Poly Vinyl Acetate (PVA) as Fill Material for Land Reclamation · Natural Organic Polymer (NOP) or...
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Chiang Mai J. Sci. 2012; 39(4) 693
Chiang Mai J. Sci. 2012; 39(4) : 693-711http://it.science.cmu.ac.th/ejournal/Contributed Paper
Poly Vinyl Acetate (PVA) as Fill Material for LandReclamationMuhammad Aqeel Ashraf*[a], Mohd. Jamil Maah [a], Ismail Yusoff [b][a] Department of Chemistry University of Malaya, Kuala Lumpur 50603, Malaysia.[b] Department of Geology, University of Malaya, Kuala Lumpur 50603, Malaysia.*Author for correspondence; e-mail: [email protected]
Received: 26 October 2011Accepted: 25 April 2012
ABSTRACTRapid industrial and commercial expansion in recent years has created the need
for more land. One of the options to create more land is to reclaim ex-mining land.Ex-mining land contains numerous mined out lakes and ponds. Numerous methods areavailable to fill these lakes and ponds. The ex-mining land consists of variety of materialsinclusive of sand, clay, slime and organic peat as well. A new landfill method which iseconomically competitive, technologically feasible and which will not contribute to anyenvironmental problems was developed. It is based on flocculation of slurry slime withNatural Organic Polymer (NOP) or Poly Vinyl Acetate (PVA), mixed with residual soil anduse the flocculated slurry slime as part of the fill material. The physical and geotechnicalproperties of the flocculated slurry were assessed by laboratory tests, including pH andEh measurements, acid neutralization capacity (ANC) determination, hydraulic conductivityand comparison and cost analysis with conventional fill materials such as sands. The flocculatedslurry slime was slightly alkaline, with pH 8.4, Eh value 171 mV, negligible ANC, and hydraulicconductivity was 8×10-5 to 7×10-4 m/s. It was also found that the material is unlikely to causesignificant change in the redox condition of the subsurface environment over a long-termperiod. Proposed method is more feasible as compared to other methods in comparisonand cost analysis. In general, the flocculated slurry slime is suitable to be used as a fill materialfor land reclamation.
Keywords: mining wasteland, construction, fill material, classical methods, flocculation,slurry slime
1. INTRODUCTIONDuring recent years, mine land
reclamation and ecological rehabilitationhas become the subject of common concernby countries in the world and is increasingly widespread. Land use is a decisionto be made by society. Land use can be
changed - society can decide to change theland use on a rehabilitated colliery fromcrops to housing or industrial estates, butmines have an obligation to ensure that nonet loss in land capability occurs . This mustbe the primary objective in rehabilitating
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mined land. Where land capability is notpreserved, society is deprived of choice.Degraded lands can potentially supportfewer land uses-no crops, for instance. Someargue that agreements with communitiesregarding land use can be made prior torehabilitation whereby a lower quality ofrehabilitation is acceptable [2, 3]. This mayoccur, for example, if the pre-mining landcapability is arable, but the community aresatisfied with grazing as a post-miningland capability. Such decisions, even whenbased on community preferences, do notpromote sustainability. Soil formation takesthousands of years and, by only restoring afraction of the original land capability, futuregenerations are deprived of the choicesthat are available to this generation [4, 5].
Malaysia is an important country inMining production, the exploitation ofmineral resources plays a decisive role innational economy development. Currently,the mining industry is responsible forproducing a sizeable proportion of thenation’s merchandise export income, andits relative importance is continuouslyincreasing. Recent available data (2000)showed that the mining industry consistedof 87 mines extracting ores of gold, tin,nickel, copper, aluminium, manganese, ironand chromium [6, 7]. In addition, thereare over 18 establishments producingnon-metallic minerals and constructionmaterials. The number of mining operationsis rapidly increasing as explorationscontinue to take place, particularly in thePerak, Penang and Selangor states. How toexploit mineral resources rationally andreduce all sorts of adverse effects on thehuman environment in its process is a majorevent in people’s livelihood .
