1.2. Consolidated-drained test (CD test)

Triaxial 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. Drainage from the specimen can be allowed or stopped, depending on the test condition.

A consolidated-drained (CD) test is a type of triaxial test where a soil specimen is contained within a chamber. This chamber is designed to apply uniform pressure around the specimen (denoted as σ3). The key aspect is that the drainage is permitted, which means any pore water generated within the soil can flow out during loading. This is important as it helps in measuring the effective stress in the soil, allowing for accurate evaluation of its strength properties.

Think of the CD test as a sponge that you place in a container filled with water. As you squeeze the sponge (applying pressure), the water can drain out, mimicking how water drains from saturated soil when pressure is applied. This drainage affects how strong the sponge (or soil) becomes during the squeezing process.

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).

In a CD test, the confining pressure (σ3) is essential because it simulates the pressures that soil experiences when buried under layers of material. Along with this, an additional stress (Δσ) is applied in the vertical direction which ultimately leads to soil failure. The relationship Δσ=Δσf indicates that when we want to fail the soil, the applied stress must equal the stress at failure (Δσf). This helps in understanding how different soil types react under pressure.

Imagine stacking books on a table, where each book represents a layer of soil. The weight of the books above (confining pressure) creates pressure on the book at the bottom. If you suddenly add a heavy object on top (additional stress), the bottom book could crack or break (soil failure). This illustrates how increasing pressures can lead to structural failure in soils.

The shear strength parameters (c and ϕ) can now be determined by plotting Mohr’s circle at failure, as shown in the figure, and drawing a common tangent to the Mohr’s circles.

During the test, we can derive important shear strength parameters known as cohesion (c) and angle of internal friction (ϕ) using graphical methods. By plotting Mohr's circle for stress states before and after failure, we can visualize how the soil behaves. A tangent drawn to these circles establishes what is known as the Mohr-Coulomb failure envelope – a graphical representation that helps predict failure conditions for soil.

Consider drawing a curve to connect points where a ball rolls down different slopes. The slope represents the soil's ability to hold together under stress. The more curves you draw, the better you can predict how the ball (or in this case, the soil) will behave under various pressures, similar to how Mohr's circles help us visualize soil strength and stability.

At failure, the relationships between major principal stresses are defined as Major Principal effective stress =σ3=Δσf=σ1 and Minor Principal effective stress =σ3=Δσ′3.

Understanding effective stress is crucial in soil mechanics. It defines how much stress the soil skeleton (actual structure of the soil) can bear after subtracting pore water pressures. At failure, the major principal effective stress is given, showing the relationship between the confining pressure, added stress at failure, and the total stress acting on the soil. It outlines how these stresses work together to influence soil behavior under load.

Imagine you are standing on a sponge that is filled with water. The weight of your body exerts pressure onto the sponge, but some of that pressure is taken up by the water inside the sponge (pore pressure). The effective stress which acts to compress or solidify the sponge is what remains after considering how much pressure the water is taking. Thus, knowing the effective stress helps us understand how soils behave under loads.

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