Elsevier Encyclopedia of Geology - vol I A-E
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Any system of soil classification involves grouping
soils into categories that possess similar properties, so
providing a systematic method of soil description
by which soils can be identified quickly. Although
soils include materials of various origins, for purposes
of engineering classification it is sufficient to consider
their simple index properties, which can be assessed
easily, such as their particle size distribution and con-
sistency limits. For instance, coarse-grained soils are
distinguished from fine on a basis of particle size,
gravels and sands being the two principal types
of coarse-grained soils. Plasticity also is used when
classifying fine-grained soils, that is, silts and clays.
Quicksands
As water flows through sands or silts and slows down,
its energy is transferred to the particles past which it is
moving that, in turn, creates a drag effect on the
particles. If the drag effect is in the same direction as
the force of gravity, then the effective pressure is in-
creased and the soil is stable. Conversely, if water
flows towards the surface, then the drag effect is
counter to gravity, thereby reducing the effective pres-
sure between particles. If the upward flow velocity is
sufficient, it can buoy up the particles so that the
effective pressure is reduced to zero. This represents
a critical condition where the weight of the submerged
soil is balanced by the upward acting seepage force.
If the upward velocity of flow increases beyond
this critical hydraulic gradient, then a quick condition
develops. As the velocity of the upward seepage force
increases further from the critical gradient, the soil
begins to boil more and more violently (Figure 1A).
At such a point structures fail by sinking into the
quicksand (Figure 1B).
Quicksands, if subjected to deformation or disturb-
ance, can undergo a spontaneous loss of strength, caus-
ing them to flow like viscous liquids. A number of
conditions must exist for quick conditions to develop.
Firstly, the sand or silt concerned must be saturated and
loosely packed. Secondly, on disturbance the constitu-
ent grains become more closely packed, which leads to
an increase in pore-water pressure, reducing the forces
acting between the grains. This brings about a reduc-
tion in strength. If the pore water can escape very
rapidly the loss in strength is momentary. Hence, the
third condition requires that pore water cannot escape
readily. This is fulfilled if the sand or silt has a low
permeability and/or the seepage path is long.
Quick conditions frequently are encountered in
excavations made in fine sands that are below the
watertable. Liquefaction of potential quicksands
also may be brought about by sudden shocks caused
by the action of heavy machinery (notably pile
driving), blasting, and earthquakes (Figure 1B).
Such shocks increase the stress carried by the pore
water, and give rise to a decrease in the effective stress
and shear strength of the soil. There is also a possibil-
ity of a quick condition developing in a layered soil
sequence containing fine sands where the individual
beds have different permeabilities.
Collapsible Soils
Soils, such as loess and brickearth, commonly are
regarded as being of aeolian origin and some believe
that the material of which they are composed was
formed initially by glacial action. These soils often
have a metastable fabric and so possess the potential
to collapse. In addition, some wind-blown silts, such
as those found in subtropical arid areas like Arizona
and the Kalahari Desert in southern Africa, have
metastable fabrics and are potentially collapsible.
Certain residual soils also are prone to collapse. For
example, when granites undergo notable chemical
weathering in subtropical regions that involves appre-
ciable leaching, then the resulting saprolite and
Table 1 Effects of transportation on sediments
Gravity Ice Water Air
Size Various Varies from clay
to boulders
Various sizes from
boulder gravel to muds
Sand size and less
Sorting Unsorted Generally
unsorted
Sorting takes place
both laterally and vertically.
Marine deposits often
uniformly sorted. River
deposits may be well sorted
Uniformly sorted
Shape Angular Angular From angular to well
rounded
Well rounded
Surface texture Striated surfaces Striated surfaces Gravel: rugose surfaces.
Sand: smooth, polished
surfaces.
Silt: little effect
Impact produces frosted surfaces
ENGINEERING GEOLOGY/Problematic Soils 555
residual soil tend to develop a metastable fabric as
fine particles are removed. As a consequence, the
weathered products are potentially collapsible.
