Civil Engineering Reference
In-Depth Information
FIGURE 7.10
Diagram illustrating lateral forces
F
in response to the base shear
V
caused by the earth-
quake. Note that the uniform static bearing pressure is altered by the earthquake such that the pressure is
increased along one side of the foundation. (
Reproduced from Krinitzsky et al. 1993, with permission from
John Wiley & Sons.
)
In terms of the analysis for rocking settlement, R. B. Seed (1991) states:
Vertical accelerations during earthquake seldom produce sufficient vertical thrust to cause
significant foundation settlements. Horizontal accelerations, on the other hand, can cause
“rocking” of a structure, and the resulting structural overturning moments can produce signif-
icant cyclic vertical thrusts on the foundation elements. These can, in turn, result in cumulative
settlements, with or without soil liquefaction or other strength loss. This is generally a poten-
tially serious concern only for massive, relatively tall structures. Structures on deep founda-
tions are not necessarily immune to this hazard; structures founded on “friction piles” (as
opposed to more solidly-based end-bearing piles) may undergo settlements of up to several
inches or more in some cases. It should be noted that the best engineering solution is generally
simply to provide a sufficiently high static factor of safety in bearing in order to allow for ample
resistance to potential transient seismic loading.
As indicated above, the best engineering solution is to provide a sufficiently high factor
of safety against a bearing capacity failure, which is discussed in Chap. 8.
7.6 PROBLEMS
The problems have been divided into basic categories as indicated below:
Liquefaction-Induced Settlement
7.1
Use the data from the example problem in Sec. 7.2.2, but assume that
a
max
/
g
0.1
and the sand contains 15 percent nonplastic fines. Calculate the settlement, using Figs. 7.1
and 7.2.
Answer:
See Table 7.3.
