2.1 Soil Properties and Laboratory Tests

2.1 Soil Properties and Laboratory Tests

Introduction to Soil as an Engineering Material

• Soil is a complex, three-phase natural material consisting of solid particles, water, and air.

• Understanding its properties is fundamental to geotechnical engineering.

• This unit systematically explores laboratory methods for determining soil characteristics that govern engineering behavior—from identification and classification to predicting performance under structural loads.

• Each test reveals specific insights: strength tests predict stability, permeability tests assess drainage, compressibility tests forecast settlement, and index properties enable systematic classification.

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1. Laboratory Tests for Fundamental Soil Properties

1.1 Strength Tests – Measuring Soil’s Resistance to Failure

Direct Shear Test

• Purpose: Determines shear strength parameters—cohesion ($\boldsymbol{c}$) and angle of internal friction ($\boldsymbol{\phi}$)—under controlled normal stress.

• Procedure:

  1. Place soil specimen in a split shear box.

  2. Apply a constant vertical (normal) load ($\boldsymbol{N}$).

  3. Gradually apply horizontal shear force until failure.

  4. Record maximum shear force at failure.

  5. Repeat with different normal loads.

• Calculations:

  • Shear stress at failure: $\boldsymbol{\tau = \frac{F}{A}}$

    where, $\boldsymbol{F}$ is shear force, and $\boldsymbol{A}$ is specimen area.

  • Plot $\boldsymbol{\tau}$ vs $\boldsymbol{\sigma_n}$ for multiple tests.

  • The intercept gives cohesion $\boldsymbol{c}$; the slope gives friction angle $\boldsymbol{\phi}$.

• Significance: Simple, quick test suitable for sandy and cohesive soils; simulates shallow foundation failure.

Triaxial Compression Test

• Purpose: Provides comprehensive shear strength data under controlled drainage and confining pressure.

• Procedure:

  1. Encase cylindrical soil specimen in a rubber membrane.

  2. Apply confining pressure ($\boldsymbol{\sigma_3}$) via chamber fluid.

  3. Apply deviator stress ($\boldsymbol{\sigma_1 - \sigma_3}$) axially until failure.

  4. Monitor pore water pressure if required.

• Test Types:

  • UU (Unconsolidated-Undrained): Quick test for total stress analysis.

  • CU (Consolidated-Undrained): Consolidate under confining pressure, then shear without drainage.

  • CD (Consolidated-Drained): Slow shearing allowing full drainage.

• Data Interpretation:

  • Plot Mohr's circles for each test.

  • Draw failure envelope tangent to circles.

  • Determine $\boldsymbol{c}$ and $\boldsymbol{\phi}$ for total or effective stress.

• Significance: Versatile, simulates various field conditions; essential for slope stability and deep foundation design.

Unconfined Compression Test

• Purpose: Quick estimation of undrained shear strength ($\boldsymbol{s_u}$) for cohesive soils ($\boldsymbol{\phi = 0}$ concept).

• Procedure:

  1. Mold cylindrical specimen (no lateral support).

  2. Apply axial load at constant strain rate until failure.

• Formula:

  $\boldsymbol{s_u = \frac{q_u}{2}}$

  where, $\boldsymbol{q_u}$ is the unconfined compressive strength.

• Significance: Rapid field/lab test for clayey soils; correlates with consistency.

Vane Shear Test

• Purpose: Determines in-situ undrained shear strength of soft clays.

• Procedure:

  1. Insert four-bladed vane into soil.

  2. Rotate vane at constant rate.

  3. Record maximum torque ($\boldsymbol{T_{max}}$) at failure.

• Formula:

  $\boldsymbol{s_u = \frac{T_{max}}{K}}$

  where, $\boldsymbol{K}$ is vane constant based on dimensions.

• Significance: Minimal soil disturbance; ideal for soft marine clays and embankment foundations.

1.2 Permeability Tests – Quantifying Water Flow Through Soil

Constant Head Test

• Purpose: Measures coefficient of permeability ($\boldsymbol{k}$) for coarse-grained soils (sand, gravel).

• Procedure:

  1. Saturate soil specimen in permeameter.

  2. Maintain constant head difference ($\boldsymbol{h}$) across specimen length ($\boldsymbol{L}$).

  3. Collect water volume ($\boldsymbol{Q}$) over time ($\boldsymbol{t}$).

• Formula:

  $\boldsymbol{k = \frac{QL}{A h t}}$

  where, $\boldsymbol{A}$ is cross-sectional area of specimen.

