Geotechnical Engineering and Soil Mechanics - Study Notes

Introduction to Geotechnical Engineering and Soil Mechanics

  • Geotechnical Engineering (also known as geotechnics) is the branch of civil engineering concerned with the engineering behavior of earth materials.
  • It uses the principles and methods of soil mechanics and rock mechanics to solve engineering problems and design engineering works.
  • Includes:
    • Investigating existing subsurface conditions
    • Determining soil properties
    • Assessing risks posed by site conditions
    • Designing earthworks and structure foundations
    • Monitoring site conditions, earthwork, and foundation construction
  • Purpose: provide the basis for safe, economical, and reliable construction by understanding how soils behave under loads and environmental conditions.

Soil Mechanics

  • A discipline of engineering science dealing with the properties and behavior of soil as a structural material.
  • All structures must be built on soils.
  • Main objective: lay down principles, theories, and procedures for the design of safe and sound structures.
  • The foundation engineer must interpret soil mechanics principles to suit field conditions.

History of Soil Mechanics

  • 1930:
    • “Soil Mechanics” established as a branch of civil engineering.
    • Karl Terzaghi is regarded as the “Father of Soil Mechanics”; he coined the term Soil Mechanics.
  • History can be divided into phases:
    • Preclassical Period of Soil Mechanics (1700–1776)
    • Classical Soil Mechanics — Phase I (1776–1856)
    • Classical Soil Mechanics — Phase II (1856–1910)
    • Modern Soil Mechanics (1910–1927)

Preclassical Period of Soil Mechanics (1700–1776)

  • Focus: natural slopes and unit weights of soils; semi-empirical earth pressure theories.
  • Key figures and dates:
    • 1717: Henri Gautier studied natural slopes of soils.
    • 1729: Bernard Forest de Belidor published a textbook proposing a theory for lateral earth pressure on retaining walls.
    • 1746: Francois Gadroy reported first laboratory model test results on a 76-mm-high retaining wall built with sand backfill.
    • 1769: Jean Rodolphe Perronet studied slope stability.

Classical Soil Mechanics — Phase I (1776–1856)

  • Predominantly French contributions; calculus-based approaches to soil problems.
  • Key milestones:
    • 1776: Charles Augustin Coulomb used calculus for maxima/minima in retaining walls.
    • 1790: Gaspard Clair Marie Riche de Prony included Coulomb’s theory in Nouvelle Architecture Hydraulique (Vol. 1).
    • 1820: Jacques Frederic Francais and Claude Navier studied inclined backfills and backfills supporting surcharge.
    • 1840: Jean Victor Poncelet extended Coulomb’s theory with a graphical method for determining lateral earth pressure.
    • 1846: Alexandre Collin provided details for deep slips in clay slopes, cuts, and embankments.

Classical Soil Mechanics — Phase II (1856–1910)

  • Emergence of experimental results from laboratory tests on sand.
  • Key milestones:
    • 1856: Henri Philibert Gaspard Darcy published work on permeability of sand filters.
    • 1856: Sir George Howard Darwin conducted laboratory tests on overturning moment for hinged walls retaining sand.
    • 1885: Joseph Valentin Boussinesq developed the theory of stress distribution.
    • 1887: Osborne Reynolds demonstrated dilatancy in sand.
    • 1898: Beresford’s uplift pressure study on the Narora Weir (Ganges) documented (Technical Paper No. 97, Govt. of India, 1902).

Modern Soil Mechanics (1910–1927)

  • Focus on clay properties and fundamental soil parameters.
  • Key milestones:
    • 1911: Albert Mauritz Atterberg defined liquid, plastic, and shrinkage limits for cohesive soils.
    • 1918, 1926: Wolmar Fellenius developed stability analysis for saturated clays.
    • 1919–1924: Karl Terzaghi developed the theory of consolidation for clays as known today.

Importance of Soil Mechanics

  • Foundations: All civil engineering structures ultimately rest on soil; soils transfer loads to underlying strata.
  • Foundation design decisions depend on soil strength:
    • If soil is strong, shallow foundations may be used.
    • If soil is weak, deep foundations (e.g., piles, wells) are required.
  • Understanding soil strength is essential to select appropriate foundation types and design methods.

Earthen Dams

  • Earthen dams are common; soil used must be suitable for construction.
  • Properties to check regularly:
    • Permeability
    • Strength
    • Density
  • Earthen dams are costly and carry high risk of failure if not designed and constructed carefully; thorough soil property assessment is essential.

Embankments

  • Embankments raise highway levels on plains and help keep foundations above the water table.
  • Embankments are typically built from soil and tested for various properties.
  • Objective: design economical embankments while ensuring stability and performance under loads and environmental conditions.

Canals and Retaining/Underground Structures

  • Canals require impermeable, adequately strong soils.
  • Retaining structures (e.g., retaining walls) rely on soil properties.
  • Key soil properties: earth pressure and shear strength guide design.
  • Soil strata are investigated by geologists to inform construction and tunnelling decisions.

