Engineering and Construction

The documented cases of bearing capacity failures during earthquakes indicate that usually the following three factors (separately or in combination) are the cause of the failure:

1. Soil shear strength. Common problems include an overestimation of the shear strength of the underlying soil. Another common situation leading to a bearing capacity failure is the loss of shear strength during the earthquake, because of the liquefaction of the soil or the loss of shear strength for sensitive clays.

2. Structural load. Another common problem is that the structural load at the time of the bearing capacity failure was greater than that assumed during the design phase. This can often be the case when the earthquake causes rocking of the structure, and the resulting structural overturning moments produce significant cyclic vertical thrusts on the foundation elements and underlying soil.

3. Change in site conditions. An altered site can produce a bearing capacity failure. For example, if the groundwater table rises, then the potential for liquefaction is increased. Another example would be the construction of an adjacent excavation, which could result in a reduction in support and a bearing capacity failure.

The most common cause of a seismic bearing capacity failure or excessive settlement is due to liquefaction of the underlying soil. Later will present the analyses used to determine if a soil will liquefy during the design earthquake.

When presenting the recommendations 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 example, the International Building Code (2006) states:

In soil reports, it is commonly recommended that for the analysis of earthquake loading, the allowable bearing pressure may be increased by a factor of 1/3. The rational behind this recommendation is that the allowable bearing pressure has an ample factor 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 earthquake
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 seismic shaking weakens them, the static values of allowable bearing pressures should not be increased for the earthquake analyses. In fact, the allowable bearing pressure may actually have to be reduced to account for the weakening of the soil during the earthquake. The remainder of this section will deal with the determination of the bearing capacity of soils that are weakened by the seismic shaking.