7.3 Laboratory tests for shear strength (direct shear, triaxial, unconfined compression)
4 min read•august 16, 2024
Shear strength tests are crucial for understanding soil behavior under stress. Direct shear, triaxial, and unconfined compression tests each offer unique insights into soil properties, helping engineers assess stability and design foundations.
These tests measure how soils resist failure when subjected to forces. By analyzing stress-strain relationships and determining key parameters like and , engineers can predict soil performance in various construction scenarios.
Soil Shear Strength Tests
Direct Shear Test
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- Measures shear force required to cause failure along a predetermined plane
- Involves placing soil sample in a and applying normal force
- Suitable for granular soils and predetermined failure plane situations (slope stability analyses, soil-structure interface studies)
- Follows specific ASTM or equivalent international standards for sample preparation, testing procedures, and data analysis
- Outputs stress-strain relationships and failure envelopes used to determine shear strength parameters (cohesion and angle of internal friction)
- Relatively simple and quick to perform compared to other tests
- Limitations include difficulty controlling drainage conditions and non-uniform stress distribution within the sample
- May overestimate friction angles due to forced failure plane
Triaxial Test
- Utilizes cylindrical soil sample enclosed in rubber membrane
- Subjects sample to confining pressure in cell filled with water or oil
- Applies axial stress through loading ram
- Offers versatility in controlling drainage conditions
- Applicable to wide range of soil types for comprehensive strength characterization
- Allows measurement of pore pressure for understanding behavior
- Enables study of stress path effects on soil behavior
- Provides superior control over stress states and drainage conditions
- Requires more complex equipment and longer testing times than direct shear tests
- Allows sample to fail along its weakest plane
- Different test types available (UU, CU, CD) providing information on total and effective stress parameters
Unconfined Compression Test
- Performed on cohesive soil samples without lateral support
- Measures axial stress required to cause failure
- Limited to cohesive soils with sufficient stand-up strength
- Provides quick estimate of undrained shear strength for short-term stability analyses
- Yields unconfined compressive strength (qu) used to estimate undrained shear strength (su) of cohesive soils
- Rapid assessment method for cohesive soils
- Limited applicability to granular materials and saturated clays
Interpreting Shear Strength Test Results
Analysis of Stress-Strain Relationships
- results presented as shear stress vs. horizontal displacement curves for different normal stresses
- results analyzed using Mohr circles and failure envelopes
- Stress-strain curves provide information on soil stiffness, strain at failure, and post-peak behavior
- Essential for understanding soil deformation characteristics
- Shape of failure envelope (linear or curved) offers insights into stress-dependency of soil strength
- Indicates applicability of different failure criteria (Mohr-Coulomb, Hvorslev)
Determination of Shear Strength Parameters
- Failure envelopes from direct shear tests used to determine cohesion and friction angle
- Triaxial tests with different types (UU, CU, CD) provide total and effective stress parameters
- results yield unconfined compressive strength (qu)
- Pore pressure measurements in triaxial tests allow determination of effective stress parameters
- Evaluation of soil behavior under different drainage conditions possible through triaxial testing
Interpretation Considerations
- Consider potential sources of error in result interpretation (sample disturbance, rate effects, boundary conditions)
- Ensure accurate representation of in-situ soil behavior
- Account for sample size effects and boundary conditions influencing test results
- Larger samples generally provide more representative results but require sophisticated testing equipment
Selecting Shear Strength Tests
Soil Type Considerations
- Direct shear tests suitable for granular soils
- Triaxial tests applicable to wide range of soil types
- Unconfined compression tests limited to cohesive soils with sufficient stand-up strength
- For saturated clays, consolidated-undrained (CU) triaxial tests with pore pressure measurements often preferred
Drainage Conditions
- Triaxial tests offer control over drainage conditions
- Drained triaxial tests (CD) suitable for long-term stability analyses of both cohesive and granular soils
- Particularly useful when time-dependent behavior is of interest
Project Requirements
- Choice between stress-controlled and strain-controlled tests depends on soil type and specific engineering application
- Strain-controlled tests often preferred for obtaining post-peak behavior
