Clay Minerals (1966) 6, 179. SOME FACTORS AFFECTING THE RELATION BETWEEN THE CLAY MINERALS IN SOILS AND THEIR PLASTICITY M. J. DUMBLETON AND G. WEST Road Research Laboratory, Ministry o[ Transport, Harmondsworth, Middlesex (Received 3 January 1966) ABSTRACT: The clay mineral composition is one of the factors that affect the physical properties of soils, and a knowledge of it is required to promote a fuller understanding of the origin of these properties. The relationships between the clay content and the plastic and liquid limits of natural montmorillonitic and kaolinitic soils and of artificial mixtures have been examined and compared. Factors affecting the relationships are discussed, and illustrated by the effect of particle aggregation on the measurement of the liquid limit of tropical red clays and on the sedimentation analysis of the Keuper Marl. The effect of muscovite and of silt-sized material on the position of soils on the Casagrande classification chart has also been examined. The clay materials encountered in civil engineering include ancient deposits and recent alluvial deposits, as well as soils formed by weathering processes at the Earth's surface; the engineer refers to all these materials as soils. Clay mineral analysis is not normally employed in the routine examination of soils for particular engineering projects. This is partly because the techniques are beyond the resources of the ordinary testing laboratory, and partly because the detailed interpretation of the results in immediate practical terms is not yet generally possible. Neverthe- less, in the study of engineering soils a consideration of the clay minerals present is useful in a number of ways : (1) In the characterization of different groups of soils having distinct engineering characteristics, for example the red and black soils of tropical Africa (Dumbleton, 1963a; Dumbleton & Newill, 1962). (2) More generally, in the formulation of local engineering soil classifications, taking into account not only the soil but also its environment, which affects both the nature of the soil and other engineering considerations (Beaven & Dumbleton, in preparation). (3) In the interpretation of engineering test results.
Transcript
Clay Minerals (1966) 6, 179.
S O M E F A C T O R S A F F E C T I N G T H E R E L A T I O N
B E T W E E N T H E C L A Y M I N E R A L S IN SOILS
A N D T H E I R P L A S T I C I T Y
M. J. D U M B L E T O N AND G. W E S T
Road Research Laboratory, Ministry o[ Transport, Harmondsworth, Middlesex
(Received 3 January 1966)
A B S T R A C T : The clay mineral composition is one of the factors that affect the physical properties of soils, and a knowledge of it is required to promote a fuller understanding of the origin of these properties. The relationships between the clay content and the plastic and liquid limits of natural montmorillonitic and kaolinitic soils and of artificial mixtures have been examined and compared. Factors affecting the relationships are discussed, and illustrated by the effect of particle aggregation on the measurement of the liquid limit of tropical red clays and on the sedimentation analysis of the Keuper Marl. The effect of muscovite and of silt-sized material on the position of soils on the Casagrande classification chart has also been examined.
The clay materials encountered in civil engineering include ancient deposits and recent alluvial deposits, as well as soils formed by weathering processes at t h e Earth 's surface; the engineer refers to all these materials as soils. Clay mineral analysis is not normally employed in the routine examination of soils for particular engineering projects. This is partly because the techniques are beyond the resources of the ordinary testing laboratory, and partly because the detailed interpretation of the results in immediate practical terms is not yet generally possible. Neverthe- less, in the study of engineering soils a consideration of the clay minerals present is useful in a number of ways :
(1) In the characterization of different groups of soils having distinct engineering characteristics, for example the red and black soils of tropical Africa (Dumbleton, 1963a; Dumbleton & Newill, 1962).
(2) More generally, in the formulation of local engineering soil classifications, taking into account not only the soil but also its environment, which affects both the nature of the soil and other engineering considerations (Beaven & Dumbleton, in preparation).
(3) In the interpretation of engineering test results.
180 M. J. Dumbleton and G. West
(4) More generally, in the search for a basic understanding of the engineering properties of soils.