Due to blooming development and rateof urbanization in Peninsular Malaysia inrecent years has created the need of more
land especially land nears to the developedareas. Many urban areas have expanded tomined out land which has numerous minedout ponds. It has been estimated that urbanpopulation will double itself every 10 to 15years and mind out lakes and ponds in theway of urban expansion will be reclaimedand utilized for the construction of industrialand industrial parks. There are numerous landreclamation projects being carried out inMalaysia to increase the total land area byreclaiming the ex-mining land. Currently,approximately 15% of the land area ofMalaysia has been reclaimed over the last35 years. The total reclaimed land area isincreasing each year. The amount of fillmaterials required also increases tremendouslyas the reclamation is ongoing and the futuresites to be reclaimed have to encroach intodeeper sea. A large number of examininglands had been reclaimed in the lastdecade for the construction purposes. Fromtime to time cases have been reported in thepress where houses or factories built overreclaimed ground had problem involvingcracks on the walls, ceilings and floors [9,10]. These problems are largely due todifferential ground settlements resultingfrom consolidation of trapped slime lenses.This is an ongoing problem and willcontinue to be as long as buildings areconstructed over improperly reclaimed land.
In Malaysia the most commonlypractised method of reclaiming mined outponds by developers of housing estates orindustrial parks is to lower the water levelof the ponds and emplace fill materialfrom one end of the pond. Lowering ofWater Level and Emplacement of FillMaterial Method does not remove the softslurry slime at the bottom of the pondsand is carried out mostly on an ad-hocbasis without any prior detailed siteinvestigation [11, 12]. Two other methods
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which are an improvement over the earliermentioned method are the DisplacementMethod and Containment Method [13, 14,15]. In the Displacement Method, fillmaterial is dumped into the pond, resultsin slurry slime being pushed to one end ofthe pond and displaced slurry slime is dug upand then carried away by trucks. In theContainment Method the soft slurry isremoved and underlying slime is containedin the pool within the pond. Geomembraneis then laid over the slime and fill material isplaced on top. To speed up the consolidationprocess, vertical sand drains are installed todrain the slime beneath the membrane[16, 17]. Currently, most of the fill materialsfor land reclamation are sands sourcedfrom seabed and the cost of the material isa significant portion of the total projectcost. If the flocculated slurry slime (withNatural Organic Polymer (NOP) or PolyVinyl Acetate (PVA) as part of the fillmaterial could be used as fill material forland reclamation, it would solve the disposalproblem and also reduce the constructioncost of the land reclamation projects [18, 19].
Land reclamation is a complex,multidisciplinary and multi-processengineering system, its implementationrequires a scientific and rational evaluationand planning of land reclamation. For themining industry, the land reclamation workscarried out earlier and have made greatprogress, but it is still faced with manyproblems to be improved in the actualimplementation process . The objectivesof this study were to investigate thesuitability of using flocculated slurry slimeas a fill material and to assess its potentialenvironmental impact. The physical,geotechnical and geochemical properties ofthe flocculated slurry slime were examined.
1.1. Description of the SiteThe study area Bestari Jaya catchment is
located at 3°242 40.413 N and 101°24256.233 E is part of District Kuala Selangorin Selangor state. Bestari Jaya has threetowns Mukim Batang Berjuntai, Mukim UluTinggi, Mukim and Tanjung Karang.The old name of Bestari Jaya is BatangBerjuntai. On 2007 the name BatangBerjuntai was renamed “Bestari Jaya” due tocensorship by the government, as “BatangBerjuntai” had phallic meanings in Malay.The Bestari Jaya is an old tin mining areafor over 10 years. The whole catchmentcovers an area of 2656.31 hectares which islocated downstream at the embankment ofKampung Bestari Jaya and University IndustrySelangor (UNISEL) main campus Figure 1.The catchment water flow downstream toRiver Ayer Hitam and River Udang whichultimately ends up with Selangor River at5 Km upstream of Batang Berjunti WaterTreatment Plant SSP2 which is major waterdistributor to federal territory (Kuala Lumpurand Putrajaya) and Selangor state as well .
