Tipos de ensayo triaxial [CD, CU, UU] - Curso Virtual Geotecnia

Introduction to Triaxial Testing

Overview of Triaxial Tests

• The discussion begins with an introduction to triaxial tests, emphasizing the importance of consolidation and drainage conditions in determining the type of test conducted.

• Three types of triaxial tests are identified: Consolidated Drained (CD), Consolidated Undrained (CU), and Unconsolidated Undrained (UU).

Key Components of Triaxial Tests

• The first letter in the test name indicates the consolidation stage, which must replicate field conditions such as pressure exerted on soil samples.

• The second letter relates to how quickly load is applied during testing, affecting shear stress generation; slow application does not generate excess pore pressure while rapid application does.

Test Setup and Parameters

• The setup involves understanding how different parameters like chamber pressure (sigma 3) affect sample consolidation and behavior under load.

• Color coding is used for mnemonic purposes to help remember different aspects related to each test type.

Understanding Stress States in Soil

Application of Chamber Pressure

• The initial application of sigma 3 during testing can either fully or partially consolidate a soil sample, impacting its response under stress.

Deviatoric Stress Application

• The second aspect focuses on applying deviatoric stress, crucial for generating principal stress states within the soil sample.

Role of Drainage Valves

Importance of Drainage Control

• Opening or closing drainage valves during testing simulates porous stone effects, allowing measurement through pore pressure transducers.

Measurement Techniques

• Modern equipment uses pore pressure transducers to measure changes in pore water pressure during loading phases.

Excess Pore Pressure Analysis

Quantifying Excess Pore Pressure

• Excess pore pressure can be quantified based on contributions from both consolidation and deviatoric loading stages.

Historical Context

• Sir Alex Skempton's contributions from the 1950s are highlighted; he was a notable figure who advanced understanding in this area through his work at Harvard and Imperial College London.

General Equation for Pore Pressure

Deriving the General Equation

• A general equation for excess pore pressure is introduced, dependent on specific parameters related to chamber stress changes.

Saturation Conditions Impact

Understanding Consolidation and Pore Pressure Analysis

Key Concepts in Pore Pressure Parameters

• The discussion begins with the importance of pore pressure parameters, which are crucial for understanding soil behavior under various conditions. These parameters represent the pressure exerted by fluids within the soil pores.

• The analysis of flow and pore pressures is essential for evaluating stability in embankments and slopes, particularly under drained or undrained conditions. This knowledge is vital for geotechnical engineering applications.

• A specific formula involving delta values (Δu and Δσ) is introduced, indicating that these values are functions of effective stress (σ3). Understanding this relationship is key to grasping triaxial test results.

Triaxial Test Insights

• The speaker emphasizes the distinction between total stresses and effective stresses during triaxial testing. Recognizing this difference helps clarify how excess pore pressure influences soil behavior.

• A question arises regarding whether certain stress values are total or effective; it’s confirmed that they are total stresses, highlighting the need to differentiate between these concepts in practical applications.

Consolidated Drained Test Overview

• The first test discussed is the consolidated drained test, which involves two main stages: consolidation and deviator stress application. Mnemonic devices are suggested to remember these stages effectively.

• An example illustrates a scenario where a 2-meter deep profile has a sample at 1.5 meters depth subjected to an applied load of 10 kPa. This setup serves as a practical illustration of consolidation principles.

Load Application Dynamics

• As time progresses during consolidation, water initially bears the load while soil remains unloaded until sufficient time passes for drainage to occur.

• The concept of maintaining constant load over time is emphasized; this allows observation of how water releases its burden on the soil structure during consolidation processes.

Effects of Consolidation Over Time

• During initial loading phases, water takes on all applied loads until consolidation occurs over time, leading to increased stress on the soil as water drains out.

• When reaching approximately 100% degree of consolidation, excess pore pressure becomes negligible (approaching zero), indicating complete transfer of load from water to soil particles.

