Geotechnical Engineering: Shear Tests Quiz
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Questions and Answers

What is a significant disadvantage of the direct shear test when evaluating soil strength?

  • It can be conducted in saturated conditions.
  • It is not suitable for completely drained conditions. (correct)
  • It provides better control over drainage compared to triaxial tests.
  • It allows for the rotation of principal stresses during the test. (correct)

Which of the following is an advantage of the triaxial shear test (TST) compared to the direct shear test (DST)?

  • It allows for better control of applied stresses. (correct)
  • It requires a larger quantity of soil sample.
  • It simplifies the measurement of shear strength.
  • It provides lesser control over drainage.

What is a key feature of a triaxial test device?

  • It allows soil specimens to be loaded in various ways. (correct)
  • It requires no drainage control during testing.
  • It applies stresses without confinement.
  • It uses only unconfined compression.

How does the triaxial shear test address the issue of stress types imposed on soil specimens?

<p>By increasing control over applied stresses. (B)</p> Signup and view all the answers

What is NOT a characteristic of the triaxial shear test when compared to the direct shear test?

<p>Lesser versatility in testing conditions. (A)</p> Signup and view all the answers

What does the Mohr–Coulomb failure criterion primarily define?

<p>Failure criteria for soil (D)</p> Signup and view all the answers

In the equation 𝜏𝑓 = 𝑐 + 𝜎 tan ∅, what does the symbol 𝜏𝑓 represent?

<p>Shear strength (C)</p> Signup and view all the answers

What role does the angle ∅ play in the context of shear strength in the Mohr–Coulomb criterion?

<p>It indicates the angle of internal friction. (A)</p> Signup and view all the answers

Which laboratory test is used to determine the shear strength parameters of soils?

<p>Direct shear test (A)</p> Signup and view all the answers

In the Mohr–Coulomb failure criterion, how is the normal stress represented?

<p>𝜎 (A)</p> Signup and view all the answers

What is the significance of the cohesion (c) in the Mohr–Coulomb failure criterion?

<p>It provides a measure of internal resistance to shear. (A)</p> Signup and view all the answers

During a direct shear test, what is applied through a metal platen?

<p>Normal stress (D)</p> Signup and view all the answers

What is the primary purpose of using the Mohr–Coulomb failure envelope?

<p>To visualize the relationship between stress and failure (B)</p> Signup and view all the answers

What is the purpose of applying shear force during a direct shear test on soil?

<p>To cause failure in the soil sample (B)</p> Signup and view all the answers

Which factor must be considered when conducting direct shear tests on clays?

<p>Pore pressures must dissipate (A)</p> Signup and view all the answers

What is one advantage of the direct shear test for granular materials?

<p>It allows for rapid drainage due to sample thickness (C)</p> Signup and view all the answers

What is a potential disadvantage of the direct shear test?

<p>Failure occurs along a predetermined failure plane (C)</p> Signup and view all the answers

What does the term 'drained' mean in the context of direct shear tests?

<p>Pore pressures are allowed to dissipate (C)</p> Signup and view all the answers

How is time for failure set in direct shear tests on clays?

<p>It is determined based on primary consolidation time (A)</p> Signup and view all the answers

Which of the following is a reason for reversing shear direction in a direct shear test?

<p>To determine the residual strength of the soil (B)</p> Signup and view all the answers

What aspect of stress conditions does the direct shear test struggle to accurately assess?

<p>The pore pressures within the specimen (A)</p> Signup and view all the answers

What is the primary function of the drainage valves during the Consolidated-Undrained (CU) Triaxial Test?

<p>To be open during consolidation but closed during shearing (B)</p> Signup and view all the answers

In a CU Triaxial Test, how do the total and effective stress parameters relate?

<p>cT does not equal c' and T does not equal ' (A)</p> Signup and view all the answers

What does the UU Triaxial Test primarily measure in cohesive soils?

<p>Undrained shear strength (D)</p> Signup and view all the answers

Which assumption is critical for the UU Triaxial Test regarding the soil sample's condition?

<p>The sample must be completely saturated (A)</p> Signup and view all the answers

In the CU Triaxial Test, when does loading of the sample begin?

<p>Once the sample has consolidated (B)</p> Signup and view all the answers

What type of test is the UU Triaxial Test commonly referred to as?

