14. Resistencia al corte

Summary

This video discusses the principles of soil mechanics, focusing on consolidated undrained testing. It explains the relationships between total, effective, and deviatoric stresses, and how these influence the failure process in materials like clays and sands. Key concepts such as pore pressure, effective stress, and shear strength are explored, with emphasis on how different sample types (dense and loose) respond to loading. The video also highlights the importance of understanding soil behavior for slope stability and short-term structural analysis in geotechnical engineering.

Takeaways

• 😀 The relationship between pore pressure and deviatoric stress is crucial in the failure analysis during the consolidation phase of soil testing.
• 😀 Effective principal stress is calculated by subtracting the pore pressure from the total principal stress in the failure zone.
• 😀 In consolidated undrained conditions, cohesion is null, and shear strength depends on the effective normal stress and the effective friction angle.
• 😀 The failure envelopes are calculated using effective stresses, and shear strength is given by the effective normal stress multiplied by the tangent of the effective friction angle.
• 😀 During undrained triaxial testing, pore pressure increases as deviatoric stress is applied until failure occurs.
• 😀 In undrained testing, the pore pressure inside the sample is influenced by both the skeleton parameter and the deviatoric stress.
• 😀 Undrained tests are particularly useful for analyzing saturated clays, where the dissipation of excess pore pressure takes longer than construction time.
• 😀 The response in undrained triaxial tests can resemble simple compression tests where friction angles are negligible, and resistance is largely attributed to cohesion.
• 😀 Dense or stiff samples exhibit peak resistance but soften afterward, while loose samples show low resistance and typically contract.
• 😀 Both dense and loose samples eventually reach a critical state where deformation occurs at constant volume and stress, known as the critical state.

Q & A

What is the relationship between pore pressure and deviatoric stress in the context of triaxial tests?

The relationship is that the total major principal stress at failure corresponds to the deviatoric stress applied during the test. The effective stress is calculated by subtracting the pore pressure from the total stress, which is essential for determining shear strength in these tests.

What is the significance of effective stress in consolidated undrained triaxial tests?

Effective stress is crucial in consolidated undrained triaxial tests as it helps determine the shear strength of the material. The effective stress is obtained by subtracting the pore pressure from the total applied stress, which is then used to calculate the shear strength and the friction angle.

How does pore pressure affect the shear strength in consolidated undrained tests?

Pore pressure affects shear strength by influencing the effective stress. During the test, as pore pressure increases, it reduces the effective stress, which directly impacts the shear strength of the material, as shear strength is typically calculated using effective stress principles.

What does the failure envelope represent in these tests, and how is it derived?

The failure envelope represents the relationship between normal stress and shear stress at failure. It is derived by considering effective stresses and the friction angle, where the shear strength is calculated by multiplying the normal stress by the tangent of the effective friction angle.

Why is the friction angle often considered negligible in consolidated undrained tests on saturated clays?

In consolidated undrained tests on saturated clays, the friction angle is often negligible because the material's shear strength is mainly governed by cohesion rather than friction. This is particularly true for saturated clays where the pore pressure is high and significantly influences the shear strength.

What is the role of pore pressure during the undrained axial loading test?

During the undrained axial loading test, pore pressure increases as the deviatoric stress is applied. This is because there is no drainage allowed, and the pore pressure buildup influences the failure behavior, contributing to the material's shear strength response under stress.

How do dense and loose materials behave differently in these types of tests?

Dense materials exhibit peak strength during the test, followed by softening, and often expand in volume. Loose materials, however, do not exhibit a peak strength and typically contract in volume. Both types eventually reach a 'critical state' where they deform at constant volume and constant stress.

What is the 'critical state' in soil testing, and how does it affect material behavior?

The critical state is the condition where the soil reaches a stable state of deformation, with constant volume and stress. This state is achieved by both dense and loose materials, where further deformation no longer causes significant changes in volume or stress.

Why are undrained triaxial tests particularly useful for clay and short-term slope stability analysis?

Undrained triaxial tests are useful for saturated clays because they provide insights into the shear strength of the material when pore pressure dissipation is slow. This is especially relevant for short-term slope stability analysis, where pore pressure buildup and dissipation can affect the material's stability in the short term.

How does the behavior of sand in undrained tests differ from that of clays?

In undrained tests, sand typically exhibits an increase in shear strength up to failure and then stabilizes, reaching a residual strength. In contrast, clays often rely more on cohesion rather than friction, and their shear strength is largely governed by this cohesive strength in undrained conditions.