How is bearing capacity of soil varied?

The bearing capacity of soil varies not only with its strength but also with the magnitude and distribution of the load. When a load Q, is applied to a soil mass is gradually increasing amounts, the soil deforms, making a load-settlement curve similar to a stress-strain curve. When the ultimate load, Qu, is reached, the rate of deformation increases. The load-settlement curve goes through a point of maximum curvature indicating failure within the soil mass. As in figure are demonstrated below:

Plate Load Test Data

Different curves depending on the character of the soil that is loaded. Dense sand and stiff clay show a sharp sudden failure while loose sand and soft clay show a more gradual transition associated with progressive failure.

Excavations of actual soil masses and model tests of foundations indicate that shear or sliding occurs on curved surfaces as shown in Figure 2.2. This is associated with a rise in the ground surface adjacent to the load and usually with a tipping of the loaded area to one side.

Typical Failure Surface beneath the Foundation

No exact mathematical methods have been derived to define the surface of failure to determine the bearing capacity of soil. Historically, over the past hundred years, large number of investigators has undertaken studies relating to foundation bearing capacity based on the shapes of observed failure zones and on certain simplification of the actual soil properties. Some of these methods have been found to give reliable results.