The behaviour of soils is described by soil mechanics, a part of soil physics and applied mechanics. It varies from fluid and solid mechanics in that soils are made up of a heterogeneous mixture of fluids (typically air and water) and particles (primarily clay, silt, sand, and gravel), although organic solids and other substances can also be found. Soil mechanics, like rock mechanics, provides the theoretical foundation for analysis in geotechnical engineering, a civil engineering subdiscipline, and engineering geology, a geology subdiscipline. Soil mechanics is a branch of engineering that studies the deformations and fluid flow within natural and man-made structures that are supported on or made of soil, as well as structures that are buried in soils. Geomechanics is a broader scientific area that examines the mechanical behaviour of all earth materials, including soils. Rock mechanics is a subset of geomechanics. The ideas of continuum and fracture mechanics are integrated into rock mechanics to measure the behaviour of a rock exposed to a stress field. Engineering rock mechanics and geological rock mechanics are the two subcategories of rock mechanics. Rock mechanics is an essential area in civil engineering since it is used in a wide range of infrastructure projects, such as dams, highways, tunnels, bridges, buildings, and slope protection.