What is the Unified Soil Classification System?
The Unified Soil Classification System (USCS) is exactly as the name implies, and one of the most important tools available to Madrid Engineering Group and the industry as a whole. USCS is the system used by geotechnical and civil engineers and geologists to describe soils based on their physical engineering properties. This means the soils are described based on the way they behave as construction materials, and not necessarily on the way they look or how they were formed. A geologist or soil scientist doing pure geology would typically want to describe a soil based on how it was formed, where it comes from, and what it is made of. A geotechnical or civil engineer needs to know how a soil will behave, most commonly based on the internal friction, the cohesiveness or stickiness, and the relative amount of various size particles or gradation of the soil.
Sandy Soils
Internal friction applies to sandy soils. Think of a column of soil, confined on all lateral faces. The sand grains at the top are held up by the friction between all the particles below. Typically, a sandy soil gets its strength only from friction between particles. The internal friction, measured as an angle, can be determined through a couple methods that use a shearing action. The soils are prepared and confined in an apparatus, and a direct shearing force is applied until they fail, or the soil begins to slip past itself. The force needed to make the soil fail can be recorded and used to calculate the internal friction angle. The test is usually referred to by the apparatus used in the test; the direct shear test using a shear box.
Clayey Soils
Cohesion applies to clayey soils, and sometimes silty soils. Clay particles are many many times smaller than sand grains. Most sand grains can be seen and distinguished by the naked eye. Clay particles are on the molecular scale. As such, they are subject to forces alien to our everyday experience. They are held together by static charges and other small forces collectively known as van der Waals forces, named for a Dutch scientist. The particles are so small that physical contact (friction) and gravity have a much diminished effect, and the sum of attractive and repulsive forces between molecules takes over. Cohesion can be determined using a similar principle to the shear box, but with an entirely different implementation. The clayey soils are confined in a cell, and a confining pressure is applied using pressurized water. Then a direct force is applied to one axis of the sample. The test is usually repeated at various confining pressures. The tabulated data of confining pressure and the load that can be supported at that confining pressure is used to calculate the cohesion of the soil. This is commonly referred to as the triaxial shear test in the industry.
A property closely related to cohesion is the plasticity of the soil. Plasticity describes the state of the soil, analogous to the phase states of matter. Water, for instance, can range from a gas to a liquid to a solid depending on temperature and pressure. Soils can range from solids to plastics to liquids depending on how much water is in them. We use the Plastic Limit and Liquid Limit tests, together known as the Atterberg Limits, to determine the range of water contents over which the soil behaves as a plastic. So below the Plastic Limit, the soil is a solid. Above the Liquid Limit, the soil is a runny, pourable liquid. In between, the soil is malleable and sticky, but still clings to itself.
Soil Gradation
The gradation, or relative amount of different sized particles, is important because as we have seen, the size of the particles is a large factor in determining the behavior of the soil. There is an exhaustive method to determine the gradation of a soil from particles on the scale of inches all the way down to the molecular scale of the clays. The soil must be collected carefully to ensure the sample is representative of the soil as a whole. The engineer is typically concerned with all particles from 3” in diameter and smaller. The soil is passed through a set of nested and progressively smaller sieves. For instance, for Florida soils we typically need a set of sieves starting with a 1” (25.4mm) opening all the way down to a No. 200 sieve with an opening of 0.0029” (0.075mm). Soils smaller than the No. 200 sieve are very hard to deal with by sieving, so we turn to another method. The hydrometer test uses a column of water to sort the particles. We mix the smaller constituents of the soil well, into a homogenous mixture, and then we watch how long it takes to fall out of suspension. We do that by measuring the density of the water. As particles settle to the bottom, the density of the water goes down. We measure that changing density over time and calculate the gradation of the very fine sands, silts, and finally clays. We typically draw a curve on semi-log plot of particle size versus percentage of soil of that (or percentage of soil smaller than) particle size.
The treatment of soils as engineered construction materials and the Unified Soil Classification System are vital to engineers, the construction industry, and of course to the population in general. The Unified Soil Classification System treats soils by their engineering properties. The internal friction angle and the cohesion of a soil can be indexed to its plasticity and gradation, or they can be measured directly. These tests, systems, and properties ensure structures are built with a quantified factor of safety, and that our public and private infrastructure lasts and performs.
