Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.

Obras y Proyectos 6, 5-10.

Behaviour and geotechnical properties of residual

soils and allophane clays
Fecha de entrega: 20 de Septiembre 2009
Fecha de aceptacin: 23 de Noviembre 2009

Laurie Wesley
Department of Civil and Environmental Engineering, the University of Auckland, Private Bag 92019, Auckland, New Zealand,
l.wesley@auckland.ac.nz

An overview of the properties of residual soils is given in the rst part En la primera parte del artculo se entrega una descripcin general de
of the paper. The different processes by which residual and sedimentary los suelos residuales. Se detallan los diferentes procesos en los cuales son
soils are formed are described, and the need to be aware that procedures formados los suelos residuales y sedimentarios, poniendo hincapi en la
applicable to sedimentary soils do not necessarily apply to residual soils necesidad de estar atento a que los procedimientos aplicados a los suelos
is emphasised. In particular, it is shown that the log scale normally sedimentarios no son necesariamente aplicables a los suelos residuales.
used for presenting oedometer test results is not appropriate or relevant En particular, se muestra que la escala logartmica generalmente usada
to residual soils. The second part of the paper gives an account of para presentar resultados de ensayos edomtricos no es apropiada o
the special properties of allophane clays. Their abnormally high water pertinente para suelos residuales. La segunda parte del artculo da
content and Atterberg limits are described, and it is shown that despite cuenta de las propiedades especiales de arcillas alofnicas. Se describen
this, their geotechnical properties are remarkably good. Methods for sus altos valores de contenido de agua y lmites de Atterberg y se muestra
control of compaction of residual soils and allophane clays are also que a pesar de esto, sus propiedades geotcnicas son sorprendentemente
described. buenas. Tambin se describen mtodos de control de compactacin para
suelos residuales y arcillas alofnicas.

Keywords: residual soils, volcanic, allophane clays, Palabras clave: suelos residual, volcnico, arcillas alofnicas,
consolidation, shear strength, compaction consolidacin, resistencia al corte, compactacin

interesting to note that very few text books, and

Introduction probably very few university courses on soil mechanics,
even mention residual soils, let alone give an adequate
Soil mechanics grew up in northern Europe and North
account of their properties.
America, and most of its concepts regarding soil
behaviour developed from the study of sedimentary
soils. In fact, most of the early concepts came from
the study of remoulded sedimentary soils and involved
investigating the inuence of stress history on their
behaviour, in the belief that this was simulating the Re-deposition
in lakes or the ocean
inuence of stresses which soils may be subject to
during their formation processes. Most text books on
soil mechanics and university courses on the subject
place considerable emphasis on stress history soils
tend to be divided into normally consolidated and over-
consolidated on this basis, and behavioural frameworks
are developed around this stress history concept. Figure 1 : Diagrammatic representation of soil formation
This might be all very well if all soils were sedimentary processes.
soils. This of course is clearly not the case. Large
areas of the earth (including large areas in the North Formation processes
Island of New Zealand) consist of residual soils, and
the application of concepts coming from sedimentary Figure 1 shows diagrammatically the physical processes
soils may or may not be relevant to these soils. It is that to the formation of sedimentary and residual soils.
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

Residual soils are formed directly from the physical and

chemical weathering of the parent material, normally
rock of some sort. Sedimentary soils are formed by
a depositional process, normally in a marine or lake
environment. Figure 2 is an attempt to summarise the
factors involved in the formation processes that inuence
the properties of the two soil types. Sedimentary soils
are seen to undergo a various additional processes
beyond the initial physical and chemical weathering of
the parent rock. It might appear from this diagram that
the factors involved in the formation of sedimentary
soils are more complex than those involved in forming
residual soils. There is some truth in this, but in practice
two important factors lead to a degree of homogeneity
and predictability with sedimentary soils that is absent
from residual soils. These factors are:

- The sorting process which take place during erosion,

transportation and deposition of sedimentary soils tend Figure: 2 Soil formation factors inuencing soil behaviour
to produce homogeneous deposits.
With residual soils, mineralogy remains an important
- Stress history is a prominent factor in determining inuence, but stress history is not a concept which
the behavioural characteristics of sedimentary soils, has much if any relevance. The physical and chemical
and leads to the convenient division of these soils into weathering processes that form these soils produce
normally and over consolidated materials. particular types of clay minerals, and particular
structures i.e. particular arrangements of the particles,
The absence of these factors with residual soils means and possibly bonding or cementing effects between
that they are generally more complex and less capable particles. These inuences are innitely more important
of being divided into tidy categories or groups. than stress history. The terms normally consolidated
and overconsolidated are therefore not directly relevant
It is perhaps helpful to consider that the behaviour of to residual soils.
a soil, whether residual or sedimentary, is dependent
on two factors, or two groups of factors. These are, Grouping and classication of residual soils
rstly the nature of the soil particles themselves (i.e.
their size, shape, and mineralogical composition) and Various attempts have been made to group or classify
secondly, the particular state in which these particles residual soils, but none are particularly useful. Some, such
exist in the ground. For convenience, these factors can as that of the British Geological Society (1990) make use
be referred to respectively as composition and structure. of soil science classications and are not very useful for
With sedimentary clays, the inuence of composition is engineering purposes. Terms such as vertisols, andosols,
well known kaolinite group clays are relatively inert etc are not normally meaningful to engineers, and the
with consequent low shrinkage/swell characteristics variation in properties within these groups is likely to
and relatively low compressibility, while montmorillinite be so large as to make the grouping of little relevance.
clays are highly active and of opposite characteristics
to the kaolinite group. Notwithstanding the inuence Focussing on the two factors discussed above, namely
of mineralogy, by far the most important attribute mineralogical composition and structure, provides a basis
of sedimentary clays in their undisturbed state (at least for dividing residual soils into groups that can be expected
according to conventional soil mechanics) is their stress to have fairly similar engineering properties. Starting with
history i.e. whether they are normally consolidated mineralogy, the following groups can be established:
or over-consolidated. This is generally given greater
importance in the literature than either mineralogy or (a) Soils without a strong mineralogical inuence
structure. those containing low activity clays): many residual soils
Wesley, L. (2009). Obras y Proyectos 6, 5-10

