Earthworks Design: ECEDE4A - Soil Classification and Compaction

Course Overview and Administrative Rules

• Course Details:

  • Institution: Vaal University of Technology

  • Department: Civil Engineering Department

  • Course Title & Code: Earthworks Design (ECEDE4A)

  • Lecturing Staff: Mr. Malambo Gwaba Pr.Eng

  • Academic Period: Week 1, Second Semester - 2026

• Rules for Assignments and Tests:

  • Late Assignments: Submissions turned in after the specified due time are treated as late and are automatically subjected to a penalty reduction of $25\%$ of the total score.

  • Non-Submission Penalty: A mark of zero ($0$) is awarded to any student who fails to submit an assignment $1\,\text{day}$ ($24\,\text{hours}$) after the due date.

  • Missed Tests: No special arrangements are made for students who fail to attend scheduled tests.

  • Sick Test Eligibility: Permission to write a substitute "sick" test is granted solely upon submission of an official doctor's medical certificate covering the exact date(s) of absence. Students must contact the Civil Engineering Department administrator to follow all mandatory administrative procedures.

  • Failure to Provide Proof: A mark of zero ($0$) is awarded for any missed test unless official permission has been explicitly granted.

• Assessment and Grading Breakdown:

  • Year Mark Weighting: $50\%$ of the total course grade.

  • Assignments: $2$ assignments contributing $10\%$ total.

  • Tests: $2$ tests contributing $25\%$ total.

  • Project: $1$ project contributing $15\%$ total.

  • Final Examination Weighting: $50\%$ of the total course grade.

Works Programme and Learning Allocation

• Learning Unit 1: Materials Selection

  • Contact / e-Learning: $4\,\text{hours}$ going through course work.

  • Self-Study: $6\,\text{hours}$.

  • Total Estimated Effort: $10\,\text{hours}$.

• Learning Unit 2: Compaction

  • Contact / e-Learning: $6\,\text{hours}$ going through course work.

  • Self-Study: $4\,\text{hours}$.

  • Total Estimated Effort: $10\,\text{hours}$.

• Learning Unit 3: Design and Construction of Embankments

  • Contact / e-Learning: $4\,\text{hours}$ going through course work.

  • Self-Study: $3\,\text{hours}$.

  • Total Estimated Effort: $7\,\text{hours}$.

• Learning Unit 4: Problem Soils

  • Contact / e-Learning: $8\,\text{hours}$ going through course work.

  • Self-Study: $6\,\text{hours}$.

  • Total Estimated Effort: $14\,\text{hours}$.

• Learning Unit 5: Soil Improvements Techniques

  • Contact / e-Learning: $8\,\text{hours}$ going through course work.

  • Self-Study: $6\,\text{hours}$.

  • Total Estimated Effort: $14\,\text{hours}$.

Rock Classification and Soil Formation Fundamentals

• Geological Rock Classification:

  • Rocks are classified into three primary geological groups:

  • Igneous Rocks: Primary rocks formed directly by the cooling and solidification of molten magma.

  • Metamorphic Rocks: Rocks altered from pre-existing rock forms under extreme heat, pressure, and chemical conditions.

  • Sedimentary Rocks: Rocks formed through the deposition, compaction, and lithification of mineral and organic particles over time.

• Definition of Soil:

  • Soil is defined as unconsolidated material composed of solid particles produced by the mechanical and chemical disintegration (weathering) of rocks.

Mechanics of Rock Weathering

• Overview of Weathering:

  • Soil formation occurs via rock weathering. Weathering is categorized into mechanical weathering and chemical weathering, though in natural field settings, most rock weathering is a combination of both.

• Mechanical Weathering (Physical Disintegration):

  • Definition: The process by which physical forces break rocks down into progressively smaller fragments without causing any chemical changes in the original material.

  • Forcing Agents:

  • Running water

  • Wind

  • Ocean waves

  • Glacier ice

  • Frost action

  • Thermal variations (heat causing expansion and contraction cycles)

  • Animals and plant growth

  • Resulting Soil Properties: The resulting soil retains the original mineral composition of the parent rock and yields predominantly coarse-grained, cohesionless soils.

