(a)5eteuo(6)PnDistancefrom surface (A)Distancefromsurface(A)L400Distance from surface (A)DistancefromSurface(A)SodiumkaoliniteSodiumFiqure20Particleswithdoublelayers[4].The water in the double layer is under an attractive force to the soil particle since this waterisattachedtotheexchangeableionswhichareinturnattractedtothesoil surface.Water isalsoattractedtothemineralsurfacebyotherforces(theforcesbetweenpolarwaterandthestray electrical charges on the mineral surface, hydrogen bonding, and van der Waals forces).Until now it isassumedthatsodium(Na)wastheexchangeableion.Ithasbeenshownthationsadsorbedon soil particlescanbereadily exchanged.A schematicexampleisgiveninfigure 21.NaNaNaNa8NaCI+4CaCl2=Clay particleCaNaNaNaNaFigure 21 Ion exchange reaction [4].As is shown in table 6,exchange of the cations hasa majoreffect on theplasticityand soonthe quality of the soil.This can be explained by the fact that a reaction as shown in figure 21results in a reduction of the thickness of the doublelayeraround the particle.It simply meansthattheratio solidvsmoistureis increasing.If two clay particles in water which are far apart, are brought towards each other, they willreachan interparticle spacing at which they begin to exert forces on each other.Since thenet charge of the particles is negative, the two particles repel each other.This starts whenthedouble layers of both particles come in contact with each other. The repulsiveforcebetween adjacent particles for any giving spacing is therefore directly related to the sizes ofthedouble layer.Anychange in the characteristics of thesoil water system that reduces thethickness of the double layers reduces this repulsive force for the same interparticle spacing.Figure 22 shows the influencesof various characteristics of the system on theelectricalpotential y, and therefore the repulsive force, at any given distance x from the particlesurface.In addition to a repulsive force between approaching clay particles, there is also a componentof attractiveforces betweenthetwo particles.This attractiveforce is the van der Waalsforce, or secondary bonding force, which acts between all adjacent pieces of material. This27
27 Figure 20 Particles with double layers [4]. The water in the double layer is under an attractive force to the soil particle since this water is attached to the exchangeable ions which are in turn attracted to the soil surface. Water is also attracted to the mineral surface by other forces (the forces between polar water and the stray electrical charges on the mineral surface, hydrogen bonding, and van der Waals forces). Until now it is assumed that sodium (Na) was the exchangeable ion. It has been shown that ions adsorbed on soil particles can be readily exchanged. A schematic example is given in figure 21. Figure 21 Ion exchange reaction [4]. As is shown in table 6, exchange of the cations has a major effect on the plasticity and so on the quality of the soil. This can be explained by the fact that a reaction as shown in figure 21 results in a reduction of the thickness of the double layer around the particle. It simply means that the ratio solid vs moisture is increasing. If two clay particles in water which are far apart, are brought towards each other, they will reach an interparticle spacing at which they begin to exert forces on each other. Since the net charge of the particles is negative, the two particles repel each other. This starts when the double layers of both particles come in contact with each other. The repulsive force between adjacent particles for any giving spacing is therefore directly related to the sizes of the double layer. Any change in the characteristics of the soil water system that reduces the thickness of the double layers reduces this repulsive force for the same interparticle spacing. Figure 22 shows the influences of various characteristics of the system on the electrical potential , and therefore the repulsive force, at any given distance x from the particle surface. In addition to a repulsive force between approaching clay particles, there is also a component of attractive forces between the two particles. This attractive force is the van der Waals’ force, or secondary bonding force, which acts between all adjacent pieces of material. This
attractiveforce between clayparticles is essentially independent of thecharacteristics of thefluidbetweentheparticles.ncentration,r0Concentration,4CDistance from particle,x2(a)eao-Valence,zValence,2zDistance from particle,(b)-Dielectricconstant,4eearaoDielectricconstant,Distancefrom particle,(c)Figure22Theeffectsofchangesinsystempropertiesondoublelayers.(a)Concentration.(b)Valence.(c)Dielectricconstant[4l5.4FlocculationanddispersionFromthediscussionsofaritisclearthattherearerepulsiveandattractiveforces.Iftheattractiveforcesaredominant,thentheparticleswilltendtomovetowardseachotherandbecomeattached,theyclustersotosayor,inotherwords,theyflocculate.Ifhoweverthenetinfluenceisrepulsivetheparticlestendtomoveawayfromeachother.theydisperseExamplesoftheflocculationanddispersiondiscussedhereareshowninfigure23aandc.Figure23Sedimentstructures.(a)Saltflocculation.(b)Nonsaltflocculation.(c) Dispersion [4].28
28 attractive force between clay particles is essentially independent of the characteristics of the fluid between the particles. Figure 22 The effects of changes in system properties on double layers. (a) Concentration. (b) Valence. (c) Dielectric constant [4]. 5.4 Flocculation and dispersion From the discussion so far it is clear that there are repulsive and attractive forces. If the attractive forces are dominant, then the particles will tend to move towards each other and become attached, they cluster so to say or, in other words, they flocculate. If however the net influence is repulsive the particles tend to move away from each other, they disperse. Examples of the flocculation and dispersion discussed here are shown in figure 23 a and c. Figure 23 Sediment structures. (a) Salt flocculation. (b) Nonsalt flocculation. (c) Dispersion [4]
