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Use of Posidonia Oceanica Ash inStabilization of Expansive SoilsMona Malekzadeh a & Huriye Bilsel ba Department of Civil Engineering , Eastern MediterraneanUniversity , Gazimagusa , Turkeyb Department of Civil Engineering , Eastern MediterraneanUniversity , Gazimagusa , Mersin , TurkeyAccepted author version posted online: 27 Mar 2013.Publishedonline: 21 Feb 2014.

To cite this article: Mona Malekzadeh & Huriye Bilsel (2014) Use of Posidonia Oceanica Ash inStabilization of Expansive Soils, Marine Georesources & Geotechnology, 32:2, 179-186, DOI:10.1080/1064119X.2012.728685

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Use of Posidonia Oceanica Ash inStabilization of Expansive Soils

MONA MALEKZADEH1 AND HURIYE BILSEL2

1Department of Civil Engineering, Eastern Mediterranean University,Gazimagusa, Turkey2Department of Civil Engineering, Eastern Mediterranean University,Gazimagusa, Mersin, Turkey

Posidonia oceanica (PO) is the most plentiful seaweed of the Mediterranean Sea,which grows all along the coastal areas, forming widespread meadows. The leafrejuvenation process of Posidonia oceanica typically occurs in fall when an increasein wave action causes the dead seaweeds to be transported and usually piled up alongthe coastal areas. This paper investigates the effect of PO ash stabilization on behav-iour of an expansive clay. The ash was obtained by combustion of crushed PO piecesin a muffle furnace at 550�C. Atterberg limits, linear shrinkage, particle size distri-bution, one-dimensional swell, and unconfined compression tests have been carriedout on natural soil as well as soil mixtures with 5% and 10% PO ash. There has beenno significant improvement in the soil properties with 5% ash inclusion, whereas 10%ash has noticeably reduced the swell amount and increased the compressive strength.It is therefore concluded that there is a potential for the use of PO ash in geotechnicalengineering applications.

Keywords one-dimensional swell, Posidonia oceanica, swelling soils, unconfinedcompressive strength

Introduction

It has been recognized for some time that problems associated with foundationsunder lightly loaded structures such as building foundations, highways, hydraulicstructures, underground utilities and embankments arise from swelling-shrinkingbehavior of expansive soils. Seasonal moisture changes may lead to large amountsof heave in wet season and shrinkage settlements in dry season, causing severedamages to structures, hence increasing the repair costs annually.

There has been considerable research done to alleviate the detrimental effects ofswelling-shrinking behaviour of soils by stabilizing expansive soils utilizing mainlychemical or mechanical stabilization techniques. Soil stabilization by chemicalmodification of soils, for upgrading and enhancing the engineering properties, isan important technique used in expansive soils. The addition of chemicals such ascement, fly ash, lime, or any combination of these, cause cementation of soil particles,

Received 16 May 2012; accepted 27 June 2012.Address correspondence to Mona Malekzadeh, Department of Civil Engineering, Eastern

Mediterranean University, Gazimagusa, Turkey. E-mail: mona.malekzadeh@cc.emu.edu.tr

Marine Georesources & Geotechnology, 32:179–186, 2014Copyright # Taylor & Francis Group, LLCISSN: 1064-119X print=1521-0618 onlineDOI: 10.1080/1064119X.2012.728685

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altering the physical and chemical properties of the soils. Especially, the use of limeadmixture has proved to have a great potential as an economical method forimprovement of the geotechnical properties of expansive soils (Basma & Tuncer1991; Bhattacharja et al. 2003; Leroueil & Vaughan 1990). However, there is a needfor new treatment methods which utilize recycled materials, providing a feasible aswell as economic solution in reducing volumetric strains of wetting-drying soilsrelated to expansive soil movements (Punthutaecha et al. 2006).

Cyprus possesses a semi-arid climate, where soils are mostly in a constant stateof partial saturation over long periods of dryness and subjected to flooding overshort periods of wet season. Swelling soils, found in different parts of the Islandcause serious problems in the behavior of light buildings associated to the seasonalcycles of wetting and drying. In Cyprus costly construction techniques are notpreferred for light structures. Recent research works have focussed on the mitigationof soil deposits having high swell-shrink potential. Locally produced lime is found tobe feasible as an additive to reduce swell, shrinkage, plasticity, compressibility andto increase shear strength (Bilsel 2002; Bilsel & Tuncer 1998; Bilsel & Oncu 2005).However, the Island is covered in many parts by expansive soils with large amountsof solluble sulfates which hinder effective use of lime stabilization.