Bestari Jaya has a tropical, humid climate,with very little variations in temperaturethroughout the year. The average temperatureof the area is 32°C during day and 23°Cat night. An annual average rainfall of2000 mm and 3000 mm with potentialevaporation of 1600 mm per year. The areaconsists of myriad ecosystems which canbe subdivided into several categories suchas degraded land, large open lakes andsmall ponds, earth drains and wetlandsarea, tin tailings (sand and slime tailings),logged peat swamp forest land in east.The contribution of storm water, peatswamp forest water and recent sand miningactivity has caused severe environmentalpollution due to drainage problem in the area.
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The area has a lot of big lakes and smallponds that are interconnected by earthdrains. Excess water from these lakes andponds is discharged to the existing earthdrains at the downstream of the lakes andponds. Flow routing had been carried outby applying the survey data. Some of thelakes flow across the downstream intoopen spaces and wetlands that run off intoRiver Ayer Hitam, meets up River Udang
Figure 1. Landuse map of ex-mining catchment, Bestari Jaya.
and ultimately ends up to River Selangorat the Jalan Timur Tambahan roadjunction, east of UNISEL Figure 1 .
The wetlands has an area of 579.7hectares is stretched along the north westernborder of the site. Several useful plant specieshave been seen in the wetland while severalharmful weed species have been also seen inthe study area that cause blocking of watercourses and water become foul due to large
masses of water leaves. This area is sandyin texture and it is representative of entireexamining area in the country. The parentmaterial is of reverine alluvium materials,with pH in range of 3.5-5.5 .
1.2. Subsoil Conditions BeforeReclamation
Prior to the reclamation work, soilinvestigation was carried out to gather subsoilinformation and engineering properties.Boreholes and trial pits were carried out atthe site area so that more continuous visualinformation on the subsoil materials can beobtained Figure 2 . Soft silty clay was
found at the mining land as well. Theaverage thickness is about 5m. Liquid limitand plastic index of the soft clay were inthe ranges of 50% to 80% and 35% to 45%respectively. Figure 3 shows the propertiesof the soft silty clay. Underlying the softsilty clay is a thick layer of medium to stiffsilty clay and medium dense silty sand layers.Hard or very dense soil layer could only beencountered at about 45m to 55m belowthe existing ground level [24, 25]. Granitebedrock was encountered in some boreholesat depth of 50m to 65m. Summary of thesubsoil profile is as shown in Table 1.
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Table 1. Typical subsoil profile.
Reduced Level (m) Soil DescriptionStandard Penetration Test
Above 0m0m to -5m
-5m to -50m
Heterogeneous mining landVery soft to soft silty clayMedium stiff or medium
dense silty clay or silty sandVery dense or hard soft layer
0 to 300 to 46 to 30
Figure 2. Typical subsoil profile from boreholes at ex- mining land.
Figure 3. Physical properties of the soft clay layer.
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1.3. The Potential Problems ofReclamation
The soft clay layer at site has low shearstrength and high compressibilitycharacteristic. Additional loading would beimposed to this soft clay layer due to thereclamation work. Two major potentialproblems were anticipated. These problemswere (1) stability and (2) long termsettlement of the reclaimed platform .Stability analysis were carried out and itwas found that with a gentle reclaimedplatform side slope and proper control onthe backfilling rate, the stability of thereclaimed platform will be under controland this should not be a major concern.The platform will become more stable inthe long term as the subsoil especially thesoft clay layer gains strength with time.Settlement analysis indicated that long termsettlement will be excessive due to thehighly compressible soft silty clayunderneath the reclaimed platform [27, 28].The settlement will take long time tocomplete. In addition, the highlyheterogeneous ex-mines at site may alsocause unexpected settlement whensubjected to fill load which causes thecollapse of large voids within the landfill.The potential biodegradation process ofsome organic materials may also contributeground settlement .
2. MATERIALS AND METHODS2.1. Materials
Flocculated slurry, sand and clay
particles were used in this work. The slurryslime samples originated from the minedout ponds were collected from Bestari Jayaex-mining catchment and flocculated byNatural Organic Polymer (NOP) or PolyVinyl Acetate (PVA) and then mixed withresidual soil . The raw flocculatedslurry had particle sizes ranging frommicrometres to larger than 1 cm in diameter.The particle shapes of the flocculatedslurry, ranged from partially rounded toangular. Polyvinyl acetate, PVA, PVAc,poly (ethenyl ethanoate), is a rubberysynthetic polymer with the formula(C4H6O2)n. It belongs to the polyvinylesters family with the general formula -[RCOOCHCH2]- Figure 4. It is a type ofthermoplastic.
