1. Magnitude of load. In many cases, the magnitude of the lateral load will be small and will
not significantly affect the design of the deep foundation. An extensive investigation or field load
tests are usually not required and the allowable lateral bearing of the soil or rock could be based on
building code values. For example, Table 18.4 presents allowable lateral bearing values for soil and
rock per the International Building Code (2009). The values listed in Table 18.4 can be increased for
each additional foot (0.3 m) of depth up to a maximum of 15 times. However, the allowable lateral
bearing values should only be applied for the upper portion of the pile that laterally deflects to the
extent that passive soil pressure is mobilized. This will be further discussed in Item 3.
For those projects where the lateral load to be carried by the deep foundation is low, the allowable
lateral bearing values from Table 18.4 can be recommended for design. In most cases, the values
in Table 18.4 will be conservative and it is often recommended that these values be applied over
the pile or pier diameter. However, there may be cases where the allowable lateral bearing values in
Table 18.4 are too high, such as when the rock is highly fractured or weathered. Likewise, the allowable
lateral bearing values in Table 18.4 may be too high for soil that is in a loose or soft state.
2. Periodic or constant load. In some cases, the deep foundation may need to only resist periodic
lateral loads, such as a wind-induced or earthquake-induced lateral loads. For periodic lateral
loads, the allowable lateral bearing capacity for a deep foundation embedded in cohesive soil would
be a short-term case and hence a total stress analysis using the undrained shear strength of the soil
would be used.
In other cases, the lateral load may be constant. An example of a constant lateral load would be
a retaining wall footing supported by a deep foundation. The backfill soil would exert a constant
lateral load onto the retaining wall and this lateral load would be transmitted to the deep foundation.
For constant lateral loads, the long-term condition could govern and an effective stress analysis
would be needed. In addition, for constant lateral loads on a deep foundation in cohesive soil,
the possibility of long-term creep of the soil must be evaluated. For example, Fig. 6.17 shows a picture
of soil movement around a concrete pier. The soil shown in Fig. 6.17 was classified as silty
clay, having a liquid limit of 56 and a plasticity index of 32. At this site, the soil was plastic enough