relationship between the atterberg limits and clay content

The Japanese Geotechnical Society

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The JapaneseGeotechnical Society

SOiLS AND FOUNDATtONS Vol. 47, No. 5, 887-896, Oet. 2007Japanese Geetechnical Society

RELATIONSHIP BETWEEN

LIMITS AND CLAYTHE

ATTERBERG

CONTENT

ENNJo PoLIDoRIi)

ABSTRACT

This study investigates the liquid limit (Casagrande's method) and plastic limit (rolling and thread method) of six in-organic soils and their respective mixtures with fine silica sand. It was observed that the liquid Iimit and plastic limit

values of the mixtures tested, except those with a low clay percentage, are linked to the respective clay size contents bya linear relationship. The Atterberg limits were subsequently recalculated using the equations of the regression lines ofthe mixtures governed by linear law with the clay percentages, The plotting of the plastic limit as a function of the liq-uid limit of these data made it possib}e to determine the relationship among the liquid limit, the plastic limit and clayfraction valid for inorganic soils that contain platey clay minerals and for clay size contents that are not too low.Hence, on the basis of the interdependence among the parameters considered (PV}., PVb, 4, CF, A), 'for a given inorgan-ic soil, knowing only two of three parameters (PVi., PVb, CF) that are measuTable using standard tests, the values ofother three parameters can be obtained. '

Key words: Atterberg limits, clay, laboratory tests,plasticity,soil classification (IGC: DllD3)

INTRODUCTION

If a clayey soil is mixed with ever increasing amounts ofwater it becomes softer and softer and a point will bereached at which the soil ceases to behave as a plasticmaterial and becomes essentially a viscous fluid, Atter-berg (1911) suggested a method for defining this change,and the water content of the soil at this point is its liquidlimit, VV'L, Likewise, Atterberg defined the change from aplastie to a semi-solid state, and the water content of thesoil at this point is its plastic limit, PVb, The methods todetermine the liquid and plastic limits, later developed byCasagrande (1932, 1958), are considered standard inter-national tests. These limits and the numerical differencebetween them, the plasticity index, 4, are very useful tocharacterize, elassify and predict fine soils engineering be-haviour. The Atterberg limits of a soil depend on its composi-tion (quantity and type of clay minerals) and so-called dy-namic factors (Veniale, 1983) such as, pH, temperature,cation exchange capacity, type and quantity of cations inthe solution, etc., which vary in space and time for natur-al soils. An example of dynamic variables can be found inthe continuous alteration of the environment by humanactivjties, such as the impact of acid rain and chemical

products used in agriculture. Dynamic factors can have astrong efiect on the liquid limit value, though such effectsmay vary according to the type of clay minerals. For ex-ample, as the concentration of salts increases, the liquidlimit of the clay mineral montmorillonite decreases mar-

i)Institute of Applied Geology, University of Urbino `Car]o Bo' `Sogesta

The manuscript for this paper was received for Teview on December 1 1Written discussions on this paper should be submitted befere May 1,Tokyo 112-OO]], Japan. Upon request the c]osing date may be extended one month.

kedly, while the Iiquid limit of the clay mineral kaoliniteis not influenced (Di Maio and Fenelli, 1994). Hence, fora giyen soil, the values of the Atterberg limits, which arethe result of the eombination of all the factors, provideinsight into that soil's plasticity characteristics for every

possible combination of the factors that infiuence theplasticity of a soil,

There are few studies in literature on the Atterberglimits of soils as a function of their clay size contents andthe results of these studies are not always in agreement.These studies have focused primarily on the liquid limit

of clay minerals mixed with silica sand (Seed et al,, 1964a, 1964b; Sivapullaiah and Sridharan, 1985; Tan et al., 1994; Nagaraj et al,, 1995; Kumar and Muir Wood, 1999). Seed et al. (1964b) in a study on the Atterberglimits of the clay minerals kaolinite, illite and montmorillonite and their respective mixtures with sand, concludedthat, fer clay percentages which are not too low, the liquid and plastic limits are both linked by a linear relationship to their clay size contents. The respective regression lines pass through the origin o'f the axes. Nevertheless, the

plastic limit values reported by Seed et al, (1964b) of the montmorillonite-sand mixtures are less than those of kaolinite-sand mixtures with the same clay percentages. Conversely, White (1949) in his study o'f the Atterberg limits of the most common clay minerals concluded that the plastic limit of montmorillonite>(illite)>kaolinite,

The same conclusion was traced later by Mitchell (1993). Several attempts to link plasticity index with liquid limit, mostly through the empirical correlations, ignoring

' Scientific Campus, Italy ([email protected].

