Absorption Water Content and Liquid Limit

© 1999 by the American Society for Testing and Materials127

© 1999 by the American Society for Testing and Materials

REFERENCE: Sridharan, A. and Nagaraj, H. B., “AbsorptionWater Content and Liquid Limit of Soils,” Geotechnical TestingJournal, GTJODJ, Vol. 22, No. 2, June 1999, pp. 121–127.

ABSTRACT: Consistency limits are extensively used in geotech-nical engineering practice. Besides the consistency limits, the plas-ticity index, liquidity index, and consistency index have been usedto correlate with engineering properties. The test procedures thathave been developed to determine liquid limit are based on thestrength criterion, and the liquid limit corresponds to a shearing re-sistance of 1.7 to 2.0 kPa. However, the mechanisms controlling thetest procedures do not simulate the mechanisms controlling the wa-ter-holding capacity of soils that the liquid limit is supposed to rep-resent. In this paper an attempt has been made to devise a test pro-cedure, known as the absorption test, which simulates themechanisms controlling the water holding-capacity of soils. Theequilibrium water content reached by the dry soil pats starting atshrinkage limit void ratio is found to have a good correlation withthe conventional liquid limit obtained from the cone penetrometermethod.

KEYWORDS: Atterberg limits, clays, laboratory tests, plasticity,soil classification

Historically, some characteristic water contents have been de-fined for soils. In 1911, Atterberg proposed the limits of consis-tency for agricultural purposes to get a clear concept of the rangeof water contents of a soil in the plastic state (Casagrande 1932).They are liquid limit (wL), plastic limit (wP), and shrinkage limit(wS). Atterberg limits for a soil are related to the amount of waterattracted to the surface of the soil particles (Lambe and Whitman1979). Therefore the limits can be taken to represent the water-holding capacity at different states of consistency. The consistencylimits as proposed by Atterberg and standardized by Casagrande(1932, 1958) form the most important inferential limits with verywide universal acceptance. These limits are found with relativelysimple tests, known as index tests, and have provided a basis for ex-plaining most engineering properties of soils met in engineeringpractice. Based on the consistency limits, different indices havebeen defined, namely, plasticity index (IP), liquidity index (IL), andconsistency index (IC). These indices are correlated with engineer-ing properties. In other words, all these efforts are primarily to clas-sify the soils and understand their physical and engineering behav-ior in terms of these limits and indices.

Liquid Limit

Liquid limit of soils is defined as the water content above whichthe soil behaves as a viscous liquid (a soil-water mixture with nomeasurable shear strength). Casagrande (1932) deduced that theliquid limit corresponds approximately to a water content at whichthe soil has a shear strength of about 2.5 kPa. Subsequent studieshave indicated that the liquid limit for all fine-grained soils corre-sponds to a shearing resistance of about 1.7 to 2.0 kPa (Wroth andWood 1978; Whyte 1982). Since the consistency of soil is associ-ated with its strength in the remolded saturated condition, it becamepossible to devise tests using the strength criterion to express thestate of consistency or limit of the soil. Based on the strength crite-rion, many methods of finding liquid limit were developed. Cur-rently two methods are in vogue for the determination of the liquidlimit of a fine-grained soil. They are:

1. Percussion cup method.2. Fall cone method.

In the percussion cup method, liquid limit is defined as the mois-ture content corresponding to a specified number of blows requiredto close a specified width of groove for a specified length(Casagrande 1932, 1958). This conventional liquid limit device hasa serious disadvantage, as concluded by Casagrande (1958) him-self. It does not provide a uniform basis of comparison for fine-grained soils that differ in their reaction when subjected to a shak-ing (dilatancy) test. In addition to this, Casagrande’s method hascertain other shortcomings such as the difficulty of cutting a groovein soils of low plasticity and the tendency of soils to slip rather thanflow. Casagrande (1958) also indicated, among other alternatives,that the static penetration test was one way to eliminate some of thedifficulties posed by the original liquid limit device. Further, thepercussion cup test has been proved to have relatively poor repro-ducibility (Whyte 1982). A more reliable and reproducible indirectmeasure of soil strength at liquid limit is water content provided bythe cone method.