• Chemical Weathering (Decomposition):

  • Definition: The process in which chemical reactions alter and decompose original rock minerals, transforming them into entirely new mineral compounds (e.g., the chemical weathering of feldspar yields clay minerals).

  • Primary Decomposition Reactions:

  • Oxidation: The chemical union of oxygen with rock minerals to form new oxide/hydroxide minerals.

  • Hydration: Water molecules enter the crystalline lattice structure of rock minerals, forming a distinct group of hydrated minerals.

  • Hydrolysis: The release of hydrogen ions ($\text{H}^+$) from water molecules, which combine chemically with rock minerals.

  • Carbonation: Occurs when carbon dioxide ($\text{CO}_2$) present in the air reacts with rock minerals in the presence of water to form carbonates. Carbonation heavily affects limestone formations.

  • Leaching: The process wherein percolating groundwater or organic acids wash out water-soluble salts and minerals from the soil matrix.

  • Resulting Soil Properties: Produces cohesive, fine-grained soils such as clays and silts.

Weinert N-Value and Regional Weathering Dynamics

• Definition and Climatic Significance:

  • The Weinert N-value quantifies whether rock weathering in a specific region is dominated by chemical decomposition or mechanical disintegration based on moisture availability.

• Weinert N-Value Thresholds:

  • $N \ll 2$: Strong chemical decomposition driven by severe moisture surplus.

  • N < 5: Preferential chemical decomposition dominated by moisture surplus.

  • $N = 5$: Equilibrium boundary where neither mechanical nor chemical weathering is preferential (coincides with major geographic markers like the Great Escarpment in South Africa).

  • N > 5: Preferential mechanical disintegration driven by moisture deficit.

  • $N \gg 10$: Strong mechanical disintegration driven by severe moisture deficit.

• Regional Examples in Southern Africa:

  • Decomposition Zones (N < 5): Durban (Dbn), Mbombela (Mbo), coastal Mozambique, and eastern regions.

  • Disintegration Zones (N > 5): Johannesburg (Jhb), Pretoria (Pta), Polokwane (Plk), Cape Town (Ctn), Lesotho, Zimbabwe, Eswatini.

  • Arid Disintegration Zones ($N \gg 10$): Arid regions of Namibia.

Categories and Classes of Soil

• Broad Soil Categories:

  • Residual Soils:

  • Formed by the in-situ weathering of rocks and deposited near their point of origin.

  • Properties closely mirror the characteristics and mineral composition of the parent rock.

  • Depth typically ranges between $5\,\text{m}$ and $20\,\text{m}$.

  • Directly associated with underlying slightly or partially weathered rock strata.

  • Transported Soils:

  • Formed by rock weathering and subsequently moved away from their origin by physical transporting agents (air, water, ice, snow).

  • Engineering properties at the deposition site are entirely different from the properties of the parent rock.

• Classification of Transported Soils:

  • Alluvial Soils: Transported and deposited by running water. Running water carries soil particles in suspension or by rolling along riverbeds, eroding hills and depositing sediments in valleys.

  • Lacustrine Deposits: Specific alluvial deposits formed within lakes.

  • Marine Deposits: Specific alluvial deposits formed when rivers transport sediments into ocean environments.

  • Aeolian Soils: Transported and deposited by wind action. Consists mainly of sand- and silt-sized particles. Particle sizes depend directly on wind velocity; finer particles are carried vast distances from their origin.

  • Colluvial Soils: Deposited by gravity-driven land movements down slopes.

  • Glacial Deposits: Transported and deposited by moving or melting ice and glaciers.

Particle Size Spectrum and Soil Identification

• Mineralogical Composition by Soil Type:

  • Gravels: Rock fragments containing occasional mineral grains of quartz, feldspar, and other minerals.

  • Sands: Composed predominantly of quartz and feldspar grains.

  • Silts: Microscopic soil fractions consisting of very fine quartz grains and flaky particles.

  • Clays: Microscopic and submicroscopic flaky particles consisting of mica, clay minerals, and colloidal material capable of base exchange.

