Secondary Consolidation And The Effect Of Surcharge Load - IJERT
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 4 Issue 11, November-2015Secondary Consolidation and the effect ofSurcharge LoadThuvaragasingam BagavasingamUniversity of MoratuwaColombo, Sri LankaAbstract— This paper analyses the behavior of soft claysduring consolidation process. The difference in rate ofconsolidation between primary and secondary is identified byOedometer test as per the Buisman’s model. Peat sample takenfrom Southern Highway project (Colombo, Sri Lanka) is usedfor laboratory tests. The rate of secondary consolidation isdiscussed in detail, with laboratory tests to derive relevant soilparameters.Other major section involved in understanding the impact ofover consolidation, to interpret the real world application ofpreloading for ground improvement. The laboratory test modelsconsidered are: stress- strain curve for loading-unloadingreloading sample and B’Jerrum Graphs model. Results arepresented in diagram to quantify the reduced rate inconsolidation for over consolidated soil samples. Further,correlation between consolation rate and Over Consolidation(OCR) are discussed in detail with aid of Mersi & Godlewskimodel.Keywords—Secondary Consolidation;Preloading; B’JerrumGraphs; OCR;I.Buismanmodel;INTRODUCTIONSecondary consolidation is a more complex progressoccurs due to the effect of creep in soft soil. This creep actionis more complex to model as the real reason for the soilparticles’ arrangement is unknown or not very obvious.Therefore, this is a research area has lot of scope to observethe properties behind this natural occurrence and themethodologies available to restrict the effect of them.Terzaghi stated that, "consolidation is any process whichinvolves decrease in water content of a saturated soil withoutreplacement of water by air". Initially when a load is appliedon the soil, the effective pressure increase will be balanced bythe pressure increase in pore water. Due to the highersurcharge pressure, water tends to overcome its intermolecularattraction and escape from these voids. Thus with thedissipation of water, the volume reduces.u – pore water pressure, t – time, z – depth, k – permeabilityof soil, M – compressibility of soil and γw - unit weight ofwater.The model predicted consolidation to be a function ofexcess pore water pressure thus ultimate consolidation takesplace with the dissipation of excess pore water. But later theobservations show that soft clays undergo significantsettlement in absence of excess pore water. Therefore the useof equation (1) is limited since it ignores the intrinsic timedependent creep effect of the soil.This phenomenon known as secondary consolidation takesplace due to the plastic movement among soil particles.Though there isn’t any clear description, to gain more stablestructure these soil particles are believed to be rearrangingwithin themselves. Terzaghi (1941) and Taylor (1942)explained secondary consolidation as the readjustment ofgrains delayed by the gradual transfer of stress from film tograin bond. The basic assumption for this mechanism is thatwhen a soil element is loaded, the total stress is shared bypressure in the free pore water, the viscous resistance withinabsorbed soil (film bond) and the bond between soil particles.During secondary consolidation, where the pore water isnegligible, the total stress is shared by the film and grain bond.The pressure on the film gradually transferred to the grainbody, and this transferring process is associated with veryslow viscous flow. When the equilibrium state is reached, theapplied load will be supported solely by grain bonds.II.LITERATURE REVIEWA. EOP & Secondary Consolidation Coefficient (Cα)In 1936 Buisman identified secondary consolidation tooccur in an approximate linear pattern, when he plottedchange of strain against logarithmic time for an oedometertest.In the first ever classical theory on consolidation, Terzaghiquantified ted the one dimensional consolidation by followingequation (1),. (1)Where coefficient of consolidation,Fig 1 : Buisman's Oedometesr test modelIJERTV4IS110642www.ijert.org(This work is licensed under a Creative Commons Attribution 4.0 International License.)695