Truly collapsible soils are those in which collapse
occurs on saturation since the soil fabric cannot sup-
port the weight of the overburden. When the saturation
collapse pressure exceeds the overburden pressure,
soils are capable of supporting a certain level of stress
on saturation and can be regarded as conditionally col-
lapsible soils. The maximum load that such soils can
support is the difference between the saturation col-
lapse and overburden pressures. These soils generally
consist of 50 to 90% silt particles, and sandy, silty, and
clayey types have been recognized, with most falling
into the silty category (Figure 2). The fabric of collaps-
ible soils generally takes the form of a loose skeleton of
Figure 1 (A) Liquefaction of sand due to the Tangshan earthquake (28 July 1976) near Dougtian, China. (B) Collapse of buildings
because of liquefaction of fine sands and silts, Niigata, Japan, 1964.
Figure 2 Particle size distribution of loess.
556 ENGINEERING GEOLOGY/Problematic Soils
grains (generally quartz) and micro-aggregates (usually
assemblages of clay particles). These tend to be separate
from each other, being connected by bonds and bridges,
with uniformly distributed pores (Figure 3). The
bridges are formed of clay-sized minerals. As grains
are not in contact, mechanical behaviour is governed
by the structure and quality of the bonds and bridges.
The structural stability of collapsible soils also is
related to the amount of weathering undergone.
Younger weakly weathered loess generally has a high
potential for collapse whereas older weathered loess is
relatively stable. In addition, highly collapsible loess
tends to occur in regions where the landscape and/or
the climatic conditions are not conducive to develop-
ment of long-term saturated conditions within the
soil. The size of the pores is all important, collapse
normally occurring as a result of pore-space reduction
taking place in pores greater than 1 mm in size and
more especially in those exceeding 10 mm in size.
Hence, collapsible soils possess porous textures with
high void ratios and relatively low densities. At their
natural low moisture content, these soils possess
high apparent strength but they are susceptible to
large reductions in void ratio upon wetting. In other
words, their metastable texture collapses as the bonds
between the grains break down when the soil is
wetted. Collapse on saturation normally only takes a
short period of time, although the more clay particles
such a soil contains, the longer the period tends to be.
From the above it may be concluded that signifi-
cant settlements can take place beneath structures
founded on collapsible soils after they have been
wetted, in some cases in the order of metres. These
have led to foundation failures. A number of tech-
niques can be used to stabilize collapsible soils; these
are summarized in Table 2.
Expansive Clays
An important characteristic of some clay soils is their
susceptibility to slow volume change that can occur
independently of loading due to swelling or shrink-
age. Some of the best examples of expansive clays
are provided by vertisols or black clays. Such soils
typically are developed in subtropical regions with
well-defined wet and dry seasons. Unfortunately, ex-
pansive clays frequently are responsible for significant
national costs due to damage caused to property. For
example, in the United States, the cost of repair of
damage to property built on expansive clays exceeds
two billion dollars annually. This frequently is twice
the cost of flood damage or landslide damage and 20
times the cost of earthquake damage. Many clay soils
in Britain, especially south-east England, possess the
potential for large volume change. However, the mild
damp climate means that significant deficits in soil
moisture developed during the summer are confined
to the upper 1.0 to 1.5 m of the soil and that the field
capacity is re-established during the winter. Even so,
deeper permanent deficits can be brought about by
Figure 3 Scanning electron photomicrograph of brickearth
from south-east Essex, showing three silt particles, two of
which are bridged by clay particles.
Table 2 Methods of treating collapsible foundations
Depth of subsoil
treatment
0–1.4 m Moistening and compaction (conventional extra heavy
impact or vibratory rollers)
1.5–10 m Over-excavation and recompaction (earth pads with or without stabilization by additives such as cement or
lime). Vibro-compaction (stone columns). Vibroreplacement. Dynamic compaction. Compaction piles.
Injection of lime. Lime piles and columns. Jet grouting. Ponding or flooding (if no impervious layer exists).
Heat treatment to solidify the soils in place.
Over 10 m Any of the aforementioned or combinations of the aforementioned, where applicable.
Ponding and infiltration wells, or ponding and infiltration wells with the use of explosive.
ENGINEERING GEOLOGY/Problematic Soils 557
transpiration from large trees, and significant damage
to property on potentially expansive clay can be
caused during notably dry summers.
