TM 5-852-4/AFM 88-19, Chap. 4
By using constants from table 4-5 and
equation 4 from paragraph 4-5.
By
performing
unconfined
compression creep tests on undistributed samples of the
foundation soil and using equation 6 of paragraph 4-5.
The first method will give a rough estimate.
An example of computations by this method using values
from table 4-5 for a silt similar to the soil under the
footing, at about the same water content, is shown in
table 4-8. Use of a value of 25 years of time, t, in these
computations is a simplification and quite conservative in
that it assumes that ground temperatures remain
throughout the year at the same level as during the
"critical period."
Since ground temperatures are
somewhat colder during a considerable portion of the
year, it is clear that the length of time required to attain
the settlements computed in table 4-8 (and in table 4-10
as well) is somewhat longer than the 25 years.
Computing the settlement
using
the
coldest
temperatures to be anticipated during the year (24F for
Zone A, etc.), with all other factors the same, results in a
(e) Making settlement estimate. Consider the
25-yr settlement of 0.2 in., 1/5 that determined in table 4-
isolated footing as a point load near the surface of a
8.
semi-infinite solid (conservative assumption because of
The second method gives a more
depth and increased bearing area).
accurate prediction for the specific case. The
1. Stress distribution. For simplicity, use
unconfined compression creep tests should be
Boussinseq's equation for point load. (See Terzaghi and
performed at the design stress level and at the predicted
198
nd
Peck , 2 Ed., p. 271.)
temperature of the foundation soil. A plot of unconfined
Vertical stress
compression creep test results for a silt at 29.5F under
2
applied stress of 50 psi (3.6 T/ft ) and a sample
computation are shown in figure 4-67 (for Zone B).
Using equation 6 and the data from the
test, the relationship between strain and time becomes,
where z = distance below base of footing.
for Zone B:
The computed stress distribution is shown in
figure 4-64.
2. Creep settlement computation.
A similar relationship must be obtained by
Assume:
tests performed on soil from each zone in the "soil
Load P is distributed uniformly over the
column" beneath the footing for the critical temperature
end of a soil column with cross section equal to the base
and the stress conditions that exist
area of the footing with stress decreasing progressively
The sum of the deformations from all the
in the column to the depth where the stress is negligible,
zones for a given time will constitute the estimate of the
as indicated in figure 4-66.
total settlement.
Vertical movement is the result of
unconfined compression creep of the frozen column of
soil directly beneath the footing. (This assumption is on
the safe side since creep-reducing effects of lateral
confinement are neglected.)
Total creep movement is the sum of
the creep of all the zones of soil in the soil column.
Creep test not performed on Zone C at
Creep in the compacted gravel is
required temperature; strain data interpolated from a
neglected.
general formula and average values to complete
Temperature distribution is as shown
example.
in figure 4-65 and as computed in table 4-7. (The
approximate distribution assumed for computation is also
shown in fig 4-66).
The amount of creep deformation can
be estimated by the following methods:
4-103
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