Soil Mechanics: Triaxial Shear Test

Triaxial Tests

• Triaxial tests are used to assess excess pore water pressures developed during shearing.
• The test continues until shear failure occurs, and post-failure behavior is observed.
• Data analysis generates stress-strain curves, and key parameters like cohesion (c) and angle of internal friction (ϕ) can be determined.

Measurement of Axial and Radial Strains

• Axial and radial strains are continuously measured during the test to monitor deformation characteristics of the soil specimen.

Shear Failure and Post-Failure Behavior

• The test continues until shear failure occurs, and post-failure behavior is observed, including the dissipation of excess pore water pressures.

Data Analysis and Interpretation

• Stress-strain curves are generated from collected data.
• Key parameters, including shear strength parameters, can be determined from the test results.
• Triaxial test results provide insights into shear strength, stress-strain behavior, and drainage characteristics of soils.

Applications of Triaxial Tests

• Triaxial testing under different drainage conditions allows engineers to simulate various field scenarios.
• The results obtained from these tests are crucial for making informed decisions in geotechnical engineering practice.

Drainage Conditions and Laboratory Tests

• Different laboratory tests are conducted to evaluate soil behavior under various drainage conditions.
• The choice of drainage condition (undrained or drained) depends on the specific engineering problem and expected field conditions.

Laboratory Tests for Different Drainage Conditions

Unconfined Compression Test

• Determines undrained shear strength (Su) of cohesive soils.
• Provides a quick assessment of undrained shear strength.
• Primarily used for cohesive soils.

Direct Shear Test

• Determines both undrained and drained shear strengths for granular soils.
• Widely used for granular soils.
• Provides shear strength parameters under different drainage conditions.

Triaxial Test

• Evaluates undrained shear strength of cohesive soils under confining pressure (Consolidated Undrained (CU) Triaxial Test).
• Assesses drained shear strength of cohesive soils under confining pressure (Consolidated Drained (CD) Triaxial Test).
• Evaluates undrained shear strength of cohesive soils without consolidation (Unconsolidated Undrained (UU) Triaxial Test).

Rowe Cell Test

• Measures compressibility and permeability of fine-grained soils under different drainage conditions.
• Suitable for assessing compressibility and permeability of fine-grained soils.

Oedometer Test

• Evaluates consolidation characteristics of soils under different drainage conditions.
• Provides data for settlement predictions and soil consolidation behavior.

Merits and Demerits of Shear Strength Tests

Unconfined Compression Test

• Merits: simplicity, quick assessment, and common use.
• Demerits: limited applicability, neglects drainage conditions.

Direct Shear Test

• Merits: versatility, ease of setup, and provision of shear strength parameters.
• Demerits: assumption of homogeneity and plane of failure.

Triaxial Test

• Merits: versatility, stress path simulation, and comprehensive results.
• Demerits: complex setup, labor-intensive, and time-consuming.

Rowe Cell Test

• Merits: suitable for fine-grained soils, controlled conditions, and measurement of compressibility and permeability.
• Demerits: limited applicability, specialized equipment required.

Oedometer Test

• Merits: consolidation characteristics, settlement predictions, and soil consolidation behavior.
• Demerits: limited shear strength information, time-consuming.

Pore Pressure Coefficient and Shear Characteristics of Different Soil Types

Pore Pressure Coefficient (A)

• Relates change in pore water pressure to change in effective stress during soil shearing.

Shear Characteristics of Soil Types

Sand

• Drained shear strength: exhibits well-defined drained shear strength characteristics.
• Undrained shear strength: cohesionless, negligible undrained shear strength.

Normally Consolidated (NC) Clays

• Drained shear strength: exhibits distinct drained shear strength characteristics.
• Undrained shear strength: significant undrained shear strength, sensitive to initial conditions.

Over-Consolidated (OC) Clays

• Drained shear strength: similar to NC clays, with cohesion intercept (c) and angle of internal friction (ϕ).
• Undrained shear strength: lower undrained shear strength compared to NC clays, reduced sensitivity.

Partially Saturated Soils

• Pore pressure generation: influenced by degree of saturation and capillary effects.
• Shear strength: lower shear strength compared to fully saturated soils due to air in voids.
• Suction effects: crucial in partially saturated soils, influencing pore pressure generation and shear strength.

Influence of Soil Structure and Strain Rate on Shear Strength

Soil Structure

• Definition: arrangement and organization of soil particles and their aggregates.
• Effects on shear strength: aggregation, particle arrangement, fabric, and cementation.

Strain Rate

• Definition: rate at which deformation occurs in a material.
• Effects on shear strength: rate dependency, thixotropy, strain rate sensitivity, and creep behavior.

Interaction of Soil Structure and Strain Rate

• Anisotropic behavior: combination of soil structure and strain rate can result in anisotropic behavior.
• Rate-dependent strength: soils with complex structures may exhibit rate-dependent strength due to the interplay of particle arrangement, cementation, and thixotropy.