In the fall cone method, liquid limit is the moisture content cor-responding to a specified depth of penetration for a cone of knowngeometry and weight. A detailed examination of the potential ofthis method has been made by Karlsson (1961), Sherwood and Ry-ley (1970), Campbell (1975), Leroueil and Le Bihan (1996), andFarrell et al. (1997). The advantages claimed in favor of the conepenetration method are easy simple operation coupled with goodreproducibility. Due to the inherent advantages, UK, India,Canada, and many European countries have identified the conepenetration method also as a standard method of determining the

A. Sridharan1 and H. B. Nagaraj2

Absorption Water Content and Liquid Limitof Soils

1 Professor, Department of Civil Engineering, Indian Institute of Science,Bangalore 560 012, India.

2 Research associate, Department of Civil Engineering, Indian Institute ofScience, Bangalore 560 012, India.

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liquid limit of fine-grained soils. There are two types of cone pen-etration testing in use. Some codes of practice like the British Stan-dard Methods of Test for Soil for Engineering Purposes (Part 2:Classification Tests: BS:1377-Part 2 (1990)), and Indian StandardMethods of Test for Soils (Part 5: Determination of Liquid andPlastic limit; IS:2720—Part 5, 1985), specify a 30°/80-g cone(British cone) and a depth of penetration of 20 mm at the liquidlimit, while countries like Canada and Sweden (Leroueil and Le Bi-han 1996; Farrell et al. 1997) adopt a 60°/60-g cone (Swedish cone)and a depth of penetration of 10 mm at the liquid limit. However,the recent studies have demonstrated that both types of cone pene-tration testing essentially give identical results (Leroueil and Le Bi-han 1996; Farrell et al. 1997).

In the case of soils of low plasticity, there is a tendency for thesoil in the cup to liquefy at the surface during the preparation of thesample for tests in the cone penetrometer. Sherwood and Ryley(1970) reported difficulties in some soils when striking off the ex-cess soil to get a smooth level surface of soil in the cup. They alsomention that no other method could give a smooth level surfacethat did not result in a “fatting up” of the surface layer.

While liquid limit is an important physical property for use insoil classification and for correlations with engineering properties,the existing methods of determining the same have their own limi-tations and practical difficulties. Further, the mechanisms control-ling the test procedures also do not simulate the mechanisms con-trolling the water-holding capacity of soils. Hence, it is desirable todevise a test procedure that can simulate the mechanisms control-ling the water-holding capacity of soils. This paper describes suchan attempt.

Basic Considerations

Plastic clays under zero confining pressure will imbibe water un-til their water absorption capacity becomes zero and reach an equi-librium condition. Croney and Coleman (1954, 1961) discuss atgreat length the quantity of water held in a soil at a given suctionand the quantity that will be transferred by a change of suction. Thiscan be determined from the knowledge of the relationship betweensoil suction and moisture content. Soil in an oven-dried conditionhas found to have a suction pressure of 106 kPa (Croney and Cole-man 1954, 1961). The same authors have conducted suction mea-surements with soil in its natural condition as well as remoldedcondition. Further, they have reported that at plastic limit and liq-uid limit two heavy clays have been found to have a suction pres-sure of 246 kPa and 0.3 to 0.98 kPa, respectively. Nelson andMiller (1992) have reported a similar range of values of suctionpressures at plastic limit, and a suction pressure value of above10 000 kPa at the shrinkage limit of soils. Further, Croney andColeman (1954, 1961) have considered the nature of the relation-ship between suction and moisture content and with the hysteresiseffect that may arise. The shape of the relationship is found to be afunction of the grading of soil, its compressibility, its density, andthe mineral nature of the soil particles. From the above discussionit is clear that the suction pressure changes significantly from anoven-dried condition to a liquid limit water content state condition.At the liquid limit state, it has negligible suction pressure and isnearer to the equilibrium water absorption state. The equilibriumwater content, however, strongly depends upon the test procedure,the way in which water is allowed to absorb, and the initial condi-tion and the state of the soil. It may be reasonable to conclude thatthe water absorbed under zero confining pressure will bear a goodcorrelation with the liquid limit water content.

