t

=

period of time after application of

is shown in figure 4-52 with an illustration of its use and

stress (σt) that failure will occur, hour

may be used as a check on these computations. The

β and B are soil constants that are temperature

nomogram is limited in its application to situations where

the shortest duration of test is longer than one hour.

dependent.

Since the cohesive strength is about one-half the

g. It should be noted that equation 3 is an

unconfined compressive strength, the value of ultimate

empirical relationship for creep strength and it is not valid

cohesive strength is one-half the σult so determined.

for small values of t since the strength would become

The value of ultimate cohesive strength must still be

infinite when time approaches zero.

For practical

reduced by an appropriate factor of safety. Factors

applications of equation 3, the value of t should not be

5

recommended in TM 5-818-1/AFM-88-3, Chapter 7

less than, say, one minute.

should be used.

h. One method of determining the ultimate

k. Allowable long-term cohesive stresses for a

bearing capacity of a foundation on permafrost is to

few specific frozen soils are listed in table 4-4. These

assume that frozen soil is a purely cohesive material and

stresses include a factor of safety of 2.0. The degrees of

use an appropriate foundation bearing capacity equation.

reduction involved in these stress values from short-term

i.

This assumption is conservative since the

strength results may be visualized in figure 4-53 which

internal friction term (p tan 0 of equation 2) is assumed

shows Mohr's envelopes for six frozen mineral soils,

to be zero. Internal friction can contribute substantial

frozen peat and ice, based on tension and unconfined

shear resistance in some frozen soils, such as

2

compression tests. The 1 T/ft allowable design stress

unsaturated frozen soils; however, the determination of

for Manchester fine sand at 31.0 F is indicated on the

the value of < requires that triaxial creep tests or similar

ordinate of the left hand diagram and may be compared

tests be performed for the specific in-situ conditions

with the envelope for the same material. The 2.0 T/ft2

under consideration, whereas a value for the cohesion

allowable design stress for the same soil at 29ƒ F is

factor can be determined by relatively simple unconfined

shown on the right hand diagram. It is emphasized that

compression creep tests using procedures described

values in table 4-4 are given for general guidance and

below. For footings, the equations developed by L.

that cohesive stress values for the specific foundation

Prandtl and K. Terzaghi can be applied using a cohesion

soil under consideration should be determined by test

value thus determined. This method is particularly

procedures previously outlined.

appropriate for frozen silts and clays and can be readily

l.

When foundations are supported on soils

applied also to frozen fine-to medium-grained saturated

containing large size particles, such as gravel or glacial

sands. For frozen gravels and tills somewhat greater

till, the working design strength will depend on the

difficulty is involved.

amount of ice present. If there is a substantial amount of

j.

To determine the strength value of the

segregated or excess ice, the behavior of the material

frozen soils for use in equation 3, a minimum of two

should be assumed to correspond to that of the frozen

unconfined compression creep test are required on

fine fractions of the soil, or of ice. If the soils are under-

undisturbed soil samples of each type of foundation soil.

saturated or the voids are completely filled but there is no

These tests must be conducted near the estimated

excess ice and full particle to particle contact exists, the

temperature or maximum seasonal temperature (critical

soils may be expected to behave like unfrozen soils with

temperature) of the natural foundation soil. To expedite

normal void ratio at relatively low stress levels. These

the creep testing, one of the creep tests should be

stress levels do not disturb essentially the original

performed at a stress level of about 60 percent of the

particle to particle bridging, which the ice here helps to

conventional unconfined compressive strength and a

maintain. However, creep behavior should be expected

second test near the 40 percent strength at the critical

at higher stress levels which involve sufficient strain

temperature. In general, the test at the 60 percent stress

deformation to force the ice matrix into positive

level should require less than eight hours to perform; the

response. It should be kept in mind that in frozen soils

test at 40 percent stress level may require as much as

the volume changes which in unfrozen soils attend

three days. Using the period of time required for each of

shearing deformation are essentially prevented, thus

the two test samples to fail and the corresponding

greatly modifying shear response.

applied creep stress for each test, the constants B and B

can be evaluated by substituting twice into equation 3

and solving the simultaneous equations. By substituting

a. Determination of the bearing capacity of

the estimated design life of the structure into equation 3

frozen foundation material on the basis of ultimate

(unless specific justification exists for assuming a

strength as described in the preceding section will not

different life span, a 25-year life should be used for a

permanent structure), a value of σult is determined. A

necessarily insure satisfactory performance, because

6

unacceptable progressive creep deformation may occur.

to

solve

equation

3

nomogramt

As described

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