One of the most important characteristics of soils is their range of particle sizes. Soil particle sizes may range from 20 cm diameter boulders to less than 0.1
m in clay particles (see also Chapter 1). Figure 3.1 shows different soils’ particles as seen with an optical microscope. The pictures of the soil particles are taken under the same magniﬁcation (
90). While one sand particle may occupy the whole image ﬁeld in one picture, no clay particles may be identiﬁed in another picture. This range of sizes has important engineering implications; for example, the interaction of soils with water is controlled by the size distribution and by the size of the smallest particles. Furthermore, particle sizes are responsible for the plasticity, hydraulic and electrical conductivity, consolidation, chemical diffusion, and shear strength behavior of soils.