• Particle Size Range Table:

  • Boulders (Very Coarse Soil): Particle size > 200\,\text{mm} (Large Boulders > 600\,\text{mm}; standard Boulders $600\,\text{mm} - 200\,\text{mm}$).

  • Cobbles (Coarse Soil): Particle size $200\,\text{mm} - 60\,\text{mm}$.

  • Gravel (Coarse Soil / Non-Cohesive): Particle size $60\,\text{mm} - 2\,\text{mm}$.

  • Coarse Gravel: $60\,\text{mm} - 20\,\text{mm}$.

  • Medium Gravel: $20\,\text{mm} - 6\,\text{mm}$.

  • Fine Gravel: $6\,\text{mm} - 2\,\text{mm}$.

  • Sand (Coarse Soil / Non-Cohesive): Particle size $2\,\text{mm} - 0.06\,\text{mm}$.

  • Coarse Sand: $2\,\text{mm} - 0.6\,\text{mm}$.

  • Medium Sand: $0.6\,\text{mm} - 0.2\,\text{mm}$.

  • Fine Sand: $0.2\,\text{mm} - 0.06\,\text{mm}$.

  • Silt (Fine Soil / Fine-Grained / Cohesive): Particle size $0.06\,\text{mm} - 0.002\,\text{mm}$.

  • Coarse Silt: $0.06\,\text{mm} - 0.02\,\text{mm}$.

  • Medium Silt: $0.02\,\text{mm} - 0.006\,\text{mm}$.

  • Fine Silt: $0.006\,\text{mm} - 0.002\,\text{mm}$.

  • Clay (Fine Soil / Fine-Grained / Cohesive): Particle size < 0.002\,\text{mm} (< 2\,\mu\text{m}).

• Engineering Grain Size Boundaries:

  • Clay / Silt Boundary: $0.002\,\text{mm}$

  • Silt / Sand Boundary: $0.075\,\text{mm}$

  • Sand / Gravel Boundary: $4.75\,\text{mm}$

  • Gravel / Cobble Boundary: $75\,\text{mm}$

  • Cobble / Boulder Boundary: $300\,\text{mm}$

• Field Identification Tests:

  • Boulders, Cobbles, Gravel: Observed directly by the naked eye.

  • Sand: Imparts a distinct gritty feel when tested against teeth.

  • Silt: Imparts a chalky feel against teeth; when dry, easily rubs off hands; exhibits dilatancy.

  • Clay: Feels sticky or soapy to the touch; soils hands; displays a shiny texture when wet.

Sieve Analysis and Grain-Size Curves

• Standard Laboratory Sieve Mesh Sizes:

  • Sieve No. 4: $4.76\,\text{mm}$ ($4.75\,\text{mm}$)

  • Sieve No. 10: $2.00\,\text{mm}$ ($2.00\,\text{mm}$)

  • Sieve No. 20: $0.84\,\text{mm}$ ($1.00\,\text{mm}$)

  • Sieve No. 40: $0.42\,\text{mm}$ ($0.600\,\text{mm}$ / $0.425\,\text{mm}$)

  • Sieve No. 60: $0.25\,\text{mm}$ ($0.212\,\text{mm}$)

  • Sieve No. 100: $0.147\,\text{mm}$ ($0.150\,\text{mm}$)

  • Sieve No. 200: $0.074\,\text{mm}$ ($0.075\,\text{mm}$)

  • Pan / Base

• Sample Sieve Analysis Data ($1000\,\text{g}$ Total Mass):

  • $6\,\text{mm}$ Sieve: Mass retained = $126\,\text{g}$; Retained = $12.6\%$; Cumulative Retained = $12.6\%$; Passing = $87.4\%$

  • $2\,\text{mm}$ Sieve: Mass retained = $86\,\text{g}$; Retained = $8.6\%$; Cumulative Retained = $21.2\%$; Passing = $78.8\%$

  • $0.6\,\text{mm}$ Sieve: Mass retained = $194\,\text{g}$; Retained = $19.4\%$; Cumulative Retained = $40.6\%$; Passing = $59.4\%$

