The classical Coulomb--Terzaghi shear strength equation, s = c + ($\sigma $ - u) tan $\phi $, depends not only upon mechanical and mineralogical factors, but to a high degree upon the total chemistry of the system. Changes in chemistry owing to natural and anthropogenic factors may change the strength properties of a given clay soil at a given water content. The classical example of slow acting chemical changes is the development of quick clay properties. Modern infiltration of different electrolytes and detergents from leaking sewer systems may change shear strength and compressibility of the ground. These processes are different in clays of expanding and non-expanding minerals. The importance of the different cation species seems to depend upon valency and polarizability. Influence of anions is more complicated and depends upon interaction with edge charges and the crystalline properties of the clay minerals.