fall into this category, especially those derived from The predominant clay mineral is allophane (frequently
the weathering of sandstones, or igneous rocks such associated with another mineral called imogolite).
as granite. These soils are likely to be fairly coarse (iii) Laterites: the term laterite is used very loosely, but
grained with a small clay fraction. Structure is likely should refer to deposits in which weathering has reached
to be an important concept in understanding the an advanced stage and has resulted in a concentration
behaviour of these soils. The weathered granite soils of iron and aluminium oxides (the sesquioxides gibbsite
of Hong Kong and Malaysia fall into this group. and goethite), which act as cementing agents. Laterials
(b) Soils with a strong mineralogical inuence, from therefore tend to consist of hard granules formed by
conventional clay minerals (i.e. those containing high this cementing action; they may range from sandy clays
activity clays): one very important worldwide group to gravels, and are used for road sub-bases or bases.
comes into this category the black cotton soils or
vertisols, also called Houston Black Clay in Texas, Table 1 shows this grouping system for residuals soils,
Tropical Black Earths of Australia, Tirs of Morocco and Table 2 attempts to list some of the more distinctive
etc. The predominant clay mineral is smectite, a group characteristics of these soil groups and indicates the
of which montmorillionite is a member. These black means by which they may possibly be identied.
cotton soils are highly plastic, highly compressible
and of high shrink/swell potential. Structural effects Following on from mineralogy, the next characteristic
are almost zero with these soils. They normally form which should be considered is structure, which refers to
in poorly drained areas, and have poor engineering specic characteristics of the soil in its undisturbed (in
properties. situ) state. Structure can be divided into two categories:
(c) Soils with a strong mineralogical inuence, coming
from special clay minerals not found in sedimentary (a) Macro-structure, or discernible structure: this
clays: the two most important clay minerals found includes all features discernible to the naked eye, such
only in certain residual soils (especially tropical residual as layering, discontinuities, ssures, pores, presence of
soils of volcanic origin) are halloysite and allophane. unweathered or partially weathered rock and other relict
These are both silicate clay minerals. Apart from silicate structures inherited from the parent rock mass.
minerals, tropical soils may contain non-silicate minerals (b) Micro-structure, or non-discernible structure: this
(or oxide minerals), in particular the hydrated forms includes fabric, inter-particle bonding or cementation,
of aluminium and iron oxide, gibbsite and goethite. aggregations of particles, pores etc. Micro-structure is
The most unusual of these minerals, in terms of more difcult to identify than macro-structure, although
understanding soil behaviour is allophane. it can be inferred indirectly from other behavioural
characteristics such as sensitivity. High sensitivity
Soils of Group (c) which contain these unusual minerals indicates the presence of some form of bonds between
include: particles which are destroyed by remoulding.

(i) tropical red clays predominant mineral is halloysite This grouping system is intended to help geotechnical
but may also contain kaolinite, with gibbsite and goethite. engineers nd their way around residual soils, and to
Halloysite particles are generally very small in size but draw attention to the properties likely to be of most
are of low activity, and soils containing halloysite as the signicance for geotechnical engineering. It is not
predominant mineral generally have good engineering intended to perform a function as a rigorous classication
properties. Red clays generally form in well drained areas system. Some comments on local or Southeast Asian
in a tropical climate having a wet and dry season. Red clays soils may be helpful at this stage.
may be referred to as lateritic soils or as latosols. There
is a wide range of engineering properties found in red Weathered Waitemata clays (Auckland, NZ) : This is an
clays, but they should not be confused with laterite itself. example of a group which does not t comfortably in
(ii) Volcanic ash soils (or andosols or andisols): any one category and this in itself tells us something
these are found in many tropical and sub-tropical about these clays. Some Waitemata clays are essentially
countries (including New Zealand) and are silts, and are not strongly inuenced by clay minerals -
formed by the weathering of volcanic glass. they belong to Group A. Others are very highly plastic
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

Table 1: A classication or grouping system for residual soils

Table 2: Characteristics of residual soils groups

Wesley, L. (2009). Obras y Proyectos 6, 5-10

clays, resulting from the presence of smectite pressure graphs, it is often informative to also
(montmorillonite) minerals - and belong in Group B. plot them as direct compression graphs using
The two types may occur in quite close proximity i.e. in linear scales. The lower part of Figure 3 shows the
interbedded layers. It appears that the weathering process linear plots. The results show the following points:
in this case is not actually creating the clay minerals; it
is simply destroying the weak bonds which lock the (i) Conventional graphs (e-logp) suggest the clays behave
clay minerals into the parent material. Waitemata clays as moderately over-consolidated soils, although there
may or not exhibit macro-structure as well as micro- is no clearly dened pre-consolidation or vertical
structural effects. yield pressure. It appears to be somewhere between
100 kPa and 500 kPa.
Weathered greywacke soils (Wellington, NZ): These
probably belong in Group A, as their properties are not (ii) When plotted using a linear scale, the picture is
strongly inuenced by their mineralogical content. They quite different. The curves are reasonably close to
are likely to exhibit signicant macro-structure effects, linear, especially over the pressure range likely to be of
dependent on their degree of weathering. engineering interest, generally about 0 to 200 kPa. The
evidence of a yield stress has largely disappeared.
Weathered granite soils (worldwide): These also belong It is not suggested that the curves in Figure 3 are
to Group A, and exhibit macro-structural effects - from representative of residual soils in general. They are
joints and presence of oating un-weathered rock presented primarily to illustrate that the standard e-log
boulders. (p) graph can be quite misleading and may imply the
Volcanic ash (allophane) soils (Worldwide): These clearly existence of pre-consolidation or yield pressures
belong to Group C. They are very strongly inuenced by when no such pressure exists. With residual soils (and
their mineral composition. They are unlikely to exhibit possibly also with sedimentary soils) it is generally
signicant macro-structure, but may exhibit some desirable to plot consolidation test results using a linear
micro-structure - signicant sensitivity for example. scale for pressure as well as the normal log scale before
drawing any conclusions about the behaviour of the
Tropical red clays (many tropical countries): These also soil. Some residual soils show quite distinct yield
belong to Group C. Those found in the island of Java, pressures, while others show steadily increasing stiffness
Indonesia (with which the author is familiar) are rather with stress level, and some demonstrate almost linear
unusual in that they exhibit neither macro-structure nor behaviour.
micro- structure, except when the weathering is not
far advanced. In this case they may show traces of the Figure 4 is presented to show the inuence of
structure of their parent material. remoulding on compression behaviour for three
different residual soils. These are respectively an
Geotechnical engineering in residual soils allophane clay, a tropical red clay, and a silt derived
from weathered Waitemata sandstone. Consolidation
In the following sections some comments will be curves are given for the soil in its undisturbed state,
made on issues of direct relevance to geotechnical its remoulded state, and after mixing it with water to
engineers, namely foundation design, slope stability and form a slurry. These last curves can be regarded as the
compaction. They are not comprehensive and should not virgin consolidation lines for the soil in its completely
be taken as generalisations applicable to all residual soils. remoulded state. It is seen that with the allophane clay
and the Waitemata silt, remoulding results in a very
Foundation design signicant change in the compression curve. These
soils clearly have a relatively stiff structure in their
Consolidation behaviour undisturbed state which is destroyed by re-moulding
(or de-structuring to use the in vogue term for this
(a) Magnitude (stress/deformation curves). Figure effect). The red clay on the other hand shows almost
3 shows typical consolidation test results from one no change in behaviour after remoulding. This is often
residual soil type - the tropical red clay found in Java, the case with red clays. They appear to exist naturally in
Indonesia. Although it is standard practice to plot a dense unstructured state close to their Plastic Limit,
consolidation test results as void ratio versus log and remoulding thus haslittle or no effect on them.
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

