TM-5-855-4
3-5. Thermal properties of rock.
a. General.
(1) Heat transfer calculations for underground structures require values of the thermal properties
of the rock or homogeneous solid assumed to represent the actual materials surrounding the structure.
These parameters include conductivity, specific heat, density, diffusivity, temperature, and moisture
content.
(2) In the calculations it was assumed that convective heat transfer associated with the percolation
of water through the rock could be neglected when compared to the conduction due to the temperature
gradients. However, moisture will generally increase conductivity, specific heat, and density and to a
lesser extent thermal diffusivity. As a result, the influence of the moisture cannot be neglected, especially
close to the ground surface, where depending on the permeability of the rock, dry and wet spells could occur
as a reflection of aboveground precipitation or drainage.
(3) Unfortunately, the available data covering thermal properties are incomplete and in some
degree discordant. This is one more reason that it is practically impossible to validate design for a given
site without a geological examination, including sampling and testing of the thermal parameters and
location of the water table.
(4) For estimating purposes, the designer is forced to exercise great care in selecting the
appropriate range of thermal properties. To achieve this goal, data from different sources are shown in
table 3-1 and discussed in the following. For more references on thermal properties, the designer is
, refered to "Soil Thermal Properties; and Annotated Bibliography (Office of Civil Defense Research
Report OCD-OS-62-58) AD 432-604."
`------b. Specific heat. For estimates, a specific heat of 0.2 Btu/lbF is recommended for any rock and for
use in the equations in this chapter, although rock specific heats as low as 0.16 Btu/lbF have been reported.
interpolation between the two values given.
c. Thermal conductivity and density.
greenstone rock in demonstration problems and are regarded as good assumptions for preliminary
estimates in many cases.
(2) A correlation for igneous rocks to known quartz, feldspar and mafic composition is shown in
figure 3-11. To find the thermal conductivity, draw a line from the representative point, concurrent with
two nearest thermal conductivity lines, and read the thermal conductivity at the intercept with the
conductivity scale. To find the density, proceed similarity with the density lines and scale.
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(3) Density of igneous and metamorphic rocks generally falls in the range from 150 to 190 lb/ft
and that of sedementary rocks in the range from 100 to 175 lb/ft3. Thermal conductivity of igneous and
quartz, 50 to 73 percent feldspar, and 5 to 12 percent mafic.
(4) The four figures 3-12 through 3-15 are presented to aid in the estimate of the thermal
conductivity of silty clay and sandy soils in the frozen and unfrozen condition. It is expected that these
charts will give conductivity values with a precision of 25 percent. The effect of density, moisture content,
freezing, and texture is clearly illustrated on these graphs. Typical thermal properties of other materials
are shown in table 3-1.
d. Temperature.
(1) At depths of 50 to 70 feet, the undisturbed temperature of earth or rock can be expected to be within
a few degrees of the mean annual air temperature for the region, in the absence of disturbing factors such
as underground fires or large subterranean streams. At greater depths, the temperature is found to be
higher, increasing at the rate of about one `F per 100 feet. Earth temperature thus determined are adequate
for AC estimates for underground spaces, although a check of the figures is desirable during the survey of
a proposed site.
(2) The analytical treatment of the steady periodic response of TD the ground temperature D feet
below the surface to the fundamental harmonic variation of the annual surface temperature indicated that
the attenuation or ratio of the amplitude diminishes exponentially as
3-18
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