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Soil Mechanics

Soil Structure and Clay Materials

Soil Compaction

Solids-Water-Air Relations and Index Properties of Solids

Soil Identification and Classification

Introduction to Soil Mechanics

Introduction to Soil and Soil Mechanics
To an Engineer, Soil is the un-aggregated or un-cemented collection of mineral and/or organic particles or fragments covering the large portion of the earth's crust.
Soil Mechanics is one of the subject of Civil Engineering involving the study of soil, its behavior and application as an engineering material. According to Terzaghi : "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 deformation and transformation of rocks regardless of whether or not they contain an admixture of an organic constituent."

Geotechnical Challenges Arising in Civil Engineering and Infrastructure Development
The major geotechnical problems that are generally occurring in civil engineering are as follows:

  • Can the soil and foundation properly support the construction project or not?
  • How will the GWT (groundwater table conditions) affect the engineering project?
  • What will be the results of excavations, grading, or filling?
  • Which is more stable ? : natural or proposed earth slope.
  • Designing of foundation systems such that it won't bear failure after construction.
  • Designing of earthen dams, retaining walls.

Importance of Soil Mechanics
Soil engineering is an applied science which deals with the applications of principles of soil mechanics in practical problems. It deals with all engineering challenges related to soils. It comprises of site investigations, design and construction of foundations, earth retaining structures, and earth structures. Soil engineering has the large application in the construction of various civil engineering works. Some of the important applications are as follows:

  1. Foundation Design and Construction
    • Bearing capacity
    • Settlement
  2. Slope Stability
    • To know the shearing strength
  3. Earth Retaining Structures
    • Permeability, factor of safety
  4. Pavement Design
    • Settlement, compressibility, density
  5. Design of Underground Structures

Historical Development of Soil Mechanics
Soil engineering was used based on the experience of past engineering work
Most designs were often based on “rule of the thumb”
During Roman times, heavy structures such as bridge, aqueducts, harbors and buildings, roads were built, where we can find the application of soil
Most theories in soil mechanics were developed after mid-1700’s

  • Coulomb
    Earth pressure theories, shear strength criterion
  • Darcy’s 
    Theory of flow through porous media
  • Stoke(1857 )
    Theory of particle suspension
  • Rankine
    Earth pressure theories
  • Boussinesq(1885)
    Pressure distribution in elastic mass
  • Atterberg(1911)
    Soil Consistency
  • Swedish Engineers(1914 – 1922)
    Stability of Slopes
  • Prandtl(1920)
    Theory of Plastic equilibrium
  • Terzaghi(1925)
    “Father of Soil Mechanics”
    The first to publish a text book on soil mechanics in The German language.
  • Casagrande
    Soil Classification
  • Proctor
    Soil Compaction
  • Seed(1966)
    Soil liquefaction

Other important names are Taylor, Skempton, Bjerrum, Lame, Rowe etc.

Soil Formation and Soil Type
The geologic cycle explains the process of rock and soil formation.

geological cycle
Geological cycle

Soils are formed either by physical disintegration or chemical decomposition of rocks. Physical disintegration or mechanical weathering of rocks occurs due to following physical processes:

  • Temperature Change 
    unequal expansion and contraction due to temp change results into the detachment of particles from rocks.
  • Wedging Action of Ice 
    water in pores and minute cracks gets frozen in very cold climate and expands which results in the breaking of rocks due to large stress developed in the cracks.
  • Spreading of Roots of Plants 
    segments of rock is forced apart by roots of trees when they grow up.

Chemical decomposition changes the chemical composition of rock i.e mineral properties changes. The following chemical processes generally occur in nature.

  • Hydration
    water combines with rock minerals to give new compounds which cause the change in volume and decomposition occurs.
  • Carbonation 
    Carbon dioxide in atmosphere combines with water to form carbonic acid which reacts with rocks and causes their decomposition
  • Oxidation 
    oxygen ions combine with minerals in rocks.
  • Solution 
    some rock minerals form the solution with water and reaction takes place to form soils.

Residual and Transported Soils
Those soils which are formed and are at the place of formation or origin are residual soils while those which are moved and deposited in other places by transporting agents like water, wind, glaciers, gravity, etc. are transported soils.
Transported soils can be classified as follows:

  • Glacial Soils
    Formed by transportation and deposition of glaciers.
  • Alluvial Soils
    Transported by running water and deposited along streams.
  • Lacustrine Soils
    Formed by deposition in quiet lakes (e.g. soils in Kathmandu valley).
  • Marine Soils
    Formed by deposition in the seas. (e.g. Hong Kong).
  • Aeolian Soils
    Transported and deposited by the wind (e.g. soils in the plateau, China).
  • Colluvial Soils
    Formed by the movement of soil from its original place by gravity, such as during landslide.




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