• Typical Values: Gravel: $\boldsymbol{10^0 - 10^{-2}}$ cm/s; Sand: $\boldsymbol{10^{-2} - 10^{-4}}$ cm/s.

Falling Head Test

• Purpose: Determines $\boldsymbol{k}$ for fine-grained soils (silt, clay).

• Procedure:

  1. Connect standpipe of area $\boldsymbol{a}$ to top of soil specimen.

  2. Allow water to flow through specimen.

  3. Record time ($\boldsymbol{t}$) for head to fall from $\boldsymbol{h_1}$ to $\boldsymbol{h_2}$.

• Formula:

  $\boldsymbol{k = \frac{aL}{At} \ln \left( \frac{h_1}{h_2} \right)}$

• Significance: Accounts for low flow rates in fine soils; essential for seepage and consolidation analysis.

1.3 Compressibility Tests – Predicting Soil Settlement

Oedometer (Consolidation) Test

• Purpose: Determines one-dimensional compressibility and rate of consolidation.

• Procedure:

  1. Place saturated soil specimen in rigid ring.

  2. Apply incremental vertical loads (typically doubling each stage).

  3. Record settlement vs time for each load.

  4. Continue through loading and unloading cycles.

• Key Parameters Determined:

  • Compression Index ($\boldsymbol{C_c}$):

    $\boldsymbol{C_c = \frac{\Delta e}{\Delta \log \sigma_v'}}$

    Empirical correlation: $\boldsymbol{C_c \approx 0.009(LL - 10)}$ for remolded clays.

  • Coefficient of Consolidation ($\boldsymbol{c_v}$): From time-settlement curves using:

    • Taylor's √t method: $\boldsymbol{c_v = \frac{0.848 H_{dr}^2}{t_{90}}}$

    • Casagrande's log t method: $\boldsymbol{c_v = \frac{0.197 H_{dr}^2}{t_{50}}}$

  • Preconsolidation Pressure ($\boldsymbol{\sigma_p'}$): Maximum past effective stress (from Casagrande construction).

• Significance: Predicts magnitude and rate of settlement for foundations; critical for soft clay sites.

1.4 Phase Relationship Tests – Defining Soil Composition

Water Content Determination

• Purpose: Measures mass of water relative to solids.

• Procedure:

  1. Weigh moist soil sample ($\boldsymbol{W_w}$).

  2. Dry in oven at 105-110°C for 24 hours.

  3. Weigh dry soil ($\boldsymbol{W_s}$).

• Formula:

  $\boldsymbol{w = \frac{W_w - W_s}{W_s} \times 100\%}$

Specific Gravity Test

• Purpose: Determines density of soil solids relative to water.

• Procedure (Density Bottle):

  1. Weigh empty bottle ($\boldsymbol{W_1}$).

  2. Add dry soil, weigh ($\boldsymbol{W_2}$).

  3. Fill with water, weigh ($\boldsymbol{W_3}$).

  4. Empty, clean, fill with water only, weigh ($\boldsymbol{W_4}$).

• Formula:

  $\boldsymbol{G = \frac{W_2 - W_1}{(W_4 - W_1) - (W_3 - W_2)}}$

Field Density Tests

• Sand Replacement Method:

  1. Excavate small hole, weigh soil ($\boldsymbol{W_{wet}}$).

  2. Fill hole with uniform sand of known density.

  3. Calculate hole volume ($\boldsymbol{V}$).

• Formula: Bulk density $\boldsymbol{\rho = \frac{W_{wet}}{V}}$; Dry density $\boldsymbol{\rho_d = \frac{\rho}{1 + w}}$

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2. Determination of Soil Properties

2.1 Index Properties – Soil Identification Characteristics

• Grain Size Distribution: Sieve and hydrometer analysis.

• Atterberg Limits: Liquid Limit ($\boldsymbol{LL}$), Plastic Limit ($\boldsymbol{PL}$), Shrinkage Limit ($\boldsymbol{SL}$).

• Plasticity Index ($\boldsymbol{PI}$): $\boldsymbol{PI = LL - PL}$.