The present paper considers the last two applications of clay mineralogy. The properties of a soil are determined by the mineralogical composition,
shape and size distribution of its component particles, the interaction of these particles with each other and with water and dissolved salts, and the effect of cementing. This paper considers the influence of clay mineral composition on the plasticity of soils, and also considers cases in which particle size distribution, particle shape, and cementing have an effect. Plasticity is the most outstanding characteristic of clay soils. It is measured by routine tests on nearly all soils before they are used in an engineering structure, and gives a good general indication of their other engineer- ing properties. An understanding of the factors which determine the plasticity of soils will aid in the interpretation of test data, and lead to a better understanding of the performance of clay soils in engineering use. The factors that affect the plasticity of a soil are also likely to affect most of its other properties of interest to the engineer.
This paper is based on work of the Road Research Laboratory, and is illustrated by examples of soils obtained during surveys of road-making materials of a number of countries. An extensive review of other work on clay mineralogy in relation to the engineering properties of clay materials has been given by Grim (1962).
R E L A T I O N B E T W E E N C L A Y C O N T E N T A N D L I Q U I D
A N D P L A S T I C L I M I T S
The plastic and liquid limits are used to define the plastic properties of a soil (British Standard 1377 : 1961); the plastic limit is the moisture content at which the soil passes from the friable to the plastic state and the liquid limit is the moisture content at which is passes from the plastic to the liquid state.
The factors which affect the plasticity of soils for the most part act simul- taneously, and it is therefore difficult to isolate the effect due to the individual factors. For instance, in addition to natural soils containing variable amounts of clay size material, clay generally comprises more than one type of clay mineral. Furthermore, samples of clay minerals of the same mineralogical type, but of different origins, may show considerable variation in physical properties. To overcome these diffi- culties, experiments were made on artificially prepared mixtures of pure clay minerals with sand and silt. These allowed a clear idea to be obtained of the relationship between clay mineral type and content and the plasticity of the material. They also provided a datum against which to compare natural soils of similar minera- logical composition.
In the figures the clay contents quoted for the artificial mixtures are the percent- ages of Surrey Finest and Supreme Kaolin. The clay contents quoted for the natural soils are the percentages of particles finer than 2 /~, determined by sedimentation after pretreatment and dispersion with sodium hexametaphosphate (British Standard 1377: 1961). This size fraction is often taken to represent the amount of clay
Clay minerals and plasticity 181
mineral present in a soil, although it may include particles of other minerals, and clay minerals may also occur as larger particles; examples will be given.
Artificial montmorillonite and kaolinite mixtures The clays used were 'Surrey Finest' (Fullers' Earth Union) from Redhill, Surrey,
consisting of 95% natural, calcium montmorillonite, and having 78% particles finer than 2/~, and 'Supreme Kaolin' (English Clays Lovering and Pochin) from St Austell, Cornwall, consisting of 95% well-ordered kaolinite, and having 96% particles finer than 2 t~. A standard sand-silt mixture was prepared by crushing quartz sand from Leighton Buzzard, Bedfordshire, to pass the No. 36 B.S. sieve and adjusting the grad- ing to contain equal proportions in the sand and silt size ranges. Mixtures of the air-dry clay with the sand-silt mixture were prepared containing 25, 50, 75, and 100% clay, based on the oven-dry weight. Their liquid and plastic limits were measured by the British Standard procedure. Each mixture was allowed to stand for at least an hour between initial mixing and testing to permit the water to become uniformly distri- buted; tests showed that there was no significant difference between determinations made after 1 hr and after 24 hr standing before testing.
The relationship between the liquid limit and the clay content for the mixtures is shown in Fig. 1. The broken lines show the relations which would hold if the addition of dry sand-silt had no effect on the plasticity of the clay; if this were the case, the liquid limit of the mixture would be reached when the moisture content of the clay in the mixture reached its liquid limit, no moisture being allowed for the effect of the sand-silt component, The observed relations, shown by the full lines, are fairly close to these, but the moisture contents are higher for lower clay contents. The extra moisture is that required to counteract the effect of the sand-silt component. As it is greater for montmorillonite mixtures than for kaolinite mixtures it seems clear that the extra water is required not simply to wet the surface of the sand-silt fraction, but that there must also be some interaction with the clay fraction, which requires additional .water.
