TM 5-852-4/AFM 88-19, Chap. 4
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
4-5. Estimation of creep deformation.
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
4-77
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