3.5Preparedbykneadingcompactiondryofoptimum;soaked otconstontvolume(flocculated structure);3.0watercontent=20.25%drydensity=105.6/bpercuft25adssr2.01.5Preparedbykneadingcompoctionwetofoptimum(dispersedstructurel;watercontent=20.3%;drydensity*105.51bpercuft1.00.5Unconsolidated-undrainedtestsConfining pressure-l.Okg persq cm0283216202448120AxialStrain-percent110FRSS107106?10514182022216WaterContent-percentFigure27Influenceofsoilstructureonstress-deformationrelationshipsforsiltyclay[5]As is shown in figure 27, there is a marked difference in behaviour between the samplescompactedwetanddryof optimummoisturecontent.Inexaminingthefigure,oneshouldrealize that for engineering purposes, 4% strain is already a very large strain. This impliesthatfor engineeringpurposes especiallythebehaviourat small strain levelsisof importance.Sincethetestsweredoneonsampleswithsimilardensitiesandmoisturecontents,thedifference inbehaviourmust be attributedtothe differencein structure.Sinceflocculatedstructures develop at the dry side of optimum moisture content, one can state thatflocculated structures havea higherstrengthand a higher stiffness thandispersed structures.6.4Influenceof compactionmethodCompaction might be achieved by means of static, vibratory or kneading rollers. One mightexpect that also the type of compaction has an influence on the soil structure and so onshrinkage, swell and strength. Experiments have shown that this is indeed the case. It isshownthat kneadingcompaction hasamuchstronger tendencytoproducedispersedstructuresthanvibratorycompaction.Vibratorycompactionontheotherhandhasastrongertendencytoproducedispersedstructuresthanstaticcompaction.Experimentshoweveralsoshowed that the influence of the compaction method is especially apparent at the wet side of32
32 Figure 27 Influence of soil structure on stress – deformation relationships for silty clay [5]. As is shown in figure 27, there is a marked difference in behaviour between the samples compacted wet and dry of optimum moisture content. In examining the figure, one should realize that for engineering purposes, 4% strain is already a very large strain. This implies that for engineering purposes especially the behaviour at small strain levels is of importance. Since the tests were done on samples with similar densities and moisture contents, the difference in behaviour must be attributed to the difference in structure. Since flocculated structures develop at the dry side of optimum moisture content, one can state that flocculated structures have a higher strength and a higher stiffness than dispersed structures. 6.4 Influence of compaction method Compaction might be achieved by means of static, vibratory or kneading rollers. One might expect that also the type of compaction has an influence on the soil structure and so on shrinkage, swell and strength. Experiments have shown that this is indeed the case. It is shown that kneading compaction has a much stronger tendency to produce dispersed structures than vibratory compaction. Vibratory compaction on the other hand has a stronger tendency to produce dispersed structures than static compaction. Experiments however also showed that the influence of the compaction method is especially apparent at the wet side of
optimum.Whencompaction isdoneatthedrysideof optimummoisturecontent,there ishardlyanydifferenceinbehaviourof staticcompactedspecimenscomparedtovibratoryandkneading compactedspecimens.Whencompaction isdoneatthewetsideofoptimum,theinfluenceof thetype of compactionis very clear.7.Compaction of cohesive soilsIn the previous chapter attention has been paid to the effect compaction can have on thecharacteristics offinegrained,cohesive,soils.Sincecompactionhas suchalargeinfluenceonthe characteristics of soils, it is important to understand at least the principles of thecompactionprocess.Theseprinciplesaredescribedhereafter.A commonly used laboratory compactiontest is the Proctor test.Thistest is used todetermine the relationship between moisture content and the dry density of the materialgiven a particular amount of compaction energy.Typical examples of moisture-densitycurvesobtainedbymeansof theProctortestareshown infigure28.160.oo oan l.sonoMediumto fineSiltySondSMDo=0.026mmLL=16.5%Cu=16PI=OSpecificGrovityofGroins,G=2.680ur136.5Modified AASHO268-.-5625OFT-LBS/1CU.FT130.5Z13--........