PROPERTIES OF VERY FINE SOIL FRACTIONS (2)

CATION EXCHANGE AND ABSORBED WATER

Cations that neutralize the net negative charge on the surface of soil particles in water are readily exchangeable with other cations. The exchange reaction depends mainly on the relative concentrations of cations in the water and also on the electrovalence of the cations. Cation exchange capacity, measured in miliequivalents of cations per gram of soil particles, is a measure of the net negative charge on the soil particles, resulting from isomorphous substitution and broken bonds at the boundaries. The values of the cation exchange capacity for the principal clay minerals are indicated in Table 1. Montmorillonite has a relatively large exchange capacity because its particles may consist of single unit sheets.Very fine particles of other minerals such as mica and quartz also carry a net negative charge in water as a result of broken bonds at the boundaries. However, even in the range of small particle sizes in which nonclay minerals occurs in soils, the exchange capacity is relatively small. 

Table 1: Cation Exchange Capacity of Principal Clay Mineral

The highly polar water molecule has the ability to form strong bonds with the surface of soil particles, as well as with the exchangeable cations that surround it. The strong short-range adsorption forces hold one to four molecular layers of water at the surface of the soil particles. This water is said to be adsorbed. It is of most importance if the particles are very small,such as films of sodium montmorillonite 1 nm thick, and is insignificant if they are large, such as 200-µm grains of quartz sand.

If a soil particle is surrounded by water, the exchangeable cations are not attached to it. Its negative electrical charge tends to attract thee cations, but the cations diffuse toward the lower cation concentration away from the particle. Therefore, the soil particle is surrounded by a domain know as an electric double layer (van Olphen 1977, Mitchell 1976). The inner layer of the double layer is the negative charge on the surface of the soil particle. The outer layer is the excess of cations and deficiency of anions with respect to the concentration in the free water not influenced by the force field of the particle. The cation concentration has a finite value near the surface of the particle and decreases exponentially with distance to the concentration of the cations in the free pore water. The thickness of double-layer water, which is determined by the valence of the exchangeable cations and by the electrolyte concentration in free pore water, can exceed 50 nm. Thick double-layer water develops with exchangeable cations of low electrovalence such as Na+, and in free pore water of low electrolyte concentration, as in freshwater rivers and lakes. On the other hand, exchangeable cations of high valence such as Ca2+ and high electrolyte concentration as in the marine environment tend to depress the thickness of the double-layer water. Thus, in general, a soil particle is covered by a 1-nm layer of absorbed water, surrounded by  to 1 to > 50 nm of double-layer water, enveloped in turn by free water. Double-layer water is most significant in sodium montmorillonite because of its very small and filmy particles.

PROPERTIES OF VERY FINE SOIL FRACTIONS (1)

MINERALOGICAL COMPOSITION

The most important grain property of fine-grained soil materials is the mineralogical composition. If the soil particles are smaller than about 0.002 mm, the influence of the force of gravity on each particle is insignificant compared with that of the electrical forces acting at the surface of the particle. A material in which the influence of the surface charges is predominant is said to be in the colloidal state. The colloidal particles of soil consist primarily of clay minerals that were derived from rock minerals by weathering, but that  have crystal structures differing from those of the parent minerals.

All the clay minerals are crystalline hydrous aluminosilicates having a lattice structure in which the atoms are arranged in several layers, similar to the pages of a book. The arrangement and the chemical composition of these layers determine the type of clay mineral.

The basic building blocks of the clay minerals are the silica tetrahedron and the alumina octahedron. These blocks from tetrahedral and octahedral layers (Figure 1), different combinations of which produce a unit sheet of the various types of clays.

Figure 1: (a) Tetrahedral Layer, (b) Octahedral Layer (After Grim, 1968)

A single particle of clay may consist of many sheets of films piled one on another. Because each sheet or film has a definite thickness but is not limited in dimensions at right angles to its thickness, clay particles are likely to exhibit flat or curved terraced surfaces. The surfaces carry residual negative electrical charges, but the broken edges may carry either positive or negative charges in accordance with the environment. 