Soil: Definition and Constituents

  • Soil is defined as:
    1) A naturally occurring material obtained from weathering, decomposition, or disintegration of rocks.
    2) Unaggregated or uncemented deposits of minerals and organic particles covering large portions of the earth’s crust.
    3) A heterogeneous mixture of fluids (air and water) and particles (clay, sand, silt, and gravel); may contain organic solids, liquids, gases, and other constituents.
  • Soil Constituents:
    • A soil mass is commonly considered to consist of solid particles and voids (interconnected pore spaces).
    • Two primary constituents: (1) Soil (solid particles) and (2) Voids (air/water present in voids).
  • Saturated vs Partially Saturated (Moist) Soil:
    • Saturated soil: only water present in voids.
    • Partially saturated / Moist soil: water and air both present in voids.

Basis of Soil Constituents and Classification

  • On the basis of constituents, soils can be categorized as:
    • Dry soil: only air in voids.
    • Saturated soil: only water in voids.
    • Partially saturated / Moist soil: both water and air in voids.

Formation of Soil and Weathering of Rocks

  • Weathering involves the conversion, decomposition, or disintegration of intact rock into smaller pieces.
  • Two types of weathering:
    • Mechanical (physical) Weathering
    • Chemical Weathering

Soil-Particle Size and Classification

  • Primary soil groups by predominant particle size:
    • Gravel
    • Sand
    • Silt
    • Clay
  • Classification systems referenced in the material:
    • MIT (Massachusetts Institute of Technology)
    • USDA (U.S. Department of Agriculture)
    • AASHTO (American Association of State Highway and Transportation Officials)
    • USCS (U.S. Army Corps of Engineers and U.S. Bureau of Reclamation)
  • General idea: particle size governs soil behavior and engineering properties.

Granular and Cohesive Soils

  • Coarse-grain soil (cohesionless): gravels and sands.
    • Characteristics: large particle sizes; generally lack significant interparticle cohesive forces.
  • Fine-grain soil (cohesive): silts and clays.
    • Characteristics: small particle sizes; exhibit intermolecular forces of attraction.

Gravel, Sand, Silt, and Clay Descriptions

  • Gravels: rocks with occasional quartz, feldspar, and other mineral grains.
  • Sand: predominantly quartz and feldspar.
  • Silts: very fine quartz grains with flake-shaped particles; micaceous fragment presence.
  • Clays: primarily flaky, microscopic to submicroscopic particles of clay minerals and mica.
  • Overall distinctions:
    • Coarse-grain soils (gravel, sand) are cohesionless.
    • Fine-grain soils (silt, clay) are cohesive.

Particle Shape

  • Particle shape affects physical properties as much as size; shape is harder to measure but important.
  • Three major categories:
    • Bulky
    • Flaky
    • Needle-shaped
  • Bulky particle shapes are described using terms like angular, subangular, subrounded, and rounded.
  • Flaky particles are thin in one dimension and common in clay minerals.
  • Needle-shaped particles are less common; examples include some coral deposits and attapulgite clays.

Identification of Soil (Field/Practical Tests)

  • Visual Examination:
    • Observe colour, gradation, and angularity to decide coarse-grain vs fine-grain soil.
  • Feel Test:
    • Take a small amount of soil, add a few drops of water, rub between fingers, observe:
    • Large particles -> sand
    • Small particles -> silt
    • Small and soapy texture -> clay
  • Rolling Test:
    • Roll a sample into a ball with water, then try to form a thread.
    • Interpret results:
    • If it crumbles without thread formation -> sandy soil
    • If ball forms and cracks appear -> silty soil
    • If a thread of about 3 mm forms -> clayey soil
  • Dry Strength Test:
    • Dry soil, form a cube, and dry at 100°C until all water is removed.
    • Interpret results:
    • Sandy soil: breaks easily
    • Silty soil: requires some pressure to break
    • Clay: breaks with difficulty
  • Dispersion Test:
    • Place dry soil in a transparent glass with water; observe settling:
    • Sandy soil: particles settle very quickly (seconds)
    • Silty soil: settles more slowly
    • Clayey soil: settles very slowly (takes a long time)
  • These tests provide quick, general ideas about soil type and are useful for initial classification in the field.

Connections to Practice and Real-World Relevance

  • Foundational principles from soil mechanics underpin design choices for foundations, earthworks, and earth-retaining structures.
  • Site investigation, soil property determination, and understanding soil behavior guide safe and economical engineering solutions.
  • Knowledge of soil types, particle size, and shape informs expectations about permeability, strength, compressibility, and settlement.

Practical Implications and Ethical/Professional Considerations

  • Accurate soil characterization is essential to prevent failures, reduce risk, and ensure public safety.
  • Proper interpretation of soil behavior under loads leads to appropriate foundation design and monitoring during construction.
  • Engineers must integrate geological investigations with geotechnical analysis for tunnels, embankments, dams, and canals.

References

  • DAS, B. M. (2014). Principles of Geotechnical Engineering, Eighth Edition, SI. Stamford, USA: Cengage Learning.
  • Various online resources cited in the original material for further reading on soil mechanics fundamentals.