2.2. Methods2.2.1.Characterization of Physical andGeotechnical Properties of FlocculatedSlurry
The tests used for physical andgeotechnical characterization of flocculatedslurry are shown in Table 2. The physicaland geotechnical tests were conducted inaccordance with either British or ASTMStandards. Some of the test procedureshave been described by Head (1982, 1984,and 1986) [31, 32, 33]. The laboratory directshear test  and triaxial consolidateddrained (CD) test  were conducted tomeasure the stress-strain and shear strengthbehaviour of the flocculated slurry.
Figure 4. Poly Vinyl Acetate (PVA)-IUPAC name: poly (1-acetyloxiethylene).
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Table 2. Tests used for characterizing physical and geotechnical properties of flocculatedslurry slime.
Property TestedMaximum/minimum dry densityMaximum/minimum void ratioParticle size distributionFinest content d<0.075 mm (%)Specific GravityShear Strength
Tests Used (British Standards)Vibratory Compaction (BS1377)Vibratory Compaction (BS1377)Mechanical Sieve (BS1377)Wet sieving though US#200 sieveDensity Bottle (BS1377)Direct shear box and trixial shear apparatus(BS4019)Constant head permeability test (BS1350)
Property TestedMaximum/minimum dry densityMaximum/minimum void ratioParticle size distributionFinest content d<0.075 mm (%)Specific GravityShear Strength
2.2.2. Characterization of GeochemicalProperties of Flocculated Slurry
The flocculated slurry was analyzedfor its elemental content, pH, acidneutralization capacity (ANC), redoxpotential (Eh), and electrical conductance(EC). The pH of the flocculated slurry at asolid:water ratio of 1:1 was measured usingHydrolab MS5 USA. The ANC of thematerial was determined by titration of a100 g sample with 0.02 N HCl. The Ehwas determined at room temperature(23±2°C) using a platinum electrode andAg/AgCl with 3.33 mol L–1 KCl as thereference electrod. The Eh that wasmeasured using Ag/AgCl with 3.33 molL–1 KCl was converted to the equivalentEh measured with a standard hydrogenelectrode (SHE), using the followingrelationship :
where t is the temperature in °C at whichthe Eh was measured.
The EC was measured using aHydrolab MS5 USA. The flocculatedslurry was submerged in the deioniseddistilled water at a solid:water ratio of 1:1.The set-up allowed oxygen diffusionthrough the water surface and therefore a
partially oxic condition prevailed. Themeasurement continued until the measuredEh and EC values stabilized. Thesemeasurements monitored the possibletime-dependent changes in the leachatecharacteristics under a partially oxiccondition . This simple set-up wasdesigned to simulate field conditions inwhich the flocculated slurry would besubmerged as the groundwater tables arecommonly near to the ground surface in allthe reclaimed sites in Malaysia.
2.2.3. Comparison and Cost AnalysisIn Malaysia, the most common method
of reclamation practised by developers forconstruction purposes is lowering of waterlevel and emplacement of fill materialmethod. Two other conventional methods ofreclamation currently practised are thedisplacement and the containment methods[36, 37]. Comparative analysis of currentlypractised methods with the proposedflocculation and admixing method hasaccomplished by the author. Similarlytheoretical comparison of reclamation costsof displacement method and containmentmethod, lowering of water andemplacement of fill material method andthe proposed flocculation and admixingmethod was carried out but cost analysis
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of the proposed method has only explainedhere due to ambiguity of results.3. RESULTS AND DISCUSSION3.1. Physical and Geotechnical Propertiesof Flocculated Slurry
The basic physical properties offlocculated slurry are summarized in Table 3.The physical properties of local sand are alsoincluded for comparison. The flocculatedslurry had a much higher specific gravityand dry density than the ordinary sand,which meant that the self-weight of theflocculated slurry was high. This is beneficialfor land reclamation as the larger the
self-weight, the larger the overburdenpressure to consolidate the soft mind out beds.
