CHAPTER 1: GEOTECHNICAL ENGG.

– A HISTORICAL PERSPECTIVE

1. Definition of Soil

 Agronomy: Upper layer of earth where plants grow.

 Geology: Unconsolidated sediments of earth’s crust (mantle/regolith).
 Geotechnical Engineering:
o Agronomist’s/Geologist’s "soil" = topsoil (contains organic matter, not suitable for
construction).
o Technical Definition: Uncemented aggregate of mineral grains + decayed organic
matter (solid particles) with liquid and gas in voids.

2. Key Definitions

 Soil Mechanics (Dr. Karl Terzaghi, 1925):

o Application of mechanics + hydraulics to problems dealing with sediments.
o Study of physical properties of soil & soil behavior under forces.
 Soils Engineering: Application of soil mechanics to practical problems.
 Geotechnical Engineering: Civil engineering subdiscipline dealing with natural materials near
earth’s surface.

3. Geotechnical Engineering: Historical Timeline

Prior to the 18th Century

 Based on experience & experimentation, not science.

 Examples:
o Chan Dynasty (China): Built dykes for irrigation (no foundation stabilization).
o Ancient Greece: Isolated pad footings, strip-and-raft foundations.
o Egypt (2700 B.C.): Built pyramids.
o China (after 68 A.D.): Built pagodas.
 Leaning Tower of Pisa (1173 A.D.):
o Tilt caused by compression of weak clay (11m deep).
o Lesson: soil bearing capacity issues.
o Stabilized (1990): 70 tons of soil removed from north side.

The Four Major Periods (1700 – 1927 A.D.)

A. Preclassical Period (1700 – 1776 A.D.)

 Focus: Natural slopes, unit weights, semi-empirical earth pressure theories.

 Key Figures:
o Henri Gautier (1717): Natural slope (sand = 31°, earth = 45°). Recommended unit
weights.
o Bernard Forest de Belidor: Proposed lateral earth pressure theory; published soil unit
weights.
o Francois Gadroy (1746): First lab tests on retaining wall with sand backfill.
o J.J. Mayniel (1808): Summarized Gadroy’s study.
o Jean Rodolphe Perronet (~1769): Studied slope stability; distinguished intact ground
vs. fills.

B. Classical Soil Mechanics – Phase I (1776 – 1856 A.D.)

 French engineers/scientists led developments.

 Key Figures:
o Charles Augustin Coulomb: Calculus for sliding surfaces; applied friction + cohesion
laws → foundation of modern theory.
o Gaspard de Prony (1790): Included Coulomb’s theory in textbook.
o Francais & Navier (1820): Special cases (inclined backfills, surcharge).
 o Jean Victor Poncelet (1840): Graphical method, symbol f for friction angle, first shallow
foundation bearing-capacity theory.
o Alexandre Collin: Deep slips in clay, failure = cohesion mobilized > existing. Surfaces
≈ arcs of cycloids.
o William Rankine (1857): Simplified earth pressure theory (Rankine’s theory).

C. Classical Soil Mechanics – Phase II (1856 – 1910 A.D.)

 Focus: Experimental lab tests on sand.

 Key Figures:
o Henri Darcy (1856): Sand filter permeability, coefficient of permeability (hydraulic
conductivity).
o George Darwin: Lab tests, overturning moment on retaining walls.
o Joseph Boussinesq (1885): Stress distribution theory (elastic, isotropic medium).
o Osborne Reynolds (1887): Demonstrated dilatancy in sand.
o Clibborn & Beresford (1901–1902): Water flow through sand, uplift pressure.

D. Modern Soil Mechanics (1910 – 1927 A.D.)

 Focus: Research on clays, fundamental soil properties.

 Key Figures:
o Albert Atterberg (1911): Defined clay-size (<2 microns), consistency limits (LL, PL,
SL), Plasticity Index.
o Jean Fontard (1909): Dam failure study; early undrained shear tests on clay.
o Arthur Bell: Lateral pressure, resistance, bearing capacity in clay (shear-box tests).
o Wolmar Fellenius (1918, 1926): Stability analysis of saturated clay slopes (φ = 0),
circular slip surfaces.
o Karl Terzaghi (1919–1924): Theory of consolidation for clays. Published
Erdbaumechanik (1925).

4. Geotechnical Engineering After 1927

 Terzaghi’s Erdbaumechanik (1925) began a new era.