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 4 Issue 11, November-2015The end of primary consolidation (EOP) is taken as thepoint where gradient of both slope intersects as shown in Fig 1(tp). The gradient of trend under secondary consolidationdefined as secondary consolidation coefficient Cα. Later in1968, Walker and Raymond found that Cα appeared to belinearly dependent on Cc, the compression index of the soil.The Cα/Cc factor explains and predicts the secondaryconsolidation behavior of geotechnical materials with andwithout surcharging ( Mersi and Godlewski 1977, 1979). Thisconcept is applied to laboratory tests and to determine thesettlement of secondary consolidation (Mersi 1986). WhenCα/Cc is less, the secondary consolidation effect would beless.stress, rather dependent on the over consolidated ratio that thesoil experienced.B. B’Jerrum Graphs and OCRFor a long time, it is believed that creep and dissipation ofpore water occur in two different phases which are namelydiscriminated as primary and secondary consolidation. But in1957, Suklje described that creep is a continuous process thateven occurs in the primary consolidation phase and presenteda isotache model – the unique relationship between effectivestress, void ratio and strain rate.B’Jerrum presented a model on 1967, similar to Suklje’smodel assuming that primary consolidation and creep strainsare not divided into separate processes. The model representsthe pre-consolidation pressure or over consolidation ratio ofvirgin clays, resulting from geological ageing. The clay whichis under same effective stress, for a longer time yields moresettlement (i.e. less void ratio). This shows the timedependency of the secondary consolidation and itsindependence from effective stress. B’Jerrum uses varioustime lines to explain how reduced creep rates resulting fromthe increased duration of loading.Fig 3 : Cα vs σv/σp (Walker 1969)The dissipation of pore water during secondaryconsolidation leads to a more stable configuration of thestructure. Such clay can support an additional load in excessof the effective overburden pressure without any significantvolume change produced by slippage of the contact points.Thus, for additional loads smaller than a certain critical valuethe clay will behave similarly to an over consolidated clay andthe instant settlements will be limited to an elasticcompression. In the above figure 3, the Walker identified thatCα is maximum when effective stress is slightly higher thanpre-consolidation stress. This observation is later proved inmany other research works. But the significant observation inabove trends is the decline in Cα on the side where preconsolidation stress higher than the present effective stress,the phenomenon known as over consolidation.Over consolidation rate (OCR) is defined as ratio betweenhighest historical effective stress and present effective stress,on a sample. Mersi & Stark (1997) develop a model toillustrate how the post surcharge rate of secondaryconsolidation (Cα”) with reference to its rate of secondaryconsolidation (Cα) changes in the pre - consolidated soil. Herethe Rs’ is given as (σ’vs/σ’vf – 1) which is basically theresidual OCR. Therefore it’s proven that effect of secondaryconsolidation significantly reduces with historical loading.Fig 2 : B'Jerrum Graph (1967)For the change of stress Δp, εt denotes the total straincomprises the effect of instant and delayed compression; εidenotes the strain only due to instant compression. Thisexplains the effect of surcharge on soil and gives an idea ofthe age of load to achieve desired void ratio.Also another important observation by Walker (1969)exposed that the Cα of a soil varies according to the ratiobetween the effective stress and effective stress at the end ofpre-consolidation (σp). (Fig 3). Later Mersi andGodlewski(1977) found that Cα is not a function on effectiveIJERTV4IS110642Fig 4 : Secondary settlement with surcharge, Mersi & Stark (1997)www.ijert.org(This work is licensed under a Creative Commons Attribution 4.0 International License.)696
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 4 Issue 11, November-2015III.EXPERIMENTS AND OBSERVATIONSObjective of laboratory experiments are to verify theclassical models of Secondary consolidation and the effect ofsurcharge. The laboratory tests are involved in tracking downthe strain variation for samples with different loading, usingthe Oedometer. The theories and tests for secondaryconsolidation, based on the assumption of one dimensionaldrain and settlement, which makes Oedometer the idealsimulation of ground conditions.For 5kN/m2 sample, the consolidation is hardly observablewhich leave higher amount of water content to be residual inthe sample. This explains the importance of preloading withsignificant surcharge. Clear difference in instant and delayedsettlement is apparent in higher loaded samples.In order to