In problems of interest to the civil engineer, clay particles are always in contact with water. The interactions among the clay particles, the water, and the various materials dissolved in the water are primarily responsible for the properties of the soil consisting of the particles.

CHARACTERISTICS OF PRINCIPAL CLAY MINERALS

Kaolinite is one of the most common clays minerals in sedimentary and residual soils (Grim 1968, Swindale 1975). A unit sheet of kaolinite, which is approximately 0.7 nm (nm = 10⁻⁹ m) thick, is composed of one aluminum octahedral layer and one silicon tetrahedral layer, joined together by shared oxygens. A typical particle of kaolinite consists of a stack sheets forming a stiff hexagonal plate with flat-faced edges. It is about 100 nm in thickness with a breadth/thickness of about 5 to 10, and a specif surface of 5 to 15 m2/g. The scanning electron microscope (SEM) photomicrograph in Figure 2 shows a range of particle sizes and shapes including terraced surfaces where packets of 0.7-nm sheets terminate. 

Figure 2: Photomicrograph of Kaolinite

Halloysite is one of the most common minerals in residual soils, particularly those derived from volcanic parent material. It is a member of the kaolin subgroup of clay minerals. A unit sheet of hydrated halloysite, including one molecular layer of water, is approximately 1 nm thick. A typical particle has the shape of a hollow tube or prism with the outside and inside diameters of 70 and 40 nm, respectively, and 300 to 500 nm long. The intersheet water in the hydrated halloysite is removed irreversibly starting 60° to 75° C.

Allophane, a major constituent of young residual soils formed from volcanic ash, is an amorphous hydrous aluminosilicate commonly associated with halloysite (Grim 1968, Fieldes and Claridge 1975, Wesley 1973). In some residual soils, the transition to halloysite from allophane, which forms at early stages of weathering, is not well defined. Allophane consists of very loosely packed chains of silica tetrahedra and alumina octahedra, cross-linked at a relatively small number of points. In the natural state, it exists as microaggregates of extremely fine particles of the order of several nanometers and specific surface areas of 250 to 800 m2/g. The allophane aggregates or clusters are relatively incompressible. They are sometimes cemented by iron or aluminum oxides, and they enclose a large amount of water. Since the aggregates are susceptible to structural breakdown upon mechanical manipulation, as by heavy equipment, a deposit of allophane may change from a granular material to a plastic sticky mass that cannot be handled easily. The natural aggregates do not readily lose water; however, when water is removed and the fabric shrinks, the process cannot be reserved. The resulting soil, with silt- and sand sized particles that are quite hard, is practically nonplastic.

Illite is the most common clay mineral in stiff clays and shales as well as in postglacial marine and lacustrine soft clay and silt deposits (Grim 1968, Radoslovic 1975, Reichenbach and Rich 1975). It is often present, sometimes interstratified with other sheet silicates, in sedimentary and residual soils, except in residual soils derived from amorphous volcanic material. Illite is also reffered to as fine-grained mica and weathered mica. The crystal structure of illite is similar to that of muscovite mica in the macroscopic form. However, in microscopic illite particles the stacking of the sheets is not so regular as in well-crystallized micas, and weathering may remove intersheet K+ from the edges of the plates. The resulting illite particles, with terraced surfaces where one or more unit sheets terminate, have frayed and tattered edges, are flexible and elastic, are 10 to 30 nm in thickness, have a breadth/thickness of 15 to 30, and have a specific surface of 80 to 100 m2/g. A SEM photomicrograph of illite particle is shown in Figure 3.

Figure 3: Photomicrograph of Illite

Chlorite is a clay mineral commonly associated with micas and illite, but is usually a minor component (Grim 1968, Bailey 1975). A unit sheet of chlorite, which is about 1.4 nm thick, consists of one biotite mica sheet in which all octahedral sites are occupied by magnesium and one brucite sheet, an octrahedral layer in which magnesium atoms are in octahedral coordination with hydroxyls. The biotite mica and brucite sheets are strongly bonded together. The unit sheets are stacked and are connected to each other by hydrogen bonding of surface oxygens of the tetrahedral layer of mica and surface hydroxyls of brucite. The size and platyness of chlorite particles are similar to those of illite. However, in contrast to illite, in which the unit sheets are bonded by potassium, the hydrogen bonding between chlorite sheets results in pseudohexagonal or euhedral-shaped platelets that are flexible but inelastic.