It should be noted that for settlement estimates

with sedimentary soils, there are various empirical
constructions or corrections for improving the
accuracy of estimates. The best known are probably
the Schmertman construction and the Skempton
and Bjerrum method. Both these methods are based
primarily on stress history concepts and are not
intended for residual soils. Therefore the use of these
methods with residual soils is highly questionable.
There are no established procedures available for
correcting consolidation curves for residual soils to
allow for sample disturbance (such as the Schmertman
method for sedimentary soils) and hence it is very
important to obtain good quality undisturbed samples
for consolidation tests.

One further factor which should be appreciated

Figure 3: Oedometer test results from a tropical red clay when attempting to predict settlement magnitudes of
foundations on residual soils is that the initial stress
With regard to the estimation of settlement magnitude, state in the ground is likely to be unknown if the water
there are two procedures commonly used in soil table is at some depth below the surface. The pore
mechanics. The rst is to use the parameters Cc and pressures above the water table will be negative (i.e. in
Cs which are obtained from the e log (p) plot, and a suction or tension state), and likely to vary between
the second method is to use mv values. For soils winter and summer. During prolonged dry periods the
which give an approximately straight line on a linear suction value may be quite large. This means that the
stress/compression plot the use of mv seems most initial effective stress in the ground is not know and
appropriate. The choice of method is a matter for likely to vary between winter and summer. This is a fact
individual judgement, based primarily on the actual soil commonly ignored in routing settlement effects. This
behaviour in consolidation tests. With residual soils the situation is illustrated in Figure 5.
mv parameter often seems more appropriate than the Cc
or Cs parameters.

Figure 5: Pore water pressure state above and below the water
table
(b) Consolidation rate: consolidation rates with residual
soils tend to be rather faster than with sedimentary
soils; as evidenced by their behaviour, both in the
laboratory and in the eld. This appears to be due to
Figure 4: Inuence of remoulding on e-log (p) graphs higher permeability associated with their undisturbed
Wesley, L. (2009). Obras y Proyectos 6, 5-10

structure. In consolidation tests the rate of pore

pressure dissipation may be too fast to allow reliable
determination of the coefcient of consolidation.
This is demonstrated in Figure 6 which shows
standard graphs of compression versus root time for
the loading increment 100 kPa to 200 kPa for three
residual soils. The normal straight line section, which is
used to determine t90 is not clearly dened. Hence, the
estimation of cv is problematical. It is usually found that
at higher stresses the graphs become more linear; the
higher stress tends to destroy the original structure and
lower the permeability.

It should be appreciated that there is an upper limit to

the value of coefcient of consolidation which can be
measured in a conventional consolidation test. Analysis
shows that the highest value of cv which can be reliably
measured with a 19mm thick sample is about 0.1 m2/day Figure 7: Inuence
(=0.012cm2/sec.). Soils with cv values greater than this Figure 6: Typical root time of remoulding on
will not show distinct straight lines on a conventional graphs from residual soils consolidation rate
compression versus root time plot. If reliable values of
cv are required for soils which behave in this way, it is Shear strength
probably best to use a different method of measurement,
such as a pore pressure dissipation test in a triaxial cell. It is not possible to make many categorical statements
regarding the shear strength of residual soils; the
Table 3 shows the wide range of cv values covered for following observations are generalisations and should
the three soils of Figure 6. be treated with some caution. It is reasonably true
to assert (excluding montmorillonite black cotton
soils) that the shear strength of residual soils, whether
Table 3: Values of cv for the three soil types in Figure 6 cover a
wide range as follows:
expressed as undrained shear strength or effective
strength parameters, is generally higher than that of
sedimentary soils. It is rare for the undrained strength
to be less than about 75 kPa, and is generally between
Soil cv m2 /day 100 and 200 kPa. Their f` values are generally above
Waitemata silts and clays 0.01 to 10 30o, and they have signicant values of the cohesion
Indonesian red clays 0.07 to 0.7 intercept c. In the case of some allophane rich
volcanic ash soils both the peak fp` and residual fr`
Volcanic ash soils 0.01 to 200 values may be higher than 35o. Figure 8 shows the
results of triaxial tests on two residual soils; the rst is
for volcanic ash soils and the second for a clay (known
These values lie above and below the value of 0.1m2/ as Middle clay) derived from weathered sandstone.
day that can be measured in the standard consolidation
test. The results from volcanic ash soil in the upper gure
show a relatively small variation in the shear strength;
Figure 7 illustrates the inuence which remoulding this is not surprising since volcanic ash soils are generally
may have on consolidation rate. The two curves are free of discontinuities and are of reasonably uniform
for the same stress increment, from 100kPa to 200kPa. composition. The lower gure shows the inuence
Remoulding destroys the soil structure responsible of structural defects (macro- structure) in the parent
for its high permeability and the much slower rate of rock that are still present in the soil. It is clear that in
consolidation produces the normal straight line on the the latter case it would be almost impossible to infer
root time plot. reasonable design parameters from results of this sort.
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

