Effective stress is a crucial concept in soil mechanics, determining how soil behaves under load. It's the difference between total stress and pore water pressure, representing the forces between soil particles.

Understanding effective stress is key to solving real-world geotechnical problems. It helps engineers predict soil strength, stability, and settlement, which is essential for designing safe and efficient foundations, retaining walls, and slopes.

Effective stress in soils

Components of soil stress

• Total stress (σ) represents sum of vertical forces acting on soil mass divided by total area
• Pore water pressure (u) denotes hydrostatic pressure exerted by water within soil voids
  • Increases linearly with depth below water table
• Effective stress (σ') signifies portion of total stress carried by soil skeleton
  • Represents inter-granular forces between soil particles
• Relationship between stresses expressed by equation $σ' = σ - u$
• Effective stress controls soil behavior (strength, compressibility, volumetric changes)

Importance in geotechnical engineering

• Fundamental concept for understanding soil mechanics
• Crucial for analyzing soil stability, settlement, and strength
• Essential for designing foundations, retaining walls, and slope stability analyses
• Applies to saturated soils
• Governs soil behavior rather than total stress or pore water pressure alone

Effective stress principle

Terzaghi's principle

• Formulated by Karl Terzaghi
• States all measurable effects of stress change in soils result from changes in effective stress
• Applies to saturated soils
• Implies soil behavior governed by effective stress rather than total stress or pore water pressure alone
• Changes in effective stress occur due to alterations in total stress, pore water pressure, or both

Applications in geotechnical engineering

• Crucial for analyzing soil stability, settlement, and strength
• Essential for designing foundations, retaining walls, and slope stability analyses
• Fundamental for interpreting and analyzing results from geotechnical tests (triaxial, direct shear)
• Critical for understanding and predicting liquefaction potential in saturated granular soils during earthquakes
• Influences hydraulic conductivity and seepage behavior in soils
  • Affects drainage and groundwater flow patterns

Calculating effective stress

Factors affecting calculations

• Soil profile knowledge required (layer thicknesses, unit weights, groundwater conditions)
• Dry soils above water table have effective stress equal to total stress (pore water pressure zero)
• Saturated soils below water table require subtracting pore water pressure from total stress
• Partially saturated soils in capillary zone may involve negative pore water pressures (suction)
• External loads (foundations, embankments) contribute to total stress
  • Must be included in effective stress calculations

Calculation methods

• Effective stress profiles constructed by calculating effective stress at various depths
• For dry soils: $σ' = σ = γ z$ (γ soil unit weight, z depth)
• For saturated soils: $σ' = σ - u = (γsat * z) - (γw * h)$ (γsat saturated unit weight, γw water unit weight, h depth below water table)
• Partially saturated soils: $σ' = (γ z) + (ua - uw)$ (ua air pressure, uw water pressure)
• External loads: $σ' = σ - u + Δσ$ (Δσ additional stress from external load)

Effective stress and soil behavior

Influence on soil strength

• Directly affects soil strength parameters (friction angle, cohesion) in drained conditions
• Governs shear strength of soils
  • Critical for stability analyses (slopes, retaining walls, foundations)
• Essential in understanding and predicting liquefaction potential (saturated granular soils during earthquakes)
• Influences results interpretation from geotechnical tests (triaxial, direct shear)

Impact on soil deformation

• Affects soil compressibility and consolidation behavior
  • Influences settlement predictions
• Changes in effective stress lead to volumetric changes in soil mass
• Controls hydraulic conductivity and seepage behavior
  • Impacts drainage and groundwater flow patterns
• Crucial for analyzing soil-structure interaction (foundations, retaining walls)
• Influences soil stiffness and stress-strain behavior under various loading conditions