dry clay particles clay particles pushed apart by water molecules
How does water make clay soils swell?
The answer is actually quite simple. Clay soils have a specific structure. You should think of them as very thin plates or, better yet, platelets. They are so thin that a bucket full of clay platelets would cover a football field if they each were laid in the ground surface. They have a very small electrical charge so that one part of each platelet has a positive charge and the opposite has a negative charge.
The same thing is true of water molecules; each water molecule has a negative charge and a positive charge. Because opposite electrical charges attract each other, the water molecules become attached to the clay platelets pushing the clay platelets apart. Thus, the clay appears to swell. Under the right conditions, the water molecules will evaporate allowing the clay platelets to come back together giving the appearance of shrinking.