<p>Quick test (C)</p> Signup and view all the answers

What happens during the unconsolidated condition of the triaxial shear test?

<p>No volume change occurs, leading to excess pore pressure. (D)</p> Signup and view all the answers

The failure envelope from CU tests on which type of soil can differ significantly from that of normally consolidated clay?

<p>Overconsolidated clay (D)</p> Signup and view all the answers

Which triaxial test provides both total and effective strength parameters?

<p>Consolidated – Undrained Test (CU). (B)</p> Signup and view all the answers

What is one of the critical assumptions about the effective stress of the soil sample in the UU test?

<p>It should be equal to its effective stress in the ground (B)</p> Signup and view all the answers

What characterizes the Consolidated-Drained (CD) Triaxial Test?

<p>Sample drainage is completed before applying vertical load. (C)</p> Signup and view all the answers

In which phase of the triaxial test is no excess pore pressure generated?

<p>During the shearing phase in drained conditions. (A)</p> Signup and view all the answers

What characterizes a consolidated-drained (CD) triaxial test?

<p>It is performed slowly to avoid excess pore water pressure generation. (B)</p> Signup and view all the answers

Which statement about the CD Triaxial Test is true?

<p>It can take up to 2 weeks for consolidation to be complete. (A)</p> Signup and view all the answers

What is the main effect of closing the drainage valve during the shearing stage?

<p>It results in an increase in excess pore pressure. (C)</p> Signup and view all the answers

What happens during the shearing phases of the CD Triaxial Test?

<p>Effective and total stresses remain equal. (C)</p> Signup and view all the answers

Which type of triaxial shear test is known as a quick test?

<p>Unconsolidated – Undrained Test (UU). (C)</p> Signup and view all the answers

Which of the following accurately describes the purpose of the Consolidated-Undrained (CU) Triaxial Test?

<p>It measures the short-term behavior of soils with excess pore pressure. (C)</p> Signup and view all the answers

What is a common feature of the Consolidated – Drained (CD) triaxial test?

<p>It allows volume change to occur without excess pressure. (A)</p> Signup and view all the answers

What occurs when excess pore pressures are present during the CU Triaxial Test?

<p>Undrained failure envelopes are generated. (A)</p> Signup and view all the answers

In the context of the CU Triaxial Test, what does the drained failure envelope represent?

<p>Soil behavior after excess pore pressure dissipates. (A)</p> Signup and view all the answers

What differentiates a Consolidated – Undrained (CU) test from a Consolidated – Drained (CD) test?

<p>CU tests measure both total and effective strength parameters. (B)</p> Signup and view all the answers

How do effective and total stresses compare during a CD Triaxial Test?

<p>They are equal due to no excess pore pressure. (D)</p> Signup and view all the answers

What is a primary feature of the CD Triaxial Test compared to other tests?

<p>It fully drains the soil before applying load. (C)</p> Signup and view all the answers

Flashcards

Shear Strength of Soil

The force a soil can withstand before failing due to shear stresses. It's the critical point where the soil breaks down and loses its strength.

Mohr-Coulomb Failure Criterion

A fundamental theory used to predict soil failure under different stress conditions. It describes the relationship between normal stress, shear stress, and the soil's inherent strengths (cohesion and friction).

Cohesion (c)

The ability of soil particles to stick together. It represents the internal resistance to shearing when there's no normal stress applied.

Angle of Internal Friction (Φ)

The angle of internal friction represents the resistance to slipping between soil particles caused by their interlock. It's measured in degrees and is a crucial factor in soil stability.

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Coulomb's Equation (τf = c + σ tan Φ)

A simplified equation describing the shear strength of soil using the Mohr-Coulomb failure criterion. It considers both cohesive and frictional components.

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Direct Shear Test

The shear strength of soil is determined by applying a controlled vertical load and then measuring the horizontal force needed to cause failure.

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Direct Shear Test Procedure

A specific test to determine the soil's shear strength in a direct shear test. It helps evaluate the soil's ability to resist shearing forces under various stress conditions.

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Mohr-Coulomb Failure Criterion in terms of Effective Stress

The Mohr-Coulomb failure criterion is modified to account for the effects of pore water pressure on soil strength. This is crucial in understanding soil behavior under saturated conditions.