, 2006; approved on May 29, 2007.2008 to the Japanese Geotcchnical Society, 4-38-2, Sengoku, Bunkyo-ku,

887

NII-Electionic



TheJapaneseGeotechnicalSociety

888 POLIDORI

the content of non-clay particles (>2 pam) are reported inliterature (Casagrande, 1948; Seed et al., 1964b; Nagarajand Jayadeya, 1983; Sivapullaiah and Sridharan, 1985;Panadian and Nagaraj, 1990). Tbis study, based on compositional factors (amountand type of clay minerals), investigates how the liquidlimit and the plastic limit vary as a function of clay sizecontents in inorganic soils with platey clay minerals. Onthe basis of the average values (using equations of theregression lines) of the experimental data collected arelationship between the Atterberg limits and the clayfractions is then investigated. Since the non-platey clayminerals such as halloysite, allophane, attapulgite havecharacteristics yery difTerent from that of platey clayminerals (e.g., high plastic lirnit, low index plasticity)(Mitchell, 1993), they are excluded from the presentresearch as organic soils are.

GEOTECHNICAL AND MINERALOGICALCHARACTERIZATION

Experiments were carried out on six inorganic soils andon their respective mixtures with fine silica sand. Three ofthe soils were composed of bentonite, one was composed

of kaolinite (commercially available), another was com-posed of 1:1 mixture of kaolinite- bentonite whereas thelast soil was a natural soil belonging to the Formation ofVaricoloured Clays (upper Cretaceous-lower Eocene)Central Italy. Their characteristics are sumrnarized inTables 1 and 2, In the soil-sand mixtures (Fig. 1) the clay

fraction, CF (2"m%2um%




ATTERBERG LIMITS AND CLAY CONTENT 889

tUres with sands. Some standards, in addition toCasagrande's method, have included the fall-conemethod. Both methods have advantages and disavan-tages. For exarnple, (with reference to this study) the conepenetration method is not suitable for very expansivesoils (Wasti and Bezirci, 1986). Grain size distribution of the silica sample and the soilswas obtained using the sieve and hydrometer methods,respectively. The percentage of clay in the mixture wasdetermined as the percentage by weight of particles fineTthan 2"m in the commercial clay and the natural soiladded to the fine silica sand, The liquid limit of the samples was determined to es-tablish a minimum of four points in order to plot the fiowline, The plastic Iimit was determined by the average offour or more water contents. This procedure was appliedto each soil and later to the mixtures with silica sand.Since the thread-rolling method is considered operator-dependent (see below), the plastic limit tests were repeat-ed, and the ayerage of the values was considered in orderto improve the alignment of the points of the mixtures

plotted on the graph. The mixtures were prepared by mixing the above-men-tioned dry components followed by, adding deionizedwater. Since the mixtures are composed of percentages inweight of the respective components, the samples were

placed in an oven at 60OC to eliminate humidity absorbedfrom the atmosphere before being mixed. The silica silt(87% silt and 13%




890 POLIDORI

produced by contact of the non-clay particles, allows thissoil tQ retain a larger amount of water and as a conse-

quence the values of the liquid Iimit will be greater thanthe values predicted by Eq. (1). In fact, the slurrie soilswith low clay size content tend to slide rather than fiow asplastic material when placed in the cup. On the basis ofthe data of the tested mixtures, it was observed that therelative increase of the liquid limit values passing frornphysical state (a) to physical state (b) occurs gradually,with initial values of CF> Ch, (probably for the geometri-cal proximity of the round particles andfor the contact ofsome particles). For the sake of clarity, these values (notproportional with CD are here defined as ``anomalous''

(Fig. 1) and they are not included in the elaboration of thedata. The present study is based (on physical state (a)) onthe values of the Atterberg limits governed by linear lawwith clay size contents, This condition is also importantin order to look for relationships between the Atterberglirnits and the intrinsic geotechnical properties of thesoils, Casagrande (1932) reported that the physical sig-nificance of the liquid limit of a non-plastic soil is fun-

damentally different from that of a plastic soil. Theauthor believes that Casagrande's consideration for non-plastic soils can also be extended to

