1.4. Unconsolidated-undrained test (UU test)

For unconsolidated-undrained triaxial tests

Major principal total stress=σ3=Δσf=σ1

Minor principal total stress =σ3

The total stress Mohr’s circle at failure can now be drawn, as shown in figure. For saturated clays, the value of σ1−σ3=Δσf is a constant, irrespective of the chamber confining pressure, σ3. The tangent to these Mohr’s circles will be a horizontal line, called the ϕ=0 condition.

In the unconsolidated-undrained (UU) test, soil samples are subjected to axial stress without allowing any drainage of pore water. This means that the volume of water in the soil remains unchanged while the stress is applied. The key point here is that the major principal total stress, denoted by σ1, is equal to the confining pressure (σ3) plus the change in stress (Δσf). In simpler terms, when we measure the stress just before the sample fails, this principle helps us determine how strong or stable the soil is under these conditions. The minor principal total stress remains equal to the confining pressure (σ3) during this test. The result from these tests can be illustrated using a Mohr's circle, which graphically represents the stresses acting on the soil sample during failure.

Think of a balloon filled with water. When you squeeze the balloon (applying stress), the water inside exerts pressure outwards. If you keep squeezing without allowing any water to escape (like in the UU test), the pressure can reach a point where the balloon bursts. Similarly, in the UU test, the soil can only hold so much stress without allowing water to drain. Just like the balloon's limit before it pops, the soil also has a point of failure that we can study using the UU test.

The shear stress for this condition is

Where B=Skempton′s pore pressure parameter

Similarly, the pore pressure ud is the result of added axial stress, Δσ, so

ud=A Δσ

Where A=Skempton′s pore pressure parameter

However, Δσ=σ1−σ3

Combining equations gives

tu=ua+ud=Bσ3+Aσ1−σ3

The pore water pressure parameter B in soft saturated soils is 1, so

tu=σ3+A(σ1−σ3).

In the UU test, we also consider the shear stress acting on the soil. Shear stress can be understood as the stress that causes layers of soil to slide over each other. The equations provided help calculate the pore pressures within the soil due to applied stresses. The Skempton's pore pressure parameters, A and B, quantify the relationship between the applied stress and the pore water pressure developed within the soil. In particular, A relates the added axial stress to pore pressure, while B relates the confining pressure to pore water pressure. When the pore water pressure parameter B equals 1, it indicates that the soil is fully saturated, leading to a straightforward formula that connects confining stress and the resulting pore water pressure.

Imagine a wet sponge. When you press down on it (adding axial stress), water is forced out, creating pressure inside. The equations are like rules that help us calculate how much water pressure builds up based on how hard we press the sponge. In the UU test, we are analyzing how the pressures within the 'sponge' of soil behave as we change the pressure on the outside. Understanding this helps engineers predict how the soil will react under different conditions.

The value of the pore water pressure parameter A at failure will vary with the type of soil. Following is a general range of the values of A at failure for various types of clayey soil encountered in nature.

When conducting the UU test, the soil type affects the pore pressure parameter A, which plays a significant role in determining the stability and behavior of soil during failure. Different types of clay soils will exhibit varying characteristics with respect to how quickly they respond to stress and how much pore pressure they generate when stressed. This variability is essential for engineers to consider so that they can accurately assess the strength and failure conditions of the soils they are working with.

Think of different kinds of clay you might play with. Some clay is very wet and gooey, making it super easy to mold, while other kinds are dry and stiff, requiring more effort to shape. Similarly, when we apply pressure to different soils, how they respond varies dramatically based on their moisture and consistency. For instance, a wet clay might generate pressure quicker, affecting how it holds together under stress. Understanding these differences in response helps engineers design safer foundations by choosing the right materials for the job.

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