Plastic Limit and Compaction Characteristics of Finegrainedsoils

Plastic limit and compaction characteristics of fine-grained soilsA. SRIDHARAN* and H. B. NAGARAJ†

*Department of Civil Engineering, Indian Institute of Science, Bangalore, India;†BMS College of Engineering, Bangalore, India

Laboratory determination of the compaction characteristics

of soils is important for use in earthwork constructions.The purpose of compacting earth fills such as earth damsand embankments (highway, railway and canal) is toproduce a soil mass that will satisfy the two basic criteria:

reduction in settlement and increase in shear strength. Forpreliminary design and assessment, correlations with theliquid limit have been attempted by various investigators.

However, from the present study it is seen that thecorrelations are not up to a satisfactory level. In this paperthe results of detailed investigations carried out are

examined to find which of the index properties correlatewell with the compaction characteristics. In the analysis,apart from the authors’ experimental results, results from

the available literature have also been considered. It isfound that the plastic limit bears a good correlation withthe compaction characteristics, namely optimum moisturecontent and maximum dry unit weight, much better than

liquid limit or plasticity index.

Keywords: clays; compaction; earth fills; embank-ments; ground improvement; laboratory tests

La determination en laboratoire des caracteristiques de

compaction des sols est importante pour les constructionsa terrassement. Les remblais de terre compactee comme leslevees de terre et les talus (autoroutes, voies ferrees etcanaux) ont pour but de produire une masse de sol qui

satisfera deux criteres de base : la reduction du tassementet l’amelioration de la resistance au cisaillement. Pour laconception preliminaire et l’evaluation, divers chercheurs

ont tente d’etablir des correlations avec la limite liquide.Cependant, cette etude montre que les correlations ne sontpas d’un niveau satisfaisant. Dans cet expose, nous

examinons les resultats de ces recherches detaillees afin detrouver celles des proprietes indice qui correspondent lemieux aux caracteristiques de compaction. Dans cette

analyse, outre les resultats experimentaux obtenus par lesauteurs, les resultats disponibles publies ont egalementete pris en compte. Nous avons trouve que la limiteplastique montre une bonne correlation avec les caracteris-

tiques de compaction, nommement la teneur en eauoptimum et le poids unitaire sec maximum, bien mieuxqu’avec l’indice de limite liquide ou l’indice de plasticite.

Notation

e void ratioGS specific gravity of soilsIP plasticity indexIS shrinkage index ¼ wL � wS

OMC optimum moisture contentSr degree of saturationw water contentwL liquid limitwP plastic limitwS shrinkage limitªd dry unit weight of soilsªd max maximum dry unit weight of soilsªw unit weight of water(ªd)P dry unit weight at plastic limit water content

Introduction

Soil compaction is the process whereby soil particles areconstrained to pack more closely together through a reductionin the air voids, and generally through mechanical means. Thepurpose of compacting earth fills such as earth dams andembankments (highway, railway and canal) is to produce asoil mass that will satisfy the two basic criteria: reduction insettlement, and increase in shear strength. Many other en-gineering structures constructed on soils, such as highways,railway subgrade and airfield pavements, also require com-paction. Compaction increases the strength characteristics ofsoils, which in turn increases the bearing capacity of founda-tions constructed over them. It also decreases the amount ofundesirable settlement of structures and increases the stabilityof slopes of embankments. Compaction plays a vital role in thepreparation of a good compacted soil liner in waste impound-ment sites to make them relatively impervious to leachatesand thereby reduce the threat of groundwater pollution. Thuscompaction is used as a practical means of achieving thedesired strength and compressibility and also hydraulicconductivity characteristics of the soils used.

The compaction characteristics of a soil as obtained froma laboratory compaction test are maximum dry unit weight

Ground Improvement (2005) 9, No. 1, 17–22 17

1365-781X # 2005 Thomas Telford Ltd

(GI 3173) Paper received 17 February 2003; last revised 22 March2004; accepted 21 April 2004

(ªd max) and optimum moisture content (OMC). In theconstruction of many earth structures, such as embank-ments, it is essential to assess the suitability of a soil withrespect to the compaction characteristics. Also, such pro-jects require large quantities of soil, and it may be difficultto obtain the desired type of soil from one borrow areaalone. To obtain the compaction characteristics fromlaboratory compaction requires considerable time andeffort. So, for a preliminary assessment of the suitability ofsoils required for the project, it is preferable to use thecorrelation of engineering properties with simple indextests.

