Civil and Building Engineering.

INTRODUCTION

Testing of soil in the laboratory, and perhaps in the field, to obtain properties is required to allow the computation of bearing capacity and settlement. The reader is referred to Chapter 3 for details on the determination of the required parameters of the soil at a site.

The problems to be addressed in this chapter can be illustrated by the shallow foundation of given lateral dimensions resting on soil, as shown in Figure 7.1a. The first of two problems facing the engineer is to find the unit vertical load on a shallow foundation, or on the base of a deep foundation, that will cause the foundation to settle precipitously or to collapse. The unit load at failure is termed the bearing capacity. With regard to bearing capacity, the following comments are pertinent:

1. Any deformation of the foundation itself is negligible or disregarded.

2. The stress-strain curve for the soil is as shown in Figure 7.1b.

3. The base of the foundation may be smooth or rough.

4. The soil is homogeneous through the semi-infinite region along and below the foundation.

5. The loading is increased slowly with no vibration.

 6. There is no interaction with nearby foundations. In spite of the constraints on computing the values, the concept of bearing capacity has been used for the design of foundations for decades and presently remains in extensive use.

Figure 7.1 Example of footing and stress-strain curve used in developing bearingcapacity equations.

Investigators have noted that the bearing capacity equations will vary if the failure is general where symmetrical failure surfaces develop below the base of the foundation; is local where a failure occurs due to excessive settlement of the foundation; or is a punch-through where the foundation punches through a strong surface layer and causes the weak soil below to fail. The equations for general shear failure are most important.

The second of the two problems addressed here is the computation of settlement of a foundation such as that shown in Figure 7.1. Two types of settlement are noted: immediate or short-term settlement and long-term settlement due to the consolidation of saturated clays. The immediate settlement of foundations on sands of loose or medium density is relatively so large that settlement controls, termed a local shear failure, as noted above, and a general bearing capacity failure does not occur. Immediate settlement of foundations on sand and clay is discussed in Chapter 9.

Equations and procedures for dealing with long-term settlement due to consolidation are presented here. The settlement of deep foundations is discussed in chapters that deal specifically with piles and drilled shafts.

The finite-element method (FEM) discussed in Chapter 5 provides valuable information to the engineer on both bearing capacity and settlement, as demonstrated by the example solution presented. FEM can now be implemented on most personal computers, rather than on large mainframes, and will play a much greater role in geotechnical engineering as methods for modeling the behavior of soil are perfected. Leshchinsky and Marcozzi (1990) performed small-scale experiments with flexible and rigid footings and noted that the
flexible footings performed better than the rigid ones. Rather than use the performance of expensive tests with full-sized footings, FEM can be used to study the flexibility of footings.