Introduction to Soil Mechanics
The term "soil" can have different meanings, depending upon the field in which it is considered.
To a geologist, it is the material in the relative thin zone of the Earth's surface within which roots occur, and
which are formed as the products of past surface processes. The rest of the crust is grouped under the
To a pedologist, it is the substance existing on the surface, which supports plant life.
To an engineer, it is a material that can be:
built on: foundations of buildings, bridges
built in: basements, culverts, tunnels
built with: embankments, roads, dams
supported: retaining walls
Soil Mechanics is a discipline of Civil Engineering involving the study of soil, its behaviour and application
as an engineering material.
Soil Mechanics is the application of laws of mechanics and hydraulics to engineering problems dealing with
sediments and other unconsolidated accumulations of solid particles, which are produced by the
mechanical and chemical disintegration of rocks, regardless of whether or not they contain an admixture of
Soil consists of a multiphase aggregation of solid particles, water, and air. This fundamental composition
gives rise to unique engineering properties, and the description of its mechanical behavior requires some of
the most classic principles of engineering mechanics.
Engineers are concerned with soil's mechanical properties: permeability, stiffness, and strength. These
depend primarily on the nature of the soil grains, the current stress, the water content and unit weight.
Formation of Soils
In the Earth's surface, rocks extend upto as much as 20 km depth. The major rock types are categorized as
igneous, sedimentary, and metamorphic.
Igneous rocks: formed from crystalline bodies of cooled magma.
Sedimentary rocks: formed from layers of cemented sediments.
Metamorphic rocks: formed by the alteration of existing rocks due to heat from igneous intrusions
or pressure due to crustal movement.
Soils are formed from materials that have resulted from the disintegration of rocks by various processes of
physical and chemical weathering. The nature and structure of a given soil depends on the processes and
conditions that formed it:
Breakdown of parent rock: weathering, decomposition, erosion.
Transportation to site of final deposition: gravity, flowing water, ice, wind.
Environment of final deposition: flood plain, river terrace, glacial moraine, lacustrine or marine.
Subsequent conditions of loading and drainage: little or no surcharge, heavy surcharge due to ice
or overlying deposits, change from saline to freshwater, leaching, contamination.
All soils originate, directly or indirectly, from different rock types.