40-50 feet tall (Figure 1). This site was representative of soils,
vegetation, and other aspects of much of the permafrost-free building
sites in the vicinity of Fairbanks. The surface at the site has a slope
of 17% at south 70° west.
Temperature sensing was by thermistors imbedded in the soil at 11
depths, one in the shallow duff layer and others at 0.5, 1, 1.5, 2, 2.5,
1
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•
Figure 1. Deciduous forest at the site; larger trees are
birch with aspen in background (19 September 1969)
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3, 4, 5, 6, and 7 feet deep. A twelfth thermistor measured air temperature
about 1 dm above the ground or snow surface. Measurement was with a
Yellow Springs Instrument Co. Telethermometer. Before being installed,
thermistors were tested in a water bath at 26° and 1°C and found to read
within 0.5°C. '
To install thermistors, a pit was dug by hand to a depth of six feet
on the afternoon of October 5, 1966, and moisture samples collected for
each horizon; the installation was complete and the pit refilled by noon
of October 6. The thermistor at 7 feet was emplaced in a small hole
(1 inch in diameter) bored into the bottom of the pit. All other sensors
were emplaced in holes 12 inches deep, bored perpendicularly into the
upper face of the pit. All bored holes were backfilled with material
from the same depth by tamping with a wooden rod. Leads from thermistors
were brought to the surface and led through a 3-foot section of electrical
conduit whose lower end was about 6 inches below the surface. This
measure was taken to prevent rodents from gnawing the insulation off the
leads. All leads plugged into a switch box that permitted readings to be
taken by plugging a single lead to the Telethermometer and switching from
one thermistor lead to the next. The terminal box was housed in a metal
can mounted atop the conduit to protect it from weather and animals
(Figure 2).
After each thermistor was emplaced, the pit was backfilled to the next
level by material approximately the same as that in which the thermistor
was placed. As backfilling progressed, the material was compacted by
trampling with the feet in 3- or 4-inch increments. This compacting
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Figure 2. Closeup of the temperature measuring station.
The switch box had been removed the previous fall.
(19 September 1969)
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proved effective, since all material taken out during excavation was
returned without an excess and slumping after 3 years was not more than
1 to 2 inches.
Temperatures were first read on October 7, about 26 hours after
installation. For the remainder of October, temperatures were read twice-
a week to determine if opening the pit had caused a temporary disruption
of the soil thermal regime. Apparently it had not because temperatures
at depth remained constant until cold weather set in. Table 1 presents
data showing moisture content and two sets of temperature measurements:
TABLE 1. DEPTH OF INSTALLATION, MOISTURE CONTENT
AND INITIAL TEMPERATURE
TEMPERATURE, °C
PROBE '
NUMBER
1
2
3
4
5
6
7
8
9
10
11
12 -
DEPTH % WATER AT
(INCHES) INSTALLATION
Air (1 dc above
surface)
2" (duff)
6"
12"
18"
24"
30"
36"
48"
60"
72"
84"
NA
25.8
•7.5
7.4
. 6.3
6.4
' 6.3
6.0
6.1
7.5
9.2
10.7
26 hrs after
installation
+5.5
+3.5
+4.5
+5.5
+5.5
+5.5
+5.5
+5.0
+5.0
+4.5
+4.0
+4.0
8 days afte'
installatioi
+2.5
+2.5
+3.0
+4.0
+4.0
+4.5
+5.0
+5.0
+5.0
+4.0
+4.0
+4.0
After October 1966, temperatures were measured weekly throughout the
year, usually between the hours of 1200 and 1300 every Monday. Tempera-
tures were read for two full years and terminated on October 14, 1968,
although the sensors were left in the ground. The switching box was
removed and used on another project.
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On January 23, 1969, a set of readings was made by hand switching
with a short section of wire through phone jacks. The reason for this
set of readings was to compare soil temperatures after the severe winter
of 1968-69 with those of the previous two years. Another set of readings
was made on'April 1 to determine maximum depth of freezing after the
severe winter and compare it with previous years. A final set of readings
was taken on September 14, 1969, after which all thermistors were removed
from the soil and the experiment terminated.
RESULTS AND DISCUSSION
Moisture content at the time of installation was low and was near
wilting point for this soil texture down to about 4 feet deep. The
summer of 1966 was extremely dry with almost no rain from August 1 through
the date of installation. Table 1 includes these data. After breakup in
1967, moisture from melting snow penetrated to 21 inches as noted in a
post-hole near the temperature measuring site.
Temperatures measured on October 7 and 14 are also shown in Table 1.
It will be noted that soil temperatures to a depth of 30 in. are gradually
decreasing as ambient temperatures decline. Below 30 in., temperatures at
both dates were identical and form the basis for the earlier statement
that opening the pit caused a minimal disturbance of the temperature regime.
Subsequent data for two years record are presented in Figures 3 and 4
with some selected data presented by Figure 5. Temperatures at each
depth were averaged monthly and grouped by 6-month intervals. Group A
represents a cooling trend in fall from October, when deep horizons are at
maximum temperatures, through March, when these horizons are nearly at
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Figure 5.
Maximum soil temperature at the onset of cooling in
autumn, (B) Minimum soil 'temperature at the end of
cooling cycle.
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their coldest. Group B represents the 6 months of warming of deep horizons
from April through September when these horizons are gaining heat through
the summer-.