The typical grain size distribution ofthe flocculated slurry is shown in Figure5. According to the unified soilclassification system (USCS), the materialcan be classified as well graded, which isideal for pavement construction, roadconstruction and land reclamation. Themean diameter of the flocculated slurry wasaround 0.5 mm, which was larger than thatof ordinary sand. Generally, fill materialwith a large mean grain size is preferred, asthe larger the mean grain size, the greater
Table 3. Basic properties of flocculated slurry slime compared with sand.
Figure 5. Typical particle size distribution curve of flocculated slurry.
Property Flocculated slurry slime Sand
Maximum dry density (Mg m-3) 2.50 1.8-2.0Minimum dry density (Mg m-3) 1.99 1.4-1.6Maximum void ratio 0.65 0.7-1.0Minimum void ratio 0.26 0.4-0.6D50 (mm) 0.50 0.1-0.4Particle size distribution Cu = 6.8, Cc = 2.4 Cu = 3.3, Cc = 1.1Main mineral contents Polyvinyl chloride QuartzSpecific gravity 3.57 2.5-2.6Particle shape Sub rounded to angular Subrounded or subangularFinest content d<0.075 mm (%) 4-7 (non cohesive) 5-10(clay or silt)pH value 8.4 Neutral
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the hydraulic conductivity and the greaterthe shear resistance .
Empirical relationships are used toestimate hydraulic conductivity fromstandard grainsize parameters . Thegrainsize diameter at which 10% of theslurry is finer (d10) is applied in a commonlyused empirical formula, initially developedby Hazen (1893);
where AH is a dimension coefficient(= 1.0 for m/d), C is an empirical constant(=860) and T is a temperature correctionfactor (=1 at 10°C). The hydraulic conductivitymeasured for the flocculated slurry was inthe range of 8×10–5 to 7×10–4 m/s dependingon the density of the material. The rangesof friction angle measured from shearstrength tests are shown in Figure 6a forthe triaxial CD tests on saturated specimensand Figure 6b for the direct shear tests ondry specimens. The friction angles measuredby the triaxial tests range from 38.9 to 41.6°for loose and 43.5 to 47.0° for dense saturatedspecimens under a confining pressure of50 to 200 kPa (Figure 6a). Direct shear testson dry flocculated slurry indicated that thefriction angles for loose specimens rangedfrom 29.0 to 40.0° (Figure 6b). Thesevalues were much higher than those forsand under the same conditions. This is a
Figure 6. Ranges of friction angle measured by: (a) triaxial CD tests on loose and densespecimens, and (b) direct shear tests.
favourable property as high friction anglewill ensure greater slope and foundationstability, which are crucial to the design ofland reclamation . It should be noted thata range of friction angles are presentedinstead of just one friction angle. This isbecause the friction angle for the flocculatedslurry is affected by the confining pressurewhich is a function of depth below theground level .
3.2. Geochemical CharacteristicsTypical acid titration curves for sand, clay
and flocculated and slurry are shown inFigure 6. The pH of the initial point forall titration curves reflects the amount offree caustic in the bulk of solution at thebeginning of titration. Titration curvesexhibit the response of the system to theaddition of acid as they show changes inpH of the bulk of the slurry after additionof acid. The amount of HCl added to thesolution is expressed in moles. It was assumedthat species trapped by red mud particlesfrom caustic solution after digestion otherthan hydroxyl ions did not affect the titrationresults to any significant extent. The mainconstituents of the caustic solution used inthe digestion that could consume protons inthe process of titration were organicscontaining carboxylic groups, carbonate
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and aluminate ions. However, the washingprocess resulted in dilution of the redmud slurry after digestion by at least 10000times as indicated by the drop in causticconcentration of the slurry. The pH ofleachate generated from the flocculatedslurry using deionised distilled water at asolid:water ratio of 1:1 was approximately8.4. This pH value is within the commonpH range for soils and groundwater. Thematerial had a rather low ANC (Figure 7) incomparison with the clay that underlies thereclaimed sites. Sand has practically negligibleANC. The ANC of a material is derivedfrom acid adsorption and dissolution ofacid-soluble minerals . The low ANCvalue indicates that flocculated slurry is not
Figure 7. pH-acid titration curves for flocculated slurry and natural soils.
