Disposal of waste fluids into the subsurface may affect the mechanical properties and strength of the rock mass in two important ways. When fluid is chemically inert, strength and ductility are reduced by increasing pore pressure. When fluid is chemically active, the strength of the rock mass is further reduced through modification of the cohesive strength of its constituent grains in contact with the fluid.

Several experiments reveal that the strength of rocks and propagation of minute surface cracks are highly dependent on the moisture content of the rock. Dilute solutions of aluminum and ferric iron salts, in addition to water, react with the surface structure of quartz and silicates and weaken the surface silicon-oxygen bonds by hydrolysis. The result is a reduction in surface energy, surface cohesion, and breaking strength. The coefficient of internal friction, however, remains unaltered.

The frictional characteristics of already-broken rocks may be significantly altered by the introduction of chemically active fluids. Because of such manmade earthquakes as those near Denver and Rangely, Colorado, it is obvious that more sophisticated tests of rock properties should be made, particularly regarding the influence of pore-fluid pressure and chemistry, if problems caused by unexpected rock failure are to be eliminated.