1.1. Types of Triaxial Tests

Triaxial compression tests can be conducted on sands and clays. Essentially, it consists of placing a soil specimen confined by a rubber membrane in a Lucite chamber. An all-round confining pressure (σ3) is applied to the specimen by means of the chamber fluid (generally water or glycerin). An added stress (Δσ) can also be applied to the specimen in the axial direction to cause failure (Δσ=Δσf at failure). Drainage from the specimen can be allowed or stopped, depending on the test condition.

The triaxial test involves a soil sample placed in a chamber that can apply pressure from all sides, using fluids like water or glycerin. This test helps us understand how soil behaves under pressure. We apply a main pressure, called confining pressure (σ3), to the soil sample. Additionally, we can apply a compressive stress (Δσ) in the upward direction to determine how much load the soil can withstand before it fails. Depending on the test condition, such as whether we allow water to drain from the sample, we can gain different insights about the soil's properties.

Imagine trying to squeeze a balloon filled with water while also pushing down on the top. The balloon represents the soil sample. Depending on how you apply pressure and allow water to escape, the balloon will behave differently, similar to how different clay types respond in triaxial tests.

For clays, three main types of tests can be conducted with Triaxial equipment:

1. Consolidated-drained test (CD test)
2. Consolidated-undrained test (CU test)
3. Unconsolidated-undrained test (UU test)

There are three primary types of triaxial tests for clays, each with different conditions for drainage during the test. The Consolidated-Drained test (CD test) allows both consolidation and drainage, providing insights about soil behavior over time. The Consolidated-Undrained test (CU test) allows for consolidation but not drainage, helping us understand how soil behaves under undrained conditions after being consolidated. Finally, the Unconsolidated-Undrained test (UU test) does not allow for either, showing how soil acts when rapidly loaded without having time to consolidate or drain.

Think of these tests like different ways to cook rice. The CD test is like cooking rice in a pot with the lid off, allowing steam (water) to escape. The CU test is like using a sealed pot that can let some steam escape, while the UU test is akin to a tightly sealed pressure cooker where no steam can escape at all. Each method gives you a different outcome based on how the water is managed.

Major Principal effective stress = σ3 = Δσf = σ1 = σ′1 Minor Principal effective stress = σ3 = Δσ′3

In triaxial tests, effective stress is a crucial concept that denotes the stress carried by the soil skeleton. The major principal effective stress (σ′1) is equal to the applied confining pressure (σ3) along with any additional stress until failure (Δσf). The minor principal effective stress (σ′3) relates directly to the confining pressure. Understanding these stresses helps engineers predict the strength and stability of soil under different loading conditions.

Imagine a sponge in a bucket of water. The weight you feel when trying to pick up the sponge is the total force exerted by both the sponge and the water inside it. However, what matters to the sponge's stability (or strength) is the pressure working directly on its structure, similar to effective stresses in soil mechanics.

The shear strength parameters (c and ϕ) can now be determined by plotting Mohr’s circle at failure, drawing a common tangent to the Mohr’s circles. This is the Mohr-Coulomb failure envelope.

Mohr’s Circle is a graphical representation of the states of stress at a point, used to determine shear strength parameters of soil. By plotting the stress conditions (σ - total stress) just before a soil sample fails, we can identify a failure envelope, defined by the cohesion (c) and the internal friction angle (ϕ). This envelope is critical for assessing how soil will behave under different loading configurations.

Visualize a tightrope walker balancing on a rope. The tension they feel from both sides is similar to how forces act on soil. As the tension increases, they reach a point where they can no longer maintain balance (failure). The Mohr-Coulomb failure envelope helps us understand the 'balance' of forces for soil.

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