INTRODUCTION

Environmental Geo-technology isemerging as an interdisciplinary science, aimingat fore-casting, analyzing and solving thegeotechnical pro-blems involving the influence ofenvironmental factors1. Absorption of gaseouspollutants released during volcanic eruptions andindustrial exhausts reduces the pH2. Although theH+ content of acid rain falling for a short duration isvery low yet its influence on the physical, chemicaland engineering properties of soil will be alarmingin near future as ever growing industrialization/

Material Science Research India Vol. 8(2), 265-271 (2011)

Effect of the Intensity of the Acid Rain on theGeotechnical Properties of Soils of Diferent Plasticity

P. SHARMA¹*, S. VYAS¹, M. KAUSHAL², N.V.MAHURE¹,N. SIVAKUMAR¹ and M. RATNAM¹

¹Central Soil and Materials Research Station, Olof Palme Marg, N. Delhi - 110 016 (India).²Department of Civil Enginnering Singhania University, Rajasthan (India)

*Corresponding author: E-mail: pan2256@gmail.com

(Received: November 20, 2011; Accepted: December 27, 2011)

ABSTRACT

Acidification of rain is caused by air pollutants like sulfur dioxide and oxides of nitrogen etc.which reduces its pH as low as 4. Although the H+ concentration of acid rain falling for a shortduration is very low in comparison to cation exchange capacity of soil yet its effect on the physical,chemical and engineering properties of soil cannot be ignored. Ever growing industrialization/pollution will increase frequency, persistence and intensity of acid rain. This will certainly enhancethe rate of leaching of cations from the soil to a great extent. Absorption of H+, SO4

2-, NO3- and CO3

2-

by the soil particles will alter its physical, chemical and engineering properties. Therefore it isimperative to examine the properties of soils eroded due to acid rain in order to augmentunderstanding of its influence on geotechnical engineering particularly when used in fields,embankment or in earthfill dams. Objective of this study was to find degree of variation in thetexture, physico-chemical and the engineering properties of soil of different plasticity when exposedto acid rains having different pH. Soil samples were collected and acidified with varying probablesimulated strength of acid rain equivalent to normality 0.005N, 0.01N, 0.02N and 0.04N of H2SO4

or HNO3. The consistency, strength and chemical characteristics of the samples were determinedfor the soil fraction passing 425 µm sieve. The acidification causes de-flocculation of soil particleswhich directly affects the consistency as well as strength properties of the soil. The paper presentsthe observed degree of variation in these properties under varying strengths of acid rain.

Key words: Environment, Consistency, Physico-Chemical,Angle of internal resistance, Cation exchange capacity.

pollution will keep on substantially decreasing itspH. The enhanced rate of leaching of cations andabsorption of H+, SO4

2-, NO3- and CO3

2- alter theseproperties3. It is thus one of the most influentialenvironmental factors which directly affect theproperties of soil4. The present study aims at findingthe variation in the texture, physico-chemical andthe engineering properties of the soils of differentplasticity when exposed to acid rain of sameconcentration. In addition the relative change in theproperties of same soil when subjected to acid rainof varying strength has also been studied.

266 SHARMA et al., Mat. Sci. Res. India, Vol. 8(2), 265-271 (2011)

EXPERIMENTAL

MaterialsThe soil samples used in this study were

obtained from Indore, M.P. (Sample A) and Nagpur,Maharashtra, (Sample B). The samples were ovendried & sieved using 4.75 mm IS sieve. The downsize material was used for further experimental work.The consistency, strength and chemicalcharacteristics of the samples were determined forthe soil fraction passing 425 µm sieve. The resultsare summarized in Table 1.

Experimental Methods1 kg of each soil sample was separately

shaken for 8 hours with 1 liter of H2SO4 and HNO3

of 0.005N, 0.01N, 0.02N and 0.04N strengths asdetailed in table 2. These samples were then keptundisturbed over night. The treated samples werethen filtered and air dried. These samples were thenused to find their consistency, strength and chemicalproperties as per Bureau of Indian Standards5-7.

RESULTS AND DISCUSSION

Soil TextureThe soil samples were analyzed before

and after acid treatment using scanning electromicroscope (SEM) for finding the texture of the soilcrystals. The SEM observation for the originalsamples A & B are presented in Fig. 1. The SEMobservation for Sample A & B treated with 0.005NH2SO4 & HNO3 are presented in Fig. 2. It is observedthat angularity of the particles is increased which

can be attributed to leaching of the cations andsubsequent deflocculation. The degree of variationincreases as the strength of acid rain increases.

Mechanical AnalysisThe results of mechanical analysis of the

soil samples before and after acid treatment arepresented in Table 3. The results clearly show thatas intensity of acidification increases thedeflocculation of soil particles which predominantlyincrease the percentage of silt and clay sizeparticles.

