This document discusses permeability in soil, which is the property that allows water to flow through soil. It defines permeability and explains its importance in engineering applications like earth dams and slope stability. Darcy's law states that flow through saturated soil is directly proportional to the hydraulic gradient. The coefficient of permeability depends on factors like particle size, pore water properties, degree of saturation, and soil structure. Laboratory tests like constant head and falling head tests are used to measure the coefficient of permeability.