Attempts have been made in the past to correlate thecompaction characteristics with the liquid limit. However,such correlations appear to be less than satisfactory. In thispaper, based both on detailed experimental investigationsand on an analysis of the published data on fine-grainedsoils, a study has been made to discover which of the indexproperties correlate well with the compaction character-istics.

Correlation of compactioncharacteristics with index properties:state of the art

Although the compaction characteristics are very impor-tant for field considerations, few attempts have been madein the past to predict them. An early attempt to predictthe compaction characteristics was made by Johnson andSallberg (1962). They developed a chart to determine theapproximate optimum moisture content of a soil using thestandard compaction test. The chart is a plot of plasticlimit against liquid limit. Different zones of optimummoisture content were indicated by means of numerouscurves. This is a useful chart in predicting only theoptimum moisture content of the soil from its liquid limitand plastic limit.

Recently Pandian et al. (1997) have proposed a method topredict the compaction characteristics in terms of the liquidlimit. They proposed the following two equations to assessthe compaction path on the dry side and wet side ofoptimum.

Dry side of optimum:

wffiffiffiffiffi

Sr

p ¼ 9:46 þ 0:2575wL (1)

Wet side of optimum:

w

S2r

¼ 10:61 þ 0:3615wL (2)

where Sr is the degree of saturation, wL is the liquid limit inpercent, and w is the water content

Pandian et al. (1997) proposed a series of predictedcompaction curves for Proctor compaction effort. For differ-ent values of degree of saturation, in the range of 50% , Sr

, 85% on the dry side of optimum and 85% , Sr , 95% onthe wet side of optimum, the water content can be calculatedusing equations (1) and (2) respectively. With the help of thepredicted set of compaction curves, the dry unit weight canbe determined, and hence the dry unit weight–watercontents path. Table1.Indexpropertiesof

soils

used

Grain

size

distribution

Maximum

dry

unitweight,

Optimum

moisture

Soilno.

Soiltype

Gs

wL:%

wP:%

wS:%

I P:%

I S:%

Sand:%

Silt:%

Clay:%

Mineralogy

ªdmax:kN

/m3

content:%

1Red

earth1

2. 70

37. 0

18. 0

14. 7

19. 0

22. 3

35. 5

38. 5

26. 0

Kaolinite,montm

orillonite,

muscovite,quartz

17. 9

16. 2

2Siltysoil1

2. 65

39. 0

29. 5

27. 4

9. 5

11. 6

36. 5

58. 5

5. 0

Illite,quartz

15. 2

24. 0

3Kaolinite1

2. 65

48. 0

35. 6

39

12. 4

9. 0

16. 0

74. 5

9. 5

Kaolinite,quartz

13. 3

31. 4

4Red

earth2

2. 70

48. 0

21. 3

15. 5

26. 7

32. 5

8. 0

57. 0

35. 0

Kaolinite,montm

orillonite,

muscovite,quartz

16. 0

21. 2

5Kaolinite2

2. 64

55. 0

31. 4

33. 1

23. 6

21. 9

1. 0

67. 0

32. 0

Kaolinite,quartz

14. 1

29. 0

6Cochin

clay

(ovendried)

2. 61

56. 4

38. 1

21. 0

18. 3

35. 4

18. 0

64. 5

17. 5

Illite

12. 9

35. 1

7Brownsoil1

2. 66

58. 5

32. 1

13. 5

26. 4

45. 0

19. 5

42. 5

38. 0

Montm

orillonite,kaolinite,

muscovite,quartz

14. 2

28. 0

8Kaolinite3

2. 65

58. 7

45. 2

46. 4

13. 5

12. 3

0. 0

88. 5

11. 5

Kaolinite,quartz

12. 2

39. 7

9Illiticsoil

2. 58

73. 4

51. 9

39. 0

21. 5

34. 4

0. 9

71. 6

27. 5

Illite,kaolinite,quartz

11. 1

44. 4

10

Black

cottonsoil1

2. 70

73. 5

35. 6

11. 9

37. 9

61. 6

13. 0

35. 5

51. 5

Montm

orillonite,quartz

13. 5

31. 2

GS,specificgravity;wL,liquidlim

it;wP,plasticlim

it;wS,shrinkagelim

it;I P,plasticityindex;I S,shrinkageindex.

