Civil Engineering Reference
In-Depth Information
FIGURE 8.4
The building suffered a liquefaction-induced bearing capacity failure during the Izmit earthquake
in Turkey on August 17, 1999. (
Photograph from the Izmit Collection, EERC, University of California, Berkeley.
)
8.1.4
One-Third Increase in Bearing Pressure for Seismic Conditions
When the recommendations are presented for the allowable bearing pressures at a site, it is
common practice for the geotechnical engineer to recommend that the allowable bearing
pressure be increased by a factor of one-third when performing seismic analyses. For exam-
ple, in soil reports, it is commonly stated: “For the analysis of earthquake loading, the allow-
able bearing pressure and passive resistance may be increased by a factor of one-third.” The
rational behind this recommendation is that the allowable bearing pressure has an ample fac-
tor of safety, and thus for seismic analyses, a lower factor of safety would be acceptable.
Usually the above recommendation is appropriate for the following materials:
1.
Massive crystalline bedrock and sedimentary rock that remains intact during the earth-
quake
2.
Dense to very dense granular soil
3.
Heavily overconsolidated cohesive soil, such as very stiff to hard clays
These materials do not lose shear strength during the seismic shaking, and therefore an
increase in bearing pressure is appropriate.
A one-third increase in allowable bearing pressure should not be recommended for the
following materials:
1.
Foliated or friable rock that fractures apart during the earthquake
2.
Loose soil subjected to liquefaction or a substantial increase in excess pore water pressure
3.
Sensitive clays that lose shear strength during the earthquake
4.
Soft clays and organic soils that are overloaded and subjected to plastic flow
These materials have a reduction in shear strength during the earthquake. Since the
materials are weakened by the seismic shaking, the static values of allowable bearing