  • $0.2\,\text{mm}$ Sieve: Mass retained = $202\,\text{g}$; Retained = $20.2\%$; Cumulative Retained = $60.8\%$; Passing = $39.2\%$

  • $0.075\,\text{mm}$ Sieve: Mass retained = $124\,\text{g}$; Retained = $12.4\%$; Cumulative Retained = $73.2\%$; Passing = $26.8\%$

  • Base / Pan: Mass retained = $268\,\text{g}$; Retained = $26.8\%$; Cumulative Retained = $100.0\%$; Passing = $0.0\%$

  • Total Mass: $1000\,\text{g}$ ($100.0\%$)

• Key Distribution Parameters:

  • $D_{10}$: Effective particle size corresponding to $10\%$ finer on the cumulative logarithmic grading curve.

  • $D_{30}$: Particle size corresponding to $30\%$ finer on the cumulative distribution curve.

  • $D_{60}$: Particle size corresponding to $60\%$ finer on the cumulative distribution curve.

• Classification of Grading Curve Profiles:

  • Curve A: Poorly-graded medium SAND (steep curve covering a narrow particle size range).

  • Curve B: Well-graded GRAVEL-SAND (smooth, continuous particle representation across sand and gravel ranges).

  • Curve C: Gap-graded COBBLES-SAND (stepped profile with missing intermediate sizes).

  • Curve D: Sandy SILT (dominated by fine silt particles with moderate sand content).

  • Curve E: Silty CLAY (dominated by fine clay particles with minor sand content).

Atterberg Limits and Soil Consistency

• Origin and Concept:

  • Developed by Swedish agriculturist Albert Atterberg to define boundary moisture levels separating different mechanical behavior states in fine-grained soils.

• Consistency States:

  • Soil behavior changes with moisture content ($w\%$) across four distinct physical states:

  • Solid State

  • Semi-Solid State

  • Plastic State

  • Viscous Liquid State

• Atterberg Limit Definitions:

  • Liquid Limit ($LL$ or $w_{LL}$): The moisture content at which soil transitions from a plastic state to a viscous liquid state and begins to flow. At the Liquid Limit, fine-grained soil possesses a small shear strength of approximately $2.5\,\text{kN/m}^2$

  • Plastic Limit ($PL$ or $w_{PL}$): The moisture content at which soil transitions into a plastic state and can be rolled without crumbling.

  • Shrinkage Limit ($SL$): The threshold moisture content below which no further soil volume reduction occurs despite further loss of moisture:     $SL = w(\%) - \Delta w(\%)$

  • Plasticity Index ($PI$ or $I_p$): The numerical range of moisture content over which a soil behaves as a plastic material:     $PI = LL - PL$

• Soil Plasticity Classification based on $PI$:

  • $PI = 0$: Nonplastic

  • $PI = 1 - 5$: Slightly plastic

  • $PI = 5 - 10$: Low plasticity

  • $PI = 10 - 20$: Medium plasticity

  • $PI = 20 - 40$: High plasticity

  • PI > 40: Very high plasticity

Engineering Principles of Soil Compaction

• Definition:

  • Soil compaction is the process of densifying soil by mechanical means to reduce air voids, bring solid particles closer together, and increase soil dry unit weight.

• Key Objectives of Compaction:

  • Increase soil shear strength.

  • Increase soil bearing capacity for foundation loads.

  • Reduce soil compressibility, minimizing settlement of structures and deformation of earth embankments.

  • Reduce soil permeability, minimizing seepage of water through the soil mass.

  • Improve overall embankment stability and lower frost action damage.

  • Reduce the degree of shrinkage and prevent cracking upon drying.