This study is aimed to assess the potential use of Posidonia oceanica (PO) ash tomitigate the swell potential and improve the shear strength of an expansive soil froma local deposit. PO is a common seaweed of the Mediterranean Sea, which grows allalong the coastal areas forming widespread meadows starting near the water surfaceto depths of 40m (Duarte 1991). Among all the aquatic plants, PO is the mostplentiful sea grass type in the basin of Mediterranean Sea, approximately covering40,000 km2 area of the seabed (Cebrian & Duarte 2001). The leaf rejuvenation cycleof PO typically occurs in fall, when an increase in wave action causes the deadseaweeds to be transported and piled up along the Mediterranean coast in winterand spring seasons (Ott 1980). There are several studies on the ecological,nutritional, and biological aspects of PO (Bell and Harmelin-Vivien 1983; Jeudyde Grissac and Boudouresque 1985; Gambi et al. 1989; Romero et al. 1992; Duarte2002). Impact of experimental research on Posidonia oceanica including the chemi-cal, physical and spectroscopic properties and possibility of recycling have been ana-lyzed by (Cocozza et al. 2010), yet there is no research regarding its application ingeotechnical engineering practices.

In the present study, dead leaves of Posidonia oceanica accumulated on thebeach were used as an alternative low-cost soil stabilization additive. The prelimi-nary research findings assess the possible use of the portion of these deposits inexcess of the amount required for preventing coastal erosion, as an innovativemethod of mitigation, and thus recycling a natural waste material found abundantlyalong the shores of Cyprus.

The ash obtained by combustion of dead Posidonia oceanica leaves at 550�C hasbeen used to study its effect on Atterberg limits, linear shrinkage, particle sizedistribution, one-dimensional swell potential, and unconfined compressive strengthof soil and ash mixtures of 5% and 10% PO ash by dry weight of soil.

Experimental Programme and Results

An experimental study was conducted to investigate the efficacy of Posidonia ocea-nica ash on alleviation of detrimental effects of expansive soils. Physical properties,

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one-dimensional swell and unconfined compressive strength behaviour of PO ash-treated specimens were determined and compared with that of untreated specimens.

Materials

The expansive soil used in this study has been obtained from the campus of EasternMediterranean University in Famagusta, North Cyprus from a depth of 1.0m.Table 1 shows the results of physical properties of the soil. Based on the indexproperties and according to USCS classification it is classified as clay with highplasticity (CH).

Posidonia oceanica was collected from the east coast of North Cyprus and trans-ported to the laboratory in plastic bags. After being washed several times to removethe soluble salts, it was air-dried and crushed with a chopper to small pieces to enablea greater amount to be packed in a muffle furnace. The ash was obtained by com-bustion in the furnace at 550�C for 24 hours. The final step was to grind the ash intopowder form using wooden pestle.

Preparation of Specimens

The untreated and PO ash added samples mixed with optimum water contentobtained from standard Proctor test (ASTM D698-70) on untreated soil, aremellowed in plastic bags for 24 hours. The untreated and PO ash-added specimenswere subjected to static compaction in the moulds of 50mm and 38mm diameterfor one-dimensional swell and unconfined compression tests respectively.

Particle Size Distribution

To monitor the effect of PO ash on particle size distribution of the studied soil, testswere performed according to ASTM D422-54T on untreated and 5% and 10%PO-ash-added soil samples. From the test results it was observed that clay size par-ticles reduced while silt size particles increased with PO ash addition, as presented inFigure 1. With 5% PO ash addition, clay size particles reduced while silt size particles

Table 1. Physical properties of the expansive soil studied

Property

Specific Gravity 2.56Sand (%) 8Silt (%) 36Clay (%) 56Liquid limit (%) 57Plastic limit (%) 28Plasticity index (%) 28Linear shrinkage (%) 20Soil classification (USCS) CHOptimum moisture content (%) 24Maximum dry density (kg=m3) 1537

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increased in equal amounts, consequently the amount of total fines remaining almostthe same. With 10% PO ash addition, however there is a marked reduction in claysize fraction and considerable increase in silt size as well as sand size particles. Thisbehavior might be attributed to a possible pozzolanic reaction between the soil andthe PO ash.

Atterberg Limits

Atterberg limits of untreated and PO-ash added samples were determined usingthe procedures outlined in ASTM D4318� 05, and the effect of PO ash has beenevaluated on 5% and 10% PO ash added samples. The plasticity index and PO ashcontent relationship is shown in Figure 2. It is observed that the plasticity indexin 5% PO ash-added samples remains almost the same as the untreated soil whilea noticeable reduction occurs with 10% PO ash addition. These findings substantiatethe results obtained in particle size distribution tests. Hence, PO ash addition of 10%changes the classification of sample from clay with high plasticity (CH) to silt withhigh plasticity (MH).

Figure 1. Relationship between fines content and PO ash content.

Figure 2. Relationship between plasticity index and PO ash content.

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Linear Shrinkage

Linear shrinkage of the untreated soil and mixtures with 5% and 10% PO ash hasbeen determined according to BS-1377:90. A reduction in shrinkage limit has beenobserved from 21% for untreated soil to 18% and 17% for 5% and 10% Posidoniaoceanica-ash added samples respectively.

