7.4 Factors affecting shear strength (drainage conditions, soil type, stress history)

Shear strength is crucial in soil mechanics, determining how soil behaves under stress. This section explores key factors that influence shear strength: drainage conditions, soil type, and stress history. Understanding these factors is essential for accurate soil analysis and design.

Drainage conditions affect pore water pressure, impacting effective stress and shear strength. Soil type determines strength mechanisms, with cohesive and cohesionless soils behaving differently. Stress history, particularly overconsolidation, shapes a soil's strength characteristics and response to loading.

Drainage Conditions and Shear Strength

Drainage and Pore Water Pressure

• Drainage conditions determine water flow in soil pores during loading affects shear strength
• Undrained conditions prevent water escape from soil pores during rapid loading leads to excess pore water pressure development
• Drained conditions allow excess pore water pressure dissipation results in effective stress changes and different shear strength behavior
• Loading rate relative to soil permeability determines drained or undrained conditions prevail
• Total stress analysis used for undrained conditions while effective stress analysis applies for drained conditions
• Critical state concept essential for understanding long-term soil behavior under different drainage conditions
• Partially drained conditions occur in intermediate loading rates require more complex analysis methods

Analysis Methods for Drainage Conditions

• Triaxial tests conducted to simulate different drainage conditions (consolidated-drained, consolidated-undrained, unconsolidated-undrained)
• Pore pressure parameters (Skempton's A and B) used to quantify pore pressure response in undrained conditions
• Effective stress principle ($\sigma' = \sigma - u$) applied to analyze drained behavior
• Consolidation theory (Terzaghi's theory) used to estimate time for pore pressure dissipation
• Stress path method employed to visualize stress changes during loading under different drainage conditions
• Numerical modeling (finite element analysis) utilized for complex drainage scenarios in geotechnical problems

Cohesive vs. Cohesionless Soil Shear Strength

Shear Strength Mechanisms

• Cohesive soils (clays) derive shear strength from friction and cohesion while cohesionless soils (sands) rely primarily on friction
• Mohr-Coulomb failure criterion describes shear strength with cohesion (c) and friction angle (φ) as key parameters
• Cohesionless soils exhibit higher permeability allows rapid drainage and predominantly drained behavior
• Cohesive soils have lower permeability often results in undrained behavior under short-term loading conditions
• Stress-strain behavior differs cohesive soils typically show more ductile behavior and cohesionless soils exhibit brittle failure
• Dilatancy tendency for dense granular materials to expand during shear more pronounced in cohesionless soils
• Critical state concept particularly important for cohesive soils defines ultimate condition where shearing occurs at constant volume and effective stress

Soil Classification and Testing

• Unified Soil Classification System (USCS) used to categorize soils based on grain size distribution and plasticity
• Atterberg limits (liquid limit, plastic limit, plasticity index) determine cohesive soil behavior
• Direct shear test commonly used for cohesionless soils measures friction angle
• Triaxial tests performed on both cohesive and cohesionless soils provide comprehensive strength parameters
• Unconfined compression test conducted on cohesive soils estimates undrained shear strength
• Vane shear test utilized for in-situ measurement of cohesive soil strength
• Cone penetration test (CPT) employed for both cohesive and cohesionless soils correlates with shear strength parameters

Overconsolidation Ratio Impact on Shear Strength

OCR and Soil Behavior

• Overconsolidation ratio (OCR) defined as ratio of maximum past effective stress to current effective stress
• Normally consolidated soils (OCR = 1) have different shear strength characteristics compared to overconsolidated soils (OCR > 1)
• Overconsolidation affects cohesion intercept and friction angle in Mohr-Coulomb failure criterion
• Critical state line in e-log p' space influenced by stress history affects ultimate shear strength of soil
• Overconsolidated clays typically exhibit higher undrained shear strength and tend to dilate during shearing
• Stress-strain behavior of overconsolidated soils generally stiffer and more brittle compared to normally consolidated soils
• Stress history affects pore pressure response during undrained loading overconsolidated soils potentially develop negative excess pore pressures

OCR Determination and Applications

• Oedometer test used to determine preconsolidation pressure and calculate OCR
• In-situ tests (CPT, pressuremeter) correlated with OCR for different soil types
• SHANSEP (Stress History and Normalized Soil Engineering Properties) method applied to estimate undrained shear strength based on OCR
• Recompression index (Cr) and compression index (Cc) influenced by OCR affect settlement calculations
• OCR consideration crucial in slope stability analysis affects choice of strength parameters
• Foundation design accounts for OCR in bearing capacity and settlement estimations
• Lateral earth pressure coefficients (K0, Ka, Kp) vary with OCR impact retaining wall design

Factors Affecting Soil Shear Strength

Soil Structure and Composition

• Soil structure including fabric and bonding between particles significantly influences shear strength especially in natural clay deposits
• Cementation both natural (calcium carbonate) and artificial (cement, lime) can increase cohesion and alter stress-strain behavior of soils
• Soil anisotropy resulting from depositional processes or stress history leads to directional variations in shear strength
• Presence of organic matter (peat, organic clay) can reduce shear strength and increase compressibility of soils
• Clay mineralogy (kaolinite, illite, montmorillonite) affects interparticle forces and shear strength characteristics
• Particle shape and size distribution influence friction angle and dilatancy behavior in granular soils
• Void ratio and relative density control shear strength of cohesionless soils denser soils exhibit higher strength

Environmental and Time-Dependent Factors

• Temperature changes affect soil strength through thermal expansion changes in pore water pressure and alterations to clay mineral properties
• Aging effects such as thixotropy in clays lead to time-dependent changes in soil strength
• Chemical factors including pH and pore fluid composition alter interparticle forces and affect shear strength particularly in clay soils
• Cyclic loading (earthquakes, machine vibrations) can cause strength degradation and liquefaction in susceptible soils
• Freeze-thaw cycles alter soil structure and strength properties in cold regions
• Biological activity (plant roots, burrowing animals) modifies soil structure and affects shear strength
• Weathering processes (physical, chemical) gradually change soil composition and strength characteristics over time