Slope stability
There are several aspects of the stability of residual soil
slopes that are of particular interest to the geotechnical
engineer. These include the following:

(a) slopes in residual soils (excluding black cotton

soils) generally remain stable at much steeper angles
than those in most sedimentary soils. Slopes of 450 or
steeper are not uncommon, and cuts can often be made
as steep as 600 without danger of slip failure,
(b) slope failures in residual soils, especially when
steep slopes are involved are unlikely to be deep seated
circular failures. They are more likely to be relatively
shallow, with fairly planar failure surfaces. However, the
volume of material involved may still be very large,
(c) slips and landslides in residual soils generally occur
during periods of heavy rainfall, and are the result of
temporary increases in the pore water pressure in the
slope,
(d) the value of c` is usually signicant and is considered
Figure 8: Triaxial test results from two types of residual soils
to be due to some form of weak bonds between
Bearing Capacity particles,
(e) the residual strength is likely to be closer to the peak
As mentioned above, the permeability and consolidation strength than is the case with many sedimentary soils,
rates with residual soils are generally high, and in especially in clays continuing allophane or halloysite,
situations where residual soils are subject to external
loading by the construction of foundations it is likely (f) with some (possibly the majority) residual soils, the
that generated pore pressures will dissipate almost presence of discontinuities may be the governing factor.
immediately and the soils will remain in the drained
state. This means that design using undrained strength Factors (c) and (f) are very important with respect to
will be conservative, as there will be some increase the use of analytical (slip circle) methods for assessing
in strength as the load on the foundation increases. stability. Factor (c) is particularly important; with
sedimentary clays of low permeability the pore pressures
However, this is not an argument against the use of can be measured and the assumption made that they
undrained strength to estimate the bearing capacity of will remain approximately the same for a long time.
the soil for foundation design purposes. During rapid With residual soils, any measurement of pore water
load application, such as during seismic loading, the soil pressure in the slope is valid only at the time it is made
will still behave in an undrained manner, and for this and is not relevant to long term stability estimates. For
reason especially, design should be based on undrained such estimates it is the worst condition likely to occur in
strength. There are also strong practical arguments the future which is of importance. Factor (f) is likely to
in favour of using undrained strength, as this can dominate the behaviour of many cut slopes in residual
be measured relatively easily and reliably. Both eld soils, and rule out the use of analytical methods. Figure
methods (e.g. Dutch penetrometer) and laboratory 8 shows an example of such a soil. Only in very rare
methods (unconned compression or vane test) situations is it likely to be possible to determine the
can be used to obtain reliable undrained strength location, orientation, and strength of discontinuities
values, whereas the measurement of drained strength with the degree of reliability needed for the use of
parameters c` and f` is more difcult and less certain. analytical methods.
Wesley, L. (2009). Obras y Proyectos 6, 5-10

The rapid changes in pore water pressure that occur with wide range of optimum water contents and maximum
residual soils mean that stability analysis must be carried dry densities. Figure 10 shows the result of a compaction
out in terms of effective stresses. The only exception to test on a volcanic ash sample from Java, Indonesia.
this might be when an embankment is constructed on The test has rst been carried out by drying the soil in
a residual soil; this situation is similar to a foundation stages from its natural water content. The soil has then
situation and undrained strength could be used. had water added to it after various degrees of drying,
and further compaction tests carried out. The results
It is worth noting that there is some evidence that pore show the very at compaction curve obtained from
water pressure in a slope will only change signicantly the natural soil, and also the very signicant inuence
as a result of periods of heavy rainfall if the cv value which drying has on the soil properties. Any value of
is greater than about 0.1 m2/day, see Kenney and Lau optimum water content can be obtained by varying the
(1984). extent of pre-drying.

Compaction of residual soils

One last property of residual soils that has caused
difculties to engineers relates to their compaction
behaviour. There are two problems, as follows:

(a) The variability of residual soils may mean a large and

rapid variation in optimum water content within short
distances in any borrow pit.
(b) Some compaction curves for residual soils, notably Figure 10: Compaction test result from a volcanic ash soil
volcanic ash soils do not show peaks indicating maximum (Indonesia)
dry densities and optimum water contents.
The behaviour illustrated in Figures 9 and 10 means
that the control of compaction by the conventional
Neither of the above problems are real problems in the method of specifying dry density and water content
sense of indicating that residual soils are more difcult to limits based on standard compaction tests is very
compact than sedimentary soils. If there is a problem, it difcult. Alternative methods of compaction
is only in the evaluation of the soils and the method to be control have been developed for such soils wich
adopted for specifying and controlling the compaction. overcome the above diculties. The simplest method
Many volcanic ash soils can be effectively compacted at is that wich is based on undrained strenght and air
water contents in the range of 100% to 180%, a fact which voids criteria and is described by Pickens (1980).
geotechnical engineers are often reluctant to accept.
The principle of the method is to specify a minimum
value of shear strenght (commonly 100 kPa to 150
kPa) and a maximum value of air voids (commonly
8 to 12%) for the compacted soil. These values
can be varied according to the nature of the job
and the soil or weather conditions at the site.