In order to make use of this water-absorbing capacity and arriveat a test procedure to quantify the same, it is necessary to standard-ize the test procedure and obtain the water-holding capacity of asoil. By its very definition, there will not be any decrease in volumeon drying a soil below the shrinkage limit. Obviously this forms thelower limit of the water-holding capacity. Starting at the lowest wa-ter-holding capacity (i.e., shrinkage limit), dry soil pats can be al-lowed to imbibe water, and their water content can be determinedwhen they reach equilibrium. The so-obtained equilibrium watercontent should be comparable with the conventional liquid limit,and, hence, it is worth examining as to whether any correlation ex-ists. Based on the above consideration, absorption tests have beencarried out starting at the shrinkage limit void ratio, allowing thesamples to imbibe water till they reach equilibrium and the resultscompared with the liquid limit water content. In the following para-graphs, the experiments conducted for this purpose have been pre-sented and the results analyzed.

Materials and Methods

A number of natural soils along with commercially availablekaolinite, bentonite, and kaolinite-bentonite mixtures covering awide range of liquid limit (31.5% , wL , 232%) were selected,tested for their physical properties, and reported in Table 1.

The specific gravity was determined using a pycnometer (stop-pered bottle having a capacity of 50 mL) as specified by the ASTMTest Method for Specific Gravity of Soils (D 854-92, 1995). Thespecific gravity values are an average of three tests; individual de-terminations differed from the mean by less than 0.01. The liquidlimit of the soils was determined by the cone penetrometer methodas specified by BS: 1377-Part 2, 1990. The liquid limit tests werecarried out to obtain a minimum of five points for plotting the flowcurve. The consistency of the soil specimens was adjusted such thatthe cone penetration ranged between 15 and 25 mm. The plasticlimit of clay specimens was determined by the rolling bead methodas outlined in the ASTM Test Method for Liquid Limit, PlasticLimit, and Plasticity Index of Soils (D 4318-93, 1995). The shrink-age limit of soil specimens was determined as per ASTM Methodfor Shrinkage Factors of Soils by the Mercury Method (D 427-83,1995). While placing the wet soil at its liquid limit water contentinto the shrinkage dish, care was taken to expel entrapped air.Cracking due to fast drying was prevented by first allowing the soilto air dry under controlled conditions, and then oven-drying to aconstant mass. The shrinkage limits reported are the average ofthree determinations; the variation between individual determina-tion was less than 0.5%.

Grain-size analysis was done as per ASTM Test Method for Par-ticle-Size Analysis of Soils (D 422-63, 1995), by wet sieving of100 g of dry soil using a 75-mm sieve. The portion retained on the75-mm sieve was oven dried and sieved using sieves of 300, 212,and 150-mm sizes. The soil passing through 75 mm was collectedcarefully, air dried, and the grain-size distribution analysis was per-formed by the hydrometer method. The results are presented inTable 1.

The mineralogical analysis of the soils was performed using anX-ray diffractometer with Cu-ka radiation. The principal clay min-erals present in the soils are given in Table 1.