Generally, the clay fraction of expansive clay
exceeds 50%, silty material varying between 20 and
40%, and sand forming the remainder. Montmoril-
lonite normally is present in the clay fraction and is the
principal factor determining the appreciable volume
changes that take place in these soils on wetting and
drying.
The depth of the active zone in expansive clays (i.e.,
the zone in which swelling and shrinkage occurs in
wet and dry seasons, respectively) can be large. For
instance, the maximum seasonal changes in the mois-
ture content of expansive clays in Romania are
around 20% at 0.4 m depth, 10% at 1.2 m depth,
and less than 5% at 1.8 m depth. The correspond-
ing cyclic movements of the ground surface are be-
tween 100 and 200 mm. Surface heaves approaching
500 mm have been recorded in some expansive clays
in South Africa. During the dry season, profiles in
some regions can dry out to depths of 15 to 20 m.
The potential for volume change in expansive clay
soils depends on the initial moisture content, initial
voids ratio, the microstructure, and the vertical stress,
as well as the type and amount of clay minerals pre-
sent. The type of clay minerals are responsible pri-
marily for the intrinsic expansiveness, whilst the
change in moisture content or suction controls the
actual amount of volume change that a soil undergoes
at a given applied pressure. Changes in soil suction
are brought about by moisture movement through
the soil, due to evaporation from its surface in dry
weather, by transpiration from plants, or alternatively
by recharge consequent upon precipitation. Alternat-
ing wet and dry seasons may produce significant ver-
tical movements as soil suction changes. The rate of
expansion depends upon the rate of accumulation of
moisture in the soil. In semi-arid regions, it is limited
by the availability of water and this, together with the
available void volume, governs the rate of penetration
of the heave front in the soil. When these expansive
clays occur above the water table, they can undergo a
high degree of shrinkage on drying. Seasonal changes
in volume also produce shrinkage cracks so that ex-
pansive clays are often heavily fissured. Sometimes
the soil is so desiccated that the fissures are wide
open and the soil is shattered or micro-shattered.
Transpiration from vegetative cover is a major
cause of water loss from soils in subtropical semi-
arid regions. Indeed, the distribution of soil suction
in soil is controlled primarily by transpiration from
vegetation and this represents one of the most signifi-
cant changes made in loading (i.e., to the state of stress
in a soil). The suction induced by the withdrawal of
moisture fluctuates with the seasons, reflecting the
growth of vegetation. The maximum soil suction
that can be developed is governed by the ability of
vegetation to extract moisture from the soil. The
level at which moisture is no longer available to plants
is termed the permanent wilting point. In fact, the
moisture content at the wilting point exceeds that of
the shrinkage limit in soils with high clay contents and
is less in those possessing low clay contents. This
explains why settlement resulting from the desiccating
effects of trees is more notable in low to moderately
expansive soils than in highly expansive ones.
These volume changes can give rise to ground move-
ments that may result in damage to buildings. Low-rise
buildings are particularly vulnerable to such ground
movements since they generally do not have sufficient
weight or strength to resist. Be that as it may, three
methods can be adopted when choosing a design solu-
tion for building on expansive soils. Firstly, a founda-
tion and structure can be provided that can tolerate
movements without unacceptable damage; secondly,
the foundation and structure can be isolated from the
effects of the soil; and thirdly, the ground conditions
can be altered or controlled. These soils also represent a
problem when they are encountered in road construc-
tion, and shrinkage settlement of embankments com-
posed of such clay soils can lead to cracking and
breakup of the roads they support (Figure 4).
Dispersive Soils
Dispersive soils occur in subtropical semi-arid regions,
normally where the rainfall is less than 850 mm annu-
ally. Dispersion occurs in such soils when the repulsive
forces between clay particles exceed the attractive
forces, thus bringing about deflocculation so that in
the presence of relatively pure water the particles repel
each other to form colloidal suspensions. In non-
dispersive soil there is a definite threshold velocity
below which flowing water causes no erosion. By con-
trast, there is no threshold velocity for dispersive soil,
the colloidal clay particles going into suspension even
in quiet water. Therefore, these soils are highly suscep-
tible to erosion and piping. Dispersive soils contain a
moderate to high content of clay material but there are
no significant differences in the clay fractions of disper-
sive and non-dispersive soils, except that soils with less
than 10% clay particles may not have enough colloids
to support dispersive piping. Dispersive soils contain a
higher content of dissolved sodium (up to 12%) in their
pore water than ordinary soils. The clay particles in
soils with high salt contents exist as aggregates and
coatings around silt and sand particles (Figure 5).
