Triaxial Test

A more complicated version of direct shear test which we talked about in the previous post, is called triaxial test, where you can also control the water drainage turning off or on, plus you can measure at what angle the sample will shear, which we will talk about more in the following paragraphs. Both of these tests are among the most common in soil mechanics, to measure soil properties, and mainly Ø. In fact, a triaxial test apparatus is the ultimate and most important test device a soil mechanics lab can have.

Triaxial Test Apparatus

In this test, a sample of cylindrical volume is used, and it is confined from its sides, and compressed. The cylindrical specimen must be carefully obtained form the site, with least possible disturbance. This is usually not possible with sands but it is with clays. After extracting the specimen, it is wrapped in  a rubber membrane and placed in cylinder. Around the specimen is filled with water and the pressure of that water can be controlled, which applies an all round pressure of σ3 at the specimen, from everywhere, including top. We say σ3 because saying σ2 would be redundant as it is also coming from side of the specimen. In addition, with a loading ram from top, a vertical stress is applied, which is σ1. Now that we can control all horizontal and vertical stresses, and also the pore water pressure in the soil (u), we can measure the strength properties of the sample.

Typical Mohr’s Circle obtained from Triaxial test – sands

So in direct shear test, we could not draw the Mohr’s circle, as we knew the failure plane, we dictated it though the plane which we slid the two pieces, and therefore it was not possible to draw a circle. But in triaxial test, we have a cylindrical specimen that can fail from any plane, in other words, it “decides” where it will fail, so we can draw a Mohr’s circle. Each circle here represents a different combination of sigma 1 and sigma 3, in other words σ1 and σ3, which are axial and confining stresses respectively. This is a major advantage to determine angle of internal friction. Because we “force” the failure plane in direct shear test, we usually obtain a little higher values for  Ø. This is because in nature, things always tend to take the easiest path possible, and in triaxial test a lower, more natural value is possible.  

Like we saw in Mohr’s circle subject before, under structural section, nothing can exist above the failure envelope in the graph, in other words, no combination of sigma 1 and sigma 3 and shear stress can happen above this line, for a particular material. If you try to do it, the material will fail and can not take that combination, before you ever reach to that point. In other words, this sloped line is the failure “limit” of that material. Below this line however, any point can be obtained with different combination of sigma 1 and 3. So all points below this line, it means that particular material is able to handle without failure.

The equation of the failure envelope is:

In the graph above, we see everything in this equation, except c. What is c? It means cohesion and for sands they are often zero, so the graph above stared at origin. But for clays c is not zero, in fact it is a major component of their shear stress. So for clays, it can look like this:

Typical Mohr’s Circle obtained from Triaxial test – clay

Another main advantage of triaxial test over direct shear test is that we can control whether the water can escape or not. This makes a big difference. When we do not let water to escape, it is called undrained test. When we let water to escape, it is called the drained test. Both versions make big difference in strength properties. Both can exist in nature. For example, when we consider loading in a short time, we consider undrained conditions, where water did not have time to escape and so we must make undrained test in the lab to simulate those conditions. When we consider long term, where water has time to escape, we consider drained conditions and perform the test accordingly. Drained tests take much, much longer time than undrained tests, as permeability of clay soils are low. Therefore, there are used less often.

Also it is useful to note that at very high stresses, such as at the bottom of dams, sand particles crush and the slope of failure envelope will decrease as in the figure above.

In the next post of this series, we will discuss “Angle of Repose”

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