derive B’Jerrum graphs , the void ratio atspecific time is extracted from each individual graph (Fig 5)and plotted against the stress applied on respective sample.The peat samples are collected from the SouthernHighway project, near Piliyandala (the highway connectSothern district Matara, to Colombo). This sample is thencleaned, free of stones, organic matters and other largeaggregates and then mixed well using beaker. As the samplewas underground water for ages, the water content was higherfor our practical purpose. Therefore the sample was left to dryfor some degree until the optimum moisture content isobtained.Fig 5 : Peat Sample as it's taken from the site & the Oedometer Testing inprogressA. Experimental study to obtain B’Jerrum graphFour samples observed under different loading (5kN/m2,7.5kN/m2, 10kN/m2 & 15kN/m2) , and the change in void ratio(settlement) with time is recorded. Readings are taken for 2weeks.Fig 6 : B'Jerrum Graph for the peat samplesSince the time interval between readings was trivialcompare to the time required for significant change in voidratio, the graphs appear to be congested. But still thedifference becomes noticeable with the high values of stressapplied.B. Determination of correlations between OCR and Cα/Cα'To examine the effect of secondary consolidation on overconsolidated soil, 4 numbers of samples are implied withdifferent equilibrium stress (σs) and OCR. The loading,unloading and re – loading trend for samples are as follow,Sample A0 10 20 40 80 88 96.8 106.8 96.8 88 80 88 96.8 106.8(kN/m2)Sample B0 10 20 40 80 120 120 180 270 180 120 80 120 180 270 (kN/m2)Fig 5 : Void ratio vs time trens for all samples in same axesSample C0 5 10 20 40 44 48.4 53.2 48.4 44 40 44 48.4 53.2(kN/m2)Sample D0 5 10 20 40 60 90 135 90 60 40 60 90 135(kN/m2)IJERTV4IS110642www.ijert.org(This work is licensed under a Creative Commons Attribution 4.0 International License.)697
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 4 Issue 11, November-2015Fig 8 : Cα’/Cα vs OCRThe general observation of ratio Cα’/Cα reduces with theincrease in OCR, satisfies the theory of Mersi & Stalk’s modelon soft clays under surcharge load. Further when consideringthe rate of decline in Cα’/Cα , the maximum rate is achieved inspecific OCR for each equilibrium stress. For 80kN/m2sample, the maximum rate is at 1.27 and for 40kN/m2 is at1.65. This phenomenon shows that Cα’ depends on theeffective stress at equilibrium. Finally this test explains theeffect of historical load on Secondary consolidation.IV.CONCULSIONFrom the lab tests it’s obvious that secondaryconsolidation is the major way of dissipating water in softclays and peats where the water content is enormously largercompare to construction lands. When referring to the voidratio vs. time graph, primary consolidation in these soilsoccurs rapidly due to the higher compressibility of the soil andthe pore water dissipates rapidly. During the lab tests, the peatsamples achieved EOP within 10 to 30 minutes. Though thelab simulation only accounts for one dimensionalcompression, it compensates the loss of radial flow byproviding more convenient drainage to flow. In contrary thewater needs to travel through much longer drainage path.After EOP (in e vs. log (time) diagram), the secondaryconsolidation appears to occur in a constant rate. This explainswith the fact that Cα/Cc will remain specific unless the soilproperties are disturbed. Further in detail it’s found out thatCα/Cc changes with the loading arrangements of the samples.Referring to the literature review, Cα is also a time dependentfactor, given that the considered time span is very longduration. However this wasn’t been a critical issue in theaspects of geotechnical engineering, since however by meansof ground improvement techniques the value of Cα will beminimized. And therefore, the change in Cα over the longspan of time can be neglected.Fig 7 : Over consolidated peat samples’ Stress vs Strain graphsEach loading was kept for 3 days and individual Stress VsStrain diagram was plotted to derive the Cα and Cα’ values ofthe specific loading. Over consolidated peat showed a sharpdecline in consolidation rate. These consolidation ratesderived from each graph, for the loading phase (Cα) and there-loading phase (Cα’) are plotted to according OCR for eachsample as shown in Fig 8.IJERTV4IS110642Also another important characteristic which has to beentaken under consideration is the surface drying effect of thesoft clays and peat. In order to prevent this effect duringexperiment, the sample is kept saturated. When in field groundimprovement procedure, it’s vital to ensure that the surface issaturated to facilitate the load distribution along the soilprofile.www.ijert.org(This work is licensed under a Creative Commons Attribution 4.0 International License.)698