Montmorillonite, the most common member of a group of clay minerals known as smectites, is the dominant clay mineral in some clays and shales and in some residual soils derived from volcanic ash (Grim 1968, Mering 1975). Relatively pure seams of montmorillonite are found in some deposits, Wyoming bentonite being the best-known example. A unit sheet of montmorillonite is similar to that of the micas. In montmorillonite, octahedral Al is partially replaced by Mg atoms. Each isomorphous replacement produces a unit negative charge at the location of the substituted atom, which is balanced by exchangeable cations, such as Ca2+ and Na+ situated at the exterior of the sheets. In a packet of montmorillonite in the anhydrous state, where as many as 10 unit sheets are in contact, the stacking of the sheets is disordered in the sense that the hexagonal cavities of the adjacent surfaces of two neighboring sheets are not matched face-to-face. In a hydrous environment, water molecules penetrate between the sheets and separate them by 1 nm (i.e., four molecular layers of water).

In sodium montmorillonite, the exchange capacity is satisfied by Na cations. If the electrolyte concentration in water is less tahn 0.3 N, further separation of unit sheets takes place to more than 3nm by diffuse doublelayer repulsion. The resulting particles of sodium montmorillonite in water are thin films 1 nm thick, with breadth-to-thickness ratios in excess of 100 and specific surfaces as much as 800 m2/g. In calcium montmorillonite, the electrostatic attraction of the Ca cations links the successive sheets together and prevents seperation beyond 1 nm. The resulting domains of 8 to 10 unit sheets, which are separated from each other by up to four molecular layers of water, experience minimal swelling. Among clay minerals, sodium montmorillonite has the smallest and most filmy particles, as shown by the SEM photomicrograph in Figure 4.

Figure 4: Photomicrograph of Montmorillonite

Attapulgite is a fibrous clay mineral composed of silica chains linked together by oxygens along their longitudinal edges to form single laths or bundles of laths (grim 1968, Henin and Caillere 1975). Water molecules fill the interstices between the chains. The particles of attapulgite are relatively rigid, 5 to 10 nm thick, 10 to 20 nm wide, and 0.1 to 1µm in length, as shown in the SEM, Figure 5. The specific surface of the bundles is about 150 m2/g, but it can be much higher of dispersed single laths. Attapulgite, frequently associated with carbonate rocks, is not a very common clay mineral in soil deposits. However, when present, it results in unusual physical properties such as a very high plastic limit, a very high plasticity index, and high frictional resistance.

Figure 5: Photomicrograph of Attapulgite

MIXED-LAYER CLAY MINERALS

Particles in some soils are composed of regularly or randomly interstratified unit sheets of two or more types of clay minerals (MacEwan and Ruiz-Amil 1975). For example, a marine or lacustrine clay deposit may include regularly alternating montmorillonite-illite particles, and a volcanic ash residual soil may contain randomly interstratified halloysite and hydrated halloysite. These mixed-layer clay minerals often represent an intermediate  phase in the transformation, by weathering or through a diagenetic process, of one mineral to another mineral, such as that of mica to montmorillonite or vice versa. However, the physical properties of interstratified clay minerals are not well defined, inasmuch as a wide range of combinations is possible. Mixed-layer clay mineral particles possess properties generally representative of the constituent minerals.