- Need for anisotropic consolidation or K0 conditions may dictate use of specialized triaxial testing equipment or procedures
Advantages vs Limitations of Shear Strength Tests
Test Efficiency and Simplicity
- Direct shear tests relatively simple and quick to perform
- Triaxial tests require more complex equipment and longer testing times
- Unconfined compression tests provide rapid assessment of undrained shear strength for cohesive soils
Control and Measurement Capabilities
- Triaxial tests offer superior control over stress states and drainage conditions
- Allow measurement of pore pressure for understanding effective stress behavior
- Enable study of stress path effects on soil behavior
- Direct shear and unconfined compression tests limited in controlling drainage conditions
Applicability to Soil Types
- Direct shear tests suitable for granular soils and predetermined failure plane situations
- Triaxial tests applicable to wide range of soil types
- Unconfined compression tests limited to cohesive soils with sufficient stand-up strength
Result Accuracy and Representation
- Direct shear tests may overestimate friction angles due to forced failure plane
- Triaxial tests allow sample to fail along its weakest plane
- Sample size effects and boundary conditions can influence test results
- Larger samples generally provide more representative results but require more sophisticated testing equipment
Key Terms to Review (21)
ASTM Standards: ASTM Standards refer to a set of technical guidelines and specifications developed by ASTM International, which outline the procedures for testing materials and products in various industries. These standards are crucial for ensuring consistency, reliability, and safety in engineering practices, particularly in construction and materials testing.
BS Standards: BS Standards, or British Standards, are officially recognized guidelines and specifications that ensure quality, safety, and efficiency across various sectors in the UK. These standards play a crucial role in laboratory testing methods for shear strength, ensuring consistency and reliability in techniques like direct shear, triaxial tests, and unconfined compression tests.
Clay: Clay is a fine-grained natural soil material that becomes plastic when wet and hardens when dried or fired. This unique property allows clay to play a crucial role in various soil classification systems, soil composition, and structure, as well as settlement calculations, shear strength testing, and slope stability analysis.
Cohesion: Cohesion is the property of soil that describes the attraction between soil particles, which contributes to the soil's strength and stability. This internal binding force is essential in understanding how soil behaves under different conditions, including how it interacts with moisture, external loads, and other forces acting on it.
Compaction: Compaction is the process of densifying soil by reducing the volume of air within its voids through mechanical means, thereby increasing its density and strength. This process plays a critical role in geotechnical engineering by enhancing soil properties, reducing settlement, and improving load-bearing capacity.
Density: Density is a physical property defined as the mass of a substance per unit volume, often expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). It is a crucial factor in understanding how materials behave under various conditions, particularly in geotechnical engineering where it affects compaction processes and shear strength tests.
Direct Shear Test: The direct shear test is a laboratory procedure used to measure the shear strength of soil by applying a horizontal shear force along a predetermined plane. This test helps in understanding how different soil types behave under various loading conditions, and it plays a vital role in evaluating the shear strength parameters that influence stability in geotechnical engineering. The results from the direct shear test can be compared to other tests like triaxial and unconfined compression tests to provide a comprehensive view of soil behavior.
Drained conditions: Drained conditions refer to a state in which soil is able to freely allow water to flow through it, leading to effective stress being solely a function of the applied external loads. In this scenario, excess pore water pressure dissipates quickly, allowing soil particles to interact directly under the influence of these loads. This concept is crucial in understanding shear strength in soils and how different laboratory tests, like direct shear and triaxial tests, behave under varying moisture conditions.
Effective Stress: Effective stress is the stress that contributes to the strength and stability of soil, representing the difference between total stress and pore water pressure within the soil. This concept is crucial in understanding how soil behaves under various conditions, particularly in the context of fluid movement, consolidation, and strength properties of soils.