resistant to acid attack.Another important property of the
flocculated slurry is the Eh value. The initialEh value (measured with an Ag/AgClreference electrode) for the flocculatedslurry was 171 mV at a solid:water ratio of1:1. The deionised distilled water added tothe flocculated slurry originally had ameasured Eh of 289 mV. Therefore, it canbe concluded that when placed in thereclaimed site, the material will create aslightly reducing condition. The Eh wouldcontinue to decrease rapidly within days afterplacing in water and would graduallystabilize thereafter, as illustrated by Figure 8.It was found that the values stabilized after120 days. The decrease was accompanied
Figure 8. Changes of measured redox potential and electrical conductance of submergedflocculated slurry.
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by a rise in EC, which could be attributedmainly to the time-dependent mineraldissolution. These observations were dueto the presence of traces of sulphideminerals, or other reduced minerals presentin the slurry . These minerals couldgradually oxidize under oxic conditions andrelease H+ as well as other ions into thepore water. As a result, there was a marginaldrop in pH from the initial pH 8.4 to afinal pH 7.9 .
3.3. Methods Comparison with NewlyDeveloped Method3.3.1.Lowering of Water Level andEmplacement of Fill Material
This method of reclamation is widelypractised by developers of housing estatesor industrial parks in Malaysia. Reclamationis often carried out on ad-hoc basis withoutany prior site investigation. The water inmined out pond is lowered and fill material,usually tailing sand from nearby dumps ispushed in from one end of the pond [43, 44,45]. The soft slurry slime at the bottom ofthe pond is not removed and some of theslurry slime would seep into the voids of thefill material and others, on the pond bottomare consolidated. It is almost impossible topredict the total time needed for thedevelopment of a competent, consolidatedground as classical methods for determinationof consolidation deal with small strainsettlement .
This method of reclamation involvesdewatering in the mined out ponds bypumping out and disposing off thesupernatant. Any indiscrimate extraction ofwater can cause drawdown slope instability,particularly if the sides of the pond aresteep. Secondly, slurry slime is soft with nobearing strength or shearing strength;the soft slurry slime would seep into thevoids of the fill material, resulting in localised
soft spots. Another shortcoming with thismethod is that it is almost impossibleto predict the total time needed forthe development of a competent andconsolidated ground in areas reclaimedwith this method .
3.3.2. Displacement MethodRaju and Hoffman (1996) used this
method for reclamation project in theSelayang area in Kuala Lumpur sometimein the 1980s . Soft slurry slime wasdisplaced by pushing in sand fill in aprescribed direction. A layer of geomembraneis laid over the entire pond surface and sunkby laying sand bags or dipping fill materialover it. The slurry slime is then displaced asmore fill material is dropped onto thesurface of the geomembrane. When the slurryslime is displaced, a mud wave is developedin most of the sand fill. This mud wavewill increase the height of the fill materialrequired to displace the slurry slime. Afterdisplacing the slurry slime to one end of thepond, the slime is excavated. Fill material isthen pushed in from one end of the pond.
A major disadvantage is the incompletedisplacement of slurry slime within a troughof karstic limestone bedrock topography.In the reclamation project carried out inthe Selayang area by Raju and Hoffman(1996), it was reported that the entrapmentof slurry slime within the troughs of karsticlimestone was one of the reason for thecreation of localised soft spots in thereclaimed ground . As a result of floors,drains and other poorly supported parts ofbuildings constructed over such areasuffered damages and cracks were formed.A second disadvantage is that the displacedslurry slime has to be discarded and quiteoften, they are just dumped into anotherpond nearby or into some nearby drainagesystems .
