If you haven’t clicked on the Foundations & Retaining Structures tab, please do so first to read and see some interesting youtube videos and then come back here to read about shallow foundations.
The next two videos actually highlight the interaction of soil under shallow foundations. Terzaghi (Called the father of soil mechanics) from his vast experience presented his theory of ultimate bearing capacity of soil. Pictorially shown here (Apologies for my bad graphics).
If you want to understand more watch the next two videos.
First one is by ASCE describing the types of soil failure when disturbed.
The second one also shows the soil failure with the failure planes and wedges.
Now you have a basic understanding of how the soil acts. The Geotechnical engineer who is the soil expert will study the soil and come up with a recommendation of what ultimate bearing capacity of the soil is for the spread footing. Ultimate bearing capacity is the bearing capacity at which the soil fails. The Geotech then uses a factor of safety to come up with the allowable bearing capacity of the soil. This allowable bearing capacity (in pounds per square foot or psf) is what the structural engineer must use to design his spread footings or wall footings. The foundation design must make sure that this allowable bearing capacity is not exceeded.
Failure of soil will lead to settlement of the structure. Depending on the soil type, the settlement can either be sudden or gradual. Building settlement causes cracks in the walls ceilings and in some cases large settlements makes it dangerous for people to live in.
The structural engineer uses the loads that come from the building on to the foundation, say for example 1000 lb concentric load on a column and takes the allowable bearing capacity value that the soils report gives to come up with a size of the spread footing. For example, if the allowable bearing capacity is 1000 psf then the footing size will be, Area = 1000 lb/1000psf = 1 sq.ft , In other words, a 1ft x 1ft footing will be adequate. This is obviously a footing for a small porch column or something similar. And good structural engineers never design to the capacity of the soil. They always make the footing larger than required so there is room for adding more, for example, say the addition of a second story on a house increases the loads on this column or accounting for a change in building materials that may increase the loads on this column.
If the project is too small to hire a geotechnical engineer then the structural engineer can look at the local building codes to see what the code recommends for allowable bearing capacity for that particular region of code jurisdiction. Usually, the code recommended values are on the extremely conservative side.