Table 2: Details of sample designation

S. No. Description Strength of acid Designation

1. Original Soil Sample + 1 lit Distilled Water (DW) DW A B2. 1 kg Soil + 50 ml 0.1 N H2SO4 + 950 ml DW 0.005 N AS-1 BS-13. 1 kg Soil +100 ml 0.1 N H2SO4 + 900 ml DW 0.01 N AS-2 BS-24. 1 kg Soil + 200 ml 0.1 N H2SO4 + 800 ml DW 0.02 N AS 3 BS 35. 1 kg Soil + 400 ml 0.1 N H2SO4 + 600 ml DW 0.04 N AS-4 BS-46. 1 kg Soil + 50 ml 0.1 N HNO3 + 950 ml DW 0.005 N AN-1 BN-17. 1 kg Soil + 100 ml 0.1 N HNO3 + 900 ml DW 0.01 N AN-2 BN-28. 1 kg Soil + 200 ml 0.1 N HNO3 + 800 ml DW 0.02 N AN-3 BN-39. 1 kg Soil + 400 ml 0.1 N HNO3 + 600 ml DW 0.04 N AN-4 BN-4

Table 1: Characteristics oforiginal samples A and B

Characteristic Sample

A B

Liquid limit (WL ) % 67.30 37.50Plastic limit (WP ) % 26.39 20.30Plastic Index (Ip) % 40.91 17.20Shrinkage limit % 6.18 15.29Free Swell Index % 66.57 48.62Particles finer than 0.002mm % 22.90 13.20Particle 0.002mm-0.075mm % 47.70 57.50Particle 0.075mm-0.425mm % 26.10 23.80Particle 0.425 mm-2.00 mm % 2.20 3.40Particle 2.00 mm - 4.75 mm % 1.10 2.10pH 7.11 6.80Organic Matter % 0.70 0.54Effective Cohesion c (kg./cm2) 0.45 0.31Effective Ø (degree) 12.44 15.20

267SHARMA et al., Mat. Sci. Res. India, Vol. 8(2), 265-271 (2011)

Table 3: Results of Grain size Analysis of soil before and after acid treatment

Soil Particle Size (mm)

Sample Below 0.002 0.002 to 0.075 0.075 to 0.425 0.425 to 2.0 2.0 to 4.75(%) (%) (%) (%) (%)

A 22.90 47.70 26.10 2.20 1.10AS - 1 24.00 53.60 20.80 1.10 0.50AS - 2 25.30 57.40 14.60 1.80 0.90AS - 3 26.40 57.80 13.70 1.30 0.80AS - 4 26.70 59.10 12.30 1.10 0.80AN - 1 25.80 51.40 20.90 0.90 1.00AN - 2 26.60 57.40 13.30 1.80 0.90AN - 3 27.40 59.50 10.30 1.90 0.90AN - 4 28.60 57.90 10.90 1.80 0.80B 13.20 57.50 23.80 3.40 2.10BS - 1 14.10 60.30 20.60 3.10 1.90BS - 2 15.80 61.50 18.20 2.90 1.60BS - 3 16.30 63.90 15.90 2.60 1.30BS - 4 17.00 65.70 14.20 2.00 1.10BN - 1 14.60 60.50 19.90 3.00 2.00BN - 2 16.20 62.60 16.80 2.60 1.80BN - 3 17.10 64.20 14.90 2.40 1.40BN - 4 17.60 65.30 13.80 2.10 1.20

Table 4: Physico-Chemical Properties of soilsamples before and after acid treatment

Sample pH Organic Matter %

A 7.11 0.70AS - 1 5.98 0.63AS - 2 4.89 0.00AS - 3 4.15 0.00AS - 4 3.60 0.00AN - 1 6.15 0.37AN - 2 5.38 0.00AN - 3 4.60 0.00AN - 4 3.17 0.00B 6.80 0.54BS - 1 5.90 0.43BS - 2 4.56 0.00BS - 3 4.02 0.00BS - 4 3.65 0.00BN - 1 6.20 0.28BN - 2 5.00 0.00BN - 3 4.25 0.00BN - 4 3.46 0.00

Physico-Chemical PropertiesOn treating the soil samples with different

concentrations of H2SO4 or HNO3 the physico-chemical properties of the soil samples gotchanged. The results are presented in Table 4. ThepH of the samples reduced as the concentration ofthe acid increased. On acid treatment, the organicmatter got digested and thereby its percentagereduced.

Consistency CharacteristicsThe values of Liquid limit (WL), Plastic limit

(WP), Plastic Index (Ip), Shrinkage limit and FreeSwell Index for the soil samples before and afteracid treatment are presented in Table 5. The degreeof variation in the consistency characteristics of thesoil depends on factors like type of soil, electricalcharge of exchangeable cations absorbed by soilparticles and concentration of cations in soil water3.