ROLE OF ISOMORPHIC SUBSTITUTION

Isomorphic substitution is the replacement of a cation in the mineral structure by another cation of lower electrovalence. The difference in the valences leads to a negative charge, and the difference in size of the cations produces a distortion of the mineral structure. Both effects tend to decrease the resistance of a mineral structure to chemical and mechanical weathering. Quartz is a space-lattice silicate composed of silica tetrahedrons, (SiO4)^-4 linked together by primary valence bonds to form a three-dimensional network with the formula SiO2. There is no isomorphous substitution in quartz, and each silica tetrahedron is firmly and equally braced in all directions. As a result, quartz has no planes of weakness and is very hard and highly resistant to mechanical and chemical weathering. Quartz is not only the most common mineral in sand-and silt-sized particles of soils, but quartz or amorphous silica is frequently present in colloidal (1 to 100 nm) and molecular (< 1 nm) dimensions (Mitchell 1975). In some young soils of volcanic origin, clay-sized particles may consist essentially of silica, an appreciable proportion of which is amorphous. The boundary between quartz and amorphous silica, however, is not distinct, inasmuch as physical processes such as grinding may render crystalline quartz amorphous.

In feldspar, some of the silicon are replaced by aluminum. This results in a negative charge and in distortion of the crystal structure, because Al atoms are larger than Si atoms. The negative charge is balanced by taking in cations such as K+, Na+, Ca2+ in orthoclase, albite, and anorthite feldspars, respectively. The distortion of the lattice and the inclusion of the cations cause cleavage planes that reduce the resistance of feldspars to mechanical and chemical weathering. For these reasons, feldspars are not so common as quartz in the sand-, silt-, and clay-sized fractions of soils, even though feldspars are the most common constituent of the earth's crust.

Common micas such as muscovite and biotite are often present in the silt- and sand-sized fractions of soils. In a unit sheet of mica, which is 1 nm thick, two tetrahedral layers are linked together with one octahedral layer. In muscovite, only two of every three octahedral sites are occupied by aluminum cations, whereas in biotite all sites are occupied by magnesium. In well-crystallized micas one fourth of the tetrahedral Si4+ are replaced by Al3+. The resulting negative charge in common micas is balanced by intersheet potassiums. In a face-to-face stacking of sheets to form mica plates, the hexagonal holes on opposing tetrahedral surfaces are matched to enclose the intersheet potassium. Perfect cleavage occurs along the K-plane, however, as the intersheet bond strength of (KO12)^-23 is only 1/12 of the bond strength of (SiO4)^-4 inside the sheets.

This is very little isomorphous substitution in kaolinite, the particles of which are formed by hydrogen bonding of unit sheets. The edge charge is important, because the particles are thick and the edge surface area is significant. The edge charge, which results from broken bonds and exposed oxygens at the edges of the particles, and give the edge a net positive charge. When the pH is high, H+ ions disassociate from the edge, which then becomes negatively charged. In mica and illite a major part of the negative charge resulting from isomorphous substitution is neutralized by K atoms which also connect the successive unit sheets to each other to form particles.

SIZE AND SHAPE OF SOIL PARTICLES

The size of the particle that constitute soils may vary from that boulders to that of large molecules. Grains larger than approximately 0.06 mm can be inspected with the naked eye or by means of a hand lens. They constitute the very coarse and coarse fractions of the soils. Grains ranging in size from about 0.06 mm to 2 µ (1 µ = 1 micron = 0.001 mm = 1×10⁻⁶ m) can be examined only under the microscope. They represent the fine fraction.

Grain smaller than 2 µ constitute the very fine fraction (clay size fraction, CF). Grains having a size between 2 µ and about 0.1 µ can be differentiated under the microscope, but their shape can not be discerned. The shape of grains smaller than about 1 µ can be determined by means of an electron microscope. Their molecular structure can be investigated by means of X-ray analysis.

The process of separating a soil aggregate into fractions, each consisting of grains within a different size range, is known as mechanical analysis. By means of mechanical analysis, it has been found the most natural soils contain grains representative of two or more soil fractions. The general character of mixed-grained soils is determined almost entirely by the character of the smallest soil constituents.

Very coarse fractions, for example gravel, consist of rock fragments each composed of one or more minerals. The fragments may be angular, subangular, rounded, or flat. They may be fresh, or they may show signs of considerable weathering. They may be resistant or crumbly. 

