Soil mechanics is the study of engineering behaviour of soil when it is used either as a construction material or as a foundation material. This is relatively young discipline of civil engineering, systematized in its modern form by Karl Von Terzaghi (1925), who is rightly regarded as the Father of Modern Soil Mechanics.
An understanding of the principles of mechanics is essential to the study of soil mechanics. A knowledge and application of the principles of other basics sciences such as physics and chemistry should would also be helpful in the understanding of soil behaviour. Further, laboratory and field research have contributed in no small measure to the development of soil mechanics as a discipline.
The application of the principles of soil mechanics to the design and construction of foundations for various structures is known as foundation engineering. Geotechnical Engineering may be considered to include both soil mechanics and foundation engineering. In fact, according to Terzaghi, it is difficult to draw a distinct line of demarcation between soil mechanics and foundation engineering; the latter starts where the former ends.
History of development of soil mechanics
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, 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 an important development in soil mechanics. He also published, in 1925, the first treatise on Soil Mechanic, a term coined by him. 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.
Different Internation Conferences have been held under the auspices of the International Society of Soil Mechanics and Foundation Engineering at Harvard (USA) 1936, Rotterdam (The Netherlands) 1948, Zurich (Switzerland) 1953, London (U.K.) 1957, Paris (France) 1961, Montreal (Canada) 1965, Mexico City (Mexico) 1969, Moscow (U.S.S.R) 1973, Tokyo (Japan) 1977, Stockholm (Sweden) 1981, San Francisco (U.S.A.) 1985 and Rio de Janeiro (Brazil) 1989. The thirteenth was held in New Delhi in 1994, the fourteenth in Hamburg, Germany, in 1997, and the fifteenth in Istanbul, Turkey in 2001. The sixteenth is proposed to be held in Osaka, Japan, 2005.
These conferences have given a big boost to research in the field of Soil Mechanics and Foundation engineering.
Field of Application of Soil Mechanics
The knowledge of soil mechanics has application in many fields of civil engineering such as:
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 may consist of the design of flexible or rigid pavements. Flexible pavements depend more on the sub-grade 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 required the knowledge of slope stability analysis; deep excavations may need temporary support – ‘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 design and construction of embankments and earth dams.
The knowledge of soil mechanics, assuming the soil to be an ideal material elastic, isotropic, and homogeneous material – coupled with the experimental determination of soil properties, is helpful in predicting the behaviour of soil in the field.
Soil being a particulate and heterogeneous material, doesn’t lend itself to simple analysis. Further, the difficulty is enhanced by the fact that soil strata vary in extent as well as in depth even in a small area.
A thorough knowledge of soil mechanics is a prerequisite to be a successful foundation engineer. It is difficult to draw a distinguishing line between Soil Mechanics and Foundation Engineering; the later starts where the former ends.