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