For a given eroding fluid the boundary between the
flocculated and deflocculated states depends on the
558 ENGINEERING GEOLOGY/Problematic Soils
value of the sodium adsorption ratio, the salt concen-
tration, the pH value, and the mineralogy. The sodium
adsorption ratio (SAR) is used to quantify the role of
sodium where free salts are present in the pore water
and is defined as:
Na
SAR ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
0:5ðCa þ MgÞ
p
½1
with units expressed in meq/litre of the saturated ex-
tract. It has been suggested that an SAR value greater
than 10 is indicative of dispersive soils, between 6 and
10 as intermediate, and less than 6 as non-dispersive.
However, dispersion has occurred in soils with values
lower than 6. The presence of exchangeable sodium is
the main chemical factor contributing towards disper-
sive behaviour in soil. This is expressed in terms of the
exchangeable sodium percentage (ESP):
ESP ¼
exchangeable sodium
cation exchange capacity
100 ½2
where the units are given in meq/100 g of dry clay.
A threshold value of ESP of 10% has been recom-
mended, above which soils that have their free salts
leached by seepage of relatively pure water are prone
to dispersion. Soils with ESP values above 15% are
highly dispersive. Those with low cation exchange
values (15 meq/100 g of clay) have been found to
be completely non-dispersive at ESP values of 6% or
below. Similarly, soils with high cation exchange cap-
acity values and a plasticity index greater than 35%
swell to such an extent that dispersion is not significant.
Severe erosion or worse, serious piping damage to
embankments and piping failures of earth dams have
occurred when dispersive soils have been used in their
construction. Indications of piping take the form of
small leakages of muddy coloured water from an earth
dam after initial filling of the reservoir. The pipes
become enlarged rapidly and this can lead to failure
of the dam (Figure 6). Experience, however, indicates
Figure 5 Scanning electron photomicrograph illustrating the
fabric of dispersed soil from Natal, South Africa.
Figure 4 Breakup along the CapeTown Johannesburg road, South Africa, due to expansive clay.
ENGINEERING GEOLOGY/Problematic Soils 559
that if an earth dam is built with careful construction
control and incorporates filters, then it should be safe
enough, even if it is constructed with dispersive soil.
Alternatively, hydrated lime, pulverised fly ash, gyp-
sum, or aluminium sulphate have been used to treat
dispersive soils used in earth dams.
Humid Tropical Zone Soils
In humid tropical regions, weathering of rock is more
intense and extends to greater depth than in other parts
of the world. Residual soils develop in place as a conse-
quence of weathering, primarily chemical weathering.
The mineralogy of residual soils is partly inherited from
the parent rock from which they were derived and
partly produced by the processes of weathering.
Hence, the mineralogy varies widely, as does grain
size and unit weight. The particles and their arrange-
ment evolve gradually as weathering proceeds. In ad-
dition, weathering of parent rock in situ may leave
behind relict structures that may offer weak bonding
even in extremely weathered material. Low strength
along relict discontinuities may be attributable to par-
ticles being coated with low-friction iron/manganese
organic compounds.
Reproducible results from some standard tests
may be difficult to obtain from residual tropical
soils. Different results can be obtained depending
upon whether the soil is pre-dried prior to testing
or kept close to its natural moisture content. Also,
disaggregation of the soil structure, especially in rela-
tion to particle size analysis has proved problematic.
Consequently, conventional index tests frequently
have been modified in an attempt to make them
more applicable for use with tropical residual soils.
Of course, it would be wrong to assume that all trop-
ical soils behave differently from those found in other
climatic regions. For instance, alluvial clays and sands
behave in the same manner and have similar geotech-
nical properties, regardless of the climatic conditions
of the region of deposition.
