Engineering and Construction

Introduction

This section will discuss deep foundations subjected to lateral loads. If the soil is unable to provide sufficient lateral support, then failure of the structure could occur. In terms of the amount of lateral resistance, the most important factor is the shear strength of the soil or rock. Obviously solid rock can impart much more lateral resistance than a soft clay. Other important factors in the design of deep foundations subjected to lateral loads are as follows:

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 (2006). 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 to simply flow around the pier. Thus if the cohesive soil is plastic enough, the constant lateral load may cause the deep foundation to slowly deflect laterally with the clay creeping around the pile or pier and opening a gap such as shown in Fig. 6.17.

3. Allowable lateral movement. The allowable lateral deflection of the deep foundation will    govern the amount of lateral resistance exerted by the soil. This is because, especially for cohesive soil, a considerable amount of lateral deformation is required in order to achieve passive pressure. If the deep foundation is very flexible and can sustain a large lateral deflection, then a greater depth of soil will be subjected to passive pressure. On the other hand, if the acceptable amount of lateral deflection is low, then only the upper few feet of soil may mobilize passive soil resistance.