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2.4 Index properties of soils (particle size, Atterberg limits, specific gravity)

4 min readLast Updated on August 16, 2024

Soil index properties are crucial for understanding soil behavior without direct testing. These properties, including particle size, Atterberg limits, and specific gravity, help engineers classify soils and predict their performance in various applications.

Particle size distribution reveals soil composition, while Atterberg limits indicate consistency states of fine-grained soils. Specific gravity helps calculate important soil parameters. Together, these properties form the foundation for soil classification and engineering design.

Soil Index Properties

Key Concepts and Applications

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  • Soil index properties provide insight into engineering behavior without direct measurement of strength or permeability
  • Include particle size distribution, Atterberg limits, specific gravity, void ratio, porosity, and unit weight
  • Determined through standardized laboratory tests
  • Essential for soil classification systems (Unified Soil Classification System)
  • Used to estimate other soil parameters (hydraulic conductivity, compressibility) through empirical correlations
  • Crucial for predicting soil behavior in geotechnical applications (foundation design, slope stability analysis, earthwork construction)
  • Vary significantly with depth and location, requiring thorough site investigation and multiple tests

Importance in Geotechnical Engineering

  • Enable engineers to identify and classify soils efficiently
  • Serve as indicators of soil composition and behavior
  • Facilitate comparison between different soil samples
  • Aid in the selection of appropriate construction materials and techniques
  • Help in estimating soil properties that are more complex or expensive to measure directly
  • Provide a basis for preliminary design calculations and risk assessments
  • Assist in identifying potential geotechnical issues (liquefaction susceptibility, expansive soils)

Particle Size Distribution

Sieve Analysis for Coarse-Grained Soils

  • Quantitative description of particle size range and proportion in soil sample
  • Expressed as curve on semi-logarithmic plot
  • Used for gravel and sand particles
  • Process involves:
    • Drying soil sample
    • Passing through stack of sieves with decreasing mesh sizes
    • Weighing retained portions on each sieve
  • Sieves typically range from 75 mm to 0.075 mm openings
  • Results plotted as percent passing vs. particle size
  • Provides information on soil gradation (well-graded, poorly graded, gap-graded, uniformly graded)

Hydrometer Test for Fine-Grained Soils

  • Employed for silt and clay particles
  • Based on Stokes' law of particle settling in fluid suspension
  • Process involves:
    • Dispersing soil in water with deflocculating agent
    • Measuring density of suspension at various time intervals
    • Calculating particle sizes based on settling velocities
  • Hydrometer readings taken over 24-48 hour period
  • Results combined with sieve analysis for complete particle size distribution

Interpretation and Parameters

  • Key parameters derived from particle size distribution:
    • D10: Particle size for which 10% of soil is finer
    • D30: Particle size for which 30% of soil is finer
    • D60: Particle size for which 60% of soil is finer
    • Coefficient of uniformity: Cu=D60/D10Cu = D60 / D10
    • Coefficient of curvature: Cc=(D30)2/(D10D60)Cc = (D30)^2 / (D10 * D60)
  • Shape of curve indicates soil gradation
  • Applications:
    • Soil classification
    • Estimating permeability
    • Assessing susceptibility to phenomena (liquefaction, internal erosion)
    • Designing filters and drainage systems

Significance of Atterberg Limits

Consistency States and Determination

  • Define boundaries between different consistency states of fine-grained soils
  • Include liquid limit (LL), plastic limit (PL), and shrinkage limit (SL)
  • Liquid limit:
    • Water content at transition from plastic to liquid state
    • Determined using Casagrande cup or fall cone test
  • Plastic limit:
    • Water content at transition from semi-solid to plastic state
    • Determined by rolling threads of soil until crumbling occurs
  • Plasticity index (PI):
    • Difference between liquid limit and plastic limit
    • Indicates range of water content for plastic behavior
    • PI=LLPLPI = LL - PL

Applications in Soil Classification and Engineering

  • Used to classify fine-grained soils in engineering classification systems (USCS, AASHTO)
  • Correlate with important engineering properties:
    • Compressibility
    • Shear strength
    • Hydraulic conductivity
  • Activity of clay:
    • Ratio of plasticity index to clay fraction
    • Indicates soil's potential for volume change
    • Used to identify expansive soils
  • Influenced by factors:
    • Mineralogy (kaolinite, illite, montmorillonite)
    • Organic content
    • Pore fluid chemistry
  • Valuable indicators of soil composition and behavior
  • Used in empirical correlations for estimating other soil properties

Specific Gravity of Soil Solids

Definition and Measurement

  • Ratio of density of soil solids to density of water at standard temperature (typically 20°C)
  • Determined using pycnometer method:
    • Involves measuring mass of soil, water, and soil-water mixture in calibrated flask
    • Calculated using formula: Gs=(Ws)/(Ws+WwWsw)Gs = (Ws) / (Ws + Ww - Wsw) Where: Ws = Weight of dry soil Ww = Weight of water in pycnometer Wsw = Weight of soil-water mixture
  • Typical values:
    • Most inorganic soils: 2.60 to 2.80
    • Quartz-rich soils: ~2.65
    • Iron-rich minerals: up to 3.0
    • Organic soils: often below 2.0

Importance in Geotechnical Calculations

  • Crucial parameter in various geotechnical calculations:
    • Void ratio
    • Degree of saturation
    • Unit weight relationships
  • Used in hydrometer analysis to calculate particle sizes based on Stokes' law
  • Applications:
    • Identifying different soil layers in profiles
    • Detecting anomalies in soil composition
    • Estimating soil phase relationships
    • Calculating soil parameters (porosity, void ratio)
  • Variations can indicate changes in soil composition
  • Essential for accurate soil characterization and engineering design
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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