``anomalous'' values

of the Atterberg limits. In fact, as these values increase(in a relative sense), so do the values of the residual fric-tion angle because both mainly depend on the character-istics of the granular phase. The values of Cth for the soils plotted in Fig. 2 areoverestimated because the calculation of e, was only

based on the added sand (the components >2 pam foundin the soils were not taken into consideration), The equa-tion in Fig. 2 (for the average values of assumed e,, p, andp,), permits to obtain the Cli value as a function of thewater content (or vice versa) to have the same volume thatis when the clay water system will fill the voids in thegranular phase and the round particles will be in contactwith one another. It is graphically shown by the markedline and it was obtained by inserting the selected values

300

2so

2DOAgev

150l100

50

eo CmCm 20 40

4f

li

tttal

into the equation (CFfp,)+(va!p.,)=[(100-CF)fp,] e,(that is equivalent to Eq, (4), setting PV= PVL,yl100), Tbedetermination of e, of the fraction >2um of a soil iscurnbersome and time consuming. Nevertheless, on thebasis of the data collected in this study and insights intoclayey soils reported in Mitchell (l993), the liquid limitshould be determined in the soil fraction with a clay frac-tion greater than 20-25% so that the relationship PZ.-CFis governed by a linear law also for soils that contain lessexpansive clay minerals (kaolinite) and with high valuesof e,.

Ptastic Limit The data of the plastic limit (Figs. 1 and 3) show lesslinearity and relatively little variation (especially accerd-ing to the type of clay minerals) in comparison with thedata of the liquid limit. Note how small the range of theplastic limit values is compared with that of the liquidlimit values. Except for the mixtures with low clay frac-tion (see "anomalous" values Fig, 3), the best link be-tween the plastic limit and the clay percentages is a linearrelationship, The regression lines of the plotted mixturesintersect the PVb axis randomly between 8.4% and 11.9%,The equations shown in Fig. 3 refer to the average valueof the intercept, PVb=10%. Hence, the general equationis:

PI- == (k2 CF)+ 1O (7)

CF(%)60

80 100

Fig. 2, Retationship between vvater cont.ent, W and amount or c}ay,

C. needed to fi11 the voids in a granular soil, e,=void ratio of the

granular phase. p,, p. != density of round particles and clay size particles, respectively. 1, 4 and 6=liquid Limit regression lines oE soil-

sand miximres, see Fig. 1

where the }Vb-slope k2 depends on the type of clay miner--als (chemical state enclosed) contained into the soil (as oc-cur, in a very amplified way, for kT to PVL). To understand the reasons for the non-zero interceptvalue in Eq. (7) further investigations are necessary. Atterberg limits (for the rnixtures with CF ranging 10%) were subsequently recalculated using the equations(from Figs. 1 and 3) of the regression lines of the mixturesgoverned by linear law with the clay size contents in orderto obtain abetter correlation. Thesedata, P7b against IVLare plotted in Fig. 4, The dashed lines of regression definethe variation of the plastic limit as a function of the liquidlimit in mixtures with the same percentage of clay, Theselines are parallel (slope=O.04) and if they are extended,

60se

.. 40ge-l-

3020

-- ptg'.,o

r"ol-

, -.- o 2e

:. ''7ro.m,eiog.;L'

ta:er,e' v" ...--1" 2. Wp=O,34CF+10 ...'. ...:-Ar

3- YV,=O,35CF-O ..' ..' .f.ti 4- yv,!e,ssCF"O ..r ..' -e./..i:. : U:::'2k' :[:1: . .t{-2'1.'il'.i.tt

-'J'" ' '

l..is-.zr'-i314"-"' 3.-- . tt- t -'-t" at-

40CF{%)60

../.60'too

Fig.3. P]astic limit, EV, as f"nction of clay fraction CF (




ATTERBERG LIMITS AND CLAY CONTENT 891

60

50

-

40

8 i"

l: .tt....kettti,

.3L i i riol'`"["

"D

7: a --so cF(%




892 POLIDORI

represented by a straight line passing through the origin(t4 = lb !CF), The regression lines that represent the activ-ity of soils tested (Fig, 5) have non-zero intercept values(b= - 1O, intercept average value), Hence, the activity ofa clayey soil can be redefined as follows:

A=[O,96 PVL-(O.26 CF+10)]ICF (11) Figures 6 and 7 show how the activity (measured andcalculated) varies as a function of the clay fraction for theinvestigated soils. It can be observed that for a given soil,the activity against CF is not constant as suggested by therelationship proposed by Skempton (1953) (and in Fig. 6,the activity values should not decrease wjth an increase ofclay size content in the mixtures). The activity (Fig, 7)should reach its maximum value when CF=100%; itdrops slightly and almost linearly with high clay contents,whereas there is a marked decrease in activity in the mix-tures with low clay percentages. In this case, the dashedlines are theoretical, because the Atterberg limits (forCFs20% and 30% for kaolinite) are not linearly propor-tional to their respective clay size contents,

In routine assessments of soil properties, it is usuallyassumed that the fraction




ATTERBERG LIMITS AND CLAY CONTENT S93

esTab]e

3. I"dex propenies from literature

va ag A1,311.301.l71.901.04

BKK11

300

13 38 21 9.5 48 35.6 11.5 58,7 45.2

S 39 ,29.5 17.S S6.4 38.1

B= bentonite; K= kaolinite; I=illite; CF

geVa

60

- 408V-

20

o --o

, PVL and PV:. are in %

20

ge:x.2.s.-a

200

4e 80 100

WL (%)60

rioo1

ol

oa)dOO

300

200

2eoYVL{%)

1/

deol

oi

]-T-300

Mc

.

O.5 -line

Fig. 10, P]asticite, chart (after Polidori, 2003). C-line and O.5C-line correspond to 10D% CF and 50% (]F (5e%) with low and high plnsticity respectiyely. OL, OH=organic soils with low and high plasticity respectively. Low plnsticity (L) and high plasticity (H> based on ASTM standard (D 2487). NPC=soils with non-plate)' clay minerals

unk ' . xgdipe

O.NPC

l

b)

- T100 , T 200WL(%)

l l-Tt -tf3oe

Fig. 11. I,ocation on plasticity chart {see Fig. 10) of l25 soil samples from literutuTe (Seed et a}., 1964m; Lupini et al., 1981; Skempton, 1985; Wnsti and Bezirci, 1986; Burland, 1990; Di Ma;o and Fenel]i,

1994): a) 4 measured against -1.. Casagrallde's A-line also shown and b) 1} calculated with Eq. (Y} against FVL

the chart shown in Fig. 10, all the lb- VVL values of the in-

organic soils containing platey clay minerals andCF< 1OO% should lie above the C-line and the distance ofthe points from the C-line should be inversely propor-tional to clay percentage of the respective soils, The O,5C-line allows us to distinguish the points that fall below theline, clays (C), from the points lying above the line in thesilt zone (M), In turn, the silt and clay zones can be sub-divided in groups with low (L) or high (H) plasticity, ac-cording to the ASTM standard (D 2487) when the liquidlimit value is less than or greater than 50%, respectively.In the original paper it has been demonstrated that the siltzone is found above the clay zone, The residual inorganic soils (NPC) composed of non-platey clay minerals (allophane, halloysite, attapulgite)should lie below the C-line because their characteristics(high plastic limit, low plasticity index) are very differentfrom those of platey clay mineTals for which the plasticitychart was developed, The soils (NPC) that contain bothplatey and non-platey clay minerals as well as the organicsoils (O) can Iie above or below the C-line according tocharacteristics of the soil constituents. Clearly, for thesesoils the proposed quantitative relationshjp is not valid. The original plasticity chart was calibrated using alsesome of the experimental data shown in this paper andavailable in Pelidori




894 POLIDORI

(obviously if the organic substance and non-platey clayminerals are absent in the soils). In Fig. 12, the measured and calculated values of theplasticity index for the soils plotted on Fig, 11 are com-pared and the following considerations can be made.- In agreement with this study, from Figs. 12 and 13 it can be inferred that the measured 4 (and W}) against CF have non-zero intercept values. The plotted data (Fig. 13) do not lie according to the hypothetical U-line and the hypothetical O,5C-line shown, In addition, the O.5C-line is the line that best separates the soils with CF50%,

- For 60% of the soils (75 of the 125 soils plotted) the average value of the difference between both the 4 values (measured and calculated) is 1.3 units (ranging 3units).- For 90% of the soils (112 of the 125 plotted) the average value of this difference is 2.5 units (ranging 6 units).- tn the remaining 13 samples this discrepancy is often

very rnarked (for 5 soils ranging from 10 to 17 units).