• Liquidity Index ($\boldsymbol{LI}$): $\boldsymbol{LI = \frac{w - PL}{PI}}$.

• Consistency Index ($\boldsymbol{CI}$): $\boldsymbol{CI = \frac{LL - w}{PI}}$.

2.2 Engineering Properties – Predicting Soil Behavior

Shear Strength Parameters

• Total Stress Analysis: $\boldsymbol{\tau = c + \sigma \tan \phi}$

• Effective Stress Analysis: $\boldsymbol{\tau = c' + \sigma' \tan \phi'}$ where $\boldsymbol{\sigma' = \sigma - u}$

Compressibility Parameters

• Coefficient of Volume Change: $\boldsymbol{m_v = \frac{\Delta e / (1 + e_0)}{\Delta \sigma_v'}}$

• Compression Ratio: $\boldsymbol{CR = \frac{C_c}{1 + e_0}}$

Permeability

• Factors Affecting k: Particle size, void ratio, degree of saturation, soil structure.

• Typical Ranges:

  • Clean gravel: 1-100 cm/day

  • Fine sand: 0.01-1 cm/day

  • Silt: 0.001-0.01 cm/day

  • Clay: <0.0001 cm/day

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3. Soil Classification Systems

3.1 Descriptive Classification

• Format: [Color] [Secondary component] [Primary component] [Additional features]

• Example: "Brown silty CLAY with occasional gravel"

• Advantages: Simple, intuitive; good for field identification.

3.2 Textural Classification

• Basis: Percentages of sand, silt, clay.

• Tool: USDA textural triangle.

• Classes: Sand, Loamy Sand, Sandy Loam, Loam, Silt Loam, Silt, Sandy Clay Loam, Clay Loam, Silty Clay Loam, Sandy Clay, Silty Clay, Clay.

3.3 ISI Classification (IS:1498)

• Coarse-grained soils (>50% retained on 75µ IS sieve):

  • Gravel (G): >50% of coarse fraction retained on 4.75mm sieve

  • Sand (S): >50% of coarse fraction passes 4.75mm sieve

  • Gradation: W (well-graded), P (poorly-graded)

• Fine-grained soils (>50% passes 75µ sieve):

  • Inorganic: M (silt), C (clay)

  • Organic: O

  • Plasticity: L (LL<35), I (35<LL<50), H (LL>50)

• Example: SW = Well-graded sand

3.4 MIT Classification

• Similar to USCS but developed at Massachusetts Institute of Technology.

• Uses grain size distribution and plasticity characteristics.

• Common groups: GW, GP, SW, SP, ML, CL, MH, CH.

3.5 USCS (Unified Soil Classification System)

• Most widely used system worldwide.

• Basis: Grain size distribution and Atterberg limits.

• Flowchart Decision Process:

  1. Determine % passing #200 sieve (0.075mm).

  2. If >50% retained → Coarse-grained (Prefix: G or S).

  3. If >50% passes → Fine-grained (Prefix: M, C, or O).

  4. Use gradation criteria ($\boldsymbol{C_u}$, $\boldsymbol{C_c}$) for coarse soils.

  5. Use plasticity chart for fine soils.

• USCS Plasticity Chart

  • A-line Equation: $\boldsymbol{PI = 0.73(LL - 20)}$

  • Above A-line: Clay (C)

  • Below A-line: Silt (M)

• Additional Designations:

  • L: LL < 50 (low plasticity)

  • H: LL ≥ 50 (high plasticity)

• Complete USCS Group Symbols: e.g., CL, CH, ML, SM, GP-GC, etc.

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4. Sieve Analysis – Procedure, Calculations, and Interpretation

4.1 Laboratory Procedure

• Sample Preparation:

  • Oven-dry representative sample.

  • Break up aggregations without crushing particles.

• Sieve Stack Assembly:

  • Arrange sieves in descending order (largest opening at top).

  • Include pan at bottom.

  • Common sizes: 4.75mm, 2.36mm, 1.18mm, 600µm, 425µm, 300µm, 212µm, 150µm, 75µm.

• Sieve Shaking:

  • Place sample on top sieve.

  • Shake mechanically for 10-15 minutes.

• Weighing:

  • Weigh retained material on each sieve.

  • Ensure total recovered weight is within 1% of original.