Drying brings about changes in the properties of
residual clay soils in that it initiates two important
effects, namely, cementation by the sesquioxides and
aggregate formation on the one hand, and loss of
water from hydrated clay minerals on the other. In
the case of halloysite, the latter causes an irreversible
transformation to metahalloysite. Drying can cause
almost total aggregation of clay size particles into silt
and sand size ranges, and a reduction or loss of plasti-
city. Cycles of wetting and drying may increase the
stiffness of the soil fabric, which increases its shear
strength and decreases its compressibility.
Laterite can be regarded as a highly weathered ma-
terial that forms as a result of the concentration of
hydrated oxides of iron and aluminium in such a way
that the character of the deposit in which they occur is
affected. These oxides may be present in an unhar-
dened soil, as a hardened layer, as concretionary no-
dules in a soil matrix or in a cemented matrix enclosing
Figure 6 Failed earth dam constructed of dispersive soil, showing piping outlets on the downstream side, near Ramsgate, South
Africa.
560 ENGINEERING GEOLOGY/Problematic Soils
other materials. When a hardened crust is present near
the surface, then the strength of laterite beneath
decreases with increasing depth.
Red clays and latosols are residual ferruginous soils
formed primarily by chemical weathering of the
parent rock. This results in the release of iron and
aluminium sesquioxides, increasing loss of silica and
increasing dominance of new clay minerals such as
smectites, allophane, halloysite and, with increasing
weathering, kaolinite. The microstructure also is de-
veloped by chemical weathering processes and con-
sists of an open-bonded fabric of silt and sand size
peds, which are formed mainly of clay minerals and
fine disseminated iron oxides. Relatively weak bonds
exist between the peds and are formed of iron oxides
and/or amorphous aluminium silicate gels. Such soils
differ from laterite in that they behave as clay and do
not possess strong concretions. They do, however,
grade into laterite.
Allophane-rich soils or andosols are developed
from basic volcanic ashes in high temperature-rainfall
regions. Allophane is an amorphous clay mineral.
These soils have very high moisture contents, usually
in the range 60% to 80% but values of up to 250%
have been recorded; and corresponding high plasti-
city. The soils also are characterized by very low dry
densities and high void ratios (sometimes as high as
6). Moisture content does affect the strength of ando-
sols significantly as the degree of saturation can have
an appreciable affect on cementation. Soils contain-
ing halloysite, or its partially dehydrated form meta-
halloysite, have high moisture contents (30% to
65%) and can possess high plasticity. Some of these
soils are susceptible to collapse.
Soils of Hot Arid Regions
Most soils in arid regions consist of the products of
physical weathering of rock material. This breakdown
process gives rise to a variety of rock and mineral
fragments that may be transported and deposited
under the influence of gravity, wind, or water. Many
arid soils are of aeolian origin and sands frequently are
uniformly sorted. Uncemented silty soil may possess a
metastable fabric and hence be potentially collapsible.
The precipitation of salts in the upper horizons of
an arid soil, due to evaporation of moisture from
the surface, commonly means that some amount of
cementation has occurred, which generally has been
concentrated in layers, and that the pore water is
likely to be saline. High rates of evaporation in hot
arid areas may lead to ground heave due to the
precipitation of minerals within the capillary fringe.
Where the watertable is at a shallow depth the soils
may possess a salty crust and be chemically aggressive
due to the precipitation of salts from saline ground-
water. Occasional wetting and subsequent evap-
oration frequently are responsible for a patchy
development of weak, mainly carbonate, and occa-
sionally gypsum cement, often with clay material de-
posited between and around the coarser particles.
These soils, therefore, may undergo collapse, espe-
cially where localized changes in the soil–water
regime are brought about by construction activity.
Collapse is attributed to a loss of strength in the bind-
ing agent and the amount of collapse undergone
depends upon the initial void ratio. Loosely packed
aeolian sandy soils, with a density of less than 1.6 Mg
m
3
, commonly exhibit a tendency to collapse.
