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Soil Classification & Properties

Aug 14, 2025

Overview

This lecture reviews core concepts and problem-solving techniques in geotechnical engineering relevant for the FE Civil exam, focusing on soil classification, phase relations, lab/field tests, effective stress, bearing capacity, and slope stability.

Soil Classification & Index Properties

  • Soil classification uses systems like USCS and AASHTO, based on grain size and plasticity.
  • Sieve size #4 separates gravel/sand; #200 separates sands/silts/clays (fines).
  • Well-graded soils have a wide range of particle sizes; poorly/uniformly graded soils mostly one size.
  • Uniformity coefficient (Cu = D60/D10) and coefficient of curvature (Cc = D30²/(D10×D60)) help classify grading.
  • Plasticity Index (PI = Liquid Limit - Plastic Limit) indicates soil's plasticity.
  • For fine-grained soils, location above/below the "A-line" in the plasticity chart helps distinguish between clays and silts.

Phase Relations & Soil Properties

  • Soils consist of three phases: solids, water, and air.
  • Water content = (weight of water / weight of solids) × 100%.
  • Specific gravity (Gs) = (mass of solids / volume of solids) / (density of water).
  • Degree of saturation (S) = (volume of water / volume of voids) × 100% or S = w × Gs / e.
  • Use phase diagrams to organize and solve for unknowns in soil problems.

Lab & Field Testing

  • Proctor test (standard or modified) determines maximum dry density of soil.
  • Nuclear density gauge and sand cone are used in the field to measure in-place density.
  • Sieve analysis provides particle size distribution for classification.

Compaction & Relative Density

  • Relative compaction = (field dry unit weight / lab maximum dry unit weight) × 100%.
  • Relative density (Dr) formula: Dr = [(γ_field - γ_min) / (γ_max - γ_min)] × (γ_max / γ_field).
  • Specifications typically require certain levels of relative compaction for construction acceptance.

Effective Stress & Shear Strength

  • Effective vertical stress: σ' = (total stress) – (pore water pressure).
  • For soils below the water table, subtract water’s unit weight from total stress for effective stress.
  • Shear strength in sands (non-cohesive): Ï„ = σ_n × tan(Ï•); in clays (cohesive): Ï„ = c + σ_n × tan(Ï•).
  • Triaxial test uses principal stresses to determine friction angle (Ï•): sinÏ• = radius / average normal stress in Mohr’s circle.

Retaining Walls & Bearing Capacity

  • Lateral earth pressure: P_a = ½ k_a γ H² (ka is the active earth pressure coefficient).
  • Surcharge adds a uniform horizontal pressure to the wall.
  • Passive earth pressure (kp) provides resistance; kp >> ka for typical soils.
  • Bearing pressure under footings: check with both vertical load and overturning moment (eccentricity).
  • Use q_toe and q_heel to confirm entire footing stays in compression; compare to allowable bearing capacity.

Settlement & Consolidation

  • Compression index (Cc) and recompression index (Cr) estimate expected settlement from loading.
  • Cc ≈ 0.009 × (LL–10); Cr ≈ Cc/6.
  • Settlement calculation may require partitioning between recompression and virgin compression depending on preconsolidation pressure.

Slope Stability

  • Factor of safety = (resisting forces) / (driving forces).
  • Mobilizing force is W × sin(α); resisting force includes normal force × tan(Ï•) plus cohesion × length.
  • A factor of safety above 1 indicates stability.

Soil Stabilization & General Concepts

  • Soil stabilization methods include mechanical compaction, chemical admixtures, and geotextiles.
  • Clearing and grubbing increase erosion risk and are not stabilization methods.

Key Terms & Definitions

  • USCS — Unified Soil Classification System used for soil identification.
  • Uniformity Coefficient (Cu) — D60/D10, indicates grading range.
  • Plasticity Index (PI) — Difference between liquid and plastic limit.
  • Proctor Test — Lab test to determine soil’s maximum dry density.
  • Effective Stress (σ') — Stress carried by soil skeleton: total stress minus pore water pressure.
  • Bearing Capacity — Maximum stress soil can support under a foundation without failure.

Action Items / Next Steps

  • Review index property definitions and soil classification charts (especially USCS/AASHTO).
  • Practice drawing and using phase diagrams for phase relation problems.
  • Memorize key formulas: Cu, Cc, PI, effective stress, compaction, and factor of safety.
  • Solve additional practice problems, especially in settlement/consolidation and slope stability.
  • Update notes with any corrections or new examples as you review.

Full transcript