One-Dimensional Swell

Samples for one-dimensional swell have been prepared by static compaction atmaximum dry density of the expansive soil. Figure 3 shows the swell percentageversus logarithm of time for untreated samples and samples with different PO ashcontents. Table 2 summarizes the primary swell, secondary swell and time takenfor primary swell to be completed, which indicates that with 5% PO ash contentthe primary swell is slightly increased while time taken for completion has beenreduced more than 10 fold. The significant increase in the rate of swelling arises asa result of increase in particle sizes, thus increase in the porosity of the PO ash-treatedspecimens. With 10% PO ash addition, there is a noticeable reduction in both primaryand secondary swell amounts and time of swell in comparison to the untreated sam-ple. These observations further verify the increase in particle sizes due to possible poz-zolanic reaction between PO ash and the expansive soil. The variation of primary andsecondary swell amounts with respect to PO ash contents is also presented in Figure 4.

Figure 3. One-dimensional swell versus time.

Table 2. One-dimensional swell test results of untreatedand PO ash-treated soils

PropertiesUntreated

soil5%PO

10%PO

Primary swell (%) 4.5 5.6 3.2Secondary swell (%) 0.6 0.4 0Primary swell time (min) 100 7 7

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Unconfined Compression Test

Unconfined compression test is performed on untreated and 5% and 10% POash-added samples according to the ASTM D2166-06. Axial stress versus axial strainrelationships of the untreated soil and the PO ash-treated samples can be observed inFigure 5. It can be deduced that with 5% and 10% PO ash additions there is a sig-nificant reduction in strain values at failure indicating a brittle behavior while theuntreated samples exhibit a more ductile behavior. A significant enhancement ofunconfined compressive strength is also evident with the addition of 10% PO ash,yet the loss of peak strength is more significant than the 5% PO ash-added samples.Figure 6 gives the relationship of unconfined compressive strength with different POash contents, where it is indicated that with 5% PO ash addition the unconfinedcompressive strength is almost the same as the untreated soil, whereas a twofoldincrement is observed with 10% PO ash inclusion. The findings herein can also beattributed to the particle size increments due to pozzolanic reaction between thePO ash and the soil.

Figure 5. Axial stress versus axial strain curves.

Figure 4. Relationship between one-dimensional swell and PO ash content.

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Conclusions

A series of tests were performed to investigate the effect of Posidonia oceanica ash onphysical and engineering properties of expansive soils and its potential use as anadmixture for mitigation of swelling soils. The major conclusions derived from theanalyses of test results are summarized as follows:

1. PO ash addition significantly reduced the clay size fraction while silt size and sandsize fractions increased. This behaviour is very significant in 10% PO ash-addedsamples, in which the clay size particles reduced by 48%, while silt size and sandsize particles increased by 42% and 50% respectively.

2. The reduction in plasticity index of treated samples substantiated the aboveconclusion on particle size changes. The plasticity index decreased by 18% with10% PO ash addition. Even though the particle sizes increased with ash treatment,the soil classification could only change from CH to MH, as the total fines con-tent reduced only by 8%. The potential for further reduction in plasticity index,however, is experimentally observed and therefore will be addressed in a moreextensive research programme, using higher PO ash contents.

3. One- dimensional swell test results showed that with 5% PO ash inclusion theprimary swell amount remained almost the same as the untreated soil while timetaken for completion had a 10-fold reduction. This significant increase in the rateof swelling can be attributed to the increase in particle sizes, and a subsequentincrease in the porosity of the PO ash treated specimens. With 10% PO ashaddition, there is a noticeable reduction in both primary and secondary swellamounts and time of swell in comparison to the untreated samples. These obser-vations further validate the increase in particle sizes due to pozzolanic reactionbetween PO ash and the expansive soil.

4. Based on the unconfined compression test results, it can be inferred that with 5%PO ash addition the unconfined compressive strength is almost the same as theuntreated soil, whereas a twofold increment is observed with 10% PO ashaddition. Therefore, a significant improvement in unconfined compressivestrength occurred with the addition of 10% PO ash, while the samples attaineda more brittle behaviour when treated. These findings can also be attributed tothe particle size increments and reduction in plasticity due to pozzolanic reactionbetween the PO ash and the soil.

Figure 6. Relationship between unconfined compressive strength and PO ash content.

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As an overall conclusion, recycling a fraction of Posidonia oceanica wasteabundantly found along the coastline of North Cyprus (in excess of the amountrequired for coastal erosion protection) can be an environmentally friendly and feas-ible solution for soil improvement. However, further research is needed to achieve amore feasible method of producing PO ash and to investigate the optimum amountfor more efficient improvement of expansive soils.

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