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3.3.3.Containment MethodThe containment method does not
displace the slurry slime in the pond.Instead the slime is contained with thepond and is compressed in situ. Thismethod has advantage of requiring lesserfill material for the back filling of the pond.Yee (2010) stated that this method requiresthe removal of topmost 500 mm layer ofthe soft material . The initial layer of thefill material has to be placed carefully so asnot to exceed the bearing strength of theunderlying soft slime. Geo-membrane is oftenused as a separation and reinforcement layer.Fill material is placed by sand pumping andconveyer belt system in uniform layers withdepth not exceeding the bearing capacity ofthe underlying slime. The underlying soft slimeis allowed to consolidate, whereby resultingin an increase in the shear and bearingstrengths. When a stable condition is attained,side tipping can be used to speed up theworks. Often vertical drains are installed tospeed up the consolidation of the slime .
Like the displacement method, there aresome shortcomings in the containmentmethod as well. Yee (2010) stated that topmost500 mm of slurry which has no shear andbearing strengths has to be removed.However investigations carried out by writershoed that slurry slime in the test ponds wasthicker . As such a substantial amount ofslurry has to be removed and this can result inenvironmental problems. The initial layer ofthe sand fill has to be placed very carefully soas not to exceed the bearing capacity of theunderlying material. The sand fill process hasto be carried out in uniform layers with depthsnot exceeding the bearing capacity. Such sandfilling requirements are difficult to achieve .
3.3.4.Admixing of Flocculated SlurrySlime with Soils
An alternative method which is
economically competitive, technologicallyfeasible and which will not contribute to anyenvironmental problem is to flocculate theslurry slime with natural organic polymer(NOP) or poly vinyl acetate (PVA) and usethe flocculated slurry slime as part of the fillmaterial. Tests carried out by Chow and Wengshown that slurry slime with NOP or PVAhave higher engineering strength and bettersettlements and a shorter time to achievecomplete settlement . Chow and Wengexplained that NOP or PVA flocculated slurryslime admixed with residual soil or tailing sanddistinctly have better physical( i,e. higherincrease of increase in the solids concentration,higher rate of decrease in the voids ratio andmoisture content) and higher engineeringstrengths (i,e. higher shear strength and higherbearing strength. The flocculated slurry slimeis mixed with either residual granitic orschistose soil, depending on their availability,or with tailing sand which are often found inthe tailing dumps in the vicinity of the ponds.The admixed material is then emplaced in thepond as part of the fill material .
In this proposed method, a holdingpond is excavated beside the pond ear-marked for reclamation. An ideal site for thelocation of the holding pond is on slightlyhigher ground.
Material excavated from the holdingpond is used for the construction of bundsresulting in a higher holding-pond capacity.Alternatively the excavated material can beused as a source of admixing material .
Slurry and very soft slime from thebottom of the pond designated forreclamation is pumped out using submersiblesuction pumps until the layer of soft slime isreached. Mixing is best achieved by passingthe slurry and the flocculating reagents alonga 20m to 50m long sluice box fitted withtransverse riffles. The flocculated slurry is thenallowed to settle in the holding pond and the
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clear supernatant is allowed to be drained offinto a nearby pond or into the drainage system.
The still wet flocculated slurry is thenadmixed, preferably, with granitic soil orschistose soil. The minimum admixed ratiosfor granitic soil: NOP-flocculated slurry,granitic soil: PVA flocculated slurry, Schistosesoil: NOP-flocculated slurry, Schistose soil:PVA-flocculated slurry admixtures arebetween 0.75-1.00. If tailing sand is used foradmixing, the minimum admixing ratios fortailing sand: NOP-flocculated slurry, tailingsand: PVA-flocculated slurry re between 3.0and 5.0. The admixed material is then pushedback into the designated pond in layers ofabout 500mm thick, taking steps to ensurethat the initial layer of admixed material doesnot cause a shear failure in the underlying softslime .
Following this, fill material comprisingeither tailing sand or residual soil is emplacedover the admixed material. This method ofreclamation does not involve the dumping ofslurry slime into another pond as they formpart of the admixture. As such, there will notbe environmental problems.
3.4. Comparative Cost of ReclamationsA theoretical comparison of reclamation
costs of displacement method andcontainment method, lowering of water andemplacement of fill material method and theproposed flocculation and admixing methodwas carried out. Costing was carried out forthree lakes with size of 3 hectares. Comparisonwas also carried out for various pond waterdepths and is illustrated in Table 4.