 Karl Terzaghi (1883–1963): Father of Modern Soil Mechanics
o Born in Prague; Mechanical Eng. degree (1904), Doctorate (1912).
o Research at Robert College (1918–1925).
o Taught at MIT, Vienna, Harvard.
o Led 1936 Harvard conference.
o Declared foundation failures not “acts of God.”

5. End of an Era: Other Key Pioneers

 Ralph B. Peck (1912–2008):

o Worked with Terzaghi (Chicago Subway).
o Professor at University of Illinois.
o Known as “godfather of soil mechanics.”
o Pioneered tunnel/dam techniques (Channel Tunnel).
o His death marked the end of an era.

Unit Summary: Key Points

 Soil Mechanics coined by Terzaghi (1925).

 Modern understanding began in the 18th century.
 Four historical periods:
1. Preclassical (1700–1776)
2. Classical Phase I (1776–1856)
3. Classical Phase II (1856–1910)
4. Modern (1910–1927)
 Karl Terzaghi = Father of modern soil mechanics.
CHAPTER 2: SOIL FORMATION – ORIGIN OF SOIL AND GRAIN SIZE

🌍 INTRODUCTION

 Soils: formed by weathering of rocks (mechanical disintegration + chemical decomposition).

 Geologic cycle: erosion → transportation → deposition → upheaval.
 Types of soil formation:
o Residual soil (sedentary): remains at place of origin, above parent rock.
o Transported soil: deposited away from origin.

ORIGIN OF SOIL

1. Residual Soil

 Properties vary from top to bottom.

 Top: fine material → Bottom: large stone fragments.
 Bottom layer resembles parent rock.
 Thickness: limited to a few meters.

2. Transported Soil

 Properties differ completely from parent rock.

 Deposits: thick, usually uniform.
 Most soils engineers deal with are transported.

THREE TYPES OF ROCKS

1. Igneous Rock
o Formed by solidification of molten magma.
o Extrusive (surface cooling) or Intrusive/Plutons (cooling beneath surface).
2. Sedimentary Rock
o Formed from deposits (gravel, sand, silt, clay).
o Compacted by overburden pressure.
o Cemented by agents (iron oxide, calcite, dolomite, quartz).
o Form detrital sedimentary rocks.
3. Metamorphic Rock
o Formed by heat + pressure (without melting).
o New minerals formed, grains sheared → foliated texture.

🚛 TRANSPORTATION OF SOIL

1. Water Transported (Alluvial Deposits)

o Carried in suspension/rolling along bed.
o Deposits:
 Alluvial (valleys, rivers)
 Lacustrine (lakes)
 Marine (seas, oceans)
2. Wind Transported (Aeolian Deposits)
o Particle size depends on wind velocity.
o Sand dunes: arid regions, coastal sandy areas.
o Loess deposits: low density, high compressibility, low bearing capacity, high
permeability.
3. Glacier Deposited
o Glaciers carry soil of all sizes (fine grains → boulders).
o Mixed with ice → transported far.
o Drift: deposits by glaciers.
o Till: direct deposits from melting glaciers.
4. Gravity Deposited (Colluvial Soils)
o Short distance transport by gravity.
o Found at foot of cliffs/slopes.
o Example: Talus (irregular, coarse particles).
 5. Combined Action
o Soil may be transported by multiple agents sequentially (gravity → wind → water →
glacier).

⚖ GRAIN SIZE

Soil Particle Types

 Gravel: rock fragments, quartz, feldspar.

 Sand: quartz + feldspar.
 Silt: microscopic fine quartz, micaceous mineral fragments.
 Clay: flake-shaped micro/submicroparticles (mica, clay minerals).

Classification Systems

1. Massachusetts Institute of Technology (MIT)

2. U.S. Department of Agriculture (USDA)
3. American Association of State Highway and Transportation Officials (AASHTO)
4. Unified Soil Classification System (USCS)

🔬 MECHANICAL ANALYSIS OF SOIL

 Purpose: Determine % particle size distribution by dry weight.

 Methods:
1. Sieve Analysis: for particles > 0.075 mm.
2. Hydrometer Analysis: for particles < 0.075 mm (based on sedimentation).

📈 PARTICLE-SIZE DISTRIBUTION CURVE

 Parameters:
1. Effective size (D10) → diameter at 10% finer.
2. Uniformity coefficient (Cu = D60 / D10).
3. Coefficient of gradation (Cc = (D30²) / (D10 × D60)).
4. Sorting coefficient (S0) → another uniformity measure.