Sincetherepulsiveforces arehighlydependenton thecharacteristics of thesystem andtheattractiveforcesarenot,atendencytowardsflocculationordispersionmaybecausedbyanalteration inthesystemcharacteristics,whichaltersthedoublelayerthickness.Atendencytoward flocculation is usuallycaused by decreasing the doublelayerthicknesswhich results by increasing one or more of the following.electrolyteconcentration(seealsofigure22a),ion valence (seefigure 22b),temperature.Flocculation is also promoted by a decrease of the following:dielectric constant,size of hydrated ion,pH,anionadsorption.There arehoweversituations that theclayparticles comeveryclosetogether.This isusuallythecase in deposits.In that particular case a different type of attractive forces becomeimportant which may result in the non salt flocculation as shown in figure 23 b.This isbecause of the phenomenon schematically shown in figure 24.alongthefacesof theclaymineral thenetchargeisnegativeat the edges, where the clay mineral is broken, the net charge is positiveFigure24Netchargesalongthefacesandbrokenedgesofaclaymineral.Innonsaltflocculatedstructuresthepositiveedges linktothenegativefaces.The engineering behaviour of a soil depends very much on its structure.In general aflocculated structure has a higher strength and a lower compressibility than a dispersedstructure. This is due to the fact that in the flocculated structure there is interparticleattractionand it ismoredifficulttodisplaceparticleswhentheyareinadisorderlyarray.ThehigherpermeabilityintheflocculatedsoilresultsfromlargerchannelsavailableforflowIt should be noted that physical working a soil, this is called remolding, can change itsstructure,flocculated structurestend to changetodispersed structures.AnotoriousexampleofasoilthatissensitivetoremoldingisNorwegianquickcay.Thisclayhasaflocculatedstructure when it developed under water.Movement of the earth caused the deposit to riseabovewaterlevel.Rainwaterthenleachedthecationspresentatthefaceswhich impliedthattheattractiveforcesthatkepttheflocculated structuretogetherdisappeared,thesoilbecamea very unstable house of cards. When this soil is remolded by subjecting it e.g.to vibrationforces, it looses its strength immediately.6.EffectsofcompactiononthestructureofasoilanditsengineeringpropertiesNormally engineers don't work with the soil as it is. They might e.g.excavate the soil to makea cut or build an embankment.Compaction is always needed to givethe material therequiredstrengthtocarrytheloadsthatwillbeapplied.Differencesinstructureasaresultofcompaction can have a pronounced effect on the engineering properties of a soil. Therefore itisnecessarytohaveatleastanideaoftheseeffects.Todescribetheseeffects,useismadeofaratherdatedbutstill excellentpaperbySeedandChan[5].29
29 Since the repulsive forces are highly dependent on the characteristics of the system and the attractive forces are not, a tendency towards flocculation or dispersion may be caused by an alteration in the system characteristics, which alters the double layer thickness. A tendency toward flocculation is usually caused by decreasing the double layer thickness which results by increasing one or more of the following. - electrolyte concentration (see also figure 22 a), - ion valence (see figure 22 b), - temperature. Flocculation is also promoted by a decrease of the following: - dielectric constant, - size of hydrated ion, - pH, - anion adsorption. There are however situations that the clay particles come very close together. This is usually the case in deposits. In that particular case a different type of attractive forces become important which may result in the non salt flocculation as shown in figure 23 b. This is because of the phenomenon schematically shown in figure 24. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - along the faces of the clay mineral the net charge is negative at the edges, where the clay mineral is broken, the net charge is positive Figure 24 Net charges along the faces and broken edges of a clay mineral. In non salt flocculated structures the positive edges link to the negative faces. The engineering behaviour of a soil depends very much on its structure. In general a flocculated structure has a higher strength and a lower compressibility than a dispersed structure. This is due to the fact that in the flocculated structure there is interparticle attraction and it is more difficult to displace particles when they are in a disorderly array. The higher permeability in the flocculated soil results from larger channels available for flow. It should be noted that physical working a soil, this is called remolding, can change its structure, flocculated structures tend to change to dispersed structures. A notorious example of a soil that is sensitive to remolding is Norwegian quick clay. This clay has a flocculated structure when it developed under water. Movement of the earth caused the deposit to rise above water level. Rainwater then leached the cations present at the faces which implied that the attractive forces that kept the flocculated structure together disappeared, the soil became a very unstable house of cards. When this soil is remolded by subjecting it e.g. to vibration forces, it looses its strength immediately. 6. Effects of compaction on the structure of a soil and its engineering properties Normally engineers don’t work with the soil as it is. They might e.g. excavate the soil to make a cut or build an embankment. Compaction is always needed to give the material the required strength to carry the loads that will be applied. Differences in structure as a result of compaction can have a pronounced effect on the engineering properties of a soil. Therefore it is necessary to have at least an idea of these effects. To describe these effects, use is made of a rather dated but still excellent paper by Seed and Chan [5]. + + + + + +