18

Sridharan and Nagaraj

Scope for improvement

Johnson and Sallberg (1962) developed a chart to predictonly the optimum moisture content, but did not suggestany correlative chart or method to predict maximum dryunit weight. Without knowing the maximum dry unitweight the prediction of compaction characteristics is notcomplete.

The method suggested by Pandian et al. (1997) predictsthe compaction characteristics in terms of liquid limit alone.Using liquid limit alone in predicting engineering propertieshas limitations, in that soils having the same liquid limit butdifferent plasticity characteristics will behave differently.This investigation aims to clarify this aspect.

Methodology

Materials

As soil type is important in studying the compactioncharacteristics of soils, 10 soils including both natural soilsand commercially available kaolinites were selected, cover-ing a wide range of liquid limits. The soils selected were inpairs, with each pair having similar liquid limit values butdifferent plasticity properties. This was done to bring outthe effect of plasticity characteristics on the compactioncharacteristics of fine-grained soils. The physical propertiesof the 10 soils are reported in Table 1.

The 10 soils were characterised for their physical proper-ties—specific gravity, liquid limit, plastic limit, shrinkagelimit and grain size distribution—using standard proceduresas specified in BS 1377 Part 2 (BSI, 1990a). The test resultsare reported in Table 1.

A mineralogical analysis of each soil was performed usingan X-ray diffractometer. The principal clay minerals presentin each soil type are given in Table 1.

The compaction test was done for the 10 soils using thestandard Proctor mould (BSI, 1990b). For each soil a mini-mum of five trials were done with varying initial watercontent to get the compaction curve.

Results and discussion

Figure 1 shows a typical plot of dry unit weight againstmoisture content for a pair of soils having nearly the sameliquid limit, but with different plasticity characteristics. Itcan be seen from the figure that, although the liquid limit isnearly the same, the compaction curves are vastly different(including the optimum moisture content and maximum dryunit weight). Further, from Fig. 1 it can also be observed thatthe soil with the higher plastic limit has a higher optimummoisture content and a lower maximum dry unit weightthan the soil having a lower plastic limit, although the liquidlimit of both the soils is nearly the same. Similar behaviouris observed for the other pairs of soils (Table 1). Henceprediction of compaction characteristics from the liquid limitalone is not justified.

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Fig. 2 (a) Optimum moisture content and (b) maximum dry unit weightagainst liquid limit for soils both from the present study and from theliterature

19

Plastic limit and compaction characteristics of fine-grained soils

In order to check the validity of the above finding,compaction data from the literature (McRae, 1958; Johnsonand Sallberg, 1960; Foreman and Daniel, 1986; Sridharan etal., 1990; Benson and Trast, 1995) were also plotted alongwith the data from present study. Fig. 2(a) shows a plot ofoptimum moisture content against the liquid limit from thecompaction data of the literature and the present study. Itcan be seen from the figure that, although there is anincreasing trend of optimum moisture content with anincrease in the liquid limit, there is considerable scatter,particularly at higher values of the liquid limit. SimilarlyFig. 2(b) shows a plot of maximum dry unit weight againstthe liquid limit, which shows a decreasing trend withincrease in the liquid limit, but again with a lot of scatter.Thus the compaction data from the literature also supportthe present study that correlation of the compaction charac-teristics with the liquid limit is not justified.

Figure 3 shows typical compaction curves for a pair ofsoils having nearly the same plasticity index. It can be seenthat the compaction characteristics are vastly different.Similar results for other pairs of soils can be observed fromTable 1. An attempt has been made to correlate the optimummoisture content with the plasticity index, using data bothfrom the authors’ experimental results and from the litera-ture. Poor correlation has been obtained.

As the plastic limit is an important parameter, it was feltthat the compaction characteristics could be well correlatedwith it. Figs 4(a) and 4(b) show plots of dry unit weightagainst moisture content for two soil pairs having nearly thesame plastic limit. From the figures it is clear that thecompaction curves are very similar, and the compactioncharacteristics (optimum moisture content and maximumdry unit weight) are almost the same. Further, to verify thisobservation, the compaction test results from the presentstudy were plotted relating dry unit weight and themoisture content, as shown in Fig. 5. The plastic limit watercontents of the soils are marked on the respective compac-tion curves. It can be seen that the compaction curves of the10 soils are placed in the order of their plastic limits, andthat the optimum moisture content of each soil is slightlyless than the respective plastic limits. Figures 6(a) and 6(b)

show plots of the optimum moisture content against theplastic limit and the maximum dry unit weight against theplastic limit using data both from the present study andfrom the literature. The correlation of the compaction charac-teristics with the plastic limit is good. Correlation of theoptimum moisture content with the plastic limit, as obtainedfrom Fig. 6(a), is given by the following equation, with acorrelation coefficient of 0.99:

