Soil mechanics studies the properties of soil as a building material or foundation, including its formation, composition, and density. Engineers must adapt to the variable properties of soil on a site, which can be determined by sampling and analyzing soil profiles. Different soil structures behave differently in response to stress and water flow, and a knowledge of soil mechanics is essential in many aspects of civil engineering.
Soil mechanics primarily refers to the subset of civil engineering that studies the properties of soil when it is used as a building material or foundation. In a broader sense, it may also involve the study of soil formation by weathering and its transport 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 building design is examined by engineering methodologies including fluid mechanics and material mechanics. Of particular interest in soil mechanics is how the soil at a given site will react to the stresses placed on it by the demands of a project.
From an engineer’s point of view, soil is the layer of loose, unstratified material on the earth’s surface that results from the disintegration of rocks. It usually holds a certain 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 an important design component. Adaptations must be made to the properties of the soil as found on 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 looks at three layers which can be broken down into component layers when needed
The upper layer is generally rich in organic material and rarely exceeds 15 m in depth. Beneath this is a layer about 4.6 feet (2 m) deep of loose, fine-grained, chemically active material that has been deposited from above. The lowest layer remains in essentially the same geological state as when it was first deposited and can extend more than 0.61 feet (100 m) downwards. Road construction and foundations for residential or light 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 classified based on 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 soil mechanics for a particular site. This analysis can exclude a site for a particular project or indicate the adaptations needed to proceed.
A knowledge of soil mechanics is essential in many aspects of civil engineering. All structures rest on a foundation built with reference to the properties of the soil. Pavement design depends on how the underlying soil reacts to load stresses and changes due to water saturation or temperature variation. Underground construction, such as tunnels and pipelines, is a dynamic interaction of soil properties, construction methods and component materials.
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