# Stress Increase in Soils

We have looked at bearing capacity of the soil in the previous section but checking only that is not enough. We must also make sure that, even if the soil can bear the pressure, it will not settle too much.  Before calculating settlement amount in soil however, we must first know the stresses in soil.

We have already seen total and effective stresses that exist in natural soil a few sections before in this book. But when we introduce new loads to the soil such as by building structures on it, the stresses increase beyond those existing natural levels. This new stress increase will cause a certain settlement in the soil.

In other words, we must know how much stress at what depth and what horizontal location increased in soil below as a result of new loads that we introduce. There are various methods to estimate stress increase in soils.

The most straightforward of them, called 2:1 method, is shown below:

In this method, the stress below a footing gets distributed gradually over a larger area which widens as 2 vertical to 1 horizontal distance below the footing. This means, the vertical stress on the soil decreases with increasing depth.

There are other more complex methods, developed by researchers. One of the most commonly known methods, developed by Boussinesq in 1883, mathematically estimates the stresses at a certain depth and horizontal location. For these estimates to be valid, he assumed that the soil medium is perfectly elastic (no permanent deformations upon stress removal), homogeneous and isotropic (transmits stress equally in all directions). Although we will not present the equations here, the graphical representation of those equations looks similar to the figure below. Note that this graph is drawn just to present the concept to show that as we go on a further curve, the percentage of q reduces dramatically, and it not specifically given here the shape of loading:

Soil strength 20