Bowles

CHAPTER

4
BEARING CAPACITY
OF FOUNDATIONS

4-1

INTRODUCTION
The soil must be capable of carrying the loads from any engineered structure placed upon it
without a shear failure and withthe resulting settlements being tolerable for that structure.
This chapter will be concerned with evaluation of the limiting shear resistance, or ultimate
bearing capacity qu\u of the soil under afoundation load. Chapter 5 will be concerned with
estimation of settlements. A soil shear failure can result in excessive building distortion and
even collapse. Excessive settlements can result instructural damage to a building frame,
nuisances such as sticking doors and windows, cracks in tile and plaster, and excessive wear
or equipment failure from misalignment resulting from foundationsettlements.
Seldom has a structure collapsed or tilted over from a base shear failure in recent times.
Most reported base failures have occurred under embankments or similar structures where a
lowfactor of safety was deemed acceptable. Most structural distress attributed to poor foundation design is from excessive settlements. Even here, however, structural collapse seldom
occurs. This may inpart be due to settlements being time-dependent, so that when cracks or
other evidence first appears, there is sufficient time to take remedial measures.
It is necessary to investigate both base shearresistance and settlements for any structure.
In many cases settlement criteria will control the allowable bearing capacity; however, there
are also a number of cases where base shear (in which abase punches into the ground—
usually with a simultaneous rotation) dictates the recommended bearing capacity. Structures
such as liquid storage tanks and mats are often founded on soft soils, whichare usually more
susceptible to base shear failure than to settlement. Base shear control, to avoid a combination base punching with rotation into the soil, is often of more concern than settlement...