Specific Surfaces of Soil Materials

One concept that appears at the end of Chapter 1 of Soils in Construction is that of the specific surfaces of soil materials, including data from Lambe and Whitman’s classic text Soil Mechanics. Additional information on this topic can be found in Tsytovich, which is reproduced below. (Tsytovich refers to this as “unit surface area,” but the concept is the same.)

A factor of importance in the evaluation of the properties of solid soil particles is their mineral composition. Thus, some minerals, such as quartz and feldspar, interact only slightly with the surrounding water, whereas other minerals, for instance, montmorillonite, can interact substantially more actively and in a different way. The smaller the particles of a soil, the greater their unit surface area (per cm³ or per gram) and the larger the number of centres of interaction with the surrounding water and in contacts between solid particles proper. For instance, particles of kaolin (a clay mineral) have a unit surface area of 10 m²/g, whereas those of montmorillonite have a very large unit surface area of 800 m²/g, which inevitably affects the properties of natural soils containing particles of montmorillonite. The presence of particles of mica (which are very slippery and have only a negligible shear resistance) has an essential effect on the physical properties of the soils containing such particles; this circumstance must always be taken into account.

It can be shown that, for spherical particles, the unit surface area can be computed using the formula

$A_{us} = \frac{3}{r \rho}$

where r is the radius of the particle and $\rho$ is the density of the material. We thus can see that, for a uniform particle density (which is nearly the case for inorganic soil particles) the unit surface area is inversely proportional to the radius or diameter of the particle. Thus smaller particles have a higher unit surface area and larger particles a lower one. The units, as presented by Tsytovich and others, are tricky, as they combine SI or MKS units with CGS ones.