Natural montmorillonitic and kaolinitic soils Examples of soils containing clay minerals of one particular type, either mont-
morillonite or kaolinite, were selected. The relations between their plastic and liquid limits and their clay contents, as determined by sedimentation, are compared in Figs. 2 and 3 with the relations found for the artificial mixtures, represented in the figures by full lines.
Materials containing montmorillonite are Shown in Fig. 2. The comparison shows that there is a fair measure of agreement between the results for the artificial Surrey Finest mixtures and for the montmorillonitic soils from different parts of the world. This was best illustrated by the Kenya black clays, of which there were samples covering a range of clay contents. Also shown are Jamaican soils formed on shale and on alluvium (Dumbleton, West & Newill, 1966), a Borneo soil formed on
182 1 6 0 i
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120
100
E 80
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M. J. Durnbleton and G. West
i i
MONTMORILLONITE Surrey Finest
/J
[
4o i i /
2 0
/ E / " I / ~"//'KAOLIN[ T
./~" Supreme / . " I Koo l i n
2 5 5 0 7 5 1 0 0
C t a y content (%)
FIG. 1. Pure clay minerals--relation between liquid limit and clay content for mixtures with quartz sand and silt.
basalt (Dumbleton, 1963b, 1965), and soils from Tanganyika and Melbourne, Australia (Russam & Dumbleton, 1964).
Most of the kaolinitic soils examined contained disordered kaolinite or meta- halloysite, with higher plastic and liquid limits than the more well-ordered St Austell kaolinite, which is of hydrothermal origin. They usually also contain iron oxide that affects their behaviour. For these reasons there is a greater divergence between the results for natural soils and for the artificial mixtures (Fig. 3) than in the case of the montmorillonitic materials.
The relation for tropical red clays from Kenya (broken curve, Fig. 3) is similar to that for Supreme Kaolin except that the lines are higher; this shows the higher plastic and liquid limits of the disordered kaolinite minerals (metahalloysite) in these soils. These red clays are greatly influenced by the presence of iron oxide, mainly in the form of hematite, which introduces difficulties in the measurement of the plastic and liquid limits (see below), and which is probably responsible for
Clay minerals and plasticity 183
160
140
120
100 ,"E_ E
._,=,
a3 ~. 80
:"z E
'5 60 ~r
J
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- ~ Surrey Finest mixtures --0-- Kenya black clays
Jamaica soil on shale �9 Jamaica alluvial soil
Borneo soil on bosoltic rock ~- Tanoanyika X Melbourne
/ Liquid til it / / ~ / j /e//~
/
/ :/e/&
/ tic / ~ Limit
20 40 60 80 100 Clay content (%)
FIG. 2. MontmoriUonitic soils--relation between liquid and plastic limits and clay content.
the considerable scatter of the points. Also shown are soils from Borneo containing disordered kaolinite and goethite, and soils from Jamaica and Tanganyika.
Another factor which has been shown to affect the plasticity of soils is the size distribution of the clay particles within the less than 2 /~ range (White, 1949). In addition, fine-grained particles which are not clays in the mineralogical sense can also produce plasticity, as is illustrated by a bauxite from Jamaica, containing 70% of gibbsite and also hematite, but no clay minerals (Fig. 3). The effect of the shape of the sand-silt fraction on the plasticity of mixtures with clay has been noted by Robertson (1951), and has recently been studied in more detail by Dumble- ton & West (in preparation). These factors should be borne in mind when examining the relationship between the mineralogy of soils and their plasticity.
184 7~0
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M. J. Dumbleton and G. West
- O ~ Supreme Kaolin mixtures Well ordered kaolinite
~ - Kenya red clays. Metahalloysite and hematite 40rain mixing Borneo Disordered kaolinite and goethite
�9 Jamaica, Kaolinite
E] Jamaica. Kaotinite (bauxit ic) if" TangQnyika. Kaolinite- not well ordered, and goethite X Jamaica. Beu• (70*l*gibb~tel
t I
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e /
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/
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e j
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, I
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C l a y content (%)
BO 100
FIG. 3. Kaolinitic soils--relation between liquid and plastic limits and clay content.
