Soils are formed by weathering of rocks due to mechanical disintegration or chemical decomposition. When a rock surface gets exposed to the atmosphere for an appreciable time, it disintegrates or decomposes into small particles and thus soils are formed.
The geologic cycle consists of erosion, transportation, deposition and upheaval of soil. Exposed rocks are eroded and degraded by various physical and chemical processes. The products of erosion are picked up by agencies of transportation such as water and wind, and are carried to new locations where they are deposited. This shifting of material disturbs the equilibrium of forces on the earth and causes large-scale earth movements and upheavals. This process results in further exposure of rocks and the cycle repeats.
Deposited from suspension in running water (formed due to physical weathering). It is transported soil and the transporting agency involved is water. This type of soil is found along the banks of rivers.
Formed due to deposition from suspension in fresh still water of lakes. It is a layered soil.
Deposited from suspension in sea water.
Soil which is transported by wind.
Uniformly graded wind-blown silt, slightly cemented due to calcium compounds. When wet, it becomes soft, compressible, loses its cementing action and collapses.
Formed due to transportation by gravitational force, found in mountain valleys. Generally consists of irregular coarse particles of rocks.
Soil transported by ice. “Drift” is a general term for deposits made by glaciers directly & indirectly. “Till” refers to deposits made by melting glaciers.
Fine-graded calcium carbonate soil of marine origin, formed by decomposition of animal bones & aquatic plants; a greenish marine calcareous clay.
Chemically weathered volcanic ash, high in montmorillonite. Used as lubricant in drilling operations; highly plastic with high swelling & shrinkage.
Slightly cemented volcanic ash transported by wind or water.
Residual soil formed from basalt, containing a high percentage of montmorillonite clay. Dark in color, suitable for cotton cultivation; shows high swelling & shrinkage, high compressibility & low shear strength.
Formed due to leaching (washing out of siliceous compounds and accumulation of aluminum & iron oxides). Found in hilly areas with humid climates (e.g., Western Ghats, Eastern Ghats, North-East region).
A mixture of inorganic soil & black decomposed organic matter; dark in color. (Muck & Peat soils are also called cumulose soils.)
Highly organic soil consisting almost entirely of vegetative matter in different stages of decomposition. Color varies from black to dark brown; highly compressible.
A mixture of sand, silt & clay. (In geology called mantle or regolith; in soil engineering simply “soil.”)
The term “soil mechanics” was coined by Dr. Karl Terzaghi in 1925 when his book Erdbaumechanik was published in German. According to Terzaghi, soil mechanics is the application of the 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 rock, regardless of whether or not they contain organic constituents. Soil mechanics is therefore a branch of mechanics which deals with the action of forces on soil and with the flow of water in soil.
Soil consists of discrete solid particles which are neither strongly bonded as in solids nor as free as particles of fluids. Consequently, the behavior of soil is intermediate between that of a solid and a fluid. Soil mechanics draws heavily from both solid mechanics and fluid mechanics. As soil is inherently a particulate system, soil mechanics is also called particulate mechanics. Rock mechanics deals with the mechanics of rocks.
Every civil engineering structure—whether a building, a bridge, or a dam—is founded on or below the surface of the earth. Foundations transmit the load of the structure to the soil safely and efficiently. A shallow foundation transmits the load to upper strata of earth; a deep foundation transmits the load to strata at considerable depth below ground surface. Pile foundations are a type of deep foundation. Foundation engineering is a key branch of soil engineering.
When there isn’t enough space for soil to form a stable slope, a retaining structure is needed. Rigid retaining walls or flexible sheet-pile bulkheads hold back soil at different levels on either side and resist earth pressures.
On non-horizontal ground, the component of soil weight parallel to the surface can cause downslope movement and instability. Soil engineering provides methods to analyze both natural and man-made slopes in embankments and excavations.
Designing tunnels, shafts, and conduits requires evaluating the forces exerted by soil on these structures. Soil mechanics offers the theories and calculations needed to ensure stability and safety.
Pavements consist of surfacing (bitumen, concrete), base, subbase, and the subgrade (soil). Soil engineering studies the behavior of subgrade under traffic loads and environmental changes to inform pavement thickness and materials design.
Earth dams use soil as the primary construction material to create water reservoirs. Given the potentially catastrophic consequences of dam failure, soil engineering provides rigorous analyses of slope stability, seepage, and settlement to ensure safety.
Soil heave, subsidence, frost heave, shrinkage, and swelling all affect civil works. Soil engineering delivers in-depth study and mitigation techniques for these phenomena.