The above mentioned differences (expressed in units)between the measured and calculated values of 4 (or PVb),

geVQ

300

200

100

o

ti

fs.e.'.tt-.

pty't-;.--.'''.--.-t..i..-t'

- .t../""" i

i

ge-"

8eT

60

40:

i2e H

o-/

o

o eF- CF

l/'20

40

/'O.5Cglne '

oop

C-line

WL{%)6080

o aoo 2oo

lp(calculated}(%)

300

Fig. 12. Comparison bet"'een measured and calculated plasticity in- dcx of sampies plotted in Fig, 11

dOO

Fig. 13. Particular from Fig. 11(n). Dashed ]ines 1 (4=O,96 PPI) a"di 2 U, != O,96 PK.-IS) show expected hypothetical U-line and O,5C-line respectively, if 4 (and -) against CF was found (equall}, tinearly preportional) to pass through the origin

Table 4.

Soil

12'3*4*s*67891011'1213"141516171819*20*21222324*2526*

Index properties of inorganic soiEs froin litcrature. (c)=calculated valucs (oV PPI, l, and A with Eqs, (8), (9) and (11) respectively)

cr wr

1001008888565L5503937371OO10048363510088

S8 86 84 8443424140

37

348330.6526184234.473.S75124.2

84 84 S7,5 454546.S48

80 73Z88350362184128214.5

8019S

88

Wv43.955.2384818.535.642.523.2424937.82934.829.421.33044443529552720.93225.438PV}(c)JV}-PFV(C)or fB(c)-Il 4 4(c) A A(c)

49.949.253,940.233.926.32625.1232338.337.824.321,22]39.235.844.446.446.339.226.329.523.928.323.1-6 6-15.9

7.8-15.4

9.3 16.S- 1.9

19 26-

o.s- 8,8

10.S 8.2 e.3-

9.2 8,2- O.4-1L4-17.3

15.g O.7- 8,6

8.1- 2.9

14.9

304.127S.4488136215.9

37.9 32.5101

42 3S 19.7 16 10.2 l7.4 26.7 50 29244315333129101193.6

48172.6

50

298.1281.4472.1143.8200.547.249

99.1 61 61 19.2 7.2 20.7 25.6 27 40.8 37.2243.6303.6315.7144.8101.7185

S6.1169.7

64.9

3.042,755.S41.S43.75O.74O.652.591.13O.95O.20O.16O.21O.48O.76e.soO.332,773.663.961.542.354.611,l74.31l.352.98 B2,81 B5.36 B1.63 B3.58 BO.92 BO.98 B2.54 B1.65 B1.65 BO.19 KO,07 KO.43 KO.71 KO.77 KO.41 le.42 I2.773.533.761.722.364.401,374.241.7S

"Data from Fig. 11{a). B == belltonite; K=kaolini{e; I =: illite; CF,PVi. and PF} are in %

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ATTERBERGLIMITS AND CLAY CONTENT 895

Table 5,BentoniteIndex properties from Lupini et al.

Sand

85 70 55 40 25 -

ew132640536688

(1980. (c)=ca]etilated va]ues (of VPC,, 4 and A g'ith Eqs, (S), (9) and (11} respective]y)

1530456075100

)VL385680114140184PVp2I2023283648PV3(c) 4 4(c) A

19232g334e17365786104136

3757861071441,311,381,421.62L581.54

A(c}

1,421,421.62I.621.63

CF==%




896 POLIDORI

ptm) sieve that is excluded from Atterberg Iimits charac-terization. Nevertheless, the second method presents adisadvantage because the standard test to obtain thegrain-size distribution of a soil (hydrometer method) isrnore laborious than the standard test for determining theplastic limit.- When linearly proportional to clay size contents, the Atterberg limits are important in characterizing and classifying fine soils and to obtain correlations with the intrinsic mechanical properties of the soils, because they show the soil's behaviour dominated by the clay phase.- In a fine-grained soil (

relationship between the atterberg limits and clay content

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