Figure 11 illustrates the principle of the method in

relation to the conventional method based on water
content and maximum dry density. The requirement of
Figure 9: Compaction curves from residual soils on two sites a minimum strength means that the soil must not be too
near Auckland wet, and the requirement that the air voids not exceed
a certain value means that the soil must not be too dry.
Figure 9 shows the results of compaction tests carried
out on a number of different samples from two sites The method is easy to use and control testing involves
involving residual soils. It is evident that there is a very density and water content measurements in the usual way.
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

The values obtained are not signicant in themselves; they out on the ne fraction only, they do not give a good
are simply used to calculate the value of the air voids. At indication of the properties of the soil as a whole.
each control point, measurements are also made of shear (c) The particles of some residual soils are of a weak
strength. The simplest method of doing this is by using and fragile nature and are broken down into smaller
a hand operated shear vane, such as the Pilcon vane. particles during testing.
The actual values of optimum water content and (d) The results of these tests are inuenced by pre-drying
maximum dry density of the soil do not need to be the soil, and the plasticity limits are also dependent on
known, and it is not essential to carry out normal the amount of mixing carried out prior to testing.
compaction tests at all. Such tests may however be useful (e) Empirical relationships between either particle size
in order to know whether much drying of the soil will or Atterberg limits and other engineering properties
be needed in order to be able to effectively compact it. have been developed from sedimentary soils and are
not necessarily valid for residual soils.

There is some validity in all of these arguments, but

we should be careful in our evaluation of them; they
are certainly not valid for all residual soils on a general
basis. In the case of one important residual soil group,
namely the vertisols (or Black Cotton soils) it is likely
that none of these arguments is of any relevance at all.

Arguments (a) and (b) above are not peculiar to residual

soils; they frequently apply also to sedimentary soils,
and in any case classication tests are frequently used
for the evaluation of ll materials in which case it is the
properties of the remoulded soil which are required.

Argument (d), at least with respect to the inuence of

Figure 11: Compaction control limits using shear strength and
air voids criteria
pre-drying the soil, is not a valid argument against the
use of classication tests, since there is no difculty at
Comments on normal identication and all in carrying out the tests without pre-drying the soil.
classication tests Argument (e) above is perhaps the most important
question to be considered, especially with respect to the
The tests normally used as a starting point in the Atterberg limits. It has been the authors experience that
evaluation and classication of soils are particle size with residual soils the position which a soil occupies
measurement and the Atterberg limits. The applicability on the conventional Plasticity Chart provides a good
of such tests to residual soils is a matter of some indication of properties - probably just as good as with
contention within the profession; it is useful therefore, sedimentary soils. Soils which plot well below the A-line
to examine the arguments put forward to suggest that behave as silts while those which plot well above the A-
these tests are of less relevance to residual soils than to line behave as clays. Figure 11 show the position on the
sedimentary soils. The arguments are as follows: Plasticity Chart of the three most distinctive residual
soils - the Black Cotton soils, the tropical red clays,
(a) Classication test are carried out on the remoulded and the allophane clays.
soil, and since remoulding destroys the important
structural features which dominate the behaviour of Problems arise when attempts are made to relate specic
many residual soils the tests indicate very little about soil properties, or classication boundaries to one or
undisturbed behaviour. other of the liquid and plastic limits. For example, the
(b) Some residual soils contain a large proportion of British classication system (BS 5903: 1981) divides soils
coarse particles, and since Atterberg limits are carried up into a number of categories based on the liquid limit.
Wesley, L. (2009). Obras y Proyectos 6, 5-10

Specic empirical relationships would be those such

as:
Cc = 0.009 (L.L. 10) (1)
This relationship is for remoulded N.C. soils and thus
has no relevance to engineering situations in residual
soils. In general, these types of relationships should
hold for materials of conventional clay mineralogy. For
residual soils containing allophane or even halloysite
Figure 12: The Plasticity Chart and residual soils they may not be valid.

Such a division is not very relevant to residual soils. It is General remarks on residual soils
the position above or below the A-line which is of most
signicance, especially with tropical residual soils. If there are lessons to be learnt from geotechnical
engineering in residual soils, they are probably the
Rather than a subdivision based on the liquid limit, a following:
subdivision along the lines shown in Figure 12 would be
most relevant to residual soils. The lines drawn parallel - Geotechnical engineers ought to have open minds
to the A-line divide soils into three types labelled clay, about how soils may behave, and not assume they will
silty clay, and silt. Many residual soils behave as silty conform to preconceived patterns, especially when
clays for engineering purposes, and rightly fall into the working with residual soils.
category of silty clay on this chart. The more distinctive
- In evaluating the engineering properties of soils we
residual soil types, such as Black Cotton soils, and
ought to rst observe carefully their behaviour in the
allophane clays, would rightly be classied as clays and
eld, before looking at their behaviour in laboratory
silts respectively.
tests.
It should be noted that the inuence of increased mixing
(or even drying) of the soil on the Atterberg limits is to - While every effort should be made to develop
move the point on the plasticity chart parallel to the A- theoretical or behavioural frameworks to assist us in
line; hence if we use distance above or below the A-line understanding and interpreting soil behaviour, we ought
as our main criteria for evaluating soils this movement to recognise the limitations of such frameworks, and
is not of great signicance. Hence argument (d) above not seek to make all soils t into these frameworks.
is not very important. - Some well established procedures, such as the use of
the e-log p plot for analysing consolidation behaviour,
Empirical relationships based on particle are not necessarily appropriate for all soils, especially
size or Atterberg Limits residual soils.

There are some rather vague general relationships - With residual soils, the mode of formation is so varied
involving particle size and Atterberg limits, and there that it is unrealistic to expect them to t into a single
are specic empirical relationships. behavioural pattern.

Among the general relationships is the understanding The special properties of allophane
that as particle size decreases (or possibly as Liquid
Limit increases) the properties of a soil become less (volcanic ash) clays
favourable for engineering purposes. This is generally
true (or held to be true) if a particular soil type is being Occurrence
considered. This understanding may well apply to many
residual soils, but there is very considerable evidence There are substantial areas in the New Zeland North
that it does not apply to halloysite or allophane soils. Island where clays derived from the weathering of
Especially with allophane soils, there is no evidence volcanic ash occur. These clays tend to be rich in the
of decrease in strength or increase in compressibility clay mineral allophane, which gives them rather unusual
with either decreasing particle size or increasing L.L. and unique properties. They are often referred to as
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

brown ash by local engineers. Whether all clays referred very deep; in Indonesia the writer has encountered cuts
to as brown ash contain allophane is not known to the in these materials up to about 30 m deep, while site
writer; the term is used rather loosely and in some cases investigation drilling has shown depths of up to almost 40
may be applied to clays that do not contain allophane. metres. This thickness results from successive eruptions
The clays described here are those whose properties and associated ash showers, with weathering progressing
are inuenced primarily by their allophane content, as the thickness grows. Examination of cut exposures in
and will be referred to as allophane clays. Similar clays West Java, Indonesia, shows the individual layer thickness
occur in many parts of the world, including Indonesia, to vary generally between about 100 and 300 mm.
The Philippines, Japan, Central and South America, and
Africa. Structure

