This is physics as I love it - primal. The most important formula of physics is F=ma. Force is mass times acceleration. For us, it is better to invert it a bit -- a=F/m or zero acceleration means zero force.
Of course, nothing is simple since force and acceleration are vectors and everything must be summed. This leads to the first study in engineering, that of statics and a force-balance diagram. For, if you want something to stay still, like a building, you must balance all the forces.
You calculate it with a force-balance diagram, and if you want to learn anything here, go off and study it. Then come back. You couldn't believe the anguish in my Statics class. Some people just couldn't get it, and had to drop out of engineering. They went on to be seismologists and climate scientists. :)
So, we'll start with a soil particle, a nice shiny grain of quartz, since I like quartz. How does it live down there?
This grain does not live by himself. He is squished by the other grains. If he turned colour by the amount of squeezing, this would reflect the 'effective stress'. There could come a point where the effective stress is very high, and he could crack.
There is one thing that could relieve him of some cracking pressure (stress), and that's water. As I said in my Wikipedia explanation, we would have these quartz grains in a beaker, and fill it with water. Those seismologists would do well to read this. :) They are bitching about the lack of references, but there's nothing! Filling with water introduces some very interesting physics.