Geotechnical Know.How

The knowledge of soil mechanics has application in many fields of Civil Engineering.

Foundations: The loads from any structure have to be ultimately transmitted to a soil through the foundation for the structure. Thus, the foundation is an important part of a structure, the type and details of which can be decided upon only with the knowledge and application of the principles of soil mechanics.

Underground and Earth-retaining Structures: Underground structures such as drainage structures, pipe lines, and tunnels and earth-retaining structures such as retaining walls and bulkheads can be designed and constructed only by using the principles of soil mechanics and the concept of ‘soil-structure interaction’.

Pavement Design: Pavement Design may consist of the design of flexible or rigid pavements. Flexible pavements depend more on the subgrade soil for transmitting the traffic loads. Problems peculiar to the design of pavements are the effect of repetitive loading, swelling and shrinkage of sub-soil and frost action. Consideration of these and other factors in the efficient design of a pavement is a must and one cannot do without the knowledge of soil mechanics.

Excavations, Embankments and Dams: Excavations require the knowledge of slope stability analysis; deep excavations may need temporary supports—‘timbering’ or ‘bracing’, the design of which requires knowledge of soil mechanics. Likewise the construction of embankments and earth dams where soil itself is used as the construction material, requires a thorough knowledge of the engineering behaviour of soil especially in the presence of water. Knowledge of slope stability, effects of seepage, consolidation and consequent settlement as well as compaction characteristics for achieving maximum unit weight of the soil in-situ, is absolutely essential for efficient design and construction of embankments and earth dams.

The use of soil for engineering purposes dates back to prehistoric times. Soil was used not only for foundations but also as construction material for embankments. The knowledge was empirical in nature and was based on trial and error, and experience.

The hanging gardens of Babylon were supported by huge retaining walls, the construction of which should have required some knowledge, though empirical, of earth pressures. The large public buildings, harbours, aqueducts, bridges, roads and sanitary works of Romans certainly indicate some knowledge of the engineering behaviour of soil. This has been evident from the writings of Vitruvius, the Roman Engineer in the first century, B.C. Mansar and Viswakarma, in India, wrote books on ‘construction science’ during the medieval period. The Leaning Tower of Pisa, Italy, built between 1174 and 1350 A.D., is a glaring example of a lack of sufficient knowledge of the behaviour of compressible soil, in those days.

Coulomb, a French Engineer, published his wedge theory of earth pressure in 1776, which is the first major contribution to the scientific study of soil behaviour. He was the first to introduce the concept of shearing resistance of the soil as composed of the two components— cohesion and internal friction. Poncelet, Culmann and Rebhann were the other men who extended the work of Coulomb. D’ Arcy and Stokes were notable for their laws for the flow of water through soil and settlement of a solid particle in liquid medium, respectively. These laws are still valid and play an important role in soil mechanics. Rankine gave his theory of earth pressure in 1857; he did not consider cohesion, although he knew of its existence.

Boussinesq, in 1885, gave his theory of stress distribution in an elastic medium under a point load on the surface.

Mohr, in 1871, gave a graphical representation of the state of stress at a point, called ‘Mohr’s Circle of Stress’. This has an extensive application in the strength theories applicable to soil.

Atterberg, a Swedish soil scientist, gave in 1911 the concept of ‘consistency limits’ for a soil. This made possible the understanding of the physical properties of soil. The Swedish method of slices for slope stability analysis was developed by Fellenius in 1926. He was the chairman of the Swedish Geotechnical Commission.

Prandtl gave his theory of plastic equilibrium in 1920 which became the basis for the development of various theories of bearing capacity.

Terzaghi gave his theory of consolidation in 1923 which became an important development in soil mechanics. He also published, in 1925, the first treatise on Soil Mechanics, a term coined by him. (Erd bau mechanik, in German). Thus, he is regarded as the Father of modern soil mechanics’. Later on, R.R. Proctor and A. Casagrande and a host of others were responsible for the development of the subject as a full-fledged discipline.

The term ‘Soil’ has different meanings in different scientific fields. It has originated from the Latin word Solum. To an agricultural scientist, it means ‘‘the loose material on the earth’s crust consisting of disintegrated rock with an admixture of organic matter, which supports plant life’’. To a geologist, it means the disintegrated rock material which has not been transported from the place of origin. But, to a civil engineer, the term ‘soil’ means, the loose unconsolidated inorganic material on the earth’s crust produced by the disintegration of rocks, overlying hard rock with or without organic matter. Foundations of all structures have to be placed on or in such soil, which is the primary reason for our interest as Civil Engineers in its engineering behaviour.

Soil may remain at the place of its origin or it may be transported by various natural agencies. It is said to be ‘residual’ in the earlier situation and ‘transported’ in the latter. 

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* According to him, ‘‘Soil Mechanics is the application of the laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of soil particles produced by the mechanical and chemical disintegration of rocks regardless of whether or not they contain an admixture of organic constiuents’’.

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