Water Absorption Test

Since the magnitude of water absorption will depend upon theequilibrium suction pressure in the sample, which will depend onthe height of the sample above the water source and the properties

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of porous stone and filter paper used in the test, they have beenstandardized in the following way to get reproducible results. For each soil sample three trials of shrinkage limit tests were carried out as per standard procedure (ASTM D 427-83, 1995), andthe shrinkage limit pats thus obtained at the end of the tests wereused for the water absorption test. The size of the pats varied depending upon the shrinkage limit of the soil, but the initial size of the sample is 45 mm in diameter and 15 mm in height. A desiccator containing properly levelled fine sand (,425 mm)was taken. Two big filter papers (Whatman No. 1) were placed one above the other on the levelled sand and later saturated with distilled water. Saturated porous stones that are used in a standard oedometer test (6 cm in diameter and 0.5 cm in height)were placed on the filter paper. Another set of small-size filter papers (Whatman No. 42) were placed on each porous stone asshown in Fig. 1. The oven-dried shrinkage pats were weighed andplaced on the top of porous stones. The desiccator was coveredwith the lid on the top, and the pats were allowed to absorb water.The weight of the absorbed sample along with the porous stones

was determined at regular intervals of time. The water absorbtiontest was continued until the samples reached equilibrium as indicated by the constant weight of the water-absorbed samplesalong with saturated porous stone for three consecutive weighings.Normally the time taken to reach equilibrium is found to be be-tween 2 to 24 h. However, for many samples, readings were takeneven up to three days to understand the time-absorption behaviorbetter.

Results and Discussion

Figures 2, 3, and 4 show a typical plot of water absorbed, w (%)versus time, t, for silty soil (Soil No. 3), fine kaolinite-1 (Soil No.12), and B C soil-2 (Soil No. 18), respectively. From the figures itcan be seen that the absorption curves follow a typical rectangularhyperbolic plot. Further, the final absorbed water content getsreached almost within 400 to 500 min itself. In order to obtain thefinal absorbed water content from the initial time versus water ab-sorbed data itself, without waiting for equilibrium, the results were

SRIDHARAN AND NAGARAJ ON LIQUID LIMIT OF SOILS 129

TABLE 1—Physical properties of soils used in the study.

Grain Size Distribution(7)

Soil No. Soil Description Gs* wL, % wP, % wS, % Sand, % Silt, % Clay, % wA, % Mineralogy(1) (2) (3) (4) (5) (6) (7a) (7b) (7c) (8) (9)

1 Red earth—1, Bangalore 2.68 31.5 16.5 11.02 45.0 33.0 22.0 22.9 K,Q**2 Red earth—2, Bangalore 2.70 37.0 18.0 14.7 35.5 38.5 26.0 35.2 K,Q3 Silty soil, Ooty 2.65 39.0 29.5 27.4 36.5 58.5 5.0 38.4 I,Q4 Black soil—1, Tadapatri 2.67 47.0 NP# 23.6 … … … 48.2 M,Q5 Coarse kaolinite, Bangalore 2.65 48.0 23.2 39.0 16.0 74.5 9.5 46.6 K,Q6 Red earth—3, Devanahalli 2.70 48.0 23.6 15.5 8.0 57.0 35.0 42.0 K,Q,M7 Silty clay, Koramangala 2.70 52.5 24.6 10.1 12.1 50.0 27.9 50.5 I,Q8 Black soil—2, Tadapatri 2.68 55.7 27.7 11.7 … … … 52.6 M,Q9 Cochin clay (oven dried) 2.61 56.4 38.1 21.0 18.0 64.5 17.5 58.8 H,Mi