Coarse fractions, exemplified by sand, are made up of grains usually composed chiefly of quartz. The individual grains may be angular, subangular, or rounded. Some sands contain a fairly high percentage of mica flakes that make them very elastic or springy. In the fine and very fine fractions, any one grain usually consists of only one mineral. The particles may be angular, flake-shaped, or tubular. Rounded particles, however, are conspicuously absent. If the size of most of the grains in an aggregate of soil particles is within the limits given for any one of the soil fractions, the aggregate is called a uniform soil. Uniform very coarse or coarse soils are common, but uniform very fine or colloidal soils are very seldom encountered. All clays contain fine, very fine, and colloidal constituents, and some clays contain even coarse particles. The finest grain-size fractions of clays consist principally of flake-shaped particles.

The widespread prevalence of flake-shaped particles in the very fine fractions of natural soils is a consequence of the geological processes of soil formation. Most soils originate in the chemical weathering of rocks. The rock themselves consist partly of chemically very stable and partly of less stable minerals. Chemical weathering transforms the less stable minerals into a friable mass of very small particles of secondary minerals that commonly have a scale-like or flaky crystal form, whereas the stable minerals remain practically unaltered. Thus the process of chemical weathering reduces the rock to an aggregate consisting of fragments of unaltered or almost unaltered minerals embedded in a matrix composed chiefly of discrete scaly particles. During subsequent transportation by running water the aggregate is broken up, and the constituents are subjected to impact and grinding. The purely mechanical process of grinding does not break up the hard equidimensional grains of unaltered minerals into fragments smaller than about 10 µ (0.01 mm). On the other hand, the friable flake-shaped particles of secondary minerals, although initially very small, are readily ground and broken into still smaller particles. Hence, the very fine fractions of natural soils consist principally of flake-shaped particles of secondary minerals.

Source: Soil Mechanics in Engineering Practice by Terzaghi et al

PRINCIPAL TYPES OF SOILS

The materials that constitute the earth's crust are rather arbitrarily divided by the civil engineer into the two categories, soil and rock. Soil is a natural aggregate of mineral grains that can be separated by such gentle mechanical means as agitation in water. Rock, on the other hand, is a natural aggregate of minerals connected by strong and permanent cohesive forces. Since the terms "strong" and "permanent" are subject to different interpretations, the boundary between soil and rock is arbitrary one. As a matter of fact, there are many natural aggregates of mineral particles that are difficult to classify either as soil or as rock. In this text, however, the term soil will be applied only to materials that unquestionably satisfy the preceding definition. 

Although the terminology described in the preceding paragraph is generally understood by civil engineers, it is not in universal use. To the geologist, for example, the term rock implies all the material that constitutes the earth's crust, regardless of the degree to which the mineral particles are bound together, whereas the term soil is applied only to that portion of the earth's crust that is capable of supporting vegetation. Therefore, the civil engineer who makes use of information prepared by workers in other field must understand the sense in which the terms soils and rock are used.

On the basis of the origin of their constituents, soils can be divided into two large groups, those that consist chiefly of the results of chemical and physical rock weathering, and those that are chiefly of organic origin. If the products of rock weathering are still located at the place where they originated, they constitute a residual soil. Otherwise they constitute a transported soil, regardless of the agent that performed the transportation. 

Residual soils that have developed in semiarid or temperate climates are usually stiff and stable and do not extend to great depth. However, particularly in warm humid climates where the time of exposure has been long, residual soils may extend to depths of hundreds of meters.They may be strong and stable, but they may also consist of highly compressible materials surrounding blocks of less weathered rock. Under these circumstances they may give rise to difficulties with foundations and other types of construction. Many deposits of transported soils are soft and loose to a depth of more than a hundred meters and may also lead to serious problems.

Soils of organic origin are formed chiefly in situ, either by the growth and subsequent decay of plants such as peat mosses or by the accumulation of fragments of the inorganic skeletons or shells of organisms. Hence a soil of organic origin can be either organic or inorganic. The term organic soil ordinarily refers to a transported soil consisting of the products of rock weathering with a more or less conspicuous admixture of decayed vegetable matter.