Final Observations on Drained Conditions

• In fully consolidated conditions during drained tests, excess pore pressure equals zero due to full drainage; thus total stresses equal effective stresses throughout the process.

Understanding Effective Stress and Direct Shear Tests

Concept of Effective Stress

• The unique failure envelope is defined by effective stresses being equal to total stresses, allowing for the determination of resistance parameters such as cohesion and effective friction angle.

Direct Shear Test Overview

• The direct shear test involves consolidating a sample within a shear box where load is applied slowly, yielding effective resistance parameters or drained parameters with zero pore pressure generation.

Stages of Consolidation in Testing

• In the first stage of consolidation, there is no excess pore pressure generated; thus, it remains stable during this phase.

• If consolidation occurs without drainage, there will be no excess pore pressure due to the slow application of deviator stress.

Final Stage Analysis

• The final stage combines both total and effective stress states, allowing for an understanding of how excess pore pressures affect overall stability.

Parameters Derived from Testing

• Two conditions arise from testing: one based on effective stresses (yielding effective cohesion and friction angle), and another based on total stresses (yielding total cohesion and friction angle).

Importance of Friction Angles in Testing

• It’s crucial that the effective friction angle derived from tests should always exceed the total friction angle; if not, it indicates a significant error in calculations.

Cohesion Considerations in Overconsolidated Materials

• For overconsolidated materials, it's expected that total cohesion exceeds effective cohesion. This serves as a critical check during testing procedures.

Significance of Total vs. Effective Envelopes

• The triaxial test provides insights into both total and effective conditions; however, focus should remain on the total envelope since the effective envelope may not accurately represent soil behavior.

Excess Pore Pressure Dynamics

• When samples are not allowed to consolidate properly before loading, excess pore pressures can develop rapidly during testing phases.

Conclusion on Envelope Representations

Understanding Soil Behavior Through Triaxial Testing

Key Concepts of Circle Diagrams in Soil Mechanics

• The discussion begins with the introduction of circle diagrams, highlighting that all circles have the same diameter, which is crucial for understanding soil behavior under stress.

• The deviatoric stress is defined as the load on the piston divided by the corrected area, emphasizing that all circles maintain equal diameters and thus experience uniform deviatoric stress.

• It is noted that regardless of confinement conditions or applied loads, all circles fail at the same deviatoric stress level, indicating a lack of dependency on external factors like confinement.

• The speaker explains that since all circles fail at the same deviatoric stress, their diameters do not increase with higher stresses, contrasting this with other testing methods where larger diameters correspond to increased stresses.

• This leads to a conclusion about drawing an envelope from these points: it results in a horizontal line indicating that maximum shear occurs at the topmost point of each circle.

Implications for Shear Strength and Cohesion

• The horizontal envelope suggests that maximum shear strength occurs without any inclination; hence, friction angle equals zero. All resistance relies solely on cohesion.

• In undrained conditions during testing, resistance becomes entirely dependent on cohesion rather than friction angle due to failure occurring uniformly across samples.

• The speaker emphasizes how this concept relates to practical applications in soil mechanics and engineering practices regarding undrained strength assessments.

Triaxial Test Conditions and Their Effects

• A distinction is made between confined compression tests where confining pressure is negligible versus unconfined tests where maximum effort correlates directly with radius measurements affecting undrained resistance.

• It’s explained that triaxial tests combine loading conditions and drainage states; slow loading allows for effective drainage while rapid loading can lead to excess pore pressures affecting test outcomes.

• If drainage exists during slow loading applications, parameters derived will be effective drained parameters; conversely, rapid application may yield total parameters influenced by excess pore pressures.

Practical Applications in Engineering

• Understanding these principles helps engineers determine soil capacity under various conditions—whether total or effective—critical for foundation design and stability analysis.

• Emphasis is placed on recognizing when to apply total versus effective parameters based on loading rates and drainage capabilities during testing scenarios.

• Engineers must balance between achieving effective parameter envelopes while also considering total parameter implications when assessing material behavior under critical conditions.