-----Standord Proctor12375FT-LB/CU.FTA120E"29MOISTURE CONTENT IN PERCENT OF DRY WEIGHTFigure28Moisture-densitycurvesofacohesivesoil fordifferentcompactionefforts[6]Figure28 showstwo curves,onebeingthe result of the standardProctortestand theotherbeing the result of themodified Proctor test. In the standard Proctor test, a 5.5Ib (2.5kg)rammer is dropped from a height of 12 inch (30 cm) on a layer of soil. In total three layersarecompacted,eachwith25blows.Thetotalcompactioneffort is12375ftIbs/ft3.Thesizeof the compaction mould is 1/30 ft3. The description given here applies to the standardProctortest accordingtoAASHTO and ASTM.In themodified Proctor test, theweight of thehammer as well as the falling height is increased. In total, five layers are used. This bringsthetotalcompactionefforton56250ftIbs/ft3(4.5kghammer,45cmfallingheight,25blowsper layer).For all fine grained, cohesive, soils, similar peaked moisture - density relationships areobtained.Figure 29 shows the principles of the standard and modified Proctor test.33
33 optimum. When compaction is done at the dry side of optimum moisture content, there is hardly any difference in behaviour of static compacted specimens compared to vibratory and kneading compacted specimens. When compaction is done at the wet side of optimum, the influence of the type of compaction is very clear. 7. Compaction of cohesive soils In the previous chapter attention has been paid to the effect compaction can have on the characteristics of fine grained, cohesive, soils. Since compaction has such a large influence on the characteristics of soils, it is important to understand at least the principles of the compaction process. These principles are described hereafter. A commonly used laboratory compaction test is the Proctor test. This test is used to determine the relationship between moisture content and the dry density of the material given a particular amount of compaction energy. Typical examples of moisture – density curves obtained by means of the Proctor test are shown in figure 28. Figure 28 Moisture – density curves of a cohesive soil for different compaction efforts [6]. Figure 28 shows two curves, one being the result of the standard Proctor test and the other being the result of the modified Proctor test. In the standard Proctor test, a 5.5 lb (2.5 kg) rammer is dropped from a height of 12 inch (30 cm) on a layer of soil. In total three layers are compacted, each with 25 blows. The total compaction effort is 12375 ft lbs/ft3 . The size of the compaction mould is 1/30 ft3 . The description given here applies to the standard Proctor test according to AASHTO and ASTM. In the modified Proctor test, the weight of the hammer as well as the falling height is increased. In total, five layers are used. This brings the total compaction effort on 56250 ft lbs/ft3 (4.5 kg hammer, 45 cm falling height, 25 blows per layer). For all fine grained, cohesive, soils, similar peaked moisture – density relationships are obtained. Figure 29 shows the principles of the standard and modified Proctor test
Figure29StandardProctortest(left)andmodifiedProctortest (right)[7].Figure30showsthreemoisture-densityrelationshipsobtained onthreedifferent soilswiththe same specific gravityOnehas tobeawareof thefact thatthemoisture-density relationships of manyresidualsoils is not uniquebut changes in waysdepending on themoisture content at the startof thecompactiontest.Thepropertiesofmanyof theseresidual soilschange irreversiblyondrying,sothatif a moist samplefromaborrowpitispredriedand itscompactioncurveisdeterminedbyaddingwater(thenormalprocedure),theresultisusuallyahigherdrydensityandloweroptimummoisturecontentthan iftheProctortestswereperformedstartingatthenatural moisture contentandadding watertoobtainpointson thecompaction curve.Theprocessthat results inthefamiliarpeaked compactioncurveisquitecomplex.It involvescapillarypressures,hysteresis,poreairpressure,permeability,surfacephenomena,osmoticpressures,andtheconceptsofeffective stress,shear strength,andcompressibility.It isbelievednowthatloosemoistsoil,whetherpreparedforcompaction inthelaboratoryorinthe field, consists of lumps of particles that are held together by effective stresses caused bycapillarity.Thedrierthesoil,theharderwill bethelumps.Thecompactionprocess attemptstodeformtheselumpsandmakethemcoalesce.Agivencompactioneffortwillbemoresuccessful in doing this when the lumps are softer, as when additional water is added, thanwhen the moisture content is lowand the lumps are hard. As the soil becomes more denseduringthecompactionprocess,it becomes more difficultto pressout theair fromtheremaining pores. With additional water the compaction process can no longer expel airefficientlyandtransientporeairpressurescandevelopthat resistthecompactioneffort.All inallitisbelievedthatthisresultsinthepeakedmoisture-densityrelationshipsthataresotypicalfor cohesivesoils.34