Formation The precise structure of allophane clays is somewhat

problematic. Their extraordinarily high natural water
The formation and composition of allophane clay is contents and void ratios (described in the next section)
complex, and most of the research and literature on the clearly indicate an unusual material, and call for an
subject comes from the discipline of soil science rather explanation in terms of either structure or chemical
than soil mechanics. This research and literature has composition (or both). Various explanations have been
grown enormously in the last two or three decades since offered over the years. As mentioned above, allophane
the term allophane rst found its way into geotechnical has been described in the past as non-crystalline
literature, and it shows a number of new and interesting or amorphous, and gell-like. However, electron
ndings. Firstly, it shows that allophane seldom occurs microscopy studies over the past 10 years or so (Wada,
by itself. Instead, it is almost invariably found with other 1989 and Jacquet, 1990) show that the material in its
clay minerals, especially a mineral called imogolite. It natural state does have an ordered structure consisting
seems to be almost inseparably linked to imogolite, and of aggregations of spherical allophane particles with
many papers on allophane are in fact on allophane and imogolite threads weaving among them, or forming
imogolite rather than on allophane alone. Secondly, it bridges between them.
shows that allophane is not strictly amorphous, as early
literature asserted. Both allophane and imogolite have
some crystalline structure, albeit of a very different
nature to other clay minerals.

Allophane clays are derived primarily from the in situ

weathering of volcanic ash, although they may be derived
from other volcanic material. This parent material may
be either basic or acidic in nature. It appears that the
primary condition for allophane formation is that the
parent material be of non-crystalline (or poorly ordered
structure) composition. Volcanic ash meets this criteria;
it is formed by the rapid cooling of relatively ne-
grained pyroclastic material, the cooling process being
too rapid for the formation of well ordered crystalline
structures. In the authors experience, the parent
volcanic ash from which allophane clays are formed is
generally in the coarse silt to ne sand particle size range.

In addition to the above requirement of non-crystalline

parent material, it appears that the weathering
environment must be well drained, with water seeping 50 nm
vertically downward through the ash deposit. High
temperatures also appear to favour or accelerate the Figure 13: Electron micrograph of allophane and immogolite
formation of allophane clays. Allophane clays may be (after Wada, 1989).
Wesley, L. (2009). Obras y Proyectos 6, 5-10

Figure 13 shows an electron micrograph of the material

in its undisturbed state. The concept of approximately
spherical particles with thread-like structures spanning
between them appears to explain both the very high
natural water content, and the changes the material
undergoes on remoulding. Remoulding appears to
break up the aggregations of particles and threads
spanning between them and turns the material into a
homogeneous unstructured mass. This is generally
accompanied by some loss of strength and an increase in Figure 14. Atterberg limits of Allophane clays on the Plasticity
compressibility, as well as a reduction in permeability. Chart.

Inuence of drying
General comments on engineering properties
Drying has a very important effect on allophane clays.
Before describing particular properties the point Frost (1967) gave the rst systematic account of this
should be made that allophane clays are not problem effect for both air and oven drying on tropical soils
soils. There is still a belief among some geotechnical belonging to the allophane and halloysite group. He
engineers that the presence of allophone in a soil is showed that clays from the mountainous districts of
something to fear or be concerned about. This should Papua New Guinea with values of Plasticity Index
not be the case. Observation of these clays in their ranging from about 30 to 80 in their natural state
natural environment shows them to perform remarkably become non-plastic when air or oven dried. Wesley
well. For example, terraced riceelds in allophane clay (1973) describes similar effects from the allophane clays
areas in many countries exist on slopes as steep as 35o of Java, Indonesia. The properties of the clay described
and almost up to 40o . They are permanently saturated in this paper apply to the clay in its natural state, i.e.
by irrigation water owing from terrace to terrace. without air or oven drying, unless otherwise stated.
Many water retaining structures have been successfully
constructed from allophane clays. While they ought Identication of allophane clays
not to be a cause for concern, it is important that their
special properties be understood and taken account of There are various techniques used by soil scientists to
in planning engineering projects. identify allophane: these are primarily X-ray diffraction
and electron microscopy. Such methods are not readily
available to geotechnical engineers. For engineering
Natural water content, void ratio, and Atterberg limits purposes, sufcient indicators of the presence of
allophane are the following:
The natural water content of allophane clay covers
a very wide range, from about 50% to 300%. This - Volcanic parent material
corresponds to void ratios from about 1.5 to 8. It
appears that water content is a reasonable indication of - Very high water contents
allophane content the higher the water content the - Very high liquid and plastic limits lying well below the
greater the allophane content. Atterberg limits similarly A-line on the Plasticity Chart
cover a wide range, and when plotted on the conventional - Irreversible changes on air or oven drying - from a
Plasticity Chart invariably lie well below the A-line. This plastic to a non-plastic material.
means that according to the Unied Soil Classication
System they are silts. However they do not display If all of these apply then the soil almost certainly
the characteristics normally associated with silt the contains a signicant allophane content.
tendency to become quick when vibrated and to dilate
when deformed. At the same time they are not highly Stiness and compressibility
plastic like true clays, so they do not t comfortably into
conventional classication systems. Figure 14 shows Typical results from oedometer tests on undisturbed
a plot of the Atterberg limits on the Plasticity Chart. samples from Indonesia and New Zealand are shown
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

in Figures 15 and 16. Details of the samples are given

in Table 4.
Table 4: Details of samples used for oedometer tests.