4.2 Calculations

• Mass Retained on Each Sieve: $\boldsymbol{M_i}$

• Percent Retained: $\boldsymbol{\%R_i = \frac{M_i}{M_{total}} \times 100}$

• Cumulative Percent Retained: $\boldsymbol{\%CR_i = \sum \%R_i}$

• Percent Finer: $\boldsymbol{\%F_i = 100 - \%CR_i}$

4.3 Grain Size Distribution Curve

• Plot: % Finer (y-axis) vs Particle Diameter (x-axis, log scale).

• Key Points from Curve:

  • $\boldsymbol{D_{10}}$ = Diameter at 10% finer (Effective Size)

  • $\boldsymbol{D_{30}}$ = Diameter at 30% finer

  • $\boldsymbol{D_{60}}$ = Diameter at 60% finer

4.4 Interpretation and Coefficients

• Uniformity Coefficient:

  $\boldsymbol{C_u = \frac{D_{60}}{D_{10}}}$

  Higher $\boldsymbol{C_u}$ → wider range of particle sizes.

• Coefficient of Curvature:

  $\boldsymbol{C_c = \frac{(D_{30})^2}{D_{60} \cdot D_{10}}}$

• Gradation Classification (USCS Criteria)

  • Well-graded: $\boldsymbol{C_u > 4}$ and $\boldsymbol{1 < C_c < 3}$ for gravels (GW); $\boldsymbol{C_u > 6}$ and $\boldsymbol{1 < C_c < 3}$ for sands (SW).

  • Poorly-graded: Does not meet both criteria (GP, SP).

• Well-graded: Good particle size distribution, high density potential.

• Poorly-graded: Uniform or gap-graded, may have low density.

• Gap-graded: Missing particles in certain size ranges (flat portion on curve).

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5. Determination of Atterberg Limits

5.1 Liquid Limit (LL) – Casagrande Cup Method

• Purpose: Determines upper limit of plastic state.

• Procedure:

  1. Prepare soil paste at approximate LL consistency.

  2. Place in brass cup, level surface.

  3. Cut groove with standard tool (11mm wide at top, 2mm at bottom).

  4. Rotate crank at 2 drops/second.

  5. Record number of blows (N) required to close groove 13mm.

  6. Determine water content at that consistency.

  7. Repeat for 3-4 different water contents.

• Flow Curve: Plot water content (y-axis) vs log N (x-axis).

• LL Definition: Water content at N = 25 blows.

5.2 Liquid Limit – Fall Cone Method (Alternative)

• Procedure:

  1. Prepare soil paste.

  2. Place in cup, level surface.

  3. Release 80g, 30° cone from contact with surface.

  4. Measure penetration after 5 seconds.

  5. Repeat for different water contents.

• LL Definition: Water content at 20mm penetration.

5.3 Plastic Limit (PL)

• Purpose: Determines lower limit of plastic state.

• Procedure:

  1. Prepare soil at water content near PL.

  2. Roll on glass plate to form 3mm diameter thread.

  3. When thread crumbles at 3mm diameter, collect sample for water content.

  4. Repeat twice more.

• PL Definition: Average water content at crumbling point.

5.4 Shrinkage Limit (SL)

• Purpose: Water content below which soil volume remains constant.

• Procedure:

  1. Fill shrinkage dish with wet soil paste (known volume $\boldsymbol{V_i}$, weight $\boldsymbol{W_i}$).

  2. Dry in oven.

  3. Measure volume of dry soil pat ($\boldsymbol{V_f}$) by mercury displacement.

  4. Weigh dry soil ($\boldsymbol{W_s}$).

• Formula:

  $\boldsymbol{SL = w_i - \frac{V_i - V_f}{W_s} \cdot \rho_w \cdot 100\%}$

  Where $\boldsymbol{w_i}$ = initial water content.

5.5 Derived Indices and Their Significance

• Plasticity Index (PI): $\boldsymbol{PI = LL - PL}$. Range of water content over which soil is plastic.

• Liquidity Index (LI): $\boldsymbol{LI = \frac{w_n - PL}{PI}}$. Position of natural water content within plastic range.

• Consistency Index (CI): $\boldsymbol{CI = \frac{LL - w_n}{PI}}$.

• Shrinkage Index (SI): $\boldsymbol{SI = LL - SL}$.

• Activity (A): $\boldsymbol{A = \frac{PI}{\% \text{Clay} (<2\mu m)}}$. Indicator of clay mineral type.