6.1 ShrinkageIn [5] an investigation is reported on the shrinkage behaviour of a particular silty clay.Samples, with moisture contents lower and higher than the optimum moisture content, werecompactedtothesamedensityandwerethenstoredunderwaterforsevendays(soaked)atconstant volume.Thenthe samples weredried and allowedto shrink.The results of thisexperimentareshowninfigure25[5]MolcgWaterConerceTOFOrDeroFigure25Influenceofcompactionconditionsandsosoil structureontheshrinkagecharacteristics of a silty clay [5].The bottom part of figure 25 shows the well known moisturedensity relation that isobtained for soils when compacted with a certain amount of compaction energy.The upperpartof thefigureshowstheshrinkagethatwasobservedforthevarioussoakedspecimens.From this figure one can observe that the specimens that were compacted at the dry side oftheoptimummoisturecontentshowedtheleastshrinkage.Sinceflocculated structures showless shrinkagethandispersed structures,it isbelievedthatcompacting soils at the dry side of optimum results in flocculated structures.For practice thismeans that if onewants to limitproblemsdueto shrinkage, compactionatawatercontentlowerthantheoptimummoisturecontentisrecommended.6.2SwellingFlocculated structuresontheotherhand swell morethandispersed structures.Thereforeonemight expect that specimensthatarecompacted atthedry side of the optimummoisturecontent show more swell. Evidence that this is indeed the case is given in figure 26, whichshows thefinal watercontents after swelling for samplesof sandyclaycompactedatdifferent water contents. Some of the water that enters a compacted specimen duringswelling is required simply to fill the pores and bring the soil to a saturated condition, asdistinct from water adsorbed by the soil during the swelling process.30
30 6.1 Shrinkage In [5] an investigation is reported on the shrinkage behaviour of a particular silty clay. Samples, with moisture contents lower and higher than the optimum moisture content, were compacted to the same density and were then stored under water for seven days (soaked) at constant volume. Then the samples were dried and allowed to shrink. The results of this experiment are shown in figure 25 [5]. Figure 25 Influence of compaction conditions and so soil structure on the shrinkage characteristics of a silty clay [5]. The bottom part of figure 25 shows the well known moisture – density relation that is obtained for soils when compacted with a certain amount of compaction energy. The upper part of the figure shows the shrinkage that was observed for the various soaked specimens. From this figure one can observe that the specimens that were compacted at the dry side of the optimum moisture content showed the least shrinkage. Since flocculated structures show less shrinkage than dispersed structures, it is believed that compacting soils at the dry side of optimum results in flocculated structures. For practice this means that if one wants to limit problems due to shrinkage, compaction at a water content lower than the optimum moisture content is recommended. 6.2 Swelling Flocculated structures on the other hand swell more than dispersed structures. Therefore one might expect that specimens that are compacted at the dry side of the optimum moisture content show more swell. Evidence that this is indeed the case is given in figure 26, which shows the final water contents after swelling for samples of sandy clay compacted at different water contents. Some of the water that enters a compacted specimen during swelling is required simply to fill the pores and bring the soil to a saturated condition, as distinct from water adsorbed by the soil during the swelling process
28IncreasingIncreasingflocculationdispersionFino/water contentofter swelling2624WatercontemincreaseduePeWafer contentoftersaturotion atconstantvolumeoan181614Initialwaterconten26222016810121418MoldingWaterContent-percent1162B108IncreasingIncreasingfiocculationdispersion1021618202214101286MoldingWaterContent-percentFigure26Influenceof moldingwatercontentand soilstructureonswellingcharacteristicsofsandyclay[5]Infigure26thetotal increase in water contentduring swelling has been separated intothatwater content increaserequired to cause saturationand thatdueto swelling aftersaturationhas been attained. The marked increase in swelling tendencies of the specimens that arecompacted atthedrysideof theoptimummoisturecontent isreadilyapparent.It isthereforemost likelythatflocculated structuresaredeveloped at thedrysideof optimummoisturecontent.6.3Stress-deformationcharacteristicsFigure 27 shows the results of undrained, unconsolidated triaxial tests performed on silty claysamples that were compacted dry and wet of optimum moisture content to the same densityandwerethensoakedforsevendaysatconstantvolume.31
31 Figure 26 Influence of molding water content and soil structure on swelling characteristics of sandy clay [5]. In figure 26 the total increase in water content during swelling has been separated into that water content increase required to cause saturation and that due to swelling after saturation has been attained. The marked increase in swelling tendencies of the specimens that are compacted at the dry side of the optimum moisture content is readily apparent. It is therefore most likely that flocculated structures are developed at the dry side of optimum moisture content. 6.3 Stress – deformation characteristics Figure 27 shows the results of undrained, unconsolidated triaxial tests performed on silty clay samples that were compacted dry and wet of optimum moisture content to the same density and were then soaked for seven days at constant volume