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 3.
SOIL-STRUCTURE INTERACTION.
a. Introduction. For the dynamic analysis of the foundation, the
properties of the underlying soil should be estimated as accurately as
possible, since they have a large impact on the design. The non-linear
behavior of soils subjected to dynamic loads is simulated by one-way
springs. Equations for determining the equivalent spring constants for the
soil in both the vertical and horizontal directions are presented by Whitman
in Design Procedures for Dynamically Loaded Foundations, and the procedures
are adequately described in Stea's report. However, in the absence of more
reliable data which would be achieved through field tests and other rigorous
analyses), the values in Tables 32 and 33 should be used as estimates of the
soil properties. Note that these soil properties are correlated with the
results of a minimum of shallow test borings together with a visual
description of the soil encountered and the blow count for standard
penetrations tests.
b. Overturning Design Criteria as Related to Soils Data. The
properties of the soil grossly affect the response of the structure.
Therefore, these properties have a large impact on the foundation design.
Tables 32 and 33 provide for a particular soil, the properties in the soft
or loose condition, and the compact or hard condition. The actual
condition of the soil at a given site will be somewhere between these
extremes.
(1) In order to account for the most severe design conditions for
both overturning and strength, the structure is analyzed for both conditions
of the soil. The response of the structure on the soft condition of the
soil establishes the length of the foundation extension required to prevent
overturning, whereas the response of the compact condition of the soil
dictates the thickness of concrete and amount of reinforcing steel required
for the foundation extension to resist the bearing pressures developed in
the soil beneath the structure.
(2) Generally, rotations of the structures that approach the point
of incipient overturning (as defined in Figure 88) can be tolerated.
Therefore, for the design to be efficient, the peak response of the
structure on the soft soil should approach incipient overturning. To insure
that the structure will not overturn on the soft soil, the peak rotation of
the structure on the compact soil is limited to a percentage of the
overturning angle (defined in Figure 89). The results of several design
studies indicate that limiting the peak rotation of the structure on the
compact soil to a value of approximately 40 percent of the overturning angle
will insure that the structure will not overturn, but will approach a peak
response of incipient overturning on the soft soil.
(3) The guidelines presented in the previous paragraph are utilized
in the design of the foundation extension by performing a series of
overturning analyses. After each analysis, the dimensions of the foundation
extension are modified, according to the analysis results. This process is
repeated until the results of the analyses for both soil conditions indicate
that the structure rotates to 40 percent of its overturning angle on the
compact soil and does not overturn on the soft soil. This procedure is
generally applicable to cantilever wall barriers only. However, it can be
applied to single cell barriers provided it does not alter the foundation
dimensions to the extent that the following minima are not maintained:
2.08-307
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