• Soil Sensitivity: $\boldsymbol{S_t = \frac{s_u (\text{undisturbed})}{s_u (\text{remolded})}}$

  • Insensitive: $\boldsymbol{S_t < 2}$

  • Sensitive: $\boldsymbol{2 < S_t < 4}$

  • Extra-sensitive: $\boldsymbol{4 < S_t < 8}$

  • Quick: $\boldsymbol{S_t > 8}$

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6. Boring Log Interpretation

6.1 Components of a Standard Boring Log

• Header Information: Project name, location, boring number, coordinates, date, elevation.

• Graphical Column: Soil profile showing layer depths, descriptions.

• Test Data: SPT N-values at depth intervals.

• Sample Information: Type (disturbed/undisturbed), recovery percentage.

• Water Table: Depth at time of drilling and after stabilization.

• Remarks: Field observations, drilling difficulties, odor, etc.

6.2 Standard Penetration Test (SPT) Correlation

• Cohesionless Soils (Relative Density):

  • Very Loose: N < 4

  • Loose: 4-10

  • Medium: 10-30

  • Dense: 30-50

  • Very Dense: N > 50

• Cohesive Soils (Consistency):

  • Very Soft: N < 2

  • Soft: 2-4

  • Medium: 4-8

  • Stiff: 8-15

  • Very Stiff: 15-30

  • Hard: N > 30

6.3 Engineering Interpretation Guidelines

• Identify Soil Layers:

  • Note layer boundaries (sharp or gradual).

  • Record thickness and extent of each layer.

• Assess Soil Properties:

  • Estimate strength parameters from SPT correlations.

  • Identify compressible layers (clays with low N-values).

  • Locate potential bearing strata (dense sands, stiff clays).

• Groundwater Analysis:

  • Note seasonal variations if multiple readings.

  • Identify artesian conditions.

• Foundation Recommendations:

  • Shallow foundations: Requires competent soil within 1.5-3m.

  • Deep foundations: Needed when weak layers extend deep.

  • Special considerations: Expansive clays, collapsible soils, organic layers.

6.4 Sample Boring Log Interpretation

• Purpose: Summarize subsurface soil conditions for design and construction.

• Header Information: Includes project name, location, boring number, coordinates, date, elevation.

• Soil Profile: Graphical column shows soil layers with depth, description, and boundaries (sharp or gradual).

• Test Data: SPT N-values indicate soil density or consistency.

• Sample Information: Type (disturbed/undisturbed) and recovery percentage.

• Water Table: Depth at drilling and after stabilization; affects foundation design.

• Engineering Interpretation:

  • Identify soil layers and thickness.

  • Estimate strength (from SPT) and compressibility.

  • Locate suitable bearing strata (dense sands, stiff clays).

  • Decide foundation type: shallow or deep depending on soil strength and weak layers.

  • Consider special soils: expansive clays, collapsible soils, organic layers.

• Remarks: Field observations like drilling difficulties, color, odor, or unusual soil behavior.

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Summary and Practical Applications

Integrated Approach to Soil Investigation

• Field Exploration: Borings, sampling, in-situ testing.

• Laboratory Testing: Index, strength, compressibility, permeability tests.

• Classification: Using appropriate system (typically USCS).

• Parameter Selection: For design calculations.

• Design: Foundations, slopes, retaining structures, pavements.

Common Soil Behavior Correlations

• Clay Activity vs Swell Potential:

  • Low activity (<0.75): Low swell

  • Medium activity (0.75-1.25): Moderate swell

  • High activity (>1.25): High swell

• Gradation vs Compactability:

  • Well-graded soils achieve higher densities.

  • Uniform sands are difficult to compact.

• Plasticity vs Compressibility:

  • Higher PI generally indicates higher compressibility.

Quality Control in Laboratory Testing

• Calibrate equipment regularly.

• Use standardized procedures (ASTM, IS codes).

• Maintain consistent test conditions (temperature, humidity).

• Perform duplicate tests for critical parameters.

• Document all procedures and observations thoroughly.

This comprehensive guide provides the foundation for understanding soil properties through laboratory testing. Mastery of these concepts enables accurate soil characterization, appropriate classification, and reliable prediction of engineering behavior—essential for safe and economical geotechnical design.

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