BEARING CAPACITY ANALYSES FOR EARTHQUAKES - Geotechnical Earthquake Engineering - page 238

For the spread footings:

q ult 0.4 (1 r u ) b BN 0.4 (1 0.20)(9.7 kN/m 3 )(2.5 m)(21) 160 kPa

And using a factor of safety of 5.0 gives

q all q ult

FS 160 kPa

___ _______

5.0 32 kPa

Thus provided the strip and spread footings are at least 1.5 and 2.5 m wide, respectively,
the allowable bearing capacity is equal to 24 kPa for the strip footings and 32 kPa for the
spread footings. These allowable bearing pressures would be used to determine the size of
the footings based on the anticipated dead, live, and seismic loads.

8.4 BEARING CAPACITY ANALYSIS FOR
COHESIVE SOIL WEAKENED BY THE
EARTHQUAKE

8.4.1 Introduction

As discussed in Sec. 5.5.1, cohesive soils and organic soils can also be susceptible to a loss
of shear strength during the earthquake. Examples include sensitive clays, which lose shear
strength when they are strained back and forth. In dealing with such soils, it is often desir-
able to limit the stress exerted by the footing during the earthquake so that it is less than the
maximum past pressure vm of the cohesive or organic soils. This is to prevent the soil from
squeezing out or deforming laterally from underneath the footing.

8.4.2 Bearing Capacity Equation

As mentioned in Sec. 7.5, it is often very difficult to predict the amount of earthquake-
induced settlement for foundations bearing on cohesive and organic soils. One approach is
to ensure that the foundation has an adequate factor of safety in terms of a bearing capac-
ity failure. To perform a bearing capacity analysis, a total stress analysis can be performed
by assuming that c s u . Using Eq. (8.6), and for a relatively constant undrained shear
strength versus depth below the footing, the ultimate bearing capacity is as follows:
For strip footings:

q ult cN c 5.5 s u (8.11 a )

For spread footings:

q ult cN c ( 1 0.3 B

L ) 5.5 s u ( 1 0.3 B L )

__ __

(8.11 b )

For a given footing size, the only unknown in Eq. (8.11) is the undrained shear strength s u .
Table 5.4 presents guidelines in terms of the undrained shear strength that should be