34 Figure 29 Standard Proctor test (left) and modified Proctor test (right) [7]. Figure 30 shows three moisture – density relationships obtained on three different soils with the same specific gravity. One has to be aware of the fact that the moisture – density relationships of many residual soils is not unique but changes in ways depending on the moisture content at the start of the compaction test. The properties of many of these residual soils change irreversibly on drying, so that if a moist sample from a borrow pit is predried and its compaction curve is determined by adding water (the normal procedure), the result is usually a higher dry density and lower optimum moisture content than if the Proctor tests were performed starting at the natural moisture content and adding water to obtain points on the compaction curve. The process that results in the familiar peaked compaction curve is quite complex. It involves capillary pressures, hysteresis, pore air pressure, permeability, surface phenomena, osmotic pressures, and the concepts of effective stress, shear strength, and compressibility. It is believed now that loose moist soil, whether prepared for compaction in the laboratory or in the field, consists of lumps of particles that are held together by effective stresses caused by capillarity . The drier the soil, the harder will be the lumps. The compaction process attempts to deform these lumps and make them coalesce. A given compaction effort will be more successful in doing this when the lumps are softer, as when additional water is added, than when the moisture content is low and the lumps are hard. As the soil becomes more dense during the compaction process, it becomes more difficult to press out the air from the remaining pores. With additional water the compaction process can no longer expel air efficiently and transient pore air pressures can develop that resist the compaction effort. All in all it is believed that this results in the peaked moisture – density relationships that are so typical for cohesive soils
1332r8ng63d54NASNEAHMOISTURECONTFigure30StandardProctorcurvesforthreesoilswiththesamespecificgravity[6]InrealitycompactionisofcoursenotrealisedbymeansoftheProctortestbutbymeansofrollers.Typical rollersthat areavailable arestatic rollers,vibratoryrollers and kneadingrollers.Static rollersaresteelwheel rollers.Kneadingaction isprovidedbyrubbertyrerollersand tamping or sheep foot rollers. A vibratory roller is either a steel wheel or a tamping rolleronwhichavibratorymechanismisattached.Figure31showssomerollertypes.Figure31aStaticpneumatictyreroller(left)andstaticthreesteelwheelroller[7]35
35 Figure 30 Standard Proctor curves for three soils with the same specific gravity [6]. In reality compaction is of course not realised by means of the Proctor test but by means of rollers. Typical rollers that are available are static rollers, vibratory rollers and kneading rollers. Static rollers are steel wheel rollers. Kneading action is provided by rubber tyre rollers and tamping or sheep foot rollers. A vibratory roller is either a steel wheel or a tamping roller on which a vibratory mechanism is attached. Figure 31 shows some roller types. Figure 31a Static pneumatic tyre roller (left) and static three steel wheel roller [7]
Figure31bSelfpropelledsingledrumstaticorvibratorysmoothsteeldrumrollerandtamping roller [7].Pneumatictyre rollers normallyhave7-11pneumatictyresand thetrack of therearwheeloverlapsthetrack of thefront wheels.Theyareballasted withsand orwaterand can haveaweightbetween10and35tons.Threesteelwheelstaticrollershavetwosteeldrivingdrumsandone.steeringdrum.They.areballastedwithwaterand.have.aweightrange.of8-15tons.The self propelled single drum rollers havethe possibilitytoact as a staticas well as avibratory roller. They are available in a weight range of 3 - i7 tons. Especially the tamping(alsocalledsheepfootorpadfootroller)isveryeffectiveonclays.Compactionof siltysoilsAswithallfinegrainedsoils,thecompactionofsiltysoilsverymuchdependsonthemoisturecontent.To achieve a good compaction,themoisture content should not diverge too muchfromtheoptimummoisturecontent.Atoptimummoisturecontent,siltysoilsarerelativelyeasytocompact.Athighwatercontentandundertheinfluenceofvibration,silttransformsinamoreorlessfluidmass.CompactionofclayAlso the compaction of clay very much depends on the water content. To achieve goodresults,thewater content should stay within± 2% of the optimumwater content.Asmentioned before, clay is difficult to compact when dry