Unsaturated (or partially saturated) soil
Conventional soil mechanics theory treats soil as either fully saturated (pores filled with water) or dry (pores filled with air). However, a large number of geotechnical problems involve the presence of partially saturated soil zones where the voids between the soil particles are filled with a mixture of air and water. These zones are usually ignored in practice and the soil is assumed to be either fully saturated or completely dry. It has long been established however that the behaviour of partially saturated soils can be very different to that of fully saturated or completely dry soils.