Civil Engineering Reference
In-Depth Information
TABLE 15.4
Summary of Analyses for Soil Weakened During the Earthquake—Retaining Walls
and Deep Foundations
Topic
Discussion
Retaining
walls
The type of analysis depends on the location of weakened soil during the
earthquake. For example, there could be active wedge liquefaction, passive wedge
liquefaction, and/or liquefaction below the retaining wall footing. See Secs. 10.3
and 10.4 for the type of analysis versus the location of the weakened soil.
Deep
foundations,
vertical
capacity
Do not use a one-third increase in the allowable load capacity of piles and piers if
the soil providing support will be weakened during the earthquake. However, if the
pile or pier is end-bearing on solid material that will not be weakened during the
earthquake and only end-bearing was used in the static analysis, then a one-third
increase in allowable end-bearing pressure for seismic analyses may be appropriate.
Make sure the pile or pier is not founded in liquefiable soil or soil that will have
a significant strength loss during the earthquake shaking. Instead, deepen the piles
or piers through the zone of liquefiable soil and found the piles or piers on soil or
rock that will not be weakened during the earthquake.
Deep
foundations,
downdrag
loads
Calculate the downdrag load on the pile or pier if there is soil above the pile tip
that will be subjected to liquefaction, significant cyclic softening, or volumetric
compression during the earthquake (Sec. 11.7.3). For pile groups, calculate the
downdrag load as: (1) the downdrag load for a single pile times the number of
piles in the group, or (2) the downdrag load acting on the entire perimeter of the
group, whichever is greater.
Deep
foundations,
eccentric
loads
The earthquake will induce base shear into the foundation as illustrated in Fig. 7.10.
This in turn leads to lateral forces imposed on the structure in response to the
base shear. The result is that the uniform static bearing pressure is altered by the
earthquake such that the pressure is increased along one side of the foundation. In
essence, the static resultant force (
Q
) is offset from the centerline of the foundation
by an earthquake-induced eccentricity (
e
), as shown in Fig. 7.10. See Sec. 11.7.4 for
the analysis of deep foundations subjected to an eccentric load.
Deep
foundations,
lateral loads
Consider that liquefaction and cyclic softening of cohesive soil will reduce the
lateral resistance of the pile or pier. A conservative approach is to assume the
zone of liquefaction will not provide any lateral resistance. For level-ground
sites, passive resistance can be provided by a surface layer that is not weakened
during the earthquake and that is thick enough so it is not damaged by underlying
liquefaction (Fig. 7.6). For sloping ground, this surface layer may not protect
the deep foundation from damage due to flow slides or lateral spreading, and
other mitigation measures will be required.
15.4.2
Materials Not Weakened by the Earthquake
If the earthquake is strong enough and under the right loading conditions, it can weaken
almost every type of material. However, in general, examples of materials that are not
likely to be weakened during the earthquake are as follows:
1.
Massive crystalline bedrock.
2.
All types of rock that remain intact during the earthquake.
3.
Pyroclastic deposits, such as ash, where the particles have been welded together.
4.
Highly-cemented natural soils, such as caliche, loess, etc.