10 Black soil—3, Tadapatri 2.69 58.3 28.8 11.1 … … … 49.6 M,Q11 Brown soil, Bangalore 2.66 58.5 32.1 13.5 19.5 42.5 35.0 45.0 M,K,Q12 Fine kaolinite—1, Bangalore 2.65 58.7 45.2 46.4 0.0 88.5 11.5 53.0 K,Q13 Black soil—4, Tadapatri 2.70 72.0 35.0 11.6 … … … 68.0 M,Q14 Illitic soil, Bangalore 2.58 73.4 53.0 39.0 0.9 71.6 27.5 54.4 I,K,Q15 B C soil—1, Belgium 2.70 73.5 35.6 11.9 13.0 35.5 51.5 73.5 M,Q16 Black soil—5, Tadapatri 2.70 75.0 35.2 11.8 … … … 73.0 M,Q17 Black soil—6, Tadapatri 2.70 75.5 37.9 11.8 … … … 74.4 M,Q18 B C soil—2, Davanagere 2.71 98.0 48.7 10.8 … … … 96.8 M,Q19 Fine kaolinite—2, Shimoga 2.64 55.0 31.4 33.1 1.0 67.0 32.0 47.1 K,Q20 10% Bentonite 1 90% soil No. 19 2.65 61.0 33.0 30.5 0.9 60.3 38.8 48.3 K,Q,M21 20% Bentonite 1 80% soil No. 19 2.66 74.0 32.5 29.3 0.7 53.4 45.9 51.7 K,Q,M22 30% Bentonite 1 70% soil No. 19 2.66 87.0 35.5 28.1 0.4 46.5 53.1 61.0 K,Q,M23 40% Bentonite 1 60% soil No. 19 2.67 125.0 38.3 25.2 0.2 40.2 59.6 120.0 K,Q,M24 50% Bentonite 1 50% soil No. 19 2.68 147.0 40.8 22.5 0.1 33.5 66.4 141.5 M,K,Q25 60% Bentonite 1 40% soil No. 19 2.69 173.0 44.7 19.8 0.1 26.8 73.1 175.4 M,K,Q26 80% Bentonite 1 20% soil No. 19 2.70 232.0 48.1 12.9 0.0 13.4 86.6 207.3 M,K,Q27 Bentonite, Kolar 2.72 320.0 56.6 10.5 0.0 0.0 100.0 292.0 M

NOTE:*GS 5 Specific gravity.wL 5 Liquid limit.wP 5 Plastic limit.wS 5 Shrinkage limit.wA 5 Absorption water content.**H 5 Halloysite.I 5 Illite.K 5 Kaolinite.Mi 5 Mica.Q 5 Quartz.# NP 5 Non-plastic.



FIG. 1—Absorption test apparatus.

FIG. 2—Time versus water-absorbed relationship for silty soil.



FIG. 3—Time versus water-absorbed relationship for fine kaolinite-1.

FIG. 4—Time versus water absorbed relationship for B C soil-2.


replotted in terms of time versus time/water (t 2 t/w) absorbed asillustrated in Figs. 5, 6, and 7. It may be seen that almost a perfectlystraight line is obtained, justifying the assumption that the absorp-tion curve is hyperbolic. Further, the final absorbed water contentpredicted by the inverse slope of the straight line matches the mea-sured absorbed water content obtained for the samples upon reach-ing equilibrium. Thus, from the straight line fit of the plot of t/wversus t with the initial results of the absorption test, one can obtain

the final quantity of water absorbed on reaching equilibrium by ob-taining the inverse of the slope of the straight line, thereby reduc-ing the time of conducting the test, if need be. The results of waterabsorbed expressed as a percentage of dry weight of the sample(wA) is tabulated in Table 1.

Figure 8 is a plot of wA versus wL obtained by the cone pen-etrometer method. It is clear that dry shrinkage pats on absorbingwater reach a water content very near to the liquid limit water con-


FIG. 5—Time/water absorbed versus time relationship for silty soil.

FIG. 6—Time/water absorbed versus time relationship for fine kaolin-ite-1.

FIG. 7—Time/water absorbed versus time relationship for B C soil-2.

FIG. 8—Absorption water content versus liquid limit relationship for allsoils.


tent. Also, it can be seen that a very good correlation (r 5 0.99) ex-ists between wA and wL as given by the following equation:

wA 5 0.92wL (1)

where wA and wL are in percent.In the bulk of the soil engineering practice, the maximum liquid

limit of soil ranges up to about 100. In view of this, the absorbedwater content obtained from absorption tests for soils having a liq-uid limit up to 98% were taken and compared with the liquid limitof the soils as presented in Fig. 9. Here, also, a good correlation (r5 0.93) exists between wA and wL as given by the following equa-tion:

wA 5 0.91wL (2)

Thus, the difference between Eqs 1 and 2 is minimal. Hence, eitherof these equations can be used or one of them depending upon thevalue of the liquid limit.