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Shear strength

The force needed to cause a soil sample to fail under shear stress. It is dependent on the normal stress.

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Normal stress

The stress applied perpendicular to the shearing plane of the soil sample.

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Shear stress (τ)

The stress applied parallel to the shearing plane of the soil sample, causing the soil to deform.

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Shear strength envelope

The relationship between the shear stress needed to cause failure and the normal stress. This is represented by a graph.

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Drained direct shear test

A direct shear test where time for failure is set to be 7 times the consolidation time. This allows for pore water pressure to dissipate during shearing.

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Residual shear strength

This occurs when the shear direction is reversed, resulting in a lower shear strength. This is useful for understanding the resilience of the soil.

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Predetermined failure plane

A disadvantage of the direct shear test, where the failure occurs along a predetermined plane which may not be the weakest plane.

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Triaxial Shear Test (TST)

A laboratory test measuring a soil sample's resistance to shear forces under controlled conditions.

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Direct Shear Test (DST)

A test method to determine a soil sample's shear strength. It involves applying a normal (vertical) stress and a shear stress to the sample, measuring the force needed to cause failure.

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Why is the Triaxial Shear Test preferred over the Direct Shear Test?

A key advantage of the Triaxial Shear Test over the Direct Shear Test. It allows for controlled drainage conditions.

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What is a key limitation of the Direct Shear Test?

A limitation of the Direct Shear Test, it restricts the type of stress that can be applied, leading to less accurate results.

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What is a Triaxial Device?

A pressurised chamber where a soil sample is loaded in various ways. Most commonly, the soil is compressed axially.

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Consolidation Stage in Triaxial Test

The first stage of a triaxial test where the specimen is subjected to a confining pressure to simulate in-situ conditions. This stage allows the soil to consolidate, meaning it adjusts its volume to the applied pressure.

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Unconsolidated (Undrained) Condition in Triaxial Test

A drainage condition during the consolidation stage where the drainage valve is closed, preventing any volume change. This results in excess pore pressure building up within the soil.

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Consolidated (Drained) Condition in Triaxial Test

A drainage condition during the consolidation stage where the drainage valve is open, allowing for volume change and water to escape. This prevents excess pore pressure from building up.

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Shearing Stage in Triaxial Test

The second stage of a triaxial test where the specimen is subjected to a shear stress while applying a confining pressure. This stage simulates the actual condition of the soil under load.

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Undrained Condition during Shearing Stage

A drainage condition during the shearing stage where the drainage valve is closed. This prevents any volume change, allowing for the generation and measurement of excess pore pressure.

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Drained Condition during Shearing Stage

A drainage condition during the shearing stage where the drainage valve is open, allowing for volume change. This prevents excess pore pressure from building up.

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Consolidated-Drained (CD) Triaxial Test

A common type of triaxial test where both consolidation and shearing stages are performed under drained conditions, meaning water can freely escape. This test reveals the effective shear strength parameters of the soil.

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Consolidated-Undrained (CU) Triaxial Test

A type of triaxial test where both consolidation and shearing stages are performed under undrained conditions, meaning water cannot escape. This test reveals both the total and effective shear strength parameters of the soil.

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Unconsolidated-Undrained (UU) Triaxial Test

A triaxial test conducted on a soil sample without allowing it to consolidate before shearing. Drainage valves are closed throughout the test. This simulates a rapid loading condition, similar to an earthquake.

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Critical State Line

The critical state line represents the boundary between the normally consolidated and overconsolidated states of clay. It helps predict the behavior of clay during shear loading.

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Constant Confining Stress

This occurs when the confining pressure is kept constant during the shear test. Allows us to determine the shear strength at different confining pressures.

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Total Stress Strength (cT, ΦT)

The shear strength of soil in a CU test is measured under total stresses, taking into account both effective stresses and pore water pressure.

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Effective Stress Strength (c', Φ')

The shear strength of soil in a CU test is measured under effective stresses, taking into account only the stresses carried by the soil particles.

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Normal stress (σ)

The stress applied perpendicular to the shearing plane of the soil sample. It represents the weight or pressure acting on the soil in a direction normal to the failure plane.