Atterberg limits

Figure 15 shows the results as conventional e-log(p)

graphs and Figure 16 as compression versus stress on
a linear scale. The e-log (p) curves suggest that all the
samples have similar compressibility characteristics with
pre-consolidation pressures of varying magnitude. Figure 17: Constrained modulus (D) versus initial void ratio
However, when plotted using a linear pressure scale this
is no longer the case: only some of the samples show It is of interest to note that for these clays there does
an apparent pre-consolidation pressure. This arises not appear to be any relationship between the initial
from the structure of the soil created by the weathering void ratio and compressibility. Figure 17 shows the
process, and is perhaps best described as a vertical yield constrained modulus D measured when the sample is
pressure. Why some samples show a yield pressure and loaded from 0 to 200 kPa, and again between 1600 kPa
others do not is not known, though it may be related to and 2000 kPa, plotted against the initial void ratio. The
the original denseness of the parent material. data shows considerable scatter, but there is no clear
trend towards higher compressibility with increase in
void ratio from 2 to nearly 6.

Figure 16. Oedometer tests Figure 19: Summary of cv values

Figure 15. Oedometer test showing compression versus Figure 18: Typical root time from pore pressure dissipation
results as e-log(p) plots. pressure on a linear scale. plots from oedometer tests tests

Figure 18 shows typical root time plots from oedometer

These graphs illustrate two important points. Firstly, to tests. At low stress increments the consolidation rate is
gain a clear picture of the consolidation behaviour it clearly very rapid but becomes progressively slower as
is necessary to plot the results using a linear scale as the stress level rises. To investigate this effect in more
well as a log scale. Secondly, the portion of the graph detail, pore pressure dissipation tests were carried out
of interest in foundation design is often close to linear using a triaxial cell. Two samples from New Zealand
with respect to pressure, and favours the use of the and two from Indonesia were tested.
linear parameter mv (or constrained modulus D) for
settlement calculations rather than the log parameters A summary of the cv values obtained from these
Cc and Cs. dissipation tests is shown in Figure 19. It is seen that
Wesley, L. (2009). Obras y Proyectos 6, 5-10

the cv value decreases by approximately four orders of These are fairly similar. They show that while the in
magnitude as the stress increases from 50 to 1000 kPa. situ strength is reasonably uniform, it does have small
With the New Zealand samples, the tests were repeated uctuations over the full prole, and there are some
after remoulding the soil. It is seen that the cv value is zones with considerably higher values. These are
then consistently low and close to the end value from believed to be zones of coarser material within the
the undisturbed samples. With the Indonesian samples, ne clay. The cone resistance varies between about 1
permeability measurements were also made between and 3 MPa. Using a correlation factor (Nk) of 15 this
each consolidation stage; the results showed an identical corresponds to an undrained shear strength range of
trend to the cv values. Figure 19 shows that remoulding about 65 kPa to 200 kPa. Values of undrained strength
the soil apparently destroys the open structure of the obtained from other methods at the Kamojang site
undisturbed soil, which is believed to account for the ranged from about 50 kPa to 170 kPa, conrming the
high permeability. trend indicated by the CPT tests.

As noted earlier, with clays of this type it is not possible

to determine reliable cv values from conventional
Eective strength parameters
oedometer tests. The drainage path length is too short
The effective strength parameters f and c are
for pore pressure dissipation to control the deformation
rate. The upper limit of the cv value which can be surprisingly high for a soil of such ne grained
measured with a conventional oedometer is about composition. This is perhaps not surprising; observation
0.01cm2/sec. At the relatively low stress levels relevant of eld behaviour suggests that this must be the case.
to engineering situations, the cv value of allophane clays As mentioned earlier, in Indonesia and other tropical
is normally much higher than this. countries, terraced rice elds exist on remarkably steep
slopes in areas of allophane clay. These slopes remain
Undrained strength stable despite permanent saturation with irrigation
water, which ows from terrace to terrace.
Figure 20 shows cone penetrometer test (CPT) results
from two sites, one in Indonesia and one in New
Zealand.

Figure 21: Effective strength parameters for allophane clays

Figure 21 summarises results from laboratory tests on

samples of allophone clay from Indonesia and New
Zealand. Triaxial tests were carried out to obtain the
peak values, and ring shear tests to obtain the residual
values. Both values are remarkably high and there is
surprisingly little difference between them. Rouse et al.
(1986) have obtained similar high values from allophane
soils in Dominica.

Figure 22 shows values of the residual angle fr`

plotted against Plasticity Index. It is seen that there is
Figure 20: Cone penetrometer tests from allophone clay sites in no relationship between the two; fr` does not steadily
Indonesia and New Zealand decrease with Plasticity Index as is the case with
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

sedimentary clays. With PI values above about 80, progressively destroyed, releasing water and softening
sedimentary soils would be expected to have fr` values the soil, an effect sometimes referred to as over-
of around 10o, whereas the allophane clay has values compaction.
between 30o and 40o.

Figure 22: Residual strength friction angle from allophane clays

versus Plasticity Index.

Compaction characteristics
The compaction behaviour of typical allophane clay was Figure 23: Inuence of compactive effort on strength of compacted
illustrated earlier in Figure 10. The natural water content alllophane clays (after Kuno et al., 1978)
was 166%, and the natural curve was obtained by drying The above behaviour illustrates that difculties can
back the soil in steps from this initial water content. arise in compacting allophane soils if their properties
Fresh soil was used for each point. The test was then are not understood and taken account of in planning
repeated three times, rstly after oven drying, secondly and executing earthworks operations. Specications
after air drying, and nally after limited air drying (to w can be almost meaningless if excessive drying is allowed
= 65%). The material was then wetted up in stages, using before testing is carried out. In countries like Papua-
fresh soil for each point. The results show the dramatic New Guinea and Indonesia the wet climate in which
changes caused by drying. When dried from its natural allophane clays occur means that signicant drying
water content the compaction curve is almost at, with during excavation and compaction is not very practical.
only a very poorly dened optimum water content. On Difculties during earthworks operations are described
re-wetting, the behaviour becomes more conventional, by Parton and Olsen (1980), and Moore and Styles (1988).
with clearly dened optimum water contents and peak
dry densities. It is evident from this that almost any These problems can be overcome to some extent in
result can be obtained if the test involves drying and several ways. The rst is to recognise that soils can be
re-wetting. This result is from an Indoneisan allophane satisfactorily compacted without recourse to the rigid
clay. New Zealand allophane clays may not show such control methods associated with water content and dry
a dramatic effect because of their lower allophane density values. The second is to be clear what objective
content. is aimed for in compacting the soil. For example, the
objective with a road embankment is very different from
Figure 23 shows the effect of repeated compaction that with a water retaining embankment. With a road
on allophane soils. Some allophane clays are of high embankment it is preferable to keep the compactive
sensitivity, and others are not: this is reected in the effort to a minimum and press the soil together
curves in Figure 23. The strength of the soil has been with quite light compaction. enough to get rid of
measured after compaction using a series of different any large voids, but insufcient to destroy the natural
(but known) compactive efforts. The compactive effort structure of the soil and cause it to soften. In this
is indicated by the number of hammer blows. A cone way it is possible to retain much of the original strength
has been pushed into the soil to obtain a measure of of the material. With water retaining embankments a
strength; this is the cone index value shown in the rather more rigorous approach is needed, but even for
gure. The graphs show that in general there is a these it is desirable to carefully control the compactive
marked decrease in strength as the number of blows effort. Compaction control, involving control of
increases. Presumably the structure of the soil is being compactive effort, together with shear strength and
Wesley, L. (2009). Obras y Proyectos 6, 5-10

