**1. Introduction**

Unsaturated soils are characterized by negative pore-water pressure. In practical terms, partially saturated and unsaturated are synonymous: both terms indicate a degree of satura‐ tion lower than one; however, in specific terms, unsaturated implies the introduction of a third phase (gaseous) to the two-phase system already present in the saturated soils (liquid and solid phases only) [1].

Currently, computer programs are helpful for solving problems of water flow and facilitate both the study of transient-state flow and the characterization of unsaturated soils, which, from an analytical standpoint, are complicated and laborious tasks [2].

To demonstrate the application of the theoretical foundations presented in this chapter, an analysis of a tailings dam is presented as a case study. These structures are generally found in an unsaturated state. Thus, the primary goals of this analysis are to describe the applied methodology and to establish criteria and recommendations that can be followed to solve related problem types.