R E L A T I O N B E T W E E N L I Q U I D L I M I T A N D
P L A S T I C I T Y I N D E X
The relation between the plastic limit and the liquid limit of a soil is usually examined by considering the plasticity index, which is the difference between the two limits, and represents the range of moisture content over which the soil behaves plastically. A plot of plasticity index against liquid limit is known as the plasticity chart, and on it a large number of soils plot on or near to a line, known as the A-line, cutting the liquid limit axis at 20 % moisture and having a slope of 0-73.
The position in which a soil plots on the plasticity chart usually gives a good idea of its general engineering properties, and forms the basis of Casagrande's method (1947) of classification for fine-grained soils for engineering purposes, which has been widely adopted in civil engineering. It is therefore useful to examine some of the factors which determine the position in which a soil plots on the plasticity chart.
Clay minerals and plasticity 185
Artificial montmorillonite and kaolinite mixtures
Results for the artificial mixtures of pure clay minerals and quartz sand-silt have been plotted in Fig. 4. Each clay mineral shows a distinct relation which is near the A-line. The numbers by the points refer to the percentages of Surrey Finest and Supreme Kaolin.
100
8 0
6o
>. G
20 Supreme Kaolin
20 40 60 80 100 Liquid limit (%)
' , / , oo J
L
MONTMORILLONITE 7 5 / Surrey Fine D
Numbers by points show percentage clay
120 140 160
FJo. 4. Plasticity chart for mixtures of pure clay minerals with quartz sand and silt.
Natural montmorillonitic and kaolinitic soils Fig. 5 shows the position of naturally occurring montmorillonite-rich soils on
the plasticity chart in relation to Surrey Finest mixtures. The broken line shows the results for the Kenya black clays, and it can be seen that this is parallel with but lower than the curve for the artificial mixtures. Montmorillonitic soils from other parts of the world are also plotted, and Show the same general trend. The numbers by the points for the natural soils refer to the percentages of particles finer than 2 t~.
Fig. 6 shows the positions of naturally occurring kaolinitic soils on the plasticity chart in relation to Supreme Kaolin mixtures. A group of soils from Borneo in which the principal mineral is disordered kaolinite have plasticities which are much higher than those of soils containing similar proportions of better ordered kaolinite minerals. The Kenya red clays contain metahalloysite as their principal clay mineral together with hematite; the effect of length of mixing time in increasing their plasticity is shown, and will be discussed later.
186 M. J. Dumbleton and G. West
6C
_v t,C
I Mixtures I Kenya
Jamaica ~_ [ JamQica
Tanganyi ka Melbourne
Surrey Finest I ~ - - I I - - black clays
E) on shale �9 alluvium (+vermiculite .p arid kaolinite)
I
/ '~ show per.ntage day L
20 40 60 80 100 120 Liquid limit (%)
FIG. 5. Plasticity chart for montmorillonitic soils.
Clay minerals and plasticity 187 S O I L S W H I C H F A L L B E L O W T H E A - L I N E
Consider a soil which plots at any point P below the A-line on the plasticity chart (Fig. 7). This may be thought of either as a soil with a low plasticity index in relation to its liquid limit, or as a soil with a high liquid limit in relation to its plasticity index. The latter is probably the better way of considering the soil, because if water can be held within the soil in a way in which it is ineffective in producing plasticity (ineffective water), then the liquid and plastic limits may both be thus raised, while the plasticity index is not much affected since it is the difference between them. Materials falling below the A-line were found by Casagrande, on the basis of experience, to include organic silts and clays, micaceous soils and diatomaceous earths. In addition, hydrated halloysite soils are known to plot below the A-line. One can see how ineffective water could be held in the diatoms which make up diatomaceous earth, and also in fibrous and cellular organic matter. The cases of hydrated haUoysite, mica, and silt will be considered in greater detail.
