Triaxial test ;

The triaxial test is one of the most versatile and widely performed tests in the geotechnical laboratory. It allows shear strength and stiffness of soil and rock to be determined for use in geotechnical design.

– Procedure of triaxial test :-

The triaxial test typically involves subjecting a cylindrical specimen of soil, whose diameter ranges from 38mm to 100mm, into a cell that can be pressurized. Most of the specimens have an approximate 2:1 height to dia ratio and are sealed with a rubber membrane. The specimen preparation generally depends upon the type of soil. Cohesive soil samples are prepared directly from saturated compacted samples, either undisturbed or remolded. The specimen for cohesion-less soil is prepared with the help of the mold that maintains the required shape of the specimen.

And then the specimen is vertically covered with a thin rubber membrane and placed between two rigid ends inside a pressure chamber. The upper plate can move vertically and apply vertical stresses to the specimen. The axial stress/strain of the sample is controlled by the movement of this vertical axis. The water pressure surrounding the sample in the pressure chamber controls the confining pressure. Also, the volume change of the sample is controlled by measuring the exact volume of moving water.

There are three primary triaxial test carried out in a laboratory, depending upon the combination of loading and drainage condition ;

• Consolidated – Drained (CD)
• Consolidated – Undrained (CU)
• Unconsolidated – Undrained (UU)

Triaxial test is significant to know soil property because it determines the ability of soil to resist shear stress and strain. Different combination of confining and axial stresses can be applied. Drained and undrained test can be carried out. The types of properties of soil which can be known by triaxial test are followed ;

1. Shear strength and stiffness of soil
2. Stress/Strain behavior
3. Pore water pressure of soil

Here, from a geotechnical perspective you can’t differentiate silt and clay on the basis of grain size because it can be irrelevant to material behaviour.

Silt and clay are both the result of the physical and chemical breakdown of the minerals in rocks. They main difference is in chemical composition and particle size.

Silt is composed of silicate minerals, or those containing silicon and oxygen.

Clay is composed of metal silicates, or silicates with metals like magnesium or aluminum associated with it.

But in terms of handling the two as part of an experiment, the main discernible difference is particle size. Sand particles are larger than silt particles which are in turn larger than clay particles. Silt particles are about 0.05 to 0.002 millimeters in size. Clay particles are smaller than micrometer in size. So silt particles are too small to really be felt with your fingers like grains of sand. But wet silt will feel very smooth like finely ground baking flour. Clay particles are so small that they will simply feel sticky. You won’t be able to move them around your fingers like silt particles. This stickiness will be the main difference between the two in terms of touch.