3) Index properties

Index Properties

• Key properties used to evaluate soil behavior.

• Importance in geotechnical engineering.

Learning Objectives

• Read and interpret a Grain Size Distribution Chart.

• Assess and calculate the Uniformity Coefficient and Coefficient of Curvature.

• Understand and describe the Atterberg Limits and their order.

• Conduct Liquid Limit and Plastic Limit tests.

Questions/Answers

• Differences between soil types:

  • Gravel vs. Sand

  • Silt vs. Clay

• Characterization of index properties to evaluate soil behavior.

• Tests to determine these index properties:

  • Sieve Analysis

  • Hydrometer Analysis

  • Atterberg Limits

  • Visual Description

Sieve Analysis

• Determines particle size and distribution.

• Involves separating grains on a graded series of wire screens.

• Mechanical Analysis data example:

  • 22.5% passing #200 sieve.

  • 15% passing #50 sieve.

Mechanical Analysis

• Sieve analysis plotting:

  • Particle diameter vs. percent passing.

  • Key average sizes:

    • D50 = 0.7 mm

    • D60 = 1.1 mm

    • D30 = 0.3 mm

    • D10 = 0.12 mm

• Coefficients calculated:

  • Coefficient of Uniformity (Cu) = D60/D10

  • Coefficient of Curvature (Cz) = D30^2/(D60*D10)

Hydrometer Analysis

• Uses sedimentation principle for silt and clay size distribution.

• Involves measuring unit weight of soil-water suspension with a hydrometer.

• Calculation of the percentage of soil particles still in suspension over time.

Atterberg Limits

• Defined by Albert Atterberg in 1912.

• Five limits to characterize soil:

  • Liquid Limit (ASTM D4318)

  • Plastic Limit (ASTM D4318)

  • Shrinkage Limit (ASTM D4943)

  • Sticky Limit

  • Cohesive Limit

Atterberg Limits State

• Influence on soil behavior:

  • Plasticity Index (PI) = LL - PL

  • Shrinkage Index (SI) = PL - SL

  • Liquidity Index (LI) = (w_field - PL) / (LL - PL)

  • Consistency Index (CI) = (LL - w_field) / (LL - PL)

Plastic Limit Test

• Identifies water content at crumbling point when soil is rolled.

• Consistency should be tested at 1/8-inch diameter threads.

Liquid Limit Test

• Measures water content relation to number of blows needed to close gap:

  • Example values:

    • 43.5% = 34 blows

    • 44.5% = 27 blows

    • Average LL = 45%

Typical Values of Atterberg Limits

• Kaolinite:

  • Liquid Limit: 30-110%

  • Plastic Limit: 25-40%

  • Shrinkage Limit: 25-29%

• Illite:

  • Liquid Limit: 60-120%

  • Plastic Limit: 35-60%

  • Shrinkage Limit: 15-17%

• Montmorillonite:

  • Liquid Limit: 100-900%

  • Plastic Limit: 50-100%

  • Shrinkage Limit: 8.5-15%

• Laramie Clay:

  • Liquid Limit: 35-42%

  • Plastic Limit: 17-19%

  • Shrinkage Limit: 13-15%

Particle Shape

• Influences on weathering age:

  • Angular: Minimal weathering

  • Sub-Angular: Moderate weathering

  • Sub-Rounded: More weathering

  • Rounded: Most weathered

Soil Consistency

• Consistency tested against unconfined compressive strength (Cohesion x 2):

  • Soft: < 0.5 tons/ft²

  • Medium: 0.5 - 1.0 tons/ft²

  • Stiff: 1.0 - 2.0 tons/ft²

  • Very Stiff: 2.0 - 4.0 tons/ft²

  • Hard: > 4.0 tons/ft²

Description of Soil Sample

• Parameters to describe:

  • Water Content (LL, PL)

  • Particle Shape

  • Consistency or Strength

  • Typical Size and Range

  • Color

  • Density

  • Smell

  • Presence of Organics

  • Other significant characteristics