Grading of Soil

 Curve I – Poorly graded: uniform grain size.

 Curve II – Well graded: wide range of particle sizes.
o Cu > 4 (gravel), Cu > 6 (sand); Cc = 1–3.
 Curve III – Gap graded: missing intermediate sizes, hump in curve.

📌 USES OF PARTICLE-SIZE DISTRIBUTION CURVE

1. Soil classification (coarse-grained).

2. Estimate permeability.
3. Frost action susceptibility.
4. Design of drainage filters.
5. Index of shear strength.
6. Compressibility judgment.
7. Soil stabilization & pavement design.
8. Indicate soil deposition mode.
9. Indicate age of residual soils.
 PARTICLE SHAPE

1. Bulky Particles (mechanical weathering)

o Angular, subangular, subrounded, rounded.
o Angularity (A) defined by formula.
2. Flaky Particles
o Very low sphericity (~0.01).
o Mostly clay minerals.
3. Needle-Shaped Particles
o Less common.
o Examples: coral deposits, attapulgite clays.

📝 EXAMPLES (from problems)

1. Determining D10, D30, D60, Cu, and Cc from sieve analysis.

2. Classifying gravel, sand, silt, and clay by MIT, USDA, AASHTO systems.

📌 UNIT SUMMARY

 3 rock types: igneous, sedimentary, metamorphic.

 Soil transport: water, wind, glacier, gravity, combined.
 Soil composition: gravel, sand, silt, clay.
 Particle size analysis: sieve & hydrometer.
 Distribution curve parameters: D10, Cu, Cc, S0.
CHAPTER 3: VOLUME-WEIGHT RELATIONSHIP

🌍 INTRODUCTION

 Soil deposits consist of:

o Solids (soil particles)
o Liquids (usually water)
o Gases (usually air)
 Voids = spaces filled with water or air.
o Partially/completely filled with water.
o Remaining spaces filled with air/gas.

🏗 PHASES OF SOIL DEPOSITS

1. Completely Dry → solids + air

2. Partially Saturated → solids + water + air
3. Fully Saturated → solids + water

⚖ A. VOLUME-WEIGHT RELATIONSHIP

 Soil element has volume (V) and weight (W).

 Relationships are developed considering solids, water, and air.

📐 VOLUMETRIC RELATIONSHIPS OF SOIL

Widely used in soil engineering (5):

1. Void Ratio (e) – ratio of volume of voids to volume of solids.

2. Porosity (n) – ratio of volume of voids to total volume.
3. Degree of Saturation (S) – % of voids filled with water.
4. Percentage Air Voids (na) – % of voids filled with air.
5. Air Content (ac) – ratio of volume of air to volume of voids.
⚖ WEIGHT RELATIONSHIPS OF SOIL

Two commonly used:

1. Moisture Content (w) – ratio of weight of water to weight of solids.

2. Unit Weight (γ) – weight per unit volume of soil.

⚖ VOLUME-MASS RELATIONSHIPS

Expressed in terms of mass density:

 Bulk Density
 Dry Density
 Saturated Density
 Submerged Density
 Mass of Solids Density

⚖ VOLUME-WEIGHT RELATIONSHIPS (in terms of Unit Weights)

 Bulk Unit Weight (γ)

 Dry Unit Weight (γd)
 Saturated Unit Weight (γsat)
 Submerged Unit Weight (γ′)
 Unit Weight of Solids (γs)

🔗 RELATIONSHIPS AMONG VARIABLES

Relationship among:

 Unit weight (γ or ρ)
 Void ratio (e)
 Moisture content (w)
 Specific gravity (Gs)

(Derived by considering volume of soil solids = 1 unit volume, with voids filled by water and/or air as
shown in diagrams.)

📝 UNIT SUMMARY

 Soil deposits contain solids, liquids, gases.

 Phases of soil deposits: dry, partially saturated, fully saturated.
 5 Volumetric Relationships:
1. Void ratio
2. Porosity
3. Degree of saturation
4. Percentage air voids
5. Air content
 2 Weight Relationships:
1. Moisture content
2. Unit weight
 Volume-Mass Relationships (densities): bulk, dry, saturated, submerged, solids.
 Volume-Weight Relationships (unit weights): bulk, dry, saturated, submerged, solids.