Although the graphs are self-explanatory for the most part, several
interesting points will be discussed briefly. The most rapid cooling is
from October to November for the entire profile; this trend held for both
years as shown by A in Figures 3 and 4. However, total cooling was much
less in 1967-68 than for 1966-67 because the winter of 1967 was milder than
normal. By November 1966 the soil had frozen to a depth of 2 1/2 feet.
From November through the cooling phase, cooling progressed slowly until
February, when final depth of freezing reached 5 feet. It appears that
the entire profile cools and warms as a unit with different portions
responding at different rates; i.e., the amplitude decreases with depth.
During the mild winter of 1967 total depth of freezing was only 2 1/2 in.
and was reached by February, the same month as for the previous year,
which was near normal; depth of freezing after the severe winter of
1968-69 was 6 feet.
Warming curves are almost mirror images of the cooling curves with
maximum warming occurring in the interval from May to August. Part "B"
of Figure 1 shows that the entire profile was just at freezing by May and
above freezing by June. From June until September the deeper horizons
continue to warm although surface horizons (0-36 in.) start to cool with the
onset of fall weather and shorter days. Similar trends are shown by "B"
of Figure 2 for 1968 with the depth of freez ng much less. In 1968, the
deep horizons reached a slightly higher temperature than for the previous
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year, 5.5° as opposed to 4°C.. In 1968 the maximum temperature at 7 ft.
reached 5°C even though the minimum at this depth on October 68 was 1.5°C.
Figure 5 presents soil temperatures at times of the seasons when
maximum warming had occurred, shown in "A", and at maximum cooling shown
in "B". In-"A", curves for 3 years show that-temperatures for all
horizons are only a degree or so apart even though individual years had
widely different climatic conditions. 1966 was very dry with average
summer and winter temperatures. Accumulated snowfall during 1966-67
reached 6.5 dm on April 3 with about 4 dm on the ground by November 2,,
so insulation was present during cold wather. The summer of 1967 was much
wetter than normal. Weather Bureau records show the combined rainfall for
July and August was 5.5 inches above normal, although summer temperatures
were near normal. Winter temperatures for 1967-68 were milder than normal
and, although total snow accumulation was similar to the previous year,
4 dm had not accumulated until January 8, 1968.
Summer temperatures and rainfall for 1968 were near normal; records
show temperatures for June, July and August as being +1, +6, and +4 degrees
above normal and rainfall for the same period as being +0.1, +1, and -1
inches from normal. However, winter temperatures were considerably below
normal. A record for prolonged cold temperatures was set for the period
from about Christmas 1968 to near the middle of January 1969. Temperature
for January 1969 showed a -15° departure from normal because of this pro-
longed cold snap. Despite the varying seasonal temperatures throughout
the year, the final temperatures near October 1 are close. The curve for
September 1969 will probably move close to the October curve as plotted,
since at the 1-foot depth on 1968, the temperature was 10°C, yet by
October it had dropped to 5°C.
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•Part "B" of Figure 5 depicts minimum soil temperatures following
three winters with varying weather. Again, as for maximums, the final
temperatures are close, .although they do follow trends established by
the nature of the previous winters. Thus, temperatures after a mild
winter are higher than those following a severe winter. It is interesting •
that warming commences as cold weather ends, as indicated by the curves
for January 23 and April 1, 1969, even though a snow cover remains.
It should be pointed out that these measurements were made under
undisturbed conditions in birch and aspen forest. If the forest is removed,
the shape of curves and depth of freezing will be different. If a measur-
ing site is disturbed by trampling or other traffic, the depth of freezing
will increase.
Based on the first year's data, a waste disposal system was installed
for a new home built in 1967. This system consists of a steel septic tank
followed by a concrete block leaching pit. Both of these structures were
buried deep enough so that the top was at least 4 ft. deep. During an
average winter such as that of 1966-67, it was Janaury before the soil was
frozen to 4 ft. and the minimum reached at this depth for the three winters
was only -1°C. Moreover, heat in wastewater is sufficient to add to a
heat sink built up throughout the year and helps prevent freezing to
greater depths. The system referred to earlier has functioned satis-
factorily for 2 winters, one of which was abnormally cold. Although the
forest had to be removed during construction, the area overlying the
septic tank and leaching pit is not disturbed during the winter to mini-
mize loss of heat caused by trampled snow.
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CONCLUSIONS
Depth of freezing of silty soils on a forested site near Fairbanks,
not underlain by pe'rmafrost, ranged from 2 1/2 ft. for a mild winter to 6
ft. for a severe one. These depths were for .undisturbed forest; distur-
bance will increase the depth of freezing and severe trampling of snow
may cause much deeper frost penetration.
The entire profile cools and warms as a unit, but different portions
do not react at the same rate; the amplitude of change decreases with
depth. Maximum temperature measured at 7 ft. was 5.5°C and the minimum was
0.5°C. At the 1 ft. depth the coldest temperature measured was 4°C below
freezing and the warmest was 12.5°C.
A domestic waste disposal system, installed with its uppermost surface
at a depth of 4 ft., functioned satisfactorily during a severe winter. Such
a depth of installation is deemed satisfactory if the site remains undis-
turbed during the winter season. Deviation from conditions described in
this study such as presence of permafrost, traffic over the disposal
system, texture of the soil, and soil moisture at the site, will cause
significant differences in depth and time of freezing as reported here.
An additional precaution is that the slopes on which these measure-
ments were made are generally 20-25°F warmer during severe cold than are
the nearby lowlands. As cold snaps persist, a strong inversion develops
over the lowlands and slopes a few hundred feet higher in elevation
become warmer. Therefore, data for depth of freezing from slope sites
should not be extrapolated directly to valley stations.
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