The soil conditions at the site of proposed structure are commonly explored by means of test borings or test shafts. The inspector on the job examines samples of the soil as they are obtained, classifies them in accordance with local usage, and prepares a boring log or shaft record containing the name of each soil and the elevation of its boundaries. The name of the soil is modified by adjectives indicating the stiffness, color, and other attributes. At a later date the record may be supplemented by an abstract of the results of tests made on the samples in the laboratory.

The following lists of soil types includes the names commonly used for field classification.

• Sand and gravel are cohesionless aggregates of rounded subangular or angular fragments of more or less unaltered rocks or minerals. Particles with a size up to 2 mm are referred to as sand, and those with a size from 2 mm to  200 mm as gravel. Fragments with a diameter of more than 200 mm are known as boulders.

Figure 1:AIMS Angularity Index

• Hardpan is a soil that has an exceptionally great resistance to the penetration of drilling tools, usually found below the uppermost topsoil layer. Most hardpans are extremely dense, well-graded, and somewhat cohesive aggregates of mineral particles.

• Inorganic silt is a fine-grained soil with little or no plasticity. The least plastic varieties generally consist of more or less equidimensional grains of quartz and are sometimes called rock flour, whereas the most plastic types contain an appreciable percentage of flake-shaped particles and are referred to as plastic silts, Because of its smooth texture, inorganic silt is often mistaken for clay, but it may be readily distinguished from clay without laboratory testing. If shaken in the palm of the hand, a pat of saturated inorganic silt expels enough water to make its surface appear glossy. If the pat is bent between the fingers, its surface again becomes dull. This procedure is known as the shaking test. After the pat has dried, it is brittle and dust can be detached by rubbing it with the finger. Silt is relatively impervious, but if it is in loose state it may rise into a drill hole or shaft like a thick viscous fluid. The most unstable soils of this category are known locally under different names, such as bull's liver.

• Organic silt is a fine-grained more or less plastic soil with an admixture of finely divided particles of organic matter. Shells and visible fragments of partly decayed vegetable matter may also be present. The soil ranges in color from light to very dark grey, and it is likely to contain a considerable quantity of H2S, CO2, and various other gaseous products of the decay of organic matter which give it a characteristic odor. The permeability of organic silt is very low and its compressibility very high.

• Clay is an aggregate of microscopic and submicroscopic particles derived from the chemical decomposition of rock constituents. It is plastic within a moderate to wide range of water content. Dry specimens are very hard, and no powder can be detached by rubbing the surface of dried pats with the fingers. The permeability of clay is extremely low. The term gumbo is applied, particularly in the western United States, to clays that are distinguished in the plastic state by a soapy or waxy appearance and by great toughness. At higher water contents they are conspicuously sticky.

• Organic clay is a clay that owes some of its significant physical properties to the presence of finely divided organic matter. When saturated, organic clay is likely to be very compressible, but when dry its strength is very high. It is usually dark gray or black and it may have a conspicuous odor.

• Peat is a somewhat fibrous aggregate of macroscopic and microscopic fragments of decayed vegetable matter. Its color ranges between light brown and black. Peat is so compressible that it is almost always unsuitable for supporting foundations. Various techniques have been developed for carrying earth embankments across peat deposits without the risk of breaking into the ground, but the settlement of these embankments is likely to be large and to continue at a decreasing rate for many years.

If a soil is made up of a combination of two different soil types, the predominant ingredient is expressed as a noun, and the less prominent ingredient as a modifying adjective. For example, silty sand indicates a soil that is predominantly sand but contains a small amount of silt. A sandy clay is a soil that exhibits the properties of a clay but contains an appreciable amount of sand.

The aggregate properties of sand and gravel are described qualitatively by terms loose, medium, and dense, whereas those of clays are described by hard, stiff, medium, and soft. These terms are usually evaluated by the boring foreman or inspector on the basis of several factors, including the relative ease or difficulty of advancing the drilling and sampling tools and the consistency of the samples. However, since this method of evaluation may lead to a very erroneous conception of the general character of the soil deposit, the qualitative descriptions should be supplemented by quantitative information whenever the mechanical properties are likely to have an important influence on design. The quantitative information is commonly obtained by means of laboratory tests on relatively undisturbed samples, or by suitable in situ tests.