Earlier it has been brought out that several limitations and practical difficulties exist in the use of Casagrande’s apparatus or the fall cone apparatus to determine the liquid limit of a soil. Further, the mechanisms controlling these two methods do notidentify the mechanisms controlling the water-holding capacity ofsoils. Hence, the water absorption test, which is a relatively easytest, and the mechanisms involved in the test represent the water-holding capacity of soils, can be used to determine the liquid limitof soils. An additional advantage is that after carrying out a shrink-age limit test, the same soil pat could be reused to determine the

water absorbed and liquid limit from the correlation given either byEqs 1 or 2.

Conclusions

Guided by the mechanisms involved in the water absorption test,the liquid limit of a soil may be defined as the water-holding ca-pacity of the soil, and the same can be quantified as approximatelyequal to the water content absorbed by a dry soil pat at shrinkagelimit void ratio when it is allowed to absorb water under no confin-ing stress till it reaches equilibrium. The absorbed water contentthus obtained is found to have a good correlation with the conven-tional liquid limit determined by the cone penetrometer method.

References

Campbell, D. J., 1975, “Liquid Limit Determination of ArableTopsoil using a Drop-Cone Penetrometer,” Journal of Soil Sci-ence, Vol. 26, No. 3, pp. 234–240.

Casagrande, A., 1932, “Research on the Atterberg Limits of Soils,”Public Roads, Vol. 13, No. 3, pp. 121–130, 136.

Casagrande, A., 1958, “Notes on the Design of the Liquid LimitDevice,” Geotechnique, Vol. 8, No. 2, pp. 84–91.

Croney, D. and Coleman, J. D., 1954, “Soil Structure in Relation toSoil Suction (pF),” Journal of Soil Science, Vol. 5, No. 1, pp.75–84.

Croney, D. and Coleman, J. D., 1961, “Pore Pressure and Suctionin Soil,” in Proceedings, Conference on Pore Pressure, Butter-worths, London, pp. 31–37.

Farrell, E. R., Schuppener, B., and Wassing, B., 1997, “ETC5 Fall-Cone Study,” Ground Engineering, Vol. 30, No. 1, pp. 33–36.

Karlsson, R., 1961, “Suggested Improvements in the Liquid LimitTest, with Reference to Flow Properties of Remoulded Clays,”Proceedings, Fifth International Conference on SM & FE, Paris,Vol. 1, pp. 171–184.

Lambe, T. W. and Whitman, R. V., 1979, Soil Mechanics, John Wi-ley & Sons, New York, p. 31.

Leroueil, S. and Le Bihan, J. P., 1996, “Liquid Limit and FallCones,” Canadian Geotechnical Journal, Vol. 33, No. 5, pp.793–798.

Nelson, J. D. and Miller, D. J., 1992, “Expansive Soils—Problemsand Practice in Foundation and Pavement Engineering,” JohnWiley & Sons, New York, p. 110.

Scherrer, H. V., 1961, “Determination of Liquid Limit by the StaticCone Penetration Test,” Proceedings, Fifth International Con-ference on SM & FE, Paris, Vol. 1, pp. 319–322.

Sherwood, P. T. and Ryley, M. D., 1970, “An Investigation of aCone-Penetrometer Method for the Determination of the LiquidLimit,” Geotechnique, Vol. 20, No. 2, pp. 203–208.

Whyte, I. L., 1982, “Soil Plasticity and Strength—A New Ap-proach using Extrusion,” Ground Engineering, Vol. 15, No. 1,pp. 16–24.

Wroth, C. P. and Wood, D. M., 1978, “The Correlation of IndexProperties with Some Basic Engineering Properties of Soils,”Canadian Geotechnical Journal, Vol. 15, No. 2, pp. 137–145.


FIG. 9—Absorption water content versus liquid limit relationship forsoils having liquid limit less than 100%.


Absorption Water Content and Liquid Limit

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