settlement_of_shallow_foundations_on_granular_soils (Lutenegger ang DeGroot)

7.3 Review of Load Tests
Initially, data were gathered from the literature representing reported case histories of
load/settlement behavior of shallow foundations in which sufficient results were provided to describe
the ultimate bearing capacity from the actual test results, using the slope tangent method, or in which
sufficient soil properties were given to allow the ultimate bearing capacity to be calculated. These
cases were then subdivided into four categories based on footing size as: (1) small scale laboratory
load tests (B ,; 0.305m); (2) small footing tests (B < 0.5m); (3) medium footing tests (0.5m < B <
1.5m); and (4) large footing tests (B > 1.5m). Unfortunately, there was not the same number of tests
available for each category and in fact, for large footing tests, there was a clear lack of high quality
tests available. Tables 7.1 to 7.4 summarize the case histories used in each category to develop the
nondimensional curves. The resulting curves are presented in Figures 7.13 to 7 .16.
The results presented in Figures 7.13 to 7.16 suggest that there is a definite scale effect with
progressively larger size footings producing smaller relative settlements at the same relative loading
levels. An upper bound curve is shown on each curve which describes the maximum observed
settlement behavior from all test cases and provides a conservative upper bound limit of estimated
settlement.
To illustrate the use of this approach, the observed settlements of a number of bridge
abutments as reported by Gifford eta!. (1987) will be used. The bearing capacity of several of these
abutments was evaluated by Briaud (1989) using the results of pre bored pressuremeter tests. Since
the working loads on the abutments and the foundation widths are provided by Gifford eta!. (1987),
the relative settlement may be determined. Table 7.5 presents results of the calculated factors of
safety and relative settlement for cases where sufficient data are available. The average factor of
safety under working loads for the 5 abutments is 6.9 corresponding to q/q" 11 = 0.14. The average
relative settlement based on observations is 0.30. Based on Figure 7 .16, the predicted relative
settlement for this level of loading would be on the order of 0.1 0. While this estimate of settlement
is obviously lower than the observed settlements for these abutments, adjustments to the predicted
settlement for creep should be made and it should be remembered that the estimate of ultimate
bearing capacity obtained from the PMT is usually much larger than by conventional means and
therefore the value of q/q" 11 is too low. This in tum makes the estimate of S/B too low.
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