60
50
40
>,
C,
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Borneo E3 Hydrated hal loysi te Borneo �9 Hydrated hattoysl te
(non-hydrated kaolin predominates) - - Kenya (9 Hydrated ha l loys i te
Kenya �9 . Hydrated hattoysite w i th volcanic ash and pumice)
FIG. 7. Plasticity chart for hydrated halloysite soils.
Hydrated halloysite and ferruginous kaolinite soils In addition to its chemically bound water, hydrated halloysite contains less tightly
bound water of hydration corresponding to 14% of the oven-dry weight. Normal determinations of moisture content obtained by oven drying will include this less tightly bound water. If as seems probable this water is ineffective in promoting plasticity then both plastic and liquid limits are inflated by 14% with the plasticity index remaining constant. In this case a hydrated halloysite, plotting on the dotted line in Fig. 7, would correspond to a normal mineral plotting on the A-line. However, some hydrated halloysite soils from Kenya and Borneo (Fig, 7) gave results equivalent
188 M. J. Dumbleton and G. West to 40% ineffective water rather than the theoretical maximum of 14%. This is thought to be due to water held in the tube-shaped particles of hydrated halloysite, and in the spongy iron oxide which has been observed in these soils (Robertson, 1958), and in other ferruginous kaolinitic soils (Coleman, Farrar & Marsh, 1964). In addition, some of the materials contain porous pumice-like particles which could hold ineffective water in their pores. Volcanic ash and pumice are typical parent materials for hydrated halloysite soils.
Soils containing mica minerals
Mica may exist in soils in two forms--either in a well-crystallized form, such as muscovite, or in a poorly-ordered hydrous form as the clay mineral illite. Fig. 8 shows a number of micaceous soils from Borneo (with one from Kenya, marked K) plotted on the classification chart. Those in which the mica is in the form of illite plot above the A-line, while those in which it is in the form of muscovite plot below it. However, before drawing a definite conclusion it is necessary to consider the possible effect of the proportion of silt-size material in the soils, because musco- vite also often occurs in the silt fraction. The percentage of silt-size material present in each soil is indicated by the number against the point representing it in Fig. 8. Although the silt contents of the muscovite soils are generally higher, there is a clear difference in the position with respect to the A-line of the illitic soils compared with the muscovite soils containing similar percentages of silt. It thus appears that it is mica in the form of muscovite, but not in the form of illite, that causes soils to plot below the A-line.
4 0 m _ _ • 0 Clay minera l - i l l i te �9 Clay minera l -muscov i te
~, 3 0
0 ~ 2 0
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Y 2O
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79
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! I
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Numbers by points show percentage silt
I 60 70 1 O 30 40 ,50 80
Liquid timit (%)
FIG. 8. Plasticity chart for micaceous and illitic silty soils from Borneo (except K, from Kenya).
Clay minerals and plasticity 189
Artificial mixtures containing silt-size muscovite and quartz
To throw more light on the behaviour of silty and micaceous materials, mixtures of pure clay minerals with silt-sized muscovite and silt-sized and sand-sized quartz were examined. The muscovite was obtained from a commercial supplier as a silt-sized powder. Fig. 9 shows the effect on the plasticity of the two clay minerals of adding the various components in the amounts indicated. As in the original investigation (see Fig. 4) the results show that the effect of adding silt-sized and sand-sized quartz to the clays is to reduce their plasticity (i.e. to move their position on the plasticity chart to a lower position along the A-line). It will be noted that all these mixtures plot above the A-line. For montmoriUonite, quartz sand reduced the plasticity more than the same amount of quartz silt; but for kaolinite the two additives had almost identical effects.
100
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=x 6 0
G 4o
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I M - Muscovite Q - Quartz
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M. silt
J 20 40 60 80 100
Liquid limit (%1
50% S
M. 5 ilt
120 140 160
FIG. 9. Effect of muscovite and quartz on plasticity of pure clay minerals.
The addition of silt-sized muscovite also reduced the plasticity of the days, but in addition it moved their positions on the plasticity chart below the A-line. Thus silt-sized and sand-sized quartz have similar effects, both causing a reduction in plasticity, but only muscovite causes a shift to below the A-line. An addition of 75 % of muscovite silt was required to cause montmoriUonite to plot below the A-line, due presumably to the high activity of this clay material, so that more muscovite was required before micaceous properties became apparent.