Failure criterion: A failure criterion is a set of conditions or mathematical equations used to determine the point at which a material or structure will fail under applied loads or stresses. This concept is crucial in geotechnical engineering as it helps predict the behavior of soil and rock materials during various loading conditions, especially when assessing shear strength in laboratory tests.
Friction angle: The friction angle is a measure of the internal resistance of soil to shear stress, represented by the angle at which soil particles can slide past one another. This angle is crucial for understanding how soils respond to external loads, and it plays a vital role in determining the shear strength of soils in various conditions, such as drained and undrained states.
Load Frame: A load frame is a mechanical structure designed to apply controlled loads to a test specimen during laboratory testing, particularly for determining shear strength in geotechnical engineering. It provides a stable and precise means of applying vertical and horizontal forces, ensuring accurate measurement of the specimen's response to various loading conditions. Load frames are crucial for tests like direct shear, triaxial, and unconfined compression as they facilitate consistent and repeatable test conditions.
Moisture Content: Moisture content refers to the amount of water present in a soil sample, expressed as a percentage of the dry weight of the soil. It plays a crucial role in understanding soil behavior and properties, influencing the results of site investigations, strength assessments, and stability analyses. Knowing the moisture content helps in determining the effective stress within the soil and is essential for accurate engineering applications.
Sand: Sand is a granular material composed of finely divided rock and mineral particles, typically defined as having a grain size between 0.0625 mm and 2 mm. It plays a crucial role in soil mechanics, affecting various properties like drainage, compaction, and strength of the soil, making it essential in many engineering and geological applications.
Shear Box: A shear box is a specialized apparatus used to determine the shear strength of soil materials through laboratory testing methods. This device allows engineers to simulate the conditions under which soil fails by applying controlled normal and shear stresses, making it critical for understanding soil behavior under different loading scenarios.
Silt: Silt is a fine-grained soil particle that ranges in size from 0.002 to 0.05 millimeters, falling between sand and clay on the soil texture scale. This particle size plays a significant role in soil behavior, affecting drainage, nutrient retention, and the engineering properties of the soil.
Stress-strain behavior: Stress-strain behavior describes how materials deform when subjected to external forces. In geotechnical engineering, this behavior is crucial as it illustrates the relationship between stress (force per unit area) and strain (deformation resulting from stress) in soils, which directly affects their shear strength. Understanding this relationship helps in evaluating how different soil types respond under varying conditions, particularly during laboratory tests that assess shear strength and how these factors interact with drainage conditions and stress history.
Triaxial cell: A triaxial cell is a laboratory apparatus used to test the shear strength of soil samples under controlled conditions. It allows for the application of pressure in three different directions—horizontally and vertically—simulating the in-situ stresses that soil experiences in the field. This method provides valuable data on soil behavior, particularly when assessing its stability and strength under various loading conditions.
Triaxial test: The triaxial test is a laboratory procedure used to determine the shear strength of soil by applying controlled stress conditions. It involves encasing a soil sample in a rubber membrane and subjecting it to various levels of confining pressure while applying axial stress until failure occurs. This method helps in understanding the behavior of soil under different drainage conditions and is crucial for evaluating soil stability and consolidation.
Unconfined Compression Test: The unconfined compression test is a laboratory procedure used to determine the compressive strength of cohesive soil without any lateral confinement. In this test, a cylindrical soil sample is subjected to axial loading until failure occurs, allowing for the assessment of its shear strength characteristics. This method is particularly significant when understanding how soil interacts with water and its phase relationships, as well as evaluating shear strength under varying conditions such as drainage, soil type, and stress history.
Undrained conditions: Undrained conditions refer to a situation in which the pore water within a soil does not have time to escape during loading or deformation, meaning the effective stress remains unchanged. This condition is significant in understanding how saturated soils behave under short-term loading situations, particularly when drainage cannot occur quickly enough to equalize pore pressures. Undrained behavior is critical for analyzing shear strength in saturated soils, particularly when considering how water content and drainage conditions affect soil stability.