Silts may have been affected by periodic desiccation
and be interbedded with evaporite deposits. The latter
process leads to the development of a stiffened crust
or, where this has occurred successively, to a series of
hardened layers within the formation. Loosely packed
aeolian silty soils formed under arid conditions often
undergo considerable volume reduction or collapse
when wetted. Such metastability arises from the loss
in strength of interparticle bonds resulting from in-
creases in water content. Thus, infiltration of surface
water, including that applied during irrigation, leak-
age from pipes, and rise of water table, may cause
large settlements to occur.
Low-lying coastal zones and inland plains in arid
regions with shallow water tables, are areas in which
sabkha conditions commonly develop. Sabkhas are
extensive saline flats that are underlain by sand, silt,
or clay and often are encrusted with salt. Ground-
water is saline, containing calcium, sodium, chloride,
and sulphate ions. Evaporative pumping, whereby
brine moves upward from the water table under ca-
pillary action, appears to be the most effective mech-
anism for the concentration of salt in groundwater
and the precipitation of minerals in sabkha. Salts are
precipitated at the ground surface when the capillary
fringe extends from the water table to the surface.
One of the main problems with sabkha is the de-
crease in density and strength, and increased permea-
bility that occur, particularly in the uppermost layers,
after rainfall, flash floods, or marine inundation, due to
the dissolution of soluble salts that act as cementing
materials. Changes in the hydration state of minerals,
such as calcium sulphate, also cause significant volume
change in soils. There is a possibility of differential
settlement occurring on loading due to the different
compressibility characteristics resulting from differen-
tial cementation of sediments. Excessive settlement
also can occur, due to the removal of soluble salts by
flowing groundwater. This can cause severe disruption
to structures within months or a few years. Movement
of groundwater also can lead to the dissolution of
ENGINEERING GEOLOGY/Problematic Soils 561
minerals to an extent that small caverns, channels, and
surface holes can be formed. On the other hand, heave
resulting from the precipitation and growth of crystals
can elevate the surface of a sabkha in places by as much
as 1 m.
Because sabkha soils frequently are characterized
by low strength, their bearing capacity and com-
pressibility frequently do not meet routine design re-
quirements. Various ground improvement techniques,
therefore, have been used in relation to large con-
struction projects such as vibro-replacement, dynamic
compaction, compaction piles, and underdrainage.
Soil replacement and preloading have been used when
highway embankments have been constructed.
Cementation of sediments by precipitation of min-
eral matter from the groundwater may lead to the
development of various crusts or cretes (e.g., pre-
cipitation of calcite gives rise to calcrete and of
gypsum to gypcrete). These may form continuous
sheet or isolated patch-like masses at the ground sur-
face when the water table is at or near this level, or at
some other position within the ground profile. There-
fore, duricrusts or pedocretes are soils that have been,
to a greater or lesser extent, cemented. These mater-
ials take three forms, namely, indurated (e.g., hard-
pans and nodules), non-indurated (soft or powder
forms), and mixtures of the two (e.g., nodular pedo-
cretes). Indurated pedocretes that occur at the surface
may be underlain by loose or soft material that, at
times, may be potentially expansive or collapsible.
Nodular pedocrete with a smectitic clay matrix
could have the potential to be expansive, whereas
collapsibility may be associated with cemented soils,
powder, or nodular calcretes. Small-scale karst-like
features have been recorded in some weathered
calcretes.
Soils Developed in Cold Regions
Tills are deposited by ice-sheets and consist of a vari-
able assortment of rock debris ranging from fine rock
flour to boulders; they are characteristically unsorted.
On the one hand, tills may consist predominantly of
sand and gravel with very little binder, whereas on the
other they may have an excess of clayey material.
The nature of a till deposit depends on the lithology
of the material from which it was derived, on the
position in which it was transported in the glacier,
and on the mode of deposition. The underlying bed-
rock usually constitutes up to about 80% of basal
tills, depending on its resistance to abrasion. Till
sheets can comprise one or more layers of different
material, not all of which are likely to be found at any
one locality. Shrinking and reconstituting of an
ice-sheet can complicate the sequence.
Distinction has been made between tills derived
from rock debris carried along at the base of a glacier
and those deposits that were transported within or at
the terminus of the ice. The former is referred to as
lodgement till, whilst the latter is termed ablation till.