air voids testing is generally a better approach than dam Cipanunjang (formerly spelt Tjipanundjang) in
conventional water content and dry density methods. West Java, Indonesia, built in 1928 during the Dutch
colonial period. This is a homogeneous 30 m high
The Cipanunjang dam in West Java (Wesley, 1974) is embankment with cut-off drains in the downstream
an example of successful compaction of allophane slope. It is described in detail elsewhere (Wesley, 1974),
clay; compaction here was done using steel rimmed and is still a vital part of the municipal water supply of
rollers. Some difculties were encountered due to wet the city of Bandung, the capital city of West Java. The
weather and softening of the soil, but the job was Mangamahoe Dam in New Plymouth, New Zealand,
completed satisfactorily. The writer has been involved and the embankment supporting the supply canal at
in the compaction of allophane clay at a geothermal the Kuratau power scheme (on the western shore of
power station site (Kamojang) in West Java, Indonesia. Lake Taupo, New Zealand) are further examples of
Difculties were encountered because the very wet embankments of allophane clay forming water retaining
climate at the site made it difcult to dry the soil structures. The Kamojang geothermal power station in
sufciently to achieve the target undrained shear West Java, Indonesia, is supported by a raft foundation
strength of 150 kPa. The ll was required to form a on about 35 m of allophane clay (Figure 20). There have
level platform for an electrical tansformer and switch been no problems with its performance. Wesley and
yard. The strength requirement was lowered to 90 kPa Matuschka (1988) describe these examples in greater
and the job successfully completed. The ll appeared to detail.
harden with time, presumably due to the development
of negative pore pressure in the soil.
References
Erosion resistance
British Geological Society Engineering Group Working Party
It is an interesting observation that both in their Report: Tropical Residual Soils (1990). Vol. 23, No1, 1-101
undisturbed and re-compacted state, allophane clays BS 5930 (1981). Code of Practice for Site Investigations,
are remarkably resistant to erosion. It is only when they British Standards Institute, London
are cultivated and allowed to partially dry at the surface Frost, R.J.. Importance of correct pre-testing preparation
that they become susceptible to erosion. Observation of some tropical soils. Proc. First Southeast Asian Regional
of road cuttings in Southeast Asia as well as in New Conf. on Soil Engineering, Bangkok: 44-53
Zealand (Taranaki and the central volcanic plateau)
shows that negligible erosion occurs from the cut faces. Jacket, D. (1990). Sensitivity to remoulding of some volcanic
In Indonesia, the drying of the face appears to result ash soils in New Zeland. Engineering Geology 28 (1): 1-25
in the formation of a hard crust which is resistant Kenney and Lau (1984) Temporal changes of groundwater
to erosion. It is also evident in terraced rice-elds that pressure in a natural clay slope. Canadian Geotechnical
negligible erosion takes place as the irrigation water Journal. Vol. 21, 1984
ows from one terrace to the next terrace.
Kuno, G., Shinoki, R., Kondo, T. & Tsuchiya, C. (1978). On
In relation to erodibility, the writer has investigated the construction methods of a motorway embankment by a
the question of the dispersivity of allophone clays by sensitive volcanic clay, Proc. Conf. on Clay Fills, London,
carrying out pin-hole dispersion tests on allophane pp. 149-156
clays from Indonesia and New Zealand. The results are Moore, P.J., and Styles, J.R. 1988. Some characteristics of
described by Wesley and Chan (1991). None of these volcanic ash soil . Proc. Second Int. Conf. on Geomechanics
tests showed any evidence of erosion or dispersivity. in Tropical Soils. Singapore: 161-166
Parton, I. M. and Olsen, A.J. (1980). Properties of Bay of
Signicant engineering projects Plenty Volcanic Soils. Proc. 3rd Australia New Zealand
in allophane clays Conference on Geomechanics, Welllington. Vol.1: 165-169.
Pickens, G.A. (1980). Alternative compaction specications
A number of dams and related water retaining structures for non-uniform ll materials. Procedings third Australia-
have been successfully undertaken making use of New Zeland Conference on Geomechanics, Wellington 1,
allophane clays. An early example is the water supply 231-235
Wesley, L. (2009). Behaviour and geotechnical properties of residual soils and allophane clays.
Obras y Proyectos 6, 5-10.

Wada, K. (1989). Allophane and imogolite. Chapter 21

of Minerals in Soil Environments (2nd Edition) SSSA Book
Wesley, L.D. and Matuschka, T. (1988). Geotechnical
Series No 1, 1051-1087
engineering in volcanic ash soils. Proc. Second Int. Conf.
Wesley, L.D. (1973), Some basic engineering properties of on Geomechanics in Tropical Soils, Singapore Dec. 1988.
halloysite and allophane clays in Java, Indonesia. Geotechnique Vol.1: 333-340
23, No 4: 471-494.
Wesley, L.D. and Chan S.Y. (1990). The dispersivity of
Wesley, L.D. (1974). Tjipanundjang Dam in West Java, volcanic ash soils. Proc. IPENZ Conference 1991. Vol. 1,
Indonesia. Journal of the Geotechnical Division ASCE 100/GT5: 67-76
503-522.