1. Triaxial Tests

Triaxial compression tests can be conducted on sands and clays. The Triaxial test arrangement 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.

Triaxial tests are essential for understanding how soil behaves under stress. In this test, a soil sample is placed in a chamber and is subjected to lateral pressure from the fluid around it. This lateral pressure is symbolized by σ3. Additionally, a vertical stress is applied until the sample fails, indicated by Δσ. Importantly, the test can be performed with or without allowing drainage from the sample, which influences the test conditions and results.

Imagine a balloon. If you fill it with water and then squeeze it from all sides, you're simulating the confining pressure. If you then poke it with a finger to burst it, you're applying additional stress. The way the balloon pops gives insights into how soil behaves under similar conditions.

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 variants of the triaxial test, which differ based on how the soil is allowed to behave during the test:

• Consolidated-drained test (CD test): The soil consolidates (settles under pressure) and drains excess water.
• Consolidated-undrained test (CU test): The soil consolidates but does not drain, meaning pore water pressure builds up.
• Unconsolidated-undrained test (UU test): The soil does not have time to consolidate or drain. Each test provides different information about the soil's strength and behavior under stress.

Think of a sponge. If you press it and allow the water to escape (CD test), it gets smaller and stays dry. If you press it without letting water escape (CU test), it gets tighter but retains water. If you press it quickly without giving it time to react at all (UU test), it might squirt water out but remains very compacted. Each action reveals different insights about the sponge’s material properties.

The shear strength parameters (c and ϕ) can now be determined by plotting Mohr’s circle at failure and drawing a common tangent to the Mohr’s circles. This is the Mohr-Coulomb failure envelope. (Note: For normally consolidated clay, c≈0).

In triaxial tests, the concept of shear strength plays a critical role. By analyzing the stress states during failure, we can derive two important parameters – cohesion (c) and the angle of internal friction (ϕ). We use Mohr’s circle, a graphical representation of the stress states, to visualize these relationships and define a failure envelope. For normally consolidated clay, cohesion is very low, approaching zero.

Envision pulling a rubber band. To find out how much you can stretch it before it snaps, you measure the tension and the angle at which it starts to deform significantly. The point at which it breaks tells you about its strength, similar to how we determine the limits of soil strength using Mohr's circle.

For consolidated-undrained tests, at failure,

• Major Principal total stress = σ3 = Δσf = σ1
• Minor principal total stress = σ3
• Major principal effective stress = (σ3 + Δσf) − uf = σ′1
• Minor principal effective stress = σ3 − uf = σ′3

During consolidated-undrained tests, understanding effective stress is crucial. At the point of failure, total stress consists of both the external stress applied (σ1) and the pore water pressures (uf) within the soil. The effective stress reflects the true stress that contributes to soil strength. For instance, the major principal effective stress represents the total stress minus the pore pressure, impacting the ability of the soil to support loads.

Think of a sponge again. When you squeeze it, water is pushed out – this water inside can weaken the sponge's ability to hold weight (just like pore pressure in soil). The effective stress, in this case, is the pressure your hand applies minus the 'pressure' created by water pushing back, which ultimately tells you how much weight the sponge can support.

For unconsolidated-undrained triaxial tests, the major principal total stress = σ3 = Δσf = σ1 and minor principal total stress = σ3. The total stress Mohr’s circle at failure can now be drawn. For saturated clays, the value of σ1−σ3 = Δσf is a constant, irrespective of the chamber confining pressure, σ3.

In unconsolidated-undrained tests, the behavior of the soil can be quite different. Here, the major principal total stress is equal to the difference between the stresses (Δσf), and this remains consistent across varying confining pressures. This uniformity simplifies analysis, allowing for a straightforward representation using Mohr's circles. Essentially, the soil’s behavior under rapid loading without drainage is characterized, which is critical in many real-world scenarios involving saturated soils.

Imagine pouring water over a sponge quickly. If you press down on it without letting the water escape, no matter how firmly you press (the confining pressure), the sponge will maintain a constant internal pressure from the trapped water. This consistent response indicates its behavior under stress, similar to that of saturated clay in these tests.

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