Fig. 10 shows the effect of making additions of 50% of silt-sized quartz and muscovite to two natural clay soils; brickearth from Harmondsworth, Middlesex,
190 M. J. Dumbleton and G. West
and London Clay from Heathrow, Middlesex. The different effects are markedly demonstrated by both soils, quartz silt causing a normal reduction in plasticity but muscovite silt causing a shift to well below the A-line. The effects on some of their engineering properties of adding both fine and coarse muscovite to these soils has been studied (Tubey, 1961); for British Standard compaction the dry density is reduced and the optimum moisture content is increased. The effects can be explained as being due to the plate-like shape of the muscovite particles, which lock together to produce voids containing ineffective water.
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Harmondsworth Br ickearth l London Cloy
I J
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/ / / / / /
silt
L _ _
Sand Silt Clay 5 64 31 1 37 62
/ /
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/ Hormondswor th Brickearth / ~
i .- / ~ . 0 1 50"/. Muscovite
50* / .Quar tz d " / si l t
20 30 40 50 60
Liquid [ imi t (%)
0 - - 10 70 80
FIG. 10. Effect of silt-sized muscovite and quartz on plasticity of natural clay soils.
E F F E C T O F C E M E N T I N G A N D A G G R E G A T I O N O N
S O I L P R O P E R T I E S
Any cementing or aggregation of the particles of a soil will modify the effect which the clay fraction has on the soil properties. Examples of two soils showing these effects are tropical red clays and the Keuper Marl, and these will now be discussed.
Cementing in tropical red clays An increase in the liquid limit with increasing time of mixing of a tropical red
clay (Newill, 1961) is shown in Fig. 6. This effect does not occur for all soils; similar experiments with London Clay showed that its liquid limit had not increased signi- ficantly after prolonged mixing. The red tropical clay consisted of metahalloysite, with about 14% of iron oxide in the form of hematite; the hematite has been shown to be present as a coating around the clay particles (Coleman, Farrar & Marsh,
Clay minerals and plasticity 191
1964) cementing them together and reducing their plasticity. As the clay is mixed with water, the aggregations are progressively broken down and the plasticity in- creases. A Kenya red soil containing little hematite had a much higher plasticity, even without prolonged working (Fig. 6), which is consistent with this theory.
A practical consequence of this effect is that with these materials repeatable results may not be obtained in the British Standard plasticity tests. This led to the inclusion of a note in the current standard that certain soils may require as much as 40 min continuous mixing immediately before testing to obtain repeatable results. The points shown for Kenya red soils in Fig. 3 are for 40 min mixing. A difficulty also arises in interpreting the results of plasticity tests, because, although the Kenya red clays fall near the Kenya black clays on the plasticity chart and their plasticity is increased on working, the natural unworked material in which the iron oxide bonds are intact appears far less clayey in some respects than the black mont- morillonite clays. Thus unworked red clays have a lower shrinkage potential (higher shrinkage limit) and a higher permeability than the black clays, For many appli- cations the test values obtained after only a small amount of mixing may therefore be the most appropriate.
Aggregation in the Keuper Marl
Measurement of the clay content of a soil is of ten employed to give a guide to its physical properties, and its magnitude is also a factor to be considered in assessing the origin of these properties. It is measured by sedimentation methods after a pre-treatment and dispersion procedure designed to break up any aggregation or cementing of particles. In most particle-size classification systems the clay fraction is defined as the material smaller than 2 /~, and it is usually assumed that all the clay minerals are in this fraction of the soil. This wiU not be true, however, i f clay minerals are present as particles of a size greater than 2 /z, or if the dispersion treatment is inadequate.
