TM 5-852-4/AFM 88-19, Chap. 4
(7) For engineering purposes it is very
materials used in the foundations are also affected by
important to know whether significant settlement will take
the low temperature conditions. Materials susceptible to
place upon thawing of the frozen soil. If the ice present
brittle fracture at low temperatures should not be
will produce more water upon melting than can be
specified for conditions where they may fail.
retained within the voids of the soil after thaw, then the
Consideration must be given to logistics, supply, costs
material is thaw-unstable to a degree that is dependent
and problems of field fabrication and erection. Shop
upon the amount of excess ice and the soil density. If all
fabrication may be found more economical than field
the melt water can be absorbed by the soil voids without
fabrication in spite of greater logistics problems.
significant settlement, then the soil can be considered
Availability of local materials must also be considered.
thaw stable.
Thaw-consolidation computations are
Native materials are often scarce and not very suitable
outlined in chapter 3.
and construction practices may differ because of
c. Rock. Bedrock should never be assumed
transportation problems, equipment and labor available
free of ice if the melting of such ice is possible and the
and severe weather. The relative cost and availability of
consequences significant. Numerous cases are on
all construction materials must be established early in
record where this assumption was made and substantial
the design; this will influence the selection of the
volumes of ice were later found to be present in the
foundation type to be used.
a. Wood. Wood is a satisfactory and widely
bedrock. At Thule, Greenland, this situation resulted in
used construction material because of availability, low
substantial settlement of a building; required repair
thermal conductivity, adequate durability, when
measures included installation of artificial refrigeration in
selectively used and treated if required, flexibility,
the foundation on a permanent basis. At the same
unimpaired strength at low temperature, low weight and
location the disappearance of water from a drainage
availability. However, in many arctic and subarctic areas
ditch cut in shale resulted in discovery of ice layers up to
it may be as unavailable as any other construction
20 inches thick in the rock which were being melted by
76
the drainage waters . The only safe approach is to
material.
Even when timber is obtainable in the
carry subsurface explorations into bedrock, obtaining
Subarctic, available sizes may be limited and quality low.
undisturbed frozen cores which will reveal the exact
(1) A principal use of timber in foundations
thicknesses of any ice strata present.
is in piling. Spread footings have been frequently made
(1) Structurally, the frozen water in
of wood. Heavy timbers are widely used for distributing
bedrock fissures adds substantially to the competence of
structure loads onto piles or other principal supporting
the rock, so long as the ice remains frozen.
At
members. Timber is also useful as a semi-insulating
Yellowknife, NWT, Canada, it has been stated that in
pad under a concrete footing. Rock fill wood cribs are
mining of ore by the stopping process, excavations can
useful for many purposes. Although timber piles have
safely be made in 300 foot lifts whereas the standard
lower thermal conductivity than concrete or steel piles,
height in unfrozen rock in Canada is 150 feet. Blasting
this is usually a negligible advantage for the thermal.
specialists responsible for mining of iron ore at Knob
stability of foundations. It may be assumed that no
Lake, Quebec, Canada, have stated that three times as
decay will occur in portions of piles embedded in
much powder is required for blasting of ore rock
permafrost. Within the annual frost zone, temperatures
containing 10 percent ice as for blasting of unfrozen
are low throughout the year and below freezing
deposits"'. On the other hand, ore containing this much
temperatures prevail for many months; groundwater is
moisture is very wet when thawed and very substantial
often near the surface throughout the year. These
problems have been encountered in handling,
factors slow decay below ground, though not
transporting and using such material.
guaranteeing against it. At the ground line, however,
(2) Bedrock is frequently a source of
experience shows that, except in the high Arctic, timber
severe frost heave because of mud seams in the rock or
members may be destroyed in only a few years even
concentrations of fines at or near the rock surface in
where the mean annual precipitation is low. While wood
combination with the ability of fissures in the rock to
piles therefore need preservative treatment above the
supply large quantities of water for ice segregation.
permafrost table, a coating of creosote on the portion of
a pile embedded in permafrost reduces the tangential
shear stress which can be developed below that which
2-6. Structural materials.
would apply for bare wood. In theory then, only the
Foundations of structures in areas of deep seasonal frost
upper parts of wood piles should be so treated.
and permafrost tend to experience larger and less
However, no method exists for pressure creosoting only
predictable tensile, compressive, and shear stresses
part of a pile. Therefore when wood piles require
than in more temperate regions as a result of thermal
pressure treatment, the entire length must be treated and
expansion and contraction effects, unstable conditions
tangential adfreeze working stresses
during and after the construction period, and seasonal
frost action. The properties and behavior of structural
2-26
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