Laboratory Compaction Testing (Standard Proctor Test)

• Standard Proctor Test Specifications (Ralph R. Proctor, 1933):

  • Hammer Weight: $2.5\,\text{kg}$ ($5.5\,\text{lb}$)

  • Drop Height: $12\,\text{inches}$ ($304.8\,\text{mm}$)

  • Compaction Layers: $3$ equal layers

  • Blows per Layer: $25$ blows

  • Mold Volume: $943.3\,\text{cm}^3$ ($0.001\,\text{m}^3$ or $0.03\,\text{ft}^3$)

  • Compactive Effort: $16\,778\,\text{N}\cdot\text{m}$ ($12\,375\,\text{ft}\cdot\text{lbs}$)

• Proctor Test Governing Equations:

  • Moist Density ($\rho$):     $\rho = \frac{M}{V}$     where $M$ is mass of wet soil in the mold ($\text{kg}$) and $V$ is volume of the mold ($\text{m}^3$).

  • Dry Density ($\rho_d$):     $\rho_d = \frac{\rho}{1 + \frac{w}{100}}$     where $w$ is moisture content ($\%$).

  • Zero-Air-Void Dry Unit Weight ($\gamma_{zav}$):     $\gamma_{zav} = \frac{G_s \gamma_w}{1 + w G_s}$     where $G_s$ is soil specific gravity, $\gamma_w = 9.81\,\text{kN/m}^3$ is the unit weight of water, and $w$ is moisture content as a decimal.

• Standard Proctor Laboratory Test Calculation Example:

  • Mold Volume: $V = 943.3\,\text{cm}^3 = 9.433 \times 10^{-4}\,\text{m}^3$

  • Trial 1: Wet soil mass = $1.48\,\text{kg}$, $w = 8.4\%$

  • $\rho = \frac{1.48}{9.433 \times 10^{-4}} = 1568.96\,\text{kg/m}^3$

  • $\rho_d = \frac{1568.96}{1 + 0.084} = 1447.38\,\text{kg/m}^3$

  • Trial 2: Wet soil mass = $1.88\,\text{kg}$, $w = 10.2\%$

  • $\rho = \frac{1.88}{9.433 \times 10^{-4}} = 1993.00\,\text{kg/m}^3$

  • $\rho_d = \frac{1993.00}{1 + 0.102} = 1808.53\,\text{kg/m}^3$

  • Trial 3: Wet soil mass = $2.12\,\text{kg}$, $w = 12.3\%$

  • $\rho = \frac{2.12}{9.433 \times 10^{-4}} = 2247.43\,\text{kg/m}^3$

  • $\rho_d = \frac{2247.43}{1 + 0.123} = 2001.27\,\text{kg/m}^3$

  • Trial 4: Wet soil mass = $1.82\,\text{kg}$, $w = 14.6\%$

  • $\rho = \frac{1.82}{9.433 \times 10^{-4}} = 1929.40\,\text{kg/m}^3$

  • $\rho_d = \frac{1929.40}{1 + 0.146} = 1683.60\,\text{kg/m}^3$

  • Trial 5: Wet soil mass = $1.68\,\text{kg}$ ($1.65\,\text{kg}$), $w = 16.8\%$

  • $\rho = \frac{1.68}{9.433 \times 10^{-4}} = 1780.98\,\text{kg/m}^3$

  • $\rho_d = \frac{1780.98}{1 + 0.168} = 1524.81\,\text{kg/m}^3$

  • Graphically Derived Results:

  • Maximum Dry Density ($\rho_{d(\text{max})}$): $2020\,\text{kg/m}^3$

  • Optimum Moisture Content ($w_{\text{opt}}$): $13\%$

• Zero-Air-Void Unit Weight Calculation Example ($G_s = 2.68$, $\gamma_w = 9.81\,\text{kN/m}^3$):

  • At $w = 5\%$ ($0.05$): $\gamma_{zav} = \frac{2.68 \times 9.81}{1 + (0.05 \times 2.68)} = 23.18\,\text{kN/m}^3$

  • At $w = 10\%$ ($0.10$): $\gamma_{zav} = \frac{2.68 \times 9.81}{1 + (0.10 \times 2.68)} = 20.73\,\text{kN/m}^3$

  • At $w = 15\%$ ($0.15$): $\gamma_{zav} = \frac{2.68 \times 9.81}{1 + (0.15 \times 2.68)} = 18.75\,\text{kN/m}^3$