Due to the leaching of cations anddeflocculation the consistency characteristics of thesoil reduced except shrinkage limit. The increase

268 SHARMA et al., Mat. Sci. Res. India, Vol. 8(2), 265-271 (2011)

Table 5: Results of Consistency Characteristics of soil samples before and after acid treatment

Sample WL,% WP% Ip% Shrinkage limit,% Free Swell Index%

A 67.30 26.39 40.91 6.18 66.67AS - 1 61.00 24.70 36.30 8.11 53.00AS - 2 59.20 23.40 35.80 8.30 52.00AS - 3 56.20 23.20 32.98 9.20 51.00AS - 4 55.40 22.27 31.30 9.58 49.00AN - 1 65.80 25.80 40.00 6.11 60.00AN - 2 62.80 24.60 38.20 7.30 58.00AN - 3 56.80 23.30 33.50 8.40 54.00AN - 4 54.20 22.00 32.20 10.20 52.00B 37.50 20.30 17.20 15.29 48.62BS - 1 34.20 19.10 15.10 17.22 42.60BS - 2 33.80 18.90 14.90 18.10 41.20BS - 3 31.80 17.60 14.20 19.30 40.40BS - 4 30.60 17.00 13.60 19.90 39.60BN - 1 34.60 19.60 15.00 19.41 41.90BN - 2 33.10 18.60 14.50 20.40 38.70BN - 3 31.90 17.90 14.00 21.20 35.20BN - 4 30.60 17.10 13.50 21.90 33.80

Table 6 Strength Characteristics of soilsamples before and after acid treatment

Sample Effective cohesion (c’), Effective angle of internalkg./cm2 resistance (Ø’), degree

A 0.45 16.36AS - 1 0.40 15.69AS - 2 0.31 15.01AS - 3 0.25 14.36AS - 4 0.22 13.58AN - 1 0.41 15.38AN - 2 0.32 14.35AN - 3 0.24 13.72AN - 4 0.22 13.08B 0.33 25.16BS - 1 0.20 24.62BS - 2 0.17 23.10BS - 3 0.14 22.30BS - 4 0.10 21.80BN - 1 0.24 24.44BN - 2 0.20 23.80BN - 3 0.17 23.10BN - 4 0.15 22.40

269SHARMA et al., Mat. Sci. Res. India, Vol. 8(2), 265-271 (2011)

Sample A Sample B

Fig. 1: SEM Observation for original sample A and B

Sample AS-1 Sample AN-1

Sample BS-1 Sample BN-1

Fig. 2: SEM Observation for Sample A & B treated with 0.005N H2SO4 & HNO3

270 SHARMA et al., Mat. Sci. Res. India, Vol. 8(2), 265-271 (2011)

Sample AN-1 Sample AN-2

Sample AN-3 Sample AN-4

Fig. 3: SEM Observation for Sample A treated with 0.005N, 0.01N, 0.02N and 0.04N HNO3

in values of shrinkage limit is attributed to increasein inter particulate distances due to reduction in theforces between soil particles.

Strength CharacteristicsThe strength characteristics of soil are

affected extensively by soil’s internal structure andinteraction between soil particles. The strength ofsoil originates mainly from the soil skeleton andelectric attractive force between the electric chargesabsorbed by the soil particles. The results of theeffective cohesion (c’), kg/cm2 and Effective angleof internal resistance (Ø’), degree for untreated andacid treated soil samples is presented in Table 6.The values of effective cohesion (c’) and Effectiveangle of internal resistance (Ø’) are observed to be

lesser for acid treated soil samples. This is mainlydue to reduction in the electric forces asconcentration of exchangeable cations reduced onacid treatment.

Further researchIn the present study, an approach to find

out the effect of acid rain on soils from Indore andNagpur has been established. However, the scopeof the study is being contemplated to various typesof the soils to be collected from different parts ofIndia to establish probable impact of acid rain ongeophysical properties. This will help in betterunderstanding of the situations during planning andconstruction of mega civil engineering structures.

271SHARMA et al., Mat. Sci. Res. India, Vol. 8(2), 265-271 (2011)

CONCLUSION

Acid rain results in changes in physico-chemical characteristics of soil due to cationsexchange. pH of the soil decreases while theconcentration of absorbed H+ and SO4

2- increases.Grain size distribution of the soil will shift towardsfiner fractions when subjected to acid rain. Acid rainaffects the consistency properties of different soils.Leaching of cations will reduce the attractive forcesbetween the soil particles which will lower theoverall strength of the soil. The data obtained duringthe experiments is useful in gauging the damagesto the soil that could occur due to the acid rain in

future. However the magnitude of the deteriorationwill depend on persistence and strength of acidrain.

ACKNOWLEDGEMENTS

The authors extend their sincere thanksto Dr. R.K. Parashar, Associate Prof. NCERT andMrs. Abha Sharma for valuable suggestionsextended by them. Timely help extended by Sh.Devender Singh and Sh. Anil Vashisht of CSMRSare gratefully acknowledged. We also extend oursincere gratitude to all the authors whosepublications provided us directional informationfrom time to time.

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