Soil mechanics primarily refers to the subset of civil engineering that studies the properties of soil when used as a construction material or as a foundation. In a larger sense, it may also involve the study of soil formation by weathering and its transportation by wind and water. The formation process determines many of the physical properties of soil, such as structure, composition and relative density. How these properties might affect a construction project is examined by engineering methodologies that include fluid mechanics and the mechanics of materials. Of particular concern in soil mechanics is how the soil at a given site will react to the stresses placed upon it by the demands of a project.
From the perspective of an engineer, soil is the layers of loose, unstratified material on the earth's surface that results from the disintegration of rocks. It typically holds some amount of water, may contain organic material and rests on an underlying solid layer. Soil mechanics is unique in that engineers often have little control over the material properties of a major project component. Adaptations must be made to the properties of the soil as it is found at the site.
These variable properties are a product of the geological formation process and local climatic factors. The soil mechanics of a site can be anticipated by sampling to construct a soil profile. In general, the profile examines three layers that may be broken down into component layers when necessary
The upper layer is generally rich in organic material and is seldom greater than 15 feet (4.6 m) in depth. Under this is a layer about 2 feet (0.61 m) deep of loose, fine-grained, chemically active material that has been deposited from above. The lowest layer essentially remains in the same geological state as when first deposited and may extend downward more than 100 feet (30.5 m). Road construction and foundations for light residential or commercial buildings usually depend on the properties of the secondary layer. Large earthen constructs, such as dams or levees, are typically composed of material taken from the lowest level.
Several common soil structures can be categorized by their mineral composition, chemical properties and particle arrangement. The behavior of each varies in response to compression, angular stress and water flow. Civil engineering applies physical sciences such as fluid and material mechanics to determine the soil mechanics for a particular site. This analysis may rule out a site for a particular project or indicate the adaptations necessary to proceed.
A knowledge of soil mechanics is critical in many aspects of civil engineering. All structures rest on a foundation that is built in reference to soil properties. Pavement design depends on how the underlying soil reacts to load stress and changes due to water saturation or temperature variation. Underground construction, such as tunnels and pipelines, is a dynamic interplay of soil properties, construction methods and component materials.