Lodgement till is commonly stiff, dense, and rela-
tively incompressible. Because of the overlying weight
of ice, such deposits are overconsolidated. Fissures
frequently are present in lodgement till and influence
its shear strength and therefore its stability. Ablation
till accumulates on the surface of the ice when engla-
cial debris melts out. It, therefore, is normally consoli-
dated and non-fissile. The proportion of silt and clay
size material is relatively high in lodgement till (e.g.,
the clay fraction typically varies from 15 to 40%).
Ablation till is characterized by abundant large angu-
lar stones, the proportion of sand and gravel is high,
and clay is present only in small amounts (usually less
than 10%).
The most familiar pro-glacial deposits are varved
clays. These sediments accumulated on the floors of
glacial lakes and are characteristically composed of
alternating laminae of finer and coarser grain size,
each such couplet being termed a varve. The thickness
of the individual varve is frequently less than 2 mm.
Generally, the coarser layer is of silt size and the finer
of clay size. Clay minerals tend to show a high degree
of orientation parallel to the laminae.
Varved clays have a very wide range of plasticity,
although most tend to be highly plastic and many
possess the potential to swell. They tend to be normally
consolidated or lightly overconsolidated. Furthermore,
they often undergo a notable reduction in strength
when remoulded, that is, they are medium sensitive
to sensitive. However, the remoulded strength in-
creases with time (Figure 7). The anisotropic behaviour
of varved clay is explained by the influence of their
lamination.
Quick clays were formed at the end or after the last
ice age, when meltwater from glaciers carried large
quantities of fine-grained sediments into the sea. The
material of which quick clays are composed is pre-
dominantly smaller than 0.002 mm but some deposits
seem to be poor in clay minerals, containing a high
proportion of ground down fine quartz. The open
fabric that is characteristic of quick clays has been
attributed to their initial deposition, during which
time colloidal particles interacted to form loose ag-
gregations by gelation and flocculation. This fabric
may have been retained to the present day.
Quick clays often exhibit little plasticity. However,
the most extraordinary property possessed by quick
clays is their very high sensitivity, that is, a large
proportion of their undisturbed strength is perman-
ently lost following shear (Figure 8). Consequently,
562 ENGINEERING GEOLOGY/Problematic Soils
quick clays are associated with several serious engin-
eering problems. Not only is their bearing capacity
low, with large settlements occurring under load, but
quick clays can liquefy on sudden shock. This can
result in an almost instantaneous loss in strength.
Slides, which have been referred to as mud-runs and
bottleneck slides, therefore are a common feature of
quick clay areas and sometimes have proved disas-
trous. Quick clays can be stabilized by adding salts or
lime, which increase their remoulded shear strength.
Frozen ground phenomena are found in regions that
experience a tundra climate, that is, in those regions
where the winter temperatures rarely rise above freez-
ing point and the summer temperatures are only warm
enough to cause thawing in the upper metre or so of
the soil. Beneath the upper or active zone, the subsoil
is permanently frozen and so is known as permafrost.
Because of the permafrost layer, summer meltwater
cannot seep into the ground, the active zone then
becomes waterlogged and soils on gentle slopes are
liable to flow. Generally, the depth of thaw is less, the
higher the latitude. It is at a minimum in peat or highly
organic sediments and increases in clay, silt, and sand,
to a maximum in gravel. Layers or lenses of unfrozen
ground, termed taliks, may occur, often temporarily,
in the permafrost.
The mechanical properties of frozen soil are influ-
enced largely by the grain size distribution, mineral
content, density, frozen and unfrozen water contents,
and presence of ice lenses and layering. Ice has no
long-term strength, that is, it flows under very small
loads. When loaded, stresses at the point of contact
between soil particles and ice bring about pressure
melting of the ice. Because of differences in the surface
tension of the meltwater, it tends to move into regions
of lower stress, where it refreezes. The process of ice
melting and the movement of unfrozen water are ac-
companied by breakdown of ice and bonding with
grains of soil. This leads to plastic deformation of the
ice in the voids and to a rearrangement of particle
fabric. The net result is time-dependent deformation
of the frozen soil, namely, creep. Frozen soil undergoes
appreciable deformation under sustained loading, the
magnitude and rate of creep being governed by the
composition of the soil, especially the amount of
ice present, the temperature, the stress, and the stress
history.