OMC ¼ 0:92wP (3)

One can suggest some reasons why OMC should correlatewell with the plastic limit. A close examination of the plasticlimit test procedure reveals that, when the water content isabove the plastic limit, the rolled thread will not crumble,even on reaching 3 mm diameter. When the water content isless than the plastic limit, the soil thread will crumble at adiameter more than 3 mm. This suggests that there is an

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Fig. 3. Dry unit weight against moisture content for a set of two soils withplasticity index approximately 24%

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20


optimum water content at which the soil thread crumbleswhen the diameter is 3 mm. Similarly, in the compactionprocess, there is a water content (optimum water content) atwhich the dry unit weight is maximum for any compactiveeffort. Here that optimum water content is observed near tothe plastic limit.

Similarly, from Fig. 6(b), the equation for maximum dryunit weight in terms of plastic limit is

ªd max ¼ 0:23(93:3 � wP) (4)

where ªdmax is the maximum dry unit weight in kN/m3,and wP is the plastic limit in percent.

If an assumption is made regarding the degree ofsaturation at the optimum moisture content, which is foundto be around 85% from the present study as well as fromthe data reported by Pandian et al. (1997), and about thespecific gravity of the soil (Gs), then the maximum dry unitweight (ªd max) can be obtained from the following basicrelations:

ªd ¼ Gsªw

1 þ eand e ¼ wGs

Sr

and can be stated as:

ªd max ¼ Gsªw

1 þ wGs=ð SrÞ(5)

Conclusions

A compaction study with five different pairs of soils, witheach pair having nearly the same liquid limit, has shownthat the compaction characteristics do not correlate well witheither the liquid limit or the plasticity index of the soils.

However, the plastic limit is shown to correlate well withthe compaction characteristics, namely optimum moisturecontent and maximum dry unit weight. Data from theliterature also support this correlation. Based on both thepresent study and data from the literature, the followingcorrelation equations are proposed to predict the compactioncharacteristics:

OMC ¼ 0:92wP (3)

ªd max ¼ 0:23(93:3 � wP) (4)

References

Benson C. H. and Trast J. M. (1995) Hydraulic conductivity ofthirteen compacted clays. Clays and Clay Minerals, 43, No. 6,669–681.

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Fig. 5. Dry unit weight against moisture content for all the soils used in thepresent study (dry density at plastic limit water content, (ªd)P) (ZAV (zeroair voids))

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21

Plastic limit and compaction characteristics of fine-grained soils

BSI (1990a) British Standard Methods of Test for EngineeringPurposes: Classification Tests. British Standards Institution,Milton Keynes, BS 1377: Part 2.

BSI (1990b) British Standard Methods of Test for EngineeringPurposes: Compaction-Related Tests. British Standards Institu-tion, Milton Keynes, BS 1377: Part 4.

Foreman E. D. and Daniel D. E. (1986) Permeation of compactedclay with organic chemicals. Journal of the Geotechnical Engineer-ing Division, ASCE, 112, No. 7, 669–681.

Johnson A. W. and Sallberg J. R. (1960) Factors that influence fieldcompaction of soils (compaction characteristics of field equip-ment). Highway Research Board Bulletin, No. 272, 14.

Johnson A. W. and Sallberg J. R. (1962) Factors influencingcompaction results. Highway Research Board Bulletin, No. 319,125.

McRae J. L. (1958) Index of compaction characteristics. Symposiumon Application of Soil Testing in Highway Design and Construction,ASTM Special Technical Publication No. 239, pp. 119–127.

Pandian N. S., Nagaraj T. S. and Manoj M. (1997) Re-examinationof compaction characteristics of fine-grained soils. Geotechnique,47, No. 2, 363–366.

Sridharan A., Rao S. M. and Joshi S. (1990) Classification ofexpansive soils by sediment volume method. Geotechnical Test-ing Journal, 13, No. 4, 375–380.

Discussion contributions on this paper should reach theeditor by 1 July 2005

22


Plastic Limit and Compaction Characteristics of Finegrainedsoils

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