The Term Soil

The term ‘Soil’ has various meanings, depending upon the general professional field in which it is being considered.

To an agriculturist,

soil is the substance existing on the earth’s surface, which grows and develops plant life.

To the geologist also,

soil is the material in the relatively thin surface zone within which roots occur, and all the rest of the crust is grouped under the term rock irrespective of its hardness.

To an engineer,

soil is the unaggregated or uncemented deposits of mineral and/or organic particles or fragments covering large portion of the earth’s crust. It includes widely different materials like boulders, sands, gravels, clays and silts, and the range in the particle sizes in a soil may extend from grains only a fraction of a micron (10–4 cm) in diameter up to large size boulders. Soil engineering, Soil Mechanics or Geotechnique is one of the youngest disciplines of civil engineering involving the study of soil, its behaviour and application as an engineering material.

According to Terzaghi (1948):

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

The term Soil Engineering

is currently used to cover a much wider scope implying that it is a practical science rather than a purely fundamental or mathematical one.

The term Foundation Engineering

is a branch of civil engineering, which is associated with the design, construction, maintenance, and renovation of footings, foundation walls, pile foundations, caissons, and all other structural members which form the foundations of buildings and other engineering structures (Taylor, 1948). Soil is considered by the engineer as a complex material produced by the weathering of the solid rock. The formation of soil is as a result of the geologic cycle continually taking place on the face of the earth. The cycle consists of weathering or denudation, transportation, deposition and upheaval, again followed by weathering, and so on. Weathering is caused by the physical agencies such as periodical temperature changes, impact and splitting action of flowing water, ice and wind, and splitting actions of ice, plants and animals. Cohesionless soils are formed due to physical disintegration of rocks. Chemical weathering may be caused due to oxidation, hydration, carbonation and leaching by organic acids and water. Clay minerals are produced by chemical weathering. Soil obtained due to weathering may be residual or transported. Residual soils, which remain in place directly over the parent rock, are relatively shallow in depth. The deposits of the transported soils may be considerable in depth and their homogeneity or heterogeneity depends upon the manner of their transportation and deposition. The various agencies of transporting and redepositing soils are: water, ice, wind and gravity. Water-formed transported soils are termed as alluvial, marine or lacustrine. All the material, picked up, mixed, disintegrated, transported and redeposited by glaciers either by ice or by water issuing from melting of glaciers, is termed glacial drift or simple drift. The glacial deposits in general consists of a heterogeneous mixture of rock fragments and soils of varying sizes and proportions and, except the stratified drift deposited by glacial streams, are without any normal stratification. Dune sand and loss are the wind-blown (aeoline) deposits. Loess is the wind-blown silt or silty clay having little or no stratification. Soils transported by gravitational forces are termed colluvial soils, such as talus. The accumulation of decaying and chemically deposited vegetable matter under conditions of excessive moisture results in the formation of cumulose soils, such as peat.