There are four major types of dam: Concrete arch, rock-fill, buttress and gravity. Each of these is constructed to hold back water or tailings. Their stability is dependent on a lack of leakage and structural integrity. The integrity of a dam is maintained when the boundary conditions are maintained close to the design values, and when the material cohesion of the main part of the dam is maintained. Mode shapes of all four types are characteristic of very complex structures, and STRAAM holds a database of what is normal. Tests of the dynamic characteristics of dams can quickly ascertain whether or not a dam continues to perform well.