ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 226

soils the solid phase consists of grains with intermediate space (pores) filled with
gas or/and water. The density and constitution of these components have been
discussed in chapter 2.

Solid phase
The mineral part of the soil is subdivided into sand (60 - 2000 m), silt (2 -

60 m). The sand fraction is mainly quartz (SiO 2 );

adsorption is very small because quartz is electrically neutral. Pure sand has
relatively large pores.
The silt fraction also has little adsorptive capacity. The lute fraction consists
mainly of clay minerals. Clay minerals are usually aluminium-silicates composed
of SiO 2 -tetraeders and Al(OH) 6 - octahedran, where sometimes Si 4+ is replaced by
Al 3+ , and Al 3+ by Mg 2+ or Fe 2+ ions. This creates an electrically negative value and
it has influence on the adsorptive capacity (cation exchange capacity). Other non-
crystal matter consists of oxides, hydroxides and oxi-hydroxides of Al, Fe and Si
and Al and Fe silicates. Fe oxides and organic matter usually determine the soil
colour. Fe oxides may have a large specific surface area and play an important role
in cementation or fixing of phosphate, molybdate or silicate, and also tracer
elements Cu, Pb, V, Zn, Co, etc. The organic matter consists of remnants of flora
and fauna. It plays a role in binding and buffering minerals and creates 'structure'
(smaller and larger pores).
Neutral adsorption (a-polar binding) is inversely proportional to the 6 th power of
the distance (VanderWaals forces). Adsorption of electrolytes at internal (micro)-
surfaces occur following Coulomb's law, i.e. binding forces are inversely
proportional to the square of the distance. Replacement adsorption (exchange of
ions) may work actively or weakly. The exchange capacity is determined by the
ratio of the ions and follows a certain order, such as Li < Na < K < Rb < Cs < H.
The adsorption capacity is essential for buffering nutrients for ecological processes
in soils. Adsorption by organic matter is strongly dependent on the acidity ( pH ) of
the soil, and may be significantly higher than by clay minerals.

m) and lute or clay (< 2

Liquid phase
The liquid in soil is water, which may have various conditions (e.g. temperature,
gas bubbles) and it may contain many solvents (salts, humus). It enters the soil by
infiltration (rains, rivers) or by intrusion (density driven). Osmosis or capillarity
may bind the liquid. Movement of the liquid through soil is controlled by gravity,
osmotic and capillary forces. Binding forces between ground and groundwater are
determined by mutual interaction of water molecules (hydration, usually a layer of
a few molecules), osmotic water (electric ion concentration), and capillary water
(combination of cohesive and adhesive forces). Bound water causes pressure
gradients, which influences the adsorption by plant roots. Bound water above the
groundwater table is called hanging or pendulous water. In time, it evaporates, is
absorbed by roots or precipitates towards the groundwater table.
The water in the ground under the phreatic water table is referred to as
groundwater. The phreatic or groundwater table is a hypothetical surface where the
pressure is equal to the atmospheric pressure. The actual groundwater surface