10 CHAPTER 2 PHYSICAL CHARACTERISTICS OF SOILS AND SOIL INVESTIGATIONS

Loam is a mixture of sand, silt, and clay that may contain organic material. Loess is a wind blown, uniform fine-grained soil. Mud is clay and silt mixed with water into a viscous fluid.

2.3.3 Clay MineralsMinerals are crystalline materials and make up the solids constituent of a soil.The mineral particles of fine-grained soils are platy. Minerals are classified ac-cording to chemical composition and structure. Most minerals of interest to geo-technical engineers are composed of oxygen and silicon-two of the mostabundant elements on earth. Silicates are a group of minerals with a structuralunit called the silica tetraheron. A central silica cation (positively charged ion)is surrounded by four oxygen anions (negatively charged ions), one at each cornerof the tetrahedron (Fig. 2.2a). The charge on a single tetrahedron is -4 and toachieve a neutral charge, cations must be added or single tetrahedrons must belinked to each other sharing oxygen ions. Silicate minerals are formed by additionof cations and interactions of tetrahedrons. Silica tetrahedrons combine to formsheets, called silicate sheets, which are thin layers of silica tetrahedrons in whichthree oxygen ions are shared between adjacent tetrahedrons (Fig. 2.2b). Silicatesheets may contain other structural units such as alumina sheets. Alumina sheetsare formed by combination of alumina minerals, which consists of an aluminumion surrounded by six oxygen or hydroxyl atoms in an octahedron (Fig. 2.2c,d).

The main groups of crystalline materials that make up clays are the min-erals: kaolinite, illite, and montmorillonite. Kaolinite has a structure that consists

o and = Oxygen(a) Single

and @ = Silicon

(b) A tetrahedral

Aluminumo and =Oxygen or Hydroxyl ilt =Aluminum

(c) Single octahedrons (d) Octahedral sheet

FIGURE 2.2 (a) Silica tetrahedrons, (b) silica sheets, (c) single aluminumoctahedrons, and (d) aluminum sheets.

2.3 COMPOSITION OF SOILS 11

(a) Kaolinite

Alumina sheetSi Iica sheet

Hydrogen bonds

(b) Illite

Silica sheetAlumina sheetSilica sheetPotassi um ions

Silica sheetAlumina sheetSilica sheet

+--- Layers heldtogether by vander Waalsforces andexchangeableions; easilyinfi Itrated bywater

(c) MontmorilloniteFIGURE 2.3 Structure of kaolinite, illite, and montmorillonite.

of one silica sheet and one alumina sheet bonded together into a layer about 0.72nm thick and stacked repeatedly (Fig. 2.3a). The layers are held together byhydrogen bonds. Tightly stacked layers results from numerous hydrogen bonds.Kaolinite is common in clays in humid tropical regions. Illite consists of repeatedlayers of one alumina sheet sandwiched by two silicate sheets (Fig. 2.3b). Thelayers, each of thickness 0.96 nm, are held together by potassium ions.

Montmorillonite has a structure similar to illite, but the layers are heldtogether by weak van der Waals forces and exchangeable ions (Fig. 2.3c). Watercan easily enter the bond and separate the layers in montmorillonite, causingswelling. Montmorillonite is often called a swelling or expansive clay.

2.3.4 Surface Forces and Adsorbed WaterIf we subdivide a body, the ratio of its surface area to its volume increases. Forexample, a cube of sides 1 cm has a surface area of 6 cm2 If we subdivide thiscube into smaller cubes of sides 1 mm, the original volume is unchanged but thesurface area increases to 60 cm2 The surface area per unit mass (specific surface)of sands is typically 0.01 m2 per gram, while for clays, it is as high as 1000 m2 pergram (montmorillonite). The specific surface of kaolinite ranges from 10 to 20m

2 per gram while that of illite ranges from 65 to 100 m2 per gram. The surfacearea of 45 grams of illite is equivalent to the area of a football field. Because oftheir large surfaces, surface forces significantly influence the behavior of fine-grained soils compared to coarse-grained soils.

The surface charges on fine-grained soils are negative (anions). These neg-ative surface charges attract cations and the positively charged side of watermolecules from surrounding water. Consequently, a thin film or layer of water,called adsorbed water, is bonded to the mineral surfaces. The thin film or layerof water is known as the diffuse double layer (Fig. 2.4). The largest concentrationof cations occurs at the mineral surface and decreases exponentially with distanceaway from the surface (Fig. 2.4).

