Geotechnical engineers classify soils, or more properly earth materials, for their properties relative to foundation support or use as building material. These systems are designed to predict some of the engineering properties and behavior of a soil based on a few simple laboratory or field tests, though some earlier systems were adaptations of soil-science classification systems. The most common is the Unified Soil Classification System, with three major groups:
(1) coarse-grained, sands and gravels, (2) fine-grained, silts and clays, and (3) highly organic soils (referred to as peat even when the soil is not truly a peat). The first two groups are then subdivided as follows:
Coarse grain: gravels, sands, based on the grain-size of the coarse-grained fraction. Fine grain: silts, clays, organics silts & clays, based on plasticity and organic content.
Fine-grained soils are then subdivided according to their plasticity, while coarse-grained soils are subdivided by the presence and properties of the fines or the grain-size distribution of the soil.
A full geotechnical engineering soil description will include other properties of the soil, including color, in-situ moisture content, in-situ strength, and somewhat more detail about the material properties of the soil than is provided by the USCS code.