Results in Table 4 show that lowering ofwater level and emplacement of fill materialmethod is the cheapest. It costs only RM0.74to reclaim one square foot should the depthof the pond water be 3m. The cost howeverincreases with the depth of the pond water,
rising to 1.14 per square foot for a waterdepth of 5m, RM1.54 per square foot for7m water depth, RM1.94 per square footfor 9m water depth, RM2.34 for 11m waterdepth and RM3.14 for 15m water depth.
The containment method is the mostexpensive of four reclamation methodsdiscussed above. For a pond with 3m ofwater the reclamation costs is RM4.48 persquare foot if the slurry slime to be removedis about 1m depth. The cost goes up if theslurry slime to be removed is thicker, rising toRM6.30, RM8.27, RM12.05 and RM15.83 forthe removal of 2m, 3m, 5m and 7m of slurryslime respectively. Likewise, if the depth ofwater in the pond increases, the reclamationcost goes up accordingly. Between thedisplacement method and the proposedflocculation and admixing method the formermethod is cheaper if the pond water is 3mdeep and the slurry to be removed is between5 to 7m, 9 to 11m and 15m, the displacementmethod is also cheaper if the slurry to beremoved is less then 3m, 5m, and 7m thickrespectively.
Results of the theoretical costscomparison showed that the depths of thepond and thickness of the slurry slime layerto be flocculated or removed are importantfactors which affect the cost rate ofreclamation. The size of the pond affects thetotal costs but not the cost rate. The loweringof water level and emplacement of fill materialmethod is the cheapest amongst the fourmethods discussed. However this method isalso the most inefficient method. Theproposed flocculation and admixing methodis environmentally friendly as there is nodisposal of slurry slime from the pond. Allthe slurry slime are flocculated and used aspart of the fill material, hence reducing theamount of fill material as well. The techniqueis technically feasible and economicallycompetitive.
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l of p
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4. CONCLUSIONSIn Malaysia, the most common
method of reclamation practised bydevelopers of housing estates and industrialparks is lowering of water level andemplacement of fill material method. Thismethod results in a number of technicalproblems, amongst which, is that slurryslime are entrapped in the voids of the fillmaterial and within the troughs of thekarstic limestone bedrock. Also, it is almostimpossible to predict the total timerequired to achieve a competent andconsolidated ground. Two otherconventional methods of reclamationcurrently practised are the displacementand the containment methods and bothmethods also have a number ofshortcomings. The proposed flocculationand admixing method is technicallyfeasible, economically competitive andenvironmentally friendly. It isrecommended that the slurry slime to bepumped out from a pond and flocculatedwith NOP or PVA and then the flocculatedslurry slime is admixed with residualgranitic or schistose soil or as a last resort,with tailing sand. The admixed materialsare then put back into the pond as part ofthe fill materials.
Investigations on the physical,geotechnical and geochemical characteristicsof the flocculated slurry show that the materialhas favourable engineering properties whichmake it suitable to be used for landreclamation. The flocculated slurry is ratherweathering resistant. The absence of acidicleachate generation due to redox reaction fromthe flocculated slurry also rules out thepossibility of acid generation and excessiveleaching of the material. In addition, thematerial is rather stable in its redoxcharacteristics. However, further studies thatfocus on field monitoring are still required as
there may be unexpected long-termenvironmental consequences arisen from themassive use of flocculated slurry in reclaimedsites. Time dependent reactions, such as redoxprocesses and mineral dissolution kineticsunder field condition, and the possibility ofcatalysis by indigenous microorganism, mustbe monitored. Ultimately, life-cycle assessmentapproach will be needed to identify the bestsolution for the admixing of flocculated slurrywith residual soil in Malaysia.
ACKNOWLEDGEMENTThe study reported in this study is a basic
research work carried out at AnalyticalLaboratory, Department of Chemistry andpartially at Department of Geology,University of Malaya. I take this opportunityto express my gratitude to Ministry of HigherEducation, Malaysia to provide mescholarship to complete my higher studies inMalaysia. Thanks also go to InstitutPengurusan Dan Pemantauan Penyelidikan,(IPPP) University of Malaya to provide meenough funding (Project No. PV039/2011C)for the completion of this valuable research.
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