Discussions and Closures

Discussion of “Quantitative Estimation ofClay Mineralogy in Fine-Grained Soils” byBhaskar Chittoori and Anand J. PuppalaNovember 2011, Vol. 137, No. 11, pp. 997–1008.

DOI: 10.1061/(ASCE)GT.1943-5606.0000521

K. Prakash1 and A. Sridharan21Professor, Dept. of Civil Engineering, Sri Jayachamarajendra College of

Engineering, Mysore 570 006, India (corresponding author). E-mail:kprakash60@yahoo.com

2Honorary Scientist, Indian National Science Academy; formerly, Professorof Civil Engineering, Indian Institute of Science, Bangalore 560 012,India.

The authors’ contribution is a welcome addition to the literature. Itbrings out the importance of clay mineralogy in solving engineeringproblems connected with soil improvement techniques. The engi-neering behavior of fine-grained soils is controlled by the clay min-eralogical composition of the soil. As discussed by the authors, soilsof similar plasticity characteristics can contain different dominantclay minerals. Hence, it is correctly indicated by the authors thatidentical stabilization design or treatment cannot be given to soilswith identical plasticity characteristics without the proper knowl-edge of the dominant clay minerals composing the soils. In fact, thisis the major limitation of the plasticity chart used to classify fine-grained soils. Hence, it is important to recognize the dominant claymineral composition of soils in any project dealing with fine-grained soils.

The authors have proposed a methodology of quantifying theclay mineral composition of soils, which considers montmorillonite,illite, and kaolinite to be the major and stable clay mineral types, forwhich they deserve appreciation.

The study of the paper under discussion reveals the followingpoints:• The quantification of clay minerals in the soils with the available

procedures such as X-ray diffraction, scanning electron mi-croscopy, or infrared spectroscopy is not that simple to adopt.Sophisticated instrumentation and interpretation techniques arerequired.

• The quantification of the dominant clay minerals in the soil bythe method proposed by the authors requires information such ascation exchange capacity (CEC), specific surface area (SSA),and total potassium content (TP). The determination of thesequantities, particularly SSA, is also not that simple. Hence, theproposed method is not user friendly.

• There is no definite relationship between quantities such as CECor SSA and the quantities of montmorillonite illite and kaoliniteminerals predicted.In this context, the discussers feel that the exact percentage quan-

tification of the clay minerals present in the soil is not required tojudge the clay soil behavior. They opine that it is sufficient enoughto classify the soils in to different groups based on their degree ofexpansivity, which is an indirect representation of the dominant claymineralogy of soils. Sridharan and Prakash (2000) conducted a de-tailed analysis of the swelling capabilities of a number of soils ofvarying plasticity characteristics and varying clay mineralogicalcomposition obtained from oedometer swell tests on soils, whichgive the most useful and reliable assessment of swell capacity of

soils (Chen 1975; Winterkorn and Fang 1986) from air dry tosaturated conditions under a surcharge of 7 kPa, in the compactedcondition, and from other soil parameters such as liquid limit, plas-ticity index, and activity. Their analysis indicated that liquid limit,plasticity index, and activity cannot satisfactorily predict soil expan-sivity, because they do not consider the effect of soil clay mineral-ogy. On the basis of that analysis, they proposed a very simple anduser-friendly methodology for the prediction of degree of soilexpansivity and dominant clay mineral type present in the soil basedon the free swell ratio (FSR), which compares exceedingly well withthose from the oedometer swell tests.

The FSR is defined as the ratio of the equilibrium sedimentvolume of a 10-g oven-dried soil sample passing through a 425-mm sieve placed in a 100-mL graduated measuring jar containingdistilled water (Vd) to that in carbon tetra chloride/kerosene (Vk),thoroughly mixed, and left for the sediment formation, after anequilibration period of a minimum of 24 h

FSR ¼ Vd=Vk ð1Þ

The guidelines to classify degree of expansivity of a soil based onits FSR are indicated in Table 1 and shown in Fig. 1. Table 1 showsthat the nonswelling soil is grouped as kaolinitic and the swellingsoil is grouped as montmorillonitic (with moderate, high, and veryhigh degree of expansivity). Table 1 also provides the probabledominant clay mineral composition of the soil. The classification

Table 1. Soil Classification Based on FSR

FSR Soil group Clay type

Degreeof soil

expansivityDominant claymineral type

#1.0 I Nonswelling Negligible Kaolinitic1.0–1.5 II Mixture of

swelling andnonswelling

Low Mixture ofkaolinitic andmontmorillonitic

1.5–2.0 III A Swelling Moderate Montmorillonitic2.0–4.0 III B Swelling High Montmorillonitic.4.0 III C Swelling Very high Montmorillonitic

Fig. 1. Classification of soils based on their degree of expansivity

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given in Table 1 is based on exhaustive research conducted on a numberof field soils obtained from different parts of India and on pure clayminerals (numberingmore than 70) with widely varying plasticity char-acteristics and clay mineralogical composition (Prakash and Sridharan2004). Actual claymineralogy details of the soilswere obtained fromX-ray diffraction analysis of the magnesium-saturated, glycerol-solvatedclay fraction of soils and were also predicted from the FSR method.There is an excellent and one-to-one agreement observed between theresults obtained from these two entirely different testing procedures.This shows that the FSR approach, in addition to classifying the soilon the basis of its degree of expansivity, gives additional and reliableinformation about the dominant clay mineralogy of the soil.

The paper under discussion does not contain any free swell testresults such as Vd and Vk. The discussers wish to know whetherthe authors have any such data. If they have such data, it wouldbe interesting to see the dominant clay mineral type of the soilsstudied by the authors and predicted by the FSR method.

