Fundamentals of Geotechnical Engineering and Soil Mechanics

Posted on Mar 9, 2026 in Civil Engineering

Chapter 1

Vocab

Geotechnical engineering — Deals with the engineering aspects of soils and rocks, sometimes known as geomaterials.

Soil mechanics — Application of mechanics to soils.

Rock mechanics — Application of mechanics principles to rocks.

Foundation engineering — Application of soil mechanics principles to design earth and earth-supported structures such as foundations, retaining structures, dams, etc.

Environmental geomechanics (Geoenvironmental engineering) — Branch dealing with hazardous waste disposal, landfills, groundwater contamination, and potential acid sulfate soils.

Karl Terzaghi — Father of soil mechanics. Author of Erdbaumechanik auf Bodenphysikalischer Grundlage (1925).

Soil Types

Residual soil — Formed and remains at origin.

Transported soil — Weathered soils transported by glacier, wind, water, or gravity and deposited away from origin.

Transport agent and soil name:

  • Glacier — Glacial
  • Wind — Aeolian
  • Sea — Marine
  • Lake — Lacustrine
  • River — Alluvial
  • Gravity — Colluvial
  • Humans — Fill

Soil has no tensile strength.

Soil failure = mostly shear.

Key Soil Properties

  • Permeability (k) — Ability of soil to transmit water.
  • Strength — Maximum stress before failure.
  • Stiffness — Resistance to deformation.

Soil Testing Methods

  • Trial pits (~5 m deep)
  • Boreholes (>50 m possible)
  • SPT (Standard Penetration Test)
  • CPT (Cone Penetration Test)

Rock Types

  • Igneous — From cooled magma/lava.
  • Sedimentary — From deposition.
  • Metamorphic — Altered by pressure/temperature.

Numerical Modeling Terms

  • Finite Element Method (FEM)
  • Finite Difference Method (FDM)
  • Constitutive model — Describes stress–strain behavior.
  • Boundary conditions — Define loads and displacements.
  • Software used — ABAQUS, PLAXIS, FLAC, GeoStudio.

Important Conceptual Statements

  • Soils are quite different from other engineering materials.
  • Soil is sometimes assumed to be a homogeneous, isotropic, elastic continuum (approximation).
  • Appropriate safety factors are used depending on the quality of data and degree of simplification.
  • Geotechnical engineering is a science, but its practice is an art.
  • There is a lot of judgment involved in the profession.
  • Considering uncertainty and scatter in data, it may not always make sense to calculate everything to two decimal places.

Applications of Geotechnical Engineering

  • Foundations (footings, piles)
  • Retaining walls
  • Dams
  • Sheet piles
  • Braced excavations
  • Reinforced earth
  • Slope stability
  • Ground improvement

Descriptions include:

  • Foundations transfer loads from superstructure to soil.
  • Retaining walls provide lateral support.
  • Sheet piles are continuous impervious walls made by driving interlocking sections.
  • Braced excavation supports trench walls.
  • Ground improvement techniques include compaction.

Equations

Water Content

  • Ww = weight of water
  • Ws = weight of solids

Porosity

  • Vv = volume of voids
  • Vt = total volume

Density (ρ)

Unit weight (γ)

Factor of Safety (FS)

Chapter 2

Phase Concepts

Soils contain three phases — air, water, and solid grains.

Extreme cases: Dry soil → no water (voids filled with air); Saturated soil → no air (voids filled with water).

Textbook Notes (Important)

  • Do not try to memorize the equations.
  • Understand definitions and develop phase relations from the phase diagram with Vs = 1.
  • Do not mix densities and unit weights.
  • Assume Gs = 2.6–2.8 when required.
  • Soil grains are incompressible.

γ (N/m³) = ρ (kg/m³) · g (m/s²)

γw = 9.81 kN/m³

Lab Measured vs Calculated

  • Measured in lab: density, water content, specific gravity (Gs).
  • Calculated: void ratio, porosity, degree of saturation.

Packing Void Ratios

Void ratios for different packings:

  • Loosest: 0.910
  • Denser arrangements: 0.654, 0.433
  • Densest: 0.350

Void ratio decreases as the number of contact points increases.

Chapter 3

Grain Size Boundary

0.075 mm (75 μm) separates coarse- and fine-grained soils.

Based on grain size, soils are grouped as: clays, silts, sands, gravels, cobbles, boulders.

Coarse-Grained Soils

Influencing factors: relative proportions of grain sizes, packing density, grain shape.

Grain Size Distribution

Determined by:

  • Sieve analysis (ASTM D6913)
  • Hydrometer analysis (ASTM D422)

Sieve numbers

  • No. 200 sieve = 0.075 mm
  • No. 4 sieve = 4.75 mm
  • No. 40 sieve = 0.425 mm

Grain size distribution curve: % passing vs grain size. Grain size plotted on logarithmic scale.

Definitions of D-Sizes

  • D10 = grain size corresponding to 10% passing.
  • D30 = grain size corresponding to 30% passing.
  • D60 = grain size corresponding to 60% passing.

Grain Shape

Types:

  • Angular
  • Subangular
  • Subrounded
  • Rounded

Angular grains — More interlocking, greater strength and stiffness.

Fine-Grained Soils

Clay particles:

  • 1D or 2D
  • Flakes or needles
  • Negatively charged
  • < 2 μm
  • High specific surface
  • Cohesive and sticky

Silts:

  • Nonplastic (PI ≈ 0)

Clay Mineralogy

Structural units:

  • Tetrahedron (Si surrounded by 4 O)
  • Octahedron (Al or Mg surrounded by 6 hydroxyl or O)

Common clay minerals:

Kaolinite

  • 0.72 nm layer thickness
  • Strong hydrogen bonds

Illite

  • 0.96 nm thick layers
  • Held by potassium ions

Montmorillonite (Smectite)

  • Weak van der Waals bonds
  • Swelling when water enters
  • Expansive/reactive clay

Specific surface:

  • Kaolinite = 15 m²/g
  • Illite = 80 m²/g
  • Montmorillonite = 800 m²/g

Atterberg Limits

States:

  • Brittle solid
  • Semi-solid
  • Plastic solid
  • Liquid

Shrinkage limit (SL)

Highest water content below which no further volume reduction occurs on drying.

Plastic limit (PL)

Lowest water content at which soil shows plastic behavior.

Liquid limit (LL)

Water content at which soil flows like a liquid.

Plasticity index (PI)

PI = LL − PL

USCS System

Major groups:

  • Gravel (G)
  • Sand (S)
  • Silt (M)
  • Clay (C)
  • Organic (O)
  • Peat (Pt)

Coarse-Grained

If coarse fraction > 50%.

Symbols:

  • W = well graded
  • P = poorly graded
  • M = silty
  • C = clayey

Fine-Grained

If fines > 50%.

Plasticity:

  • L = low
  • H = high

USCS Special Cases by % Fines

0–5% fines → GW, GP, SW, SP

5–12% fines → dual symbols (e.g., GW-GM)

12–50% fines → GM, GC, SM, SC

50–100% fines → ML, MH, CL, CH

Field Identification

Clays: Sticky when moist, high dry strength, slow or no dilatancy.

Silts: Gritty, low dry strength, quick dilatancy.