In the case of the tropical red clays the British Standard pre-treatment and dispersion procedure was found adequate to deal with the aggregation resulting from the, cementing effect of iron oxide, although Terzaghi (1958) has noted a discrepancy between the mineralogical analysis of a similar soil and the percentage of its particles smaller than 2 t~- The Keuper Marl is an example of a material in which aggregation is very much harder to break down. The samples studied were unweathered red and grey mudstones from the cuttings of motorways recently built in England. Their liquid and plastic limits (Fig. 11) bore no relation to their clay contents as measured by the British Standard method (Sherwood & Hollis, in preparation), but showed a strong correlation with their clay contents as deter- mined by a quantitative mineralogical method, using X-ray diffraction (Dumbleton & West, in preparation). However, the range of values of liquid and plastic limit observed seem more appropriate to the British Standard clay contents than to the values obtained by mineralogical analysis. For the thirteen samples studied the total clay mineral contents varied from 50 to 94% while the clay contents measured E
192 M. J. Dumbleton and G. West
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l (a)
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, /
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i /
(b) 0 /
0 / (
, % o N
/ / / /
10 20 30 40 50 60 70 80 go Clay content (%1
I �9 /
w
/
100
Fta. 11. Relation between: (a) the liquid and (b) plastic limits of Keuper Marl samples and their clay content as measured by two different methods.
by sedimentation varied from 12 to 36% and was not greatly increased by various chemical treatments. Visual and microscopical examination revealed that most of the coarser particles recovered during the particle-size analysis consisted of un- dispersed mudstone; persistent aggregation of the particles was, therefore, at least a contributory cause of the difference, but in addition clay particles of a size greater than 2 t~ might have been present. Both of these factors would lead to the observed small values of liquid and plastic limit in relation to the large values of clay mineral content.
C O N C L U S I O N S
The relationship between the plastic and liquid limits and the clay contents of natural soils have been compared with those for artificial mixtures of pure clay minerals with quartz sand-silt. Reasonable agreement was obtained for mont- morillonitic soils, but there were larger deviations in the case of kaolinitic soils,
C l a y m i n e r a l s a n d p l a s t i c i t y 193
due to the effect of other factors including the degree of order in the kaolinite mineral and the presence of iron oxide.
The relations between plasticity index and liquid limit for artificial montmoril- lonite and kaolinite mixtures lay close to Casagrande's A-line, and were quite distinct from each other.
The behaviour of various types of soil which plot below the A-line has been explained in terms of a concept of 'ineffective water'. Both artificial mixtures and natural silty soils containing muscovite plotted below the A-line, but artificial mix- tures containing quartz silt and natural silty soils containing illite did not. I t was concluded that in these materials it is the presence of the mineral, muscovite, rather than of silt-sized particles which causes samples to plot below the A-line.
Particle aggregation affects the liquid limits of t ropica l red clays, leading to difficulties in reproducing and interpreting standard test results. Particle aggregation in the Keuper Marl leads to difficulty in dispersing the clay particles for sedi- mentation analysis of particle size, and as a result the clay contents obtained were much less than those obtained by X-ray analysis; the values obtained by sedimenta- tion analysis seemed more appropriate to the values of liquid and plastic limit observed.
I t may be concluded that with some types of soil a correlation is found between the soil properties and the clay minerals present, but that with other types of soil additional factors are also involved. I t is only by a detailed examination of such soils that the nature and influence of these factors will be revealed, and a fuller understanding of the origin of the soil properties of these materials will be reached.
A C K N O W L E D G M E N T S The work described in this paper was carried out as part of the programme of the Road Research Board. The paper is contributed by permission of the Director of Road Research. Crown copyright. Reproduced by permission of the Controller of Her Majesty's Stationery Office.
R E F E R E N C E S CASA~RANI)E A. (1947) Proc. Am. Soc. civ. Engrs 73, 783. COLEMAN J.D., FARRAR D.M. & MARSH A.D. (1964) G(otechnique 14, 262. DUMBLETON M.J. (1963a) Dep. scient, ind. Res., Rd Res. Lab. Note No. LN/372/MJD. DUMBLETON M.J. (1963b) Proc. 2nd Asian Con[. Soil Mech. Found. Eng., Tokyo 2, 63. DUMBLETON M.J. (1965) Appendix I to K. E. CLARE & P. J. BEAVEN. Dep. scient, ind. Res.,
