Geotechnical Engineering

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.

Disturbed soil sample is one in which the structure of the soil has been changed sufficiently that tests of the structural properties of the soil will not be representatives of in-situ conditions, and only properties of soil grains can be accurately determined.

Undisturbed soil sample is one where the condition of the soil in the sample is close enough to the conditions of the soil in in-situ to allow tests of structural properties of the soil to be used to approximate the properties of the soil in-situ.