or wet. When dry, it might be neededtopulverisetheclayusinge.g.harrowdiscs especiallywhentheclayappearsinlargelumps.Pulverisation increases the effect of the added water on the compaction result.Vibratorysheepfoot rollers areveryeffectivewhen the clay isat thedryside orat optimum moisturecontent (when clay has its highest strength)asthey canprovide thehigh stresses needed tocompactthematerial.Themaximumlayerthicknessthatcanbecompactedisbetween2540 cmClay,withamoisturecontentaboveoptimumcanbecompactedwithpneumatictyrerollersor smoothdrum vibratoryrollers.Itcan becompacted in thickerlayersthan when dry ofoptimum.Vibration at high water contents however can result inamigration of moisture tothesurfaceresultinginunworkableconditions.Ouite oftenmultiplerollersareneeded whenhighcompactionlevels arereguires.First astatic tamping rolier might be applied on lifts with a thickness of 15-20 cm to precompactthe material and to crush the large lumps. Then vibratory rollers are used to achieve thedensity.Also smooth drum vibratory rollers might be used in this case since they seal thelayerwhichfacilitateswaterrunoffwhenitrains.Whenastructurewithalowpermeabilityis required, itisnot recommendedtouse smoothdrumrollers to finish the subsequent layers.Because of the smooth surface,the subsequentlayersmaynotadhereverywell toeachotherandthe interfacebetweenthelayersmightberather permeable.When applying vibration, the frequency of the vibration should preferably be between 25-30Hzwhiletheamplitudeshouldbebetween1.5-2mm.36
36 Figure 31b Self propelled single drum static or vibratory smooth steel drum roller and tamping roller [7]. Pneumatic tyre rollers normally have 7 – 11 pneumatic tyres and the track of the rear wheel overlaps the track of the front wheels. They are ballasted with sand or water and can have a weight between 10 and 35 tons. Three steel wheel static rollers have two steel driving drums and one steering drum. They are ballasted with water and have a weight range of 8 – 15 tons. The self propelled single drum rollers have the possibility to act as a static as well as a vibratory roller. They are available in a weight range of 3 – 17 tons. Especially the tamping (also called sheep foot or pad foot roller) is very effective on clays. Compaction of silty soils As with all fine grained soils, the compaction of silty soils very much depends on the moisture content. To achieve a good compaction, the moisture content should not diverge too much from the optimum moisture content. At optimum moisture content, silty soils are relatively easy to compact. At high water content and under the influence of vibration, silt transforms in a more or less fluid mass. Compaction of clay Also the compaction of clay very much depends on the water content. To achieve good results, the water content should stay within ± 2% of the optimum water content. As mentioned before, clay is difficult to compact when dry or wet. When dry, it might be needed to pulverise the clay using e.g. harrow discs especially when the clay appears in large lumps. Pulverisation increases the effect of the added water on the compaction result. Vibratory sheep foot rollers are very effective when the clay is at the dry side or at optimum moisture content (when clay has its highest strength) as they can provide the high stresses needed to compact the material. The maximum layer thickness that can be compacted is between 25 – 40 cm. Clay, with a moisture content above optimum can be compacted with pneumatic tyre rollers or smooth drum vibratory rollers. It can be compacted in thicker layers than when dry of optimum. Vibration at high water contents however can result in a migration of moisture to the surface resulting in unworkable conditions. Quite often multiple rollers are needed when high compaction levels are requires. First a static tamping roller might be applied on lifts with a thickness of 15 – 20 cm to precompact the material and to crush the large lumps. Then vibratory rollers are used to achieve the density. Also smooth drum vibratory rollers might be used in this case since they seal the layer which facilitates water run off when it rains. When a structure with a low permeability is required, it is not recommended to use smooth drum rollers to finish the subsequent layers. Because of the smooth surface, the subsequent layers may not adhere very well to each other and the interface between the layers might be rather permeable. When applying vibration, the frequency of the vibration should preferably be between 25 – 30 Hz while the amplitude should be between 1.5 – 2 mm