Hidden subsurface water percolating (or moving) through the soil has the potential to damage structures and utilities. Discovering, identifying, and mitigating it to prevent this is a challenge. The properties of soil directly influence the movement of water into (infiltration) and through it (permeability). Soils like clay, with a high PI (plasticity index is a measure of how water causes the soil to shrink/swell), have very low infiltration/permeability rates, while sandy and well-aggregated soils allow water to flow easily through it. Water will always travel the path of least resistance and keeping this fact in mind during design and construction will avoid long-term problems.