International Journal of Engineering Research & Technology (IJERT)ISSN: 2278-0181Vol. 4 Issue 11, November-2015The results obtained from B’Jerrum graphs, shows that bypreloading the instant settlement (as quoted by B”Jerrum) onsoil can be reduced. Theoretically the graphs show that aconstant load which is applied for a considerable time wouldaffect the elastic compressibility of the soil. It explains thathow the effects the instant settlement and time dependentsettlement varies. It’s clear that while increasing the stress theinstant settlement will be higher and sudden. But when aconstant stress applied for a considerable time, the instantsettlement caused by the increase in stress would be lesser.This shows how the history of load on a soil affects its internalstructure. Under the long term surcharge of load, soft claysovercome their weak bonding in between water particles andrearrange themselves with more soil particles’ interaction.This ideology is easily explainable with the aid of obtainedB’Jerrum model. But generally the practical application of thismodel is limited since B’Jerrum graphs doesn’t account forfactors affecting secondary consolidation. Therefore B’Jerrummodel serve as the fundamental to understand the effect ofsurcharge loading.From the the graphs plotted for soil undergoing series ofloading, unloading and reloading, the significant significantreduction in the rate of compression on a preloaded soil isshown. The peat which experienced a higher load than presentapplied load seems to have a compact soil structure tominimize the creep effect. But from the results obtained fromthe graphs plotted for Cα'/Cα vs. OCR shows that thisreduction depends on the Over Consolidation Rate and thestress at which soil was in equilibrium (ie. after surchargeloading). These results show the great potential of thismethodology has in practical aspect.In a real life construction environment where soft clay/peat is met , it’s possible to improve the ground strength byderiving the ideal loading pattern by choosing the optimumOCR where the least void ratio can be achieved and theappropriate surcharge loading.ACKNOWLEDGMENTFirstly, I would like to express my heartfelt gratitude andadmiration to my supervisor for this research, Prof. S.A.S.Kulathilaka (Department of Civil Engineering, University ofMoratuwa). His valuable advices and progressive guidancehelped me to carry out the research in correct path toaccomplish this task with great passion and commitment.Also I would like to thank Prof N.T.S Wijesekara(Department of Civil Engineering, University of Moratuwa)for coordinating and providing me with the best facilities tocarry out the research 642G.Mersi , T.D. Stark, M.A.Ajlouni and C.S.Chen, “Secondarycompression of peat with or without surcharging”, in Journal ofGeotechnical and Geoenvironmental Engineering, Vol. 123, No. 5,May 1997, pp. 411-421BJerrum, L. , “Engineering geology of Norwegian normallyconsolidated marine clays as related to settlements of buildings”, inGeotechnique, 1967, No.17, pp.81-118.Garlanger, J.E. , “The consolidation of soils exhibiting creep underconstant effective stress”, in Geotechnique 22, 1972, No.1, pp.71-78.Costas A. Anagnostopoulos and Innonis N. Grammatikopoulus, “ Anew model for the prediction of secondary consolidation index of lowand medium plasticity clay soils”, in European Journal of ScientificResearch, ISSN 1450 – 216X Vol.34 No.4, 2009, pp.542-549.Berre,T. and Iversen,K. , “Oedometer tests with different specimenheights on a clay exhibiting large secondary compression”, inGeotechnique 22, No.1, 1972, pp.53-70.www.ijert.org(This work is licensed under a Creative Commons Attribution 4.0 International License.)699
The model predicted consolidation to be a function of excess pore water pressure thus ultimate consolidation takes place with the dissipation of excess pore water. But later the observations show that soft clays undergo significant settlement in absence of excess pore water. Therefore the use
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n Stress-Strain in Consolidation n 1-D Consolidation Test . การทดสอบแบบ Oedometer (Consolidation) D e fo r m at i on Time (Log scale) Initial consolidation Primary consolidation �ัวคายน้ํา Secondary consolidation.
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