It can resist compression, but when it fails due to compression, it fails in shear. This means that the strength of a soil comes from its ability to resist shear forces, which refers to the soils ability to resist sliding along internal surfaces within its mass. Shear stress is defined as the intensity of force, or force prer unit area, acting tangent to the area. See figure 2. The angle phi is a constant that is dependant upon the soil, meaning that each soil has a different phi that is determined experementally.
Most soils are a mixture of the two properties.
This means that with an increased load, the shear strength of clay does not increase, but the shear strength of sand does. Because of this relationship, soils with a lot of clay are vrey poor for building on, whereas sandy soils are better to build on, but not perfect, because they have no cohesion. Here is a graph to illustrate this point, where s is the shear strenght, and sigma is the load. Most of what I have just gone over applies mainly to places where the natural slope of the soil has been changed, as in road cuts and trenches. Here is a drawing of how a soil often fails in shear.