As the soil thaws downwards the upper layers
become saturated, and since water cannot drain
through the frozen soil beneath, they may suffer a
complete loss of strength. What is more, as ice melts,
settlement occurs. Excess pore-water pressures de-
velop when the rate of ice-melt is greater than the
discharge capacity of the soil. This can lead to the
failure of slopes and foundations.
Shrinkage, which gives rise to polygonal cracking
of the ground, presents a problem when soil is sub-
jected to freezing. Individual cracks may be over 1 m
wide at their top and may penetrate to depths of 10 m.
Water that accumulates in the cracks is frozen to form
Figure 8 Moisture content, consistency indices, undrained
shear strength, and sensitivity of quick clay from near Trondheim,
Norway.
Figure 7 Examples of stress-strain curves of undisturbed and
remoulded Tees Laminated Clay tested in quick undrained tri-
axial conditions: (1) from Stockton, (2) from Cowpen Bewley,
(3) from Middlesbrough.
ENGINEERING GEOLOGY/Problematic Soils 563
ice wedges. When the ice disappears, an ice wedge
pseudomorph is formed by sediment filling the crack.
The ground also may undergo notable disturbance
due to mutual interference of growing bodies of ice
or excess pore-water pressures developed in confined
water-bearing lenses. Involutions are plugs, pockets,
or tongues of highly disturbed material, generally
possessing inferior geotechnical properties, which
have been intruded into overlying layers. Pseudo-
morphs of ice wedges and involutions usually mean
that one material suddenly replaces another. This can
cause problems in shallow excavations. Prolonged
freezing gives rise to shattering in the frozen layer.
Frost shattering may extend to significant depths
and is responsible for an increase in deformability
and permeability of the ground affected. It should
be borne in mind that periglacial conditions extended
over a much greater area of the land surface during
the Pleistocene epoch and as a result the features
mentioned are found in many present temperate
climate regions such as Britain.
There are two types of methods of construction in
permafrost, namely, passive and active methods. In
the former, the frozen ground is not disturbed and
heat from a structure is prevented from thawing the
ground below, thereby reducing its stability. In other
words, the aim is to maintain the ground beneath
buildings in a frozen state. This is accomplished either
by ventilation or by insulation. In the former case, a
building is raised on piles above the ground so that air
can circulate beneath it, thereby dissipating heat gen-
erated by the building. Alternatively, a building may
be constructed on a layer of gravel to insulate the
ground beneath. An insulating material such as poly-
styrene may be incorporated into the layer of gravel.
The ground is thawed prior to construction in the
active method. It is either kept thawed or removed
and replaced by materials not affected by frost action.
The active method is used where permafrost is thin,
sporadic, or discontinuous, and where thawed ground
has an acceptable bearing capacity. Foundations fre-
quently are taken through the active layer into the
permafrost beneath. Hence, piles often are used as
foundation structures.
Frost action in a soil obviously is not restricted to
periglacial regions. If frost penetrates down to the
capillary fringe in fine-grained soils, especially silts
then, under certain conditions, lenses of ice may be
developed. The formation of such ice lenses may, in
turn, cause frost heave and frost boil that may lead to
the breakup of roads and the failure of slopes. Such
problems are experienced in temperate climatic
zones, as well as those of tundra regions.
Peat Soils
Peat represents an accumulation of partially decom-
posed and disintegrated plant remains that have been
Figure 9 Holme Post, a cast iron pillar erected in 1851 on the south-west edge of Whittlesey Mere, near Peterborough, England, to
indicate the amount of peat shrinkage and associated subsidence caused by drainage. The post was driven through 7 m of peat into clay
until its top was flush with the ground. Within 10 years ground level had fallen 1.5 m through shrinkage. A second post was erected in
1957 with its top at the same level as that of the original post (right-hand side). Between 1851 and 1971 the ground subsided by some 4 m.
564 ENGINEERING GEOLOGY/Problematic Soils