References

Chen, F. H. (1975). Foundations on expansive soils, Elsevier, New York.Prakash, K., and Sridharan, A. (2004). “Free swell ratio and clay mineralogy

of fine-grained soils.” Geotech. Test. J., 27(2), 220–225.Sridharan, A., and Prakash, K. (2000). “Classification procedures for

expansive soils.” Geotech. Eng. Proc. ICE, 143(4), 235–240.Winterkorn, H. F., and Fang, H. Y. (1986). “Soil technology and eng-

ineering properties of soils.” Foundation engineering handbook, H. F.Winterkorn and H. Y. Fang, eds., Galgotia Book Source, New Delhi,India.

Closure to “Quantitative Estimation ofClay Mineralogy in Fine-Grained Soils” byBhaskar Chittoori and Anand J. PuppalaNovember 2011, Vol. 137, No. 11, pp. 997–1008.

DOI: 10.1061/(ASCE)GT.1943-5606.0000521

Bhaskar Chittoori, M.ASCE1; and Anand J. Puppala,M.ASCE2

1Faculty Associate-Research, Dept. of Civil Engineering, Univ. ofTexas, Arlington, TX 76019. E-mail: sinu@uta.edu

2Professor, Dept. of Civil Engineering, Univ. of Texas, Arlington, TX76019 (corresponding author). E-mail: anand@uta.edu

The writers would like to thank both discussers for their intuitivediscourse and encouragement of the clay mineralogy research pre-sented in the original paper. The discussers noted that clay miner-alogy is an important aspect in solving geotechnical engineeringproblems. This was the basis for our research and led to the devel-opment of the original paper. The writers concentrated on the dis-tribution of dominant clay mineralogy in a soil, which influencesthe overall engineering behavior of clayey soils. Hence, the origi-nal paper focused on three well-known dominant clay minerals ina fine-grained soil. The presence of these minerals provides betterunderstanding of swell-shrink behaviors of the soils, which isneeded for an enhanced stabilization design of infrastructure con-struction projects on such soils.

The discussers raised a few issues for which the writers provideresponses here. First, the discussers noted that the use of the X-raydiffraction (XRD) method for clay mineral identification can be ex-pensive, and the writers agree that it may not always be practical to

use this method in professional practice. The perplexing geotechni-cal issue is that the XRD method is often used as a forensic tool tostudy the causes of a failure in a geotechnical system rather thanfor initial site characterization during the design phase. With the ad-vent of newer and compact XRD equipment, an increase in the useof this method for better characterization of subsoils can be expected.

In this research, the writers used XRD studies to qualitativelyidentify the clay minerals present in a given geomaterial. This stepis needed because the procedure formulated in the original paperis based on the presence of kaolinite, illite, andmontmorillonitemin-erals. Hence, it is important to ensure that all three clay minerals arepresent in test soils used in the research. XRD studies were henceconducted on test soils. Another reason is to identify source(s) of po-tassium in the test soil because potassium can come from sourcesother than the illite mineral. The assumption of using potassiummeasurements to determine illite was valid, because other potassiumsources such as potassium feldspar were not found in the test soils.

The discussers noted certain difficulties with the testing proce-dures for the chemical properties such as the cation exchange ca-pacity (CEC), specific surface area (SSA), and total potassium(TP). The writers disagree with this observation and note that thetesting methods proposed in the original paper are fairly straight-forward and have often been used in both research and practice.

The discussers argued that there is no definite relationship be-tween CEC and SSA with the quantities of the minerals predicted.Fig. 1 shows a linear trend between the CEC values measured andthe percentage of montmorillonite calculated, with a correlationcoefficient (R2) value of 0.69. A similar relationship can be ob-served in Fig. 2 between SSA and percentage of montmorillonite,with an R2 value of 0.92. These relationships are expected, becausehigher CEC and SSA values indicate the presence of largeramounts of montmorillonite in a soil. This analysis shows thatthese relationships can be developed for other minerals. However,certain variability should be expected when soils with multiple clayminerals are tested.

The discussers opine that the exact percentage quantification ofthe clay minerals present in the soil is not required to judge the claysoil behavior. The discussers are of the opinion that it is sufficientto classify the soils by groups on the basis of their degree of expan-sivity, which is an indirect representation of the dominant clay

Fig. 1. Variation of percentage montmorillonite calculated with mea-sured CEC values

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mineralogy of soils. A performance test, such as a swell test, issometimes appropriate for the initial understanding of the dominantclay mineral in soil, but similar to the Atterberg limits tests, thesetests can also provide erroneous interpretations when soils contain

similar amounts of expansive clay minerals. Hence, an approxi-mate quantification method, as suggested in the original paper, willbe of value for better interpretation of clay minerals and their re-lated expansive soil behavior.

The discussers noted that the free swell ratio (FSR) method usingkerosene as amedium is useful for determining claymineral dominance(Prakash and Sridharan 2004). The writers are not able to comment onthis method, because this method has not been studied in the presentresearch; however, it is likely that this method may have potential forbetter assessments of swell behavior. Itwould be interesting to comparehow the FSR data from kerosene medium correlates with the ASTM-based swell test data from tests conducted in the water medium.

In summary, when ground improvement and other geotechnicalprojects require insights into clay mineral interactions with groundmodifications for better geotechnical design, the writers suggestthe use of the proposed chemical property–based clay mineral quan-tification method to evaluate dominant clay minerals in soils. Inclosing, the writers thank the discussers for their contributions,which have provided a much needed exchange on the need andincorporation of clay mineralogy identification of soils into a com-prehensive geotechnical site characterization.

References

Prakash, K., and Sridharan, A. (2004). “Free swell ratio and clay mineralogyof fine-grained soils.” Geotech. Test. J., 27(2), 220–225.

Fig. 2. Variation of percentage montmorillonite calculated with mea-sured SSA values

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