TM 5-852-4/AFM 88-19, Chap. 4
ε
in paragraphs 2-5a and 4-4 and as illustrated in figures
o = strain that occurs immediately upon
2-15, 4-49 and 4-50, frozen soils and ice exhibit creep
application of stress (this term can be
characteristics under long term loads down to at least as
neglected for the purpose of estimating
low as 5 to 10 percent of their rupture strengths under
creep)
m, λ, w, k = constants that depend on properties of
relatively rapid loading.
When slow, progressive
movement occurs in foundations on saturated frozen soil
material.
in absence of thawing, it is generally the result of creep.
Typical values for m, λ, w, and k for equation 4 are given
Creep is a time dependent shear phenomenon in which
for specific soils in table 4-5. These values were derived
the total volume of the stressed material remains
empirically by laboratory tests demanding quite precise
constant; i.e.
the stressed soil flows rather than
measurements of the absolute values of strain and
consolidates. In TM 5-852-1/AFM 88-19, Chapter 1,
requiring several tests for evaluation of the constants.
slope creep is defined as "extremely slow downslope
Care must be taken to use the proper units consistent
movement of surficial soil or rock debris, usually
with those given in the table.
imperceptible except by long-term observation." In that
e. A conservative method for estimating the
case the movement usually involves freeze or thaw
vertical creep of a foundation is to assume that the
action in strata near the surface and downslope
foundation is supported by a column of frozen soil having
movement of seasonally thawed soil, together with creep
a height equal to 1/2 times the least plan dimension of
of frozen materials when stress and temperature
the foundation and apply equation 4 to compute the
conditions favor this. For creep of frozen material, the
strain and hence the deformation of the soil column. In
ice filling the soil voids may be considered to be a fluid of
applying equation 4, values for stress and temperature
extremely high viscosity. Within normal pressure and
are assumed to be constant for the period of time under
time frame, consolidation of soil can only occur if air or
consideration. The average temperature of the column
other gas voids are present in the soil mass or if part of
of permafrost for the critical period of the year can be
the soil moisture is not frozen.
projected from ground temperature records or from on-
b. In general, present design practice is to
the-spot temperature measurements. The critical period
avoid the problem of creep in frozen soil foundations
of the year is that time of year when permafrost
either by supporting footings on mats of well drained
temperatures beneath the foundation are warmest. The
non-frostsusceptible gravel or other material which
magnitude and distribution of stress under the foundation
spread stresses sufficiently so that stresses on
can be approximated by using elastic theory, as outlined
underlying confined frozen materials are conservatively
5
TM 5-818-1/AFM 88-3, Chapter 7 . Since magnitude of
low, or by placing foundations at a sufficient depth in the
stress decreases with depth, it is necessary to use an
ground so that the overburden pressure effectively
equivalent constant stress in order to apply equation 4.
minimizes foundation-induced creep.
A closer approximation of the amount of creep can be
c. When analysis indicates that a footing, raft,
obtained by dividing the soil beneath the foundation into
pier or similar foundation designed on the basis of
an arbitrary number of horizontal zones and using an
ultimate strength with recommended factor of safety will
average constant stress and temperature for each zone.
develop unacceptable creep deformation over the life of
Using these average values and the thickness of each
the facility, the design must be revised to bring the
zone, equation 4 can be used to estimate the vertical
deformation within acceptable limits.
deformation of each zone. The total deformation will be
d. Various empirical equations have been
the sum of the deformations of all the zones. Where the
proposed for the prediction of creep of frozen soil in
soil is stratified, the boundaries of some of the zones
unconfined compression. At the present time, these
should be coincident with the stratum interfaces.
equations do not take into account the complex stress
f.
It is emphasized that this procedure will give
and deformation conditions of the soil beneath a
only an order of magnitude of the amount of creep and
foundation. For the first approximation of the amount of
the constants in table 4-5 apply only for the specific soils
creep that may be expected, the following empirical
listed and are given only as a guide.
107
may be applied to a
equation similar to Vialov
g. A second, more accurate, method of
foundation:
predicting creep requires performance of unconfined
compression creep tests on undisturbed samples of the
λ
σt
strain = E
=
1/m
+ Eo
(Equation 4)
foundation soil at the design stress level and at the
k
w(0 + 1)
predicted temperatures of the foundation soil and
where:
application of the following empirical equations:
σ = stress in the material under consideration,
psi
t = time that stress is to act, hr
θ = number of degrees below the freezing
point of water, F
4-81
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