A record of the color of the different strata encountered in adjacent borings reduces the risk of errors in correlating the boring logs. Color may also be an indication of a real difference in the character of the soil. For example, if the top layer of a submerged clay stratum is yellowish or brown and stiffer than the underlying clay, it was probably exposed temporarily to desiccation combined with weathering. Terms such as mottled, marbled, spotted, or speckled are used when different colors occur in the same stratum of soil. Dark or drab colors are commonly associated with organic soils.

Under certain geological conditions soils from that are characterized by one or more striking or unusual features such as a root-hole structure or a conspicuous and regular stratification. Because of these features, such soils can easily be recognized in the field and, consequently, they have been given special names by which they are commonly known. The following paragraphs contain definitions and descriptions of some of these materials.

• Till is an unstratified glacial deposit of clay, sand, gravel, and boulders.

• Tuff is a fine-grained water - or wind-laid aggregate of very small mineral or rock fragments ejected from volcanoes during explosions.

• Loess is a uniform, cohesive, wind-blown sediment, and is commonly light brown. The size of most of the particles ranges between the narrow limits of 0.01 and 0.05 mm. The cohesion is due to the presence of a binder that may be predominantly calcareous or clayey. Because of the universal presence of continuous vertical root holes, the permeability in vertical direction is usually much greater than in horizontal directions; moreover, the material has the ability to stand on nearly vertical slopes. True loess deposits have never been saturated. On saturation the bond between particles is weakened and the surface of the deposit may settle.

Figure 2: Loess

• Modified loess is a loess that has lost its typical characteristics by secondary processes, including temporary immersion, erosion and subsequent deposition, chemical changes involving the destruction of the bond between the particles, or chemical decomposition of the more perishable constituents such as feldspar. Thorough chemical decomposition produces loess loam, characterized by greater plasticity than other forms of modified loess.

• Diatomaceous earth (kieselguhr) is a deposit of fine, generally white, saliceous powder composed chiefly or wholly of the remains of diatoms. The term diatom applies to a group of microscopic unicellular marine or fresh-water algae characterized by silicified cell walls.

• Lake marl or boglime is a white fine-grained powdery calcareous deposit precipitated by plants in ponds. It is commonly associated with beds of peat.

• Marl is a rather loosely used term for various fairly stiff or very stiff marine calcareous clays of greenish color.

• Shale is a clastic sedimentary rock mainly composed of silt-size and clay-size particles. Most shales are laminated and display fissility; the rock has a tendency to split along relatively smooth and flat surfaces parallel to the bedding. When fissility is completely absent, the clastic sedimentary deposit is called mudstone or clay rock. Depending on clay mineralogy, void ratio, and degree of diagenetic bonding or weathering, compressive strength of shales may range from less than 2.5 MPa to more than 100 MPa.

• Adobe is a term applied in the southwestern United States and other semiarid regions to a great variety of light-colored soils ranging from sandy silts to very plastic clays.

• Caliche refers to layers of soil in which the grains are cemented together by carbonates deposited as a result of evaporation. These layers commonly occur at a depth of several meters below the surface, and their thickness may range up to a few meters, A semiarid climate is necessary for their formation.

• Varved clay consists of alternating layers of medium gray inorganic silt and darker silty clay. The thickness of the layers rarely exceeds 10 mm, but occasionally very much thicker varves are encountered. This constituents were transported into freshwater lakes by melt water at the close of the Ice Age. Varved clays are likely to combine the undesirable properties of both silts and soft clay.

• Bentonite is a clay with a high content of montmorillonite. Most bentonites were formed by chemical alternation of volcanic ash. In contact with water, dried bentonite swells more than other dried clays, and saturated bentonite shrinks more on drying.

Each term used in the field classification of soils includes a great variety of different materials. Furthermore, the choice of terms relating to stiffness and density depends to a considerable extent on the person who examines the soil. Consequently, the field classification of soils is always more or less uncertain and inaccurate. More specific information can be obtained only by physical tests that furnish numerical values representative of the properties of the soil.

Source: Soil Mechanics in Engineering Practice by Terzaghi et al