Direct Shear Test for Soil Mechanics

Direct Shear Test (Undrained)

• Purpose: Determines undrained shear strength for granular soils
• Procedure:
  • Soil sample is placed in a shear box and subjected to vertical stress
  • Horizontal shear displacement is applied without allowing drainage
  • Shear stress and shear displacement are recorded
• Results Obtained: Peak shear strength represents the undrained shear strength

Direct Shear Test (Drained)

• Purpose: Determines drained shear strength for granular soils
• Procedure:
  • Soil sample is placed in a shear box and subjected to vertical stress
  • Horizontal shear displacement is applied, allowing drainage during shearing
  • Shear stress and shear displacement are recorded
• Results Obtained: Peak shear strength represents the drained shear strength

Consolidated Drained (CD) Triaxial Test

• Purpose: Measures the drained shear strength (Sd) of cohesive soils
• Procedure:
  • Soil sample is consolidated under isotropic stress conditions
  • The sample is then drained, allowing excess pore water pressures to dissipate, and then sheared
  • Shear stress and axial deformation are measured
• Results Obtained: Peak shear strength (Sd) is obtained from the stress-strain curve

Measurement of Pore Water Pressure

• Importance: Measuring pore water pressure is crucial to understanding the soil's response
• Method: Pore water pressure transducers are commonly used to monitor changes in excess pore water pressure during testing

Shear Strength Parameters

• Effective Stress: The stress carried by the soil skeleton, calculated as the difference between total stress and pore water pressure
• Cohesion (c): The intercept of the shear strength envelope with the shear stress axis in drained conditions
• Angle of Internal Friction (ϕ): The slope of the shear strength envelope

Interpretation of Test Results

• The results aid in the interpretation of soil behavior under different drainage conditions
• Applications: Slope stability analysis, foundation design, and overall geotechnical engineering

Standard Test Methods

• Standardized by organizations like ASTM and IS
• Ensures consistency and comparability of results

Triaxial Test

• Purpose: Assesses the shear strength and stress-strain behavior of soils under different confining pressures and drainage conditions
• Types of Triaxial Tests:
  • Consolidated Drained (CD) Triaxial Test
  • Consolidated Undrained (CU) Triaxial Test
  • Unconsolidated Undrained (UU) Triaxial Test

Tests for Various Drainage Conditions

• Unconfined Compression Test:
  • Purpose: Determines the undrained shear strength (Su) of cohesive soils
  • Procedure: A cylindrical soil sample is loaded vertically until failure without allowing drainage
• Direct Shear Test:
  • Purpose: Determines both undrained and drained shear strengths for granular soils
  • Procedure: A soil sample is subjected to shear displacement either with (undrained) or without (drained) drainage
• Triaxial Test:
  • Purpose: Evaluates the shear strength and stress-strain behavior of soils under different confining pressures and drainage conditions
  • Procedure: Depends on the type of triaxial test (CD, CU, or UU)

Merits and Demerits of Various Shear Strength Tests

• Unconfined Compression Test:
  • Merits: Simplicity, quick assessment of undrained shear strength
  • Demerits: Limited applicability, neglects drainage conditions
• Direct Shear Test:
  • Merits: Versatility, ease of setup, provides shear strength parameters
  • Demerits: Assumes homogeneity, plane of failure
• Triaxial Test:
  • Merits: Versatility, stress path simulation, comprehensive results
  • Demerits: Complex setup, labor-intensive

Pore Pressure Coefficient (A) and Shear Characteristics

• Definition of Pore Pressure Coefficient (A): Relates the change in pore water pressure to the change in effective stress during soil shearing
• Shear Characteristics of Soil Types:
  • Sand:
    • Drained shear strength: Well-defined, follows Mohr-Coulomb failure criterion
    • Undrained shear strength: Negligible, pore pressure generation limited
  • Normally Consolidated (NC) Clays:
    • Drained shear strength: Distinct, follows Mohr-Coulomb criterion
    • Undrained shear strength: Significant, pore water pressures build up
  • Over-Consolidated (OC) Clays:
    • Drained shear strength: Similar to NC clays, with increased shear strength
    • Undrained shear strength: Lower compared to NC clays, reduced sensitivity
  • Partially Saturated Soil:
    • Pore pressure generation: Influence of degree of saturation and capillary effects
    • Shear strength: Lower compared to fully saturated soils, suction effects important

Influence of Soil Structure and Strain Rate on Shear Strength

• Influence of Soil Structure:
  • Definition: Arrangement and organization of soil particles and their aggregates
  • Effects on shear strength: Aggregation, particle arrangement, fabric, and cementation
• Influence of Strain Rate:
  • Definition: Rate of deformation in a material
  • Effects on shear strength: Rate dependency, changes in shear strength under rapid loading### Thixotropy
• Thixotropic soils exhibit a reduction in shear strength when subjected to prolonged shearing
• Shear strength may recover over time when the soil is at rest

Strain Rate Sensitivity

• Certain soils, especially clays, exhibit sensitivity to strain rate
• Rapid loading conditions can lead to increased pore water pressures and reduced effective stress, influencing shear strength

Creep Behavior

• Creep is the slow, time-dependent deformation of soils under constant stress
• Creep can influence shear strength, particularly in cohesive soils, where continued deformation may lead to consolidation and changes in effective stress

Interaction of Soil Structure and Strain Rate

Anisotropic Behavior

• The combination of soil structure and strain rate can result in anisotropic behavior
• The orientation of soil particles can impact how the soil responds to different rates of loading

Rate-Dependent Strength

• Soils with complex structures may exhibit rate-dependent strength due to the interplay of particle arrangement, cementation, and thixotropy