1.5. Conducting Tests on Unsaturated Soils

The unconfined compression test is a special type of unconsolidated-undrained Triaxial test in which the confining pressure σ3=0. In this test an axial stress, Δσ, is applied to the specimen to cause failure (that is, Δσ=Δσf).

The unconfined compression test is designed to evaluate the strength of soil, specifically clays, without applying any lateral support (confining pressure). In this context, the only applied stress is vertical. This test is crucial because it helps determine how much axial stress a soil can withstand before failing.

To perform this test, a cylindrical soil sample is placed in a compression apparatus where a force is applied until the soil fails. The failure point is characterized as the point where the vertical stress equals the unconfined compression strength, Δσf. Because there’s no lateral pressure, the conditions mimic how unsaturated soils behave in the natural environment.

Imagine pushing down on a soft sponge. If you just push down with your hand (like applying axial stress with no lateral pressure), the sponge will eventually collapse under the weight. But if you were to hold the sponge at the sides, preventing it from expanding, you would be applying confining pressure. The unconfined compression test simulates the first scenario, showing how the sponge represents soil can fail when only vertical stress is applied.

The axial stress at failure, Δσf=qu is generally referred to as the unconfined compression strength. The shear strength of saturated clays under this condition (ϕ=0).

The failure of the soil sample in the unconfined compression test is quantified as the unconfined compression strength, denoted as qu. In saturated clays, the friction angle (ϕ) equals zero during this test, which indicates that the failure occurs primarily due to the soil's cohesion, not frictional resistance. This is significant because it highlights the different behaviors of unsaturated soils compared to saturated ones; understanding these differences is crucial for accurate soil behavior predictions in various construction scenarios.

Think of qu as the maximum amount of weight a cube of sugar can hold before it crumbles under pressure. Just as the sugar's strength depends on its cohesive nature and how it interacts with moisture, the soil's strength is largely influenced by its properties of cohesion in unconfined compression tests.

With the void ratio of a soil specimen remaining constant, the unconfined compression strength rapidly decreases with the degree of saturation.

As soil goes from a dry state to a saturated state, the unconfined compression strength can drastically change. When the soil is unsaturated, there are air pockets present that help maintain structure and strength. However, as saturation increases, those pockets are filled with water, reducing the overall strength because water cannot provide the same structural support as air. Therefore, this test indicates the importance of managing water content in soil for construction and engineering projects.

Imagine a pile of sand mixed with air. When the sand is dry, it is more stable and won't easily topple over. But when you begin to wet the sand, it turns into mud, losing its stability. This situation mimics how unsaturated soils behave: as they become saturated, their strength diminishes, similar to how the sand loses its structure when wet.

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