Drying of most soils, with the exception of gypsum, using an oven for whichthe standard temperature is 105 5C, cannot remove the adsorbed water. Theadsorbed water influences the way a soil behaves. For example, plasticity, whichwe will deal with in Section 2.6, in soils is attributed to the adsorbed water. Toxicchemicals that seep into the ground contaminate soil and groundwater. The sur-face chemistry of fine-grained soils is important in understanding the migration,sequestration, re-release, and ultimate removal of toxic compounds from soils.

c

12 CHAPTER 2 PHYSICAL CHARACTERISTICS OF SOILS AND SOIL INVESTIGATIONS

Mineralsurface

c::o

:;::

~c::Q)uc::o

U

Diffuse double layer

FIGURE 2.4 Diffuse double layer.Distance

Our main concern in this book is on the physical and mechanical properties ofsoils. Accordingly, we will not deal with the surface chemistry of fine-grainedsoils. You may refer to Mitchell (1993) for further information on the surfacechemistry of fine-grained soils that are of importance to geotechnical and geoen-vironmental engineers.

2.3.5 Soil FabricSoil particles are assumed rigid. During deposition, the mineral particles are ar-ranged into structural frameworks that we call soil fabric (Fig. 2.5). Each particleis in random contact with neighboring particles. The environment under whichdeposition occurs influences the structural framework that is formed. In partic-ular, the electrochemical environment has the greatest influence on the kind ofsoil fabric that is formed during deposition.

Two common types of soil fabric-flocculated and dispersed-are formedduring soil deposition as shown schematically in Fig. 2.5. A flocculated structure,formed under a saltwater environment, results when many particles tend to orient

(a) Flocculated structure-saltwater environment (b) Flocculated structure-freshwater environment

(c) Dispersed structureFIGURE 2.5 Soil fabric.

2.3 COMPOSITION OF SOilS 13

parallel to each other. A flocculated structure, formed under a freshwater envi-ronment, results when many particles tend to orient perpendicular to each other.A dispersed structure is the result when a majority of the particles orient parallelto each other.

Any loading (tectonic or otherwise) during or after deposition permanentlyalters the soil fabric or structural arrangement in a way that is unique to thatparticular loading condition. Consequently, the history of loading and changesin the environment is imprinted in the soil fabric. The soil fabric is the brain; itretains the memory of the birth of the soil and subsequent changes that occur.

The spaces between the mineral particles are called voids, which may befilled with liquids (essentially water) and gases (essentially air). Voids occupy alarge proportion of the soil volume. Interconnected voids form the passagewaythrough which water flows in and out of soils. If we change the volume of voids,we will cause the soil to either compress (settle) or expand (dilate). Loads appliedby a building, for example, will cause the mineral particles to be forced closertogether, reducing the volume of voids and changing the orientation of the struc-tural framework. Consequently, the building settles. The amount of settlementdepends on how much we compress the volume of voids. The rate at which thesettlement occurs depends on the interconnectivity of the voids. Free water, notthe adsorbed water, and/or air trapped in the voids must be forced out for set-tlement to occur. The decrease in volume, which results in settlement of buildingsand other structures, is usually very slow in fine-grained soils and almost ceaselessbecause of their (fine-grained soils) large surface area compared with coarse-grained soils. The larger surface area of fine-grained soils compared with coarse-grained soils provides greater resistance to the flow of water through the voids.

2.3.6 Comparison of Coarse-Grainedand Fine-Grained Soils for Engineering UseCoarse-grained soils have good load-bearing capacities and good drainage qual-ities, and their strength and volume change characteristics are not significantlyaffected by change in moisture conditions. They are practically incompressiblewhen dense, but significant volume changes can occur when they are loose. Vi-brations accentuate volume changes in loose coarse-grained soils by rearrangingthe soil fabric into a dense configuration.

Fine-grained soils have poor load-bearing capacities compared with coarse-grained soils. Fine-grained soils are practically impermeable, change volume andstrength with variations in moisture conditions, and are frost susceptible. Theengineering properties of coarse-grained soils are controlled mainly by the grainsize of the particles and their structural arrangement. The engineering propertiesof fine-grained soils are controlled by mineralogical factors rather than grain size.Thin layers of fine-grained soils, even within thick deposits of coarse-grainedsoils, have been responsible for many geotechnical failures and therefore youneed to pay special attention to fine-grained soils.

In this book, we will deal with soil as a construction and a foundation ma-terial. We will not consider soils containing organic material or the parent ma-terial of soils-rock. We will label our soils as engineering soils to distinguishour consideration of soils from geologists, agronomists, and soil scientists, whohave additional interests in soils not related to construction activities.