  • At $w = 20\%$ ($0.20$): $\gamma_{zav} = \frac{2.68 \times 9.81}{1 + (0.20 \times 2.68)} = 17.12\,\text{kN/m}^3$

  • At $w = 25\%$ ($0.25$): $\gamma_{zav} = \frac{2.68 \times 9.81}{1 + (0.25 \times 2.68)} = 15.74\,\text{kN/m}^3$

• Compactive Effort Influence:

  • Increasing energy inputs (e.g., from $20$ to $25$, $30$, and $50$ blows per layer) shifts compaction curves upward and to the left, resulting in higher maximum dry density at lower optimum moisture content.

  • The line connecting peak points across differing energy levels is termed the "Line of Optimums".

Field Compaction Equipment and Application Guidelines

• Field Compaction Dynamics:

  • Field compaction relies on static or dynamic loading, depending on equipment mass and roller passes.

  • Due to differences between laboratory impact energy and field rolling energy, maximum dry density in the field typically reaches $90\%$ to $95\%$ of laboratory maximum dry density.

• Field Equipment Specifications:

  • Smooth Wheel Rollers: Weigh $5\,\text{tonnes}$ to $15\,\text{tonnes}$. Suitable for compacting sand, gravel, and broken stone. Used for highway base courses.

  • Sheep Foot Rollers: Drum weight ranges from $1.5\,\text{tonnes}$ to $10\,\text{tonnes}$. Features projection lugs shaped like sheep feet on a cylindrical drum. Suitable for compacting cohesive soils.

  • Pneumatic Rollers (Rubber-Tired): Utilizes multiple rubber tires with uneven front/rear wheel counts. Best suited for fine-grained soils and well-graded sands.

  • Grid Rollers: Features a cylindrical steel grid surface with square openings. Mass can be increased by concrete block ballasting. Compacts rock fills, sands, and gravels.

  • Tamping Rollers: Similar to sheep foot rollers but equipped with larger lug surface areas. Static weights range from $15\,\text{tonnes}$ to $40\,\text{tonnes}$. Best suited for cohesive soils.

  • Vibrating Rollers: Modified smooth wheel rollers with rotating drums that produce dynamic vibrations. Highly expensive, but offers high performance and output.

  • Vibrating Plate Rollers: Compact equipment used for small areas. Mass ranges from $100\,\text{kg}$ to $2\,\text{tonnes}$, with plate areas between $0.16\,\text{m}^2$ and $1.6\,\text{m}^2$

  • Rammers: Hand- or machine-operated tools providing high impact force to small areas. Base size typically $15\,\text{cm} \times 15\,\text{cm}$, $20\,\text{cm} \times 20\,\text{cm}$, or larger.

• Compaction Equipment Selection Matrix:

  • Rock Fill: Recommended: Impact roller, Grid roller, Vibrating roller. Can be used: Flat wheel roller.

  • Well-Graded & Poorly-Graded Sand/Gravel: Recommended: Flat wheel roller, Pneumatic tyred roller, Impact roller, Grid roller, Vibrating roller.

  • Silty Sand: Recommended: Pneumatic tyred roller, Impact roller, Vibrating roller, Padfoot roller. Can be used: Flat wheel roller.

  • Clayey Gravel: Recommended: Pneumatic tyred roller, Impact roller, Padfoot roller. Can be used: Grid roller.

  • Weak Silty Material: Recommended: Impact roller, Padfoot roller. Can be used: Pneumatic tyred roller, Grid roller.

  • Strong Clayey Material: Recommended: Impact roller, Padfoot roller.

  • Surface Seals: Recommended: Flat wheel roller, Pneumatic tyred roller. Can be used: Vibrating roller.

  • Asphalt Layers: Recommended: Flat wheel roller, Pneumatic tyred roller, Vibrating roller.

  • Stabilized Layers: Recommended: Flat wheel roller, Pneumatic tyred roller, Vibrating roller.

  • Special Operational Restriction: Pneumatic tyred rollers MUST NOT be used on Stone Mastic Asphalt layers because they tear and damage the surface.