Civil Engineering Reference
In-Depth Information
CHAPTER 13
FOUNDATION ALTERNATIVES
TO MITIGATE EARTHQUAKE
EFFECTS
13.1 INTRODUCTION
If the expected settlement or lateral movement for a proposed structure is too large, then
different foundation support or soil stabilization options must be evaluated. One alterna-
tive is soil improvement methods, such as discussed in Chap. 12. Instead of soil improve-
ment, the foundation can be designed to resist the anticipated soil movement caused by the
earthquake. For example, mat foundations or posttensioned slabs may enable the build-
ing to remain intact, even with substantial movements. Another option is a deep founda-
tion system that transfers the structural loads to adequate bearing material in order to
bypass a compressible or liquefiable soil layer. A third option is to construct a floating
foundation, which is a special type of deep foundation in which the weight of the structure
is balanced by the removal of soil and construction of an underground basement. A floating
foundation could help reduce the amount of rocking settlement caused by the earthquake.
Typical factors that govern the selection of a particular type of foundation are
presented in Table 13.1.
13.2 SHALLOWFOUNDATIONS
A shallow foundation is often selected when the structural load and the effects of the earth-
quake will not cause excessive settlement or lateral movement of the underlying soil layers.
In general, shallow foundations are more economical to construct than deep foundations.
Common types of shallow foundations are described in Table 13.2 and shown in Figs. 13.1
and 13.2.
If it is anticipated that the earthquake will cause excessive settlement or lateral move-
ment, then isolated footings are generally not desirable. This is because the foundation
can be pulled apart during the earthquake, causing collapse of the structure. Instead, a
mat foundation (Fig. 13.2) or a posttensioned slab is more desirable. This is because
such foundations may enable the building to remain intact, even with substantial
movements.
13.1