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Slow Strain Rate (CD Test)

The load applied at such a slow strain rate that particle readjustments in the specimen do not induce any excess pore pressure. Allows for complete drainage and simulates long-term soil behavior.

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Excess pore pressure (u)

The pressure exerted by water trapped within the pores of the soil sample. Excess pore pressure arises during rapid loading or changes in stress conditions.

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Effective stress (σ')

The stress that acts on the soil grains, considering the effects of pore pressure. It's the effective stress that governs the soil's strength and behavior.

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Study Notes

Soil Mechanics CE342 - Fall 2024

  • This course covers soil mechanics, specifically shear strength of soil.
  • Mohr-Coulomb failure criterion is a generally accepted theory for soil failure.
  • Shear strength (τf) is defined as the cohesion (c) plus the normal stress (σ) times the tangent of the internal friction angle (φ).
    • τf = c + σ tan φ
  • Different lab tests exist for determining shear strength parameters, such as direct shear tests and triaxial shear tests.
  • Direct shear tests are inexpensive, fast, and simple for granular materials.
    • Samples are sheared at various normal stress values.
    • Drainage can be achieved quickly.
    • Useful for determining residual strength.
  • Triaxial tests are more versatile and can better control drainage conditions.
    • They don't have the same stress rotation issues as direct shear tests.
    • More control over applied stresses.
  • Lab tests include direct shear, triaxial, simple shear, torsional/ring shear, hollow cylinder, plane strain triaxial, vane and fall cone tests.
  • Field tests include standard penetration tests, pressuremeters, vane shear tests, pocket penetrometers, and static cone penetrometers.
  • In laboratory testing, a soil sample can fail in two ways:
    • Way 1: Increasing normal stress (σ1) to failure with confining stress (σ3) constant.
    • Way 2: Applying normal stress (σ1) and then holding it constant while applying shear stress until failure.
  • Different types of triaxial tests exist, such as:
    • Consolidated-drained (CD) tests.
    • Consolidated-undrained (CU) tests.
  • Unconsolidated-undrained (UU) tests.
  • Unconfined compression tests.
  • For normally consolidated clays, the failure envelope equation is:
    • T( f= σ'( tan φ').
  • Different equations can be used for various soil types like (cohesionless, overconsolidated clay) & (normally consolidated clay).
  • Different methods for estimating undrained shear strength (Su) exist.
  • Example 12.1 shows calculation of shear and normal stresses in a direct shear test.
  • Example 12.2 shows another example calculating peak and residual shear strength from direct shear tests.
  • Example 12.3, 12.5, 12.6, 12.7, 12.10 and many more show more examples of calculations of various parameters using different tests methods.
  • Both direct shear and triaxial tests can be used to determine strength parameters.

Mohr-Coulomb Failure Criterion

  • It's a model to define failure in materials.
  • It's used with soil, particularly important for understanding shear strength.
  • Key parameters to understand include cohesion & angle of internal friction.

Shear Strength of Soil

  • Understanding how soil resists failure under stress.
  • Factors like normal stress, internal friction, and cohesion affect shear strength.
  • Many laboratory and field tests exist to precisely determine it.

Other Methods for Estimating Undrained Shear Strength

  • Methods for estimating undrained shear strength (Su) include the Torvane method and pocket penetrometer method.
  • Vane shear tests are helpful for obtaining estimates of the shear strength quickly in the field and are used extensively in the field for this purpose.

Sensitive Soils and Quick Clays

  • Some soil types show a large difference between their undisturbed and remolded shear strength.
  • This sensitivity can lead to quick clays.
  • Quick clays are soils where any slight disturbance can lead to very sudden and substantial failure.

Thixotropy of Clay

  • The ability of some clays to regain strength after disturbance.
    • The strength recovery occurs over time.

Stress Path (p'-q' Diagram)

  • An alternative way to visualize stress-strain behavior in soil.
  • It simplifies visualization by visualizing stress paths.
  • Avoiding the tedious process of drawing Mohr's circles.

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Related Documents

CE342 Lecture Notes - Shear Strength of Soil PDF

Description

This quiz explores key concepts related to direct shear tests and triaxial shear tests in geotechnical engineering. You'll discover the advantages and disadvantages of each test, as well as their unique features. Perfect for students and professionals looking to enhance their understanding of soil strength evaluation.

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