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: