PART I

Physical Properties of Soils

The subject matter of Part I is divided into four chapters. with the hydraulic and mechanical properties of soils
The first deals with the procedures commonly used to and with the experimental methods used to determine
discriminate among different soils or among different numerical values representative of these properties. The
states of the same soil. The second deals with the methods fourth chapter deals with the physical processes involved
and program of soil exploration. The third is concerned in the drainage of soils.

1
CHAPTER 1

Index Properties of Soils

ARTICLE 1 PRACTICAL IMPORTANCE OF erty of cohesionless soils is the relative density, whereas
INDEX PROPERTIES that of cohesive soils is the consistency.
The discussion of soil grain and aggregate properties
In geotechnical engineering, more than in any other field is preceded by a description of the principal types of soil,
of civil engineering, success depends on practical experi- and it is followed by a condensed review of the minimum
ence. The design of ordinary soil-supporting or soil-sup- requirements for adequate soil descriptions to be incorpo-
ported structures is necessarily based on simple empirical rated in the records of field observations.
rules, but these rules can be used safely only by the
engineer who has a background of experience. Large ARTICLE 2 PRINCIPAL TYPES OF SOILS
projects involving unusual features may call for extensive
application of scientific methods to design, but the pro- The materials that constitute the earths crust are rather
gram for the required investigations cannot be laid out arbitrarily divided by the civil engineer into the two cate-
wisely, nor can the results be interpreted intelligently, gories, soil and rock. Soil is a natural aggregate of mineral
unless the engineer in charge of design possesses a large grains that can be separated by such gentle mechanical
amount of experience. means as agitation in water. Rock, on the other hand, is
Since personal experience is necessarily somewhat lim- a natural aggregate of minerals connected by strong and
ited, the engineer is compelled to rely at least to some permanent cohesive forces. Since the terms strong and
extent on the records of the experiences of others. If permanent are subject to different interpretations, the
these records contain adequate descriptions of the soil boundary between soil and rock is necessarily an arbitrary
conditions, they constitute a storehouse of valuable infor- one. As a matter of fact, there are many natural aggregates
mation. Otherwise, they may be misleading. Conse- of mineral particles that are difficult to classify either as
quently, one of the foremost aims in attempts to reduce soil or as rock. In this text, however, the term soil will
the hazards in dealing with soils has been to find simple be applied only to materials that unquestionably satisfy
methods for discriminating among the different kinds of the preceding definition.
soil in a given category. The properties on which the Although the terminology described in the preceding
distinctions are based are known as index properties, and paragraph is generally understood by civil engineers, it
the tests required to determine the index properties are is not in universal use. To the geologist, for example, the
classification tests. term rock implies all the material that constitutes the
The nature of any soil can be altered by appropriate earths crust, regardless of the degree to which the mineral
manipulation. Vibrations, for example, can transform a particles are bound together, whereas the term soil is
loose sand into a dense one. Hence, the behavior of a applied only to that portion of the earths crust that is
soil in the field depends not only on the significant proper- capable of supporting vegetation. Therefore, the civil
ties of the individual constituents of the soil mass, but engineer who makes use of information prepared by
also on those properties that are due to the arrangement workers in other fields must understand the sense in which
of the particles within the mass. Accordingly, it is conve- the terms soil and rock are used.
nient to divide index properties into two classes: soil On the basis of the origin of their constituents, soils
grain properties and soil aggregate properties. The prin- can be divided into two large groups, those that consist
cipal soil grain properties are the size and shape of the chiefly of the results of chemical and physical rock weath-
grains and, in clay soils, the mineralogical character of ering, and those that are chiefly of organic origin. If the
the smallest grains. The most significant aggregate prop- products of rock weathering are still located at the place

3
4 INDEX PROPERTIES OF SOILS

where they originated, they constitute a residual soil. laboratory testing. If shaken in the palm of the hand, a
Otherwise they constitute a transported soil, regardless pat of saturated inorganic silt expels enough water to
of the agent that performed the transportation. make its surface appear glossy. If the pat is bent between
Residual soils that have developed in semiarid or tem- the fingers, its surface again becomes dull. This procedure
perate climates are usually stiff and stable and do not is known as the shaking test. After the pat has dried, it
extend to great depth. However, particularly in warm is brittle and dust can be detached by rubbing it with the
humid climates where the time of exposure has been long, finger. Silt is relatively impervious, but if it is in a loose
residual soils may extend to depths of hundreds of meters. state it may rise into a drill hole or shaft like a thick
They may be strong and stable, but they may also consist viscous fluid. The most unstable soils of this category are
of highly compressible materials surrounding blocks of known locally under different names, such as bulls liver.
less weathered rock (Article 47.12). Under these circum- Organic silt is a fine-grained more or less plastic soil
stances they may give rise to difficulties with foundations with an admixture of finely divided particles of organic
and other types of construction. Many deposits of trans- matter. Shells and visible fragments of partly decayed
ported soils are soft and loose to a depth of more than a vegetable matter may also be present. The soil ranges in
hundred meters and may also lead to serious problems. color from light to very dark gray, and it is likely to
Soils of organic origin are formed chiefly in situ, either contain a considerable quantity of H2S, COz, and various
by the growth and subsequent decay of plants such as other gaseous products of the decay of organic matter
peat mosses or by the accumulation of fragments of the which give it a characteristic odor. The permeability of
inorganic skeletons or shells of organisms. Hence a soil organic silt is very low and its compressibility very high.
of organic origin can be either organic or inorganic. The Clay is an aggregate of microscopic and submicro-
term organic soil ordinarily refers to a transported soil scopic particles derived from the chemical decomposition
consisting of the products of rock weathering with a more of rock constituents. It is plastic within a moderate to
or less conspicuous admixture of decayed vegetable wide range of water content. Dry specimens are very
matter. hard, and no powder can be detached by rubbing the
The soil conditions at the site of a proposed structure surface of dried pats with the fingers. The permeability
are commonly explored by means of test borings or test of clay is extremely low. The term gumbo is applied,
shafts. The inspector on the job examines samples of the particularly in the western United States, to clays that are
soil as they are obtained, classifies them in accordance distinguished in the plastic state by a soapy or waxy
with local usage, and prepares a boring log or shaft record appearance and by great toughness. At higher water con-
containing the name of each soil and the elevation of its tents they are conspicuously sticky.
boundaries. The name of the soil is modified by adjectives Organic clay is a clay that owes some of its significant
indicating the stiffness, color, and other attributes. At a physical properties to the presence of finely divided
later date the record may be supplemented by an abstract organic matter. When saturated, organic clay is likely to
of the results of tests made on the samples in the be very compressible, but when dry its strength is very
laboratory. high. It is usually dark gray or black and it may have a
The following list of soil types includes the names conspicuous odor.
commonly used for field classification. Peat is a somewhat fibrous aggregate of macroscopic
Sand and gravel are cohesionless aggregates of and microscopic fragments of decayed vegetable matter.
rounded subangular or angular fragments of more or less Its color ranges between light brown and black. Peat is
unaltered rocks or minerals. Particles with a size up to 2 so compressible that it is almost always unsuitable for
mm are referred to as sand, and those with a size from supporting foundations. Various techniques have been
2 mm to 200 mm as gravel. Fragments with a diameter developed for carrying earth embankments across peat
of more than 200 mm are known as boulders. deposits without the risk of breaking into the ground, but
Hardpan is a soil that has an exceptionally great resis- the settlement of these embankments is likely to be large
tance to the penetration of drilling tools. Most hardpans and to continue at a decreasing rate for many years.
are extremely dense, well-graded, and somewhat cohesive If a soil is made up of a combination of two different
aggregates of mineral particles. soil types, the predominant ingredient is expressed as a
Inorganic silt is a fine-grained soil with little or no noun, and the less prominent ingredient as a modifying
plasticity. The least plastic varieties generally consist of adjective. For example, silty sand indicates a soil that is
more or less equidimensional grains of quartz and are predominantly sand but contains a small amount of silt.
sometimes called rock flour; whereas the most plastic A sandy clay is a soil that exhibits the properties of a
types contain an appreciable percentage of flake-shaped clay but contains an appreciable amount of sand.
particles and are referred to as plastic silt. Because of its The aggregate properties of sand and gravel are
smooth texture, inorganic silt is often mistaken for clay, described qualitatively by the terms loose, medium, and
but it may be readily distinguished from clay without dense, whereas those of clays are described by hard, stifJ;
 ARTICLE 2 PRINCIPAL TYPES OF SOILS 5

medium, and soft.These terms are usually evaluated by decomposition produces loess loam, characterized by
the boring foreman or inspector on the basis of several greater plasticity than other forms of modified loess.
factors, including the relative ease or difficulty of advanc- Diatomaceous earth (kieselguhr) is a deposit of fine,
ing the drilling and sampling tools and the consistency generally white, saliceous powder composed chiefly or
of the samples. However, since this method of evaluation wholly of the remains of diatoms. The term diatom applies
may lead to a very erroneous conception of the general to a group of microscopic unicellular marine or fresh-
character of the soil deposit, the qualitative descriptions water algae characterized by silicified cell walls.
should be supplemented by quantitative information Lake marl or boglime is a white fine-grained powdery
whenever the mechanical properties are likely to have an calcareous deposit precipitated by plants in ponds. It is
important influence on design. The quantitative informa- commonly associated with beds of peat.
tion is commonly obtained by means of laboratory tests Marl is a rather loosely used term for various fairly
on relatively undisturbed samples (Article 11.3), or by stiff or very stiff marine calcareous clays of greenish color.
suitable in situ tests (Articles 11.4 and 11.5). Shale is a clastic sedimentary rock mainly composed of
A record of the color of the different strata encountered silt-size and clay-size particles. Most shales are laminated
in adjacent borings reduces the risk of errors in correlating and display fissility; the rock has a tendency to split along
the boring logs. Color may also be an indication of a real relatively smooth and flat surfaces parallel to the bedding.
difference in the character of the soil. For example, if the When fissility is completely absent, the clastic sedimen-
top layer of a submerged clay stratum is yellowish or tary deposit is called mudstone or clay rock. Depending
brown and stiffer than the underlying clay, it was probably on clay mineralogy, void ratio, and degree of diagenetic
exposed temporarily to desiccation combined with weath- bonding or weathering, compressive strength of shales
ering. Terms such as mottled, marbled, spotted, or speck- may range from less than 2.5 MPa to more than 100 MPa.
led are used when different colors occur in the same Adobe is a term applied in the southwestern United
stratum of soil. Dark or drab colors are commonly associ- States and other semiarid regions to a great variety of
ated with organic soils. light-colored soils ranging from sandy silts to very plas-
Under certain geological conditions soils form that are tic clays.
characterized by one or more striking or unusual features Caliche refers to layers of soil in which the grains are
such as a root-hole structure or a conspicuous and regular cemented together by carbonates deposited as a result of
stratification. Because of these features, such soils can evaporation. These layers commonly occur at a depth of
easily be recognized in the field and, consequently, they
several meters below the surface, and their thickness may
have been given special names by which they are com-
range up to a few meters. A semiarid climate is necessary
monly known. The following paragraphs contain defini-
for their formation.
tions and descriptions of some of these materials.
Varved clay consists of alternating layers of medium
Till is an unstratified glacial deposit of clay, silt, sand,
gray inorganic silt and darker silty clay. The thickness of
gravel, and boulders.
the layers rarely exceeds 10 mm, but occasionally very
Tuff is a fine-grained water- or wind-laid aggregate
much thicker varves are encountered. The constituents
of very small mineral or rock fragments ejected from
volcanoes during explosions. were transported into freshwater lakes by melt water at
Loess is a uniform, cohesive, wind-blown sediment, the close of the Ice Age. Varved clays are likely to com-
and is commonly light brown. The size of most of the bine the undesirable properties of both silts and soft clays.
particles ranges between the narrow limits of 0.01 and Bentonite is a clay with a high content of montmorillon-
0.05 mm. The cohesion is due to the presence of a binder ite (Article 4).Most bentonites were formed by chemical
that may be predominantly calcareous or clayey. Because alteration of volcanic ash. In contact with water, dried
of the universal presence of continuous vertical root holes, bentonite swells more than other dried clays, and saturated
the permeability in vertical direction is usually much bentonite shrinks more on drying.
greater than in horizontal directions; moreover, the mate- Each term used in the field classification of soils
rial has the ability to stand on nearly vertical slopes. True includes a great variety of different materials. Further-
loess deposits have never been saturated. On saturation more, the choice of terms relating to stiffness and density
the bond between particles is weakened and the surface depends to a considerable extent on the person who exam-
of the deposit may settle. ines the soil. Consequently, the field classification of soils
Mod@ed loess is a loess that has lost its typical charac- is always more or less uncertain and inaccurate. More
teristics by secondary processes, including temporary specific information can be obtained only by physical
immersion, erosion and subsequent deposition, chemical tests that furnish numerical values representative of the
changes involving the destruction of the bond between properties of the soil.
the particles, or chemical decomposition of the more per- The methods of soil exploration, including boring and
ishable constituents such as feldspar. Thorough chemical sampling, and the procedures for determining average
6 INDEX PROPERTIES OF SOILS

numerical values for the soil properties are discussed in in a given soil increases with decreasing grain size of the
Chapter 2. soil fraction.
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
ARTICLE 3 SIZE AND SHAPE OF SOIL fractions, the aggregate is called a uniform soil. Uniform
PARTICLES very coarse or coarse soils are common, but uniform very
fine or colloidal soils are very seldom encountered. All
The size of the particles that constitute soils may vary clays contain fine, very fine, and colloidal constituents,
from that of boulders to that of large molecules. and some clays contain even coarse particles. The finest
Grains larger than approximately 0.06 mm can be grain-size fractions of clays consist principally of flake-
inspected with the naked eye or by means of a hand lens. shaped particles.
They constitute the very coarse and coarse fractions of The widespread prevalence of flake-shaped particles
the soils. in the very fine fractions of natural soils is a consequence
Grains ranging in size from about 0.06 mm to 2 p (1 of the geological processes of soil formation. Most soils
p = 1 micron = 0.001 mm) can be examined only under orginate in the chemical weathering of rocks. The rocks
the microscope. They represent the fine fraction. themselves consist partly of chemically very stable and
Grains smaller than 2 p constitute the veryfinefraction partly of less stable minerals. Chemical weathering trans-
(clay size fraction, CF).Grains having a size between 2 forms the less stable minerals into a friable mass of very
p and about 0.1 p can be differentiated under the micro- small particles of secondary minerals that commonly have
scope, but their shape cannot be discerned. The shape of a scale-like or flaky crystal form, whereas the stable
grains smaller than about 1 p can be determined by means minerals remain practically unaltered. Thus the process
of an electron microscope. Their molecular structure can of chemical weathering reduces the rock to an aggregate
be investigated by means of X-ray analysis. consisting of fragments of unaltered or almost unaltered
The process of separating a soil aggregate into frac- minerals embedded in a matrix composed chiefly of dis-
tions, each consisting of grains within a different size crete scaly particles. During subsequent transportation
range, is known as mechanical analysis. By means of by running water the aggregate is broken up, and the
mechanical analysis, it has been found that most natural constituents are subjected to impact and grinding. The
soils contain grains representative of two or more soil purely mechanical process of grinding does not break up
fractions. The general character of mixed-grained soils the hard equidimensional grains of unaltered minerals
is determined almost entirely by the character of the into fragments smaller than about 10 p (0.01 mm). On the
smallest soil constituents. In this respect soils are some- other hand, the friable flake-shaped particles of secondary
what similar to concrete. The properties of concrete are minerals, although initially very small, are readily ground
determined primarily by the cement, whereas the aggre- and broken into still smaller particles. Hence, the very
gate, which constitutes most of the concrete, is inert. The fine fractions of natural soils consist principally of flake-
aggregate, or the inert portion of a mixed-grained soil, shaped particles of secondary minerals.
comprises about 80 or 90% of the total dry weight. The
decisive or active portion constitutes the remainder.
Very coarse fractions, for example gravel, consist of ARTICLE 4 PROPERTIES OF VERY FINE
rock fragments each composed of one or more minerals. SOIL FRACTIONS
The fragments may be angular, subangular, rounded, or
4.1 Mineralogical Composition
flat. They may be fresh, or they may show signs of
considerable weathering. They may be resistant or The most important grain property of fine-grained soil
crumbly. materials is the mineralogical composition. If the soil
Coarse fractions, exemplified by sand, are made up of particles are smaller than about 0.002 mm, the influence
grains usually composed chiefly of quartz. The individual of the force of gravity on each particle is insignificant
grains may be angular, subangular, or rounded. Some compared with that of the electrical forces acting at the
sands contain a fairly high percentage of mica flakes that surface of the particle. A material in which the influence
make them very elastic or springy. of the surface charges is predominant is said to be in
In the fine and very fine fractions, any one grain usually the colloidal state. The colloidal particles of soil consist
consists of only one mineral. The particles may be angu- primarily of clay minerals that were derived from rock
lar, flake-shaped, or tubular. Rounded particles, however, minerals by weathering, but that have crystal structures
are conspicuously absent. Exceptionally, the fine fraction differing from those of the parent minerals.
contains a high percentage of porous fossils, such as All the clay minerals are crystalline hydrous alumino-
diatoms or radiolaria, that produce abnormal mechanical silicates having a lattice structure in which the atoms are
properties. In general, the percentage of flaky constituents arranged in several layers, similar to the pages of a book.
 ARTICLE 4 PROPERTIES OF VERY FINE SOIL FRACTIONS 7
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 form tetrahedral and octahedral layers (Fig. 4.1),
different combinations of which produce a unit sheet of
the various types of clays.
A single particle of clay may consist of many sheets
or 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 parti-
cles are always in contact with water. The interactions
among the clay particles, the water, and the various mate-
rials dissolved in the water are primarily responsible for
the properties of the soil consisting of the particles.
The characteristics of the principal clay minerals are
Figure 4.2 Photomicrograph of kaolinite.
described in the following paragraphs.

4.2 Characteristics of Principal Clay Minerals 1 nm thick. A typical particle has the shape of a hollow
Kaolinite is one of the most common clay minerals in tube or prism with outside and inside diameters of 70
sedimentary and residual soils (Grim 1968, Swindale and 40 nm, respectively, and is 300 to 500 nm long. The
1975). A unit sheet of kaolinite, which is approximately intersheet water in the hydrated halloysite is removed
0.7 nm (nm = m) thick, is composed of one alumi- irreversibly starting at 60" to 75C.
num octahedral layer and one silicon tetrahedral layer, Allophane, a major constituent of young residual soils
joined together by shared oxygens. A typical particle of formed from volcanic ash, is an amorphous hydrous alu-
kaolinite consists of a stack of sheets forming a stiff minosilicate commonly associated with halloysite (Grim
hexagonal plate with flat-faced edges. It is about 100 nm 1968, Fieldes and Claridge 1975, Wesley 1973). In some
in thickness with a breadthhhickness of about 5 to 10, residual soils, the transition to halloysite from allophane,
and a specific surface of 5 to 15 m2/g. The scanning which forms at early stages of weathering, is not well
electron microscope (SEM) photomicrograph in Fig. 4.2 defined. Allophane consists of very loosely packed chains
shows a range of particle sizes and shapes including of silica tetrahedra and alumina octahedra, cross-linked
terraced surfaces where packets of 0.7-nm sheets at a relatively small number of points. In the natural state,
terminate. it exists as microaggregates of extremely fine particles
Hulloysite is one of the most common minerals in of the order of several nanometers and specific surface
residual soils, particularly those derived from volcanic areas of 250 to 800 m2/g. The allophane aggregates or
parent material. It is a member of the kaolin subgroup clusters are relatively incompressible. They are some-
of clay minerals. A unit sheet of hydrated halloysite, times cemented by iron or aluminum oxides, and they
including one molecular layer of water, is approximately enclose a large amount of water. Since the aggregates
are susceptible to structural breakdown upon mechanical
manipulation, as by heavy equipment, a deposit of allo-
phane 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
la (6) reversed. The resulting soil, with silt- and sand-sized
0 Oxygen 0 Hydroxyl particles that are quite hard, is practically nonplastic.
SiJicon Aluminum, Magnesium, Illite is the most common clay mineral in stiff clays
Iton
and shales as well as in postglacial marine and lacustrine
Figure 4.1 (a) Tetrahedral layer. (b)Octahedral layer. (After soft clay and silt deposits (Grim 1968, Radoslovich 1975,
Grim 1968) Reichenbach and Rich 1975). It is often present, some-
8 INDEX PROPERTIES OF SOILS

times interstratified with other sheet silicates, in sedimen- Montmorillonite, the most common member of a group
tary and residual soils, except in residual soils derived of clay minerals known as smectites, is the dominant clay
from amorphous volcanic material. Illite is also referred mineral in some clays and shales and in some residual
to as fine-grained mica and weathered mica. soils derived from volcanic ash (Grim 1968, Mering
The crystal structure of illite is similar to that of musco- 1975). Relatively pure seams of montmorillonite are
vite mica in the macroscopic form. However, in micro- found in some deposits, Wyoming bentonite being the
scopic illite particles the stacking of the sheets is not so best-known example. A unit sheet of montmorillonite is
regular as in well-crystallizedmicas, and weathering may similar to that of the micas. In montmorillonite,octahedral
remove intersheet K+ from the edges of the plates. The A1 is partially replaced by Mg atoms. Each isomorphous
resulting illite particles, with terraced surfaces where one replacement (Article 4.3) produces a unit negative charge
or more unit sheets terminate, have frayed and tattered at the location of the substituted atom, which is balanced
edges, are flexible and elastic, are 10 to 30 nm in thick- by exchangeable cations, such as Ca and Na+ situated
ness, have a breadthhhickness of 15 to 30, and have a at the exterior of the sheets. In a packet of montmorillonite
specific surface of 80 to 100 m2/g. A SEM photomicro- in the anhydrous state, where as many as 10 unit sheets
graph of illite particles is shown in Fig. 4.3. are in contact, the stacking of the sheets is disordered in
Chlorite is a clay mineral commonly associated with the sense that the hexagonal cavities of the adjacent sur-
micas and illite, but is usually a minor component (Grim faces of two neighboring sheets are not matched face-to-
1968, Bailey 1975). A unit sheet of chlorite, which is face. In a hydrous environment,water molecules penetrate
about 1.4 nm thick, consists of one biotite mica sheet in between the sheets and separate them by 1 nm (Le., four
which all octahedral sites are occupied by magnesium and molecular layers of water).
one brucite sheet, an octahedral layer in which magnesium In sodium montmorillonite, the exchange capacity is
atoms are in octahedral coordination with hydroxyls. The satisfied by Na cations. If the electrolyte concentration
biotite mica and brucite sheets are strongly bonded in water is less than 0.3 N, further separation of unit
together. The unit sheets are stacked and are connected sheets takes place to more than 3 nm by difluse double-
to each other by hydrogen bonding of surface oxygens layer repulsion (Article 4.4). The resulting particles of
of the tetrahedral layer of mica and surface hydroxyls of sodium montmorillonite in water are thin films 1 nm
brucite. The size and platyness of chlorite particles are thick, with breadth-to-thickness ratios in excess of 100
similar to those of illite. However, in contrast to illite, and specific surfaces as much as 800 m2/g. In calcium
in which the unit sheets are bonded by potassium, the montmorillonite, the electrostatic .attraction of the Ca cat-
hydrogen bonding between chlorite sheets results in pseu- ions links the successive sheets together and prevents
dohexagonal or euhedral-shaped platelets that are flexible separation beyond 1 nm. The resulting domains of 8 to
but inelastic. 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 is shown by
the SEM photomicrograph in Fig. 4.4.
Atrupulgite 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 Caillkre 1975). Water molecules fill the inter-
stices between the chains. The particles of attapulgite are
relatively rigid, 5 to 10 nm thick, 10 to 20 nm wide, and
0.1 to 1 pm in length, as shown in the SEM, Fig. 4.5.
The specific surface of the bundles is about 150 m2/g,
but it can be much higher for dispersed single laths.
Attapulgite, frequently associated with carbonate rocks,
is not a very common clay mineral in soil deposits. How-
ever, when present, it results in unusual physical proper-
ties such as a very high plastic limit, a very high plasticity
index, and high frictional resistance (Articles 7.2 and
19.2).
Mixed-layer Clay Minerals
Particles in some soils are composed of regularly or ran-
Figure 4.3 Photomicrograph of illite. domly interstratified unit sheets of two or more types
 ARTICLE 4 PROPERTIES OF VERY FINE SOIL FRACTIONS 9
the physical properties of interstratified clay minerals are
not well defined, inasmuch as a wide range of combina-
tions is possible. Mixed-layer clay mineral particles pos-
sess properties generally representative of the
constituent minerals.
4.3 Role of Isomorphic Substitution
Isomorphic substitution is the replacement of a cation in
the mineral structure by another cation of lower electrova-
lence. 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 sili-
cate composed of silica tetrahedrons, (Si04)-4 linked
together by primary valence bonds to form a three-dimen-
sional network with the formula Si02.There is no isomor-
phous 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
Figure 4.4 Photomicrograph of montmorillonite.
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 crystal-
line quartz amorphous.
In feldspar, some of the silicon atoms are replaced by
aluminum. This results in a negative charge and in distor-
tion of the crystal structure, because A1 atoms are larger
than Si atoms. The negative charge is balanced by taking
in cations such as K', Na', and Cat* 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 mechani-
cal 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.
Figure 4.5 Photomicrograph of attapulgite. 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
of clay minerals (MacEwan and Ruiz-Ami1 1975). For layers are linked together with one octahedral layer. In
example, a marine or lacustrine clay deposit may include muscovite, only two of every three octahedral sites are
regularly alternating montmorillonite-illite particles, and occupied by aluminum cations, whereas in biotite all sites
a volcanic ash residual soil may contain randomly inter- are occupied by magnesium. In well-crystallized micas
stratified halloysite and hydrated halloysite. These mixed- one fourth of the tetrahedral Si+4are replaced by A1+3.
layer clay minerals often represent an intermediate phase The resulting negative charge in common micas is bal-
in the transformation, by weathering or through a diage- anced by intersheet potassiums. In a face-to-face stacking
netic process, of one mineral to another mineral, such as of sheets to form mica plates, the hexagonal holes on
that of mica to montmorillonite or vice versa. However, opposing tetrahedral surfaces are matched to enclose the
10 INDEX PROPERTIES OF SOILS

intersheet potassium. Perfect cleavage occurs along the The highly polar water molecule has the ability to form
K-plane, however, as the intersheet bond strength of strong bonds with the surface of soil particles, as well as
( K O , Z ) - is
~ ~only 1/12 of the bond strength of (Si04)-4 with the exchangeable cations that surround it. The strong
inside the sheets. short-range adsorption forces hold one to four molecular
There is very little isomorphous substitution in kaolin- layers of water at the surface of the soil particles. This
ite, the particles of which are formed by hydrogen bonding water is said to be adsorbed. It is of most importance if
of unit sheets. The edge charge is important, because the the particles are very small, such as films of sodium
particles are thick and the edge surface area is significant. montmorillonite 1 nm thick, and is insignificant if they
The edge charge, which results from broken bonds and are large, such as 200-km grains of quartz sand.
exposed oxygens at the edges of the particles, may be If a soil particle is surrounded by water, the exchange-
positive or negative in accordance with the pH of the able cations are not attached to it. Its negative electrical
water. When the pH is low (acidic condition), excess H+ charge tends to attract the cations, but the cations diffuse
ions associate with the oxygens at the edge of the particles, toward the lower cation concentration away from the
and give the edge a net positive charge. When the pH is particle. Therefore, the soil particle is surrounded by a
high, H' ions disassociate from the edge, which then domain known as an electric double layer (van Olphen
becomes negatively charged. In mica and illite a major 1977, Mitchell 1976). The inner layer of the double layer
part of the negative charge resulting from isomorphous is the negative charge on the surface of the soil particle.
substitution is neutralized by K atoms which also connect The outer layer is the excess of cations and deficiency
the successive unit sheets to each other to form particles. of anions with respect to the concentration in the free
water not influenced by the force field of the particle. The
4.4 Cation Exchange and Adsorbed Water cation concentration has a finite value near the surface of
the particle and decreases exponentially with distance to
Cations that neutralize the net negative charge on the
the concentration of the cations in the free pore water.
surface of soil particles in water are readily exchangeable
The thickness of double-layer water, which is determined
with other cations. The exchange reaction depends mainly
by the valence of the exchangeable cations and by the
on the relative concentrations of cations in the water and
electrolyte concentration in free pore water, can exceed
also on the electrovalence of the cations. Cation exchange
50 nm. Thick double-layer water develops with exchange-
capacity, measured in milliequivalents of cations per gram
able cations of low electrovalence such as Na+, and in
of soil particles, is a measure of the net negative charge on
free pore water of low electrolyte concentration, as in
the soil particles, resulting from isomorphous substitution
freshwater rivers and lakes. On the other hand, exchange-
and broken bonds at the boundaries. The values of the
able cations of high valence such as Caf2and high electro-
cation exchange capacity for the principal clay minerals
lyte concentration as in the marine environment tend to
are indicated in Table 4.1. Montmorillonite has a rela-
depress the thickness of the double-layer water. Thus, in
tively large exchange capacity because its particles may
general, a soil particle is covered by a 1-nm layer of
consist of single unit sheets. Very fine particles of other
adsorbed water, surrounded by 1 to > 50 nm of double-
minerals such as mica and quartz also carry a net negative
layer water, enveloped in turn by free water. Double-
charge in water as a result of broken bonds at the bound-
layer water is most significant in sodium montmorillonite
aries. However, even in the range of small particle sizes
because of its very small and filmy particles.
in which nonclay minerals occur in soils, the exchange
When soil particles approach each other, as during
capacity is relatively small.
deposition or consolidation, a number of long-range and
short-range interparticle forces influence their geometri-
cal arrangement and interaction. Repulsion develops
Table 4.1 Cation Exchange Capacity of Principal when two soil particles approach each other and their
Clay Minerals double layers come in contact. The particles will remain
dispersed unless they are pushed together by an external
Cation Exchange force, such as the weight of overburden, equal to the
Mineral Capacity (meq/g) repulsive force. For example, a pressure of 500 kPa is
required to bring two face-to-face-oriented sodium mont-
Kaolinite 0.03-0.1 morillonite particles, in 0.01 N electrolyte concentration,
Illite 0.2-0.3 to within 4 nm of each other. An increase in electrolyte
Chlorite 0.2-0.3 concentration depresses double-layer thickness and
Attapulgite 0.2-0.35 allows closer approach of the particles, which aggregate
Hydrated halloysite 0.4-0.5 into pocs. The flocs of clay-mineral particles may settle
Montmorillonite 0.8-1.2 simultaneously, without segregation, with other fine parti-
cles and with silt-sized particles of quartz, mica, and
 ARTICLE 4 PROPERTIES OF VERY FINE SOIL FRACTIONS 11

feldspars. On the other hand, in a freshwater lacustrine

environment flocculation is absent, and segregated varved
clay and silt deposits develop.
Adsorbed water layers must be removed for soil parti-
cles to approach within less than 1 nm of each other.
Enormous external pressures are required to squeeze them
out. For example, a pressure of 125 MPa is required to
bring two parallel plates of montmorillonite or mica to
within 0.5 nm of each other, and a pressure of 400 MPa
is required to remove the last molecular layer of water.
Such magnitudes of contact pressure are likely to develop
only at mineral-to-mineral contact points of sand-sized
and larger particles, where in general the force per particle
can be large and the contact area very small. This is
illustrated in Figs. 4.6through 4.9, which show an assem-
blage of sand grains, close-ups of grain-to-grain contacts,
and a magnified smooth fracture surface, respectively. It
is apparent that the area of contact between grains is
extremely small and that even small forces between grains
would produce very high pressures. At the other extreme,
it is unlikely that geologic pressures due to overburden
on clays and shales or typical construction loads could Figure 4*7 PhotomicrographOf contact between grains of
quartz sand.
squeeze adsorbed water from between plates of montmo-
rillonite in which the force per particle is small and the
contact area, as compared with the size of the particles,
is very large. Edge-to-face contact in sodium montmoril-
lonite is also unlikely, because the highly flexible films
cannot be pushed together. In kaolinite, short-range edge-
to-face contact between particles is possible, because the
particles are relatively large. As a result of their small
breadth-to-thickness ratio, they are rather stiff and thus

Figure 4.8 Photomicrograph of contact between grains of

dune sand.

are capable of transmitting forces parallel to the plates.

The particle size and shape of illite suggest a behavior
intermediate between those of kaolinite and montmoril-
lonite, but closer to the former.
Mineral-to-mineral contact is established after the last
Figure 4.6 Photomicrograph of Lake Michigan beach sand molecular layer of water is desorbed and the very short
particles. range repulsion of approaching mineral surfaces is over-
12 INDEX PROPERTIES OF SOILS

Canada, suggest a possible cementing action by molecular

coatings of carbonates or silicates on the surface of soil
particles that are connected to each other. For example,
such an interparticle bonding may be responsible for the
rather high water content of the soft clays from eastern
Canada as compared with their relatively inactive mineral
composition. However, most soft and stiff clay deposits
as well as granular soils are uncemented.
4.5 Fabric
The term fabric describes the geometrical arrangement
of soil particles with respect to each other. During sedi-
mentation of very fine-grained soil particles in quiet fresh
water, the elementary mineral units tend to settle individu-
ally. The resulting sediment has a random geometrical
arrangement in which the mineral particles have no pro-
nounced preferred orientation. As overburden accumu-
lates, the platy or elongated particles tend to rotate into
more horizontal positions, whereupon the sediment
becomes more anisotropic but retains its random microfa-
bric. If the supply of sediment is seasonal, the coarser
Figure 4.9 Magnification of smooth fracture surface of sediments, which settle more quickly than the finer ones,
quartz.
soon cease to accumulate during the periods of low inflow,
and only the finer particles continue to settle out. The
come. This type of contact is required for developing result is a segregated fabric, of which a varved clay is
primary valence bonding between the particle surfaces an example. On the other hand, if the sediment accumu-
or, alternatively, for engaging microscopic roughness of lates in salt water, flocculation occurs, and all mineral
the contact surfaces, such as protruding lattice points or types with elementary particles from fine clay-size to
regions. It is also required for generating sliding fric- coarse silt-size settle simultaneously to form a random
tional resistance. unsegregated deposit.
Although cementing between mineral grains is a signif- Most fine-grained natural soil deposits consist of a
icant factor in rocks such as sandstone and even in residual mixture of mineral types that often include sheet silicates
soils at the early stages of weathering, the nature and as well as space lattice silicates and some carbonates.
extent of microscopic cementing in soils is not well estab- This is illustrated by the mineralogical composition of
lished. The existence of rather stable aggregates of clay several soft-clay deposits listed in Table 4.2. A typical
minerals in most clay and shale deposits, and the brittle soft-clay deposit includes, in addition to illite and chlorite
behavior of soils such as certain soft clays from eastern and some montmorillonite, substantial amounts of quartz

Table 4.2 Mineralogical Composition of Soft Clays

Total mineral content (%) Clay mineral content (%)

Clay Plagio- K- Montmor-

Clay Mineral Quartz clase feldspar Amphibole Calcite Dolomite Illite Chlorite illonite Other

Boston Blue 30 35 24 8 3 0 0 74 21 5 0
St. Hilaire 32 23 21 0 8 9 8 55 39 6 0
Berthierville 21 37 25 0 17 0 0 53 34 13 0
La Grande 11 29 25 7 0 10 17 70 30 0 0
Vasby 38 33 16 13 0 0 0 67 14 9 10
Pisa* 73 14 5 0 0 6 0 60 16 24+ 0

* Also contains 2% pyrite.

Mixed-layer: 17% illite/montmorillonite and 7% chlorite/montmorillonite.
 ARTICLE 4 PROPERTIES OF VERY FINE SOIL FRACTIONS 13

Figure 4.10 Fabric of undisturbed Boston blue clay. Figure 4.11 Fabric of undisturbed St. Hilaire clay.

and feldspar and some carbonates. Some of the very fine

minerals may have been eroded from shales and were about 14,000 years ago in coastal brackish water. The
probably already in an aggregated form when they sample described in Fig. 4.10 and Tables 4.2 and 4.3 was
reached the sedimentation basin. a firm to stiff clay preconsolidated by desiccation. St.
The SEM photomicrographs of natural soft-clay depos- Hilaire clay and Berthierville clay are postglacial clays
its in Figs. 4.10 through 4.15 generally suggest more deposited about 10,000years ago in the inland Champlain
or less flocculated- or aggregated-random fabrics. The Sea. La Grande clay is a postglacial clay deposited 6,000
mineralogical composition of these clays is shown in to 8,000 years ago in the marine environment of the
Table 4.2 and their index properties are presented in Table Tyrrell Sea. Vasby clay is a recent postglacial marine clay
4.3. Boston Blue Clay is a glacial outwash deposited from the eastern central coast of Sweden. Pancone clay

Table 4.3 Index Properties of Soft Clays

W O W1 WP CF a,:
No. Clay (%) (%I (%) (-2p,m%) (kPa) up:, Degrees

1 Boston Blue 27-30 34 17 40 155 3.20-3.50 30

2 St. Hilaire 6 1-68 55 23 77 83 1.40-1.57 26
3 Berthierville 57-63 46 24 36 39 1.30-1.40 27
4 La Grande 55-58 64 26 52 83 1.75-2.00 28
5 Vasby 94- 103 121 40 67 28 I .20-1.34 23
6 Pisa-Pancone 54-65 86 35 72 124 1.57-2.02 23
7 Mexico City 421-574 500 150 27 58 1.40-1.60 45

w, = natural water content (Article 6).

w I= liquid limit (Article 7.2).
wp = plastic limit (Article 7.2).
CF = clay fraction (Article 8.2).
ai, = effective vertical overburden pressure (Article 15.3)
up = preconsolidation pressure (Article 16.4).
+ = effective-stress friction angle (Article 19.2).
14 INDEX PROPERTIES OF SOILS

Figure 4.12 Fabric of undisturbed Berthierville clay. Figure 4.14 Fabric of undisturbed Vasby clay.

Figure 4.15 Fabric of undisturbed Pancone clay.

Figure 4.13 Fabric of undisturbed LaGrande clay.

The fabric of the postglacial soft clays from eastern

from Pisa was deposited 7,000 years ago in a marine Canada is very open, and their natural water content is
environment. The salt concentration in the pore water of high in comparison with their mineralogical composition
these brackish or marine deposits has been substantially and liquid limit. A slow rate of deposition accompanied
reduced, through either leaching or diffusion, following by simultaneous development of interparticle bonds by
crustal rebound that elevated the clay deposits above the carbonates and possibly silicates apparently is responsible
sea level after withdrawal of glacial ice (Bjerrum 1954, for the special fabrics (Quigley 1980). When the salt
Kenney 1964). content of these clays was subsequently reduced by leach-
 Next Page

ARTICLE 4 PROPERTIES OF VERY FINE SOIL FRACTIONS 15

Figure 4.16 Fabric of undisturbed Mexico City clay. Figure 4.17 Silt-size fraction of Mexico City clay showing
poriferous whole geometric forms and fragments of diatoms.

ing or diffusion leading to a tendency for particle disper- In contrast to the random fabric or structure of most
sion, the clays acquired the characteristics to lose most natural clays, clays that have been sheared by tectonic
of their strength and to liquefy when subjected to remold- activity, by sliding, or even by manipulation in the labora-
ing at constant water content (Bjemm 1954). tory or by construction activities, are likely to lose their
Not all soft-clay deposits have a mineralogical compo- flocculated, random structure. They are then said to have
sition similar to those in Table 4.2. Mexico City clay is a dispersed, highly oriented fabric. In this state they are
a dramatic example of soft clay derived from volcanic likely to have properties quite different from those of
material. It consists of about 5 to 10% sand-sized concre- clays with a flocculated or aggregated random fabric.
tionary particles composed of calcium carbonate; 55 to
65% silt-sized siliceous diatoms; 20 to 30% clay-sized 4.6 Organic Soils
particles ( 10% interlayered montmorillonite, with Organic substances in soil range from macroscopic
exchangeable cations that are mostly sodium, and the incompletely decomposed plant and animal residues to
remainder biogenic or volcanogenic silica); and 5 to 10% microscopic dark-colored humus. Humus includes prod-
organic material. Thus, Mexico City clay is largely com- ucts of advanced decomposition of organic residues, prod-
posed of microfossils that mainly are siliceous skeletons ucts of microbial resynthesis, precipitates of dissolved
and skeletal fragments of diatoms. Apparently the large organic compounds, and organic molecules in solution
quantities of silica released by the volcanic ash as it (Gieseking 1975a). Organic substances are composed
underwent weathering initiated a great bloom of diatoms mainly of carbon, oxygen, and hydrogen. However, dif-
in the Pleistocene lake waters of the valley of Mexico. ferent organic parent materials, various aerobic and anaer-
The undisturbed fabric of Mexico City clay is shown in obic conditions of degradation, and different degrees of
Fig. 4.16, and photomicrographs of the silt-sized fraction humification produce organic substances with a wide
are shown in Figs. 4.17 and 4.18. Figure 4.16 shows that range of molecular structure and particle morphology. A
Mexico City clay has an open flocculated-random fabric. highly poriferous and flexible cellular structure is the
The highly poriferous skeletal fragments are mainly most important characteristic of organic coarse particles,
responsible for such unusual properties as very high plas- which are either fibrous or granular. Organic fine sub-
tic and liquid limits, a very high friction angle, and great stances, usually smaller than 100 km, consist of irregu-
loss of strength upon manipulation. Similar high values larly shaped organic skeletons such as cell fragments and
of plastic limit and friction angle are displayed by volca- tissue parts, as well as of globular organic precipitates
nic ash residual soils of Japan, which contain allophane smaller than 1 km, and of 3- to 9-nm organic polymolec-
and halloysite. ules. Organic fine substances are negatively charged and