1. INTRODUCTIONThe overconsolidation ratio (OCR) is a geotechnicalparameter, related to historical changes in the state ofstress in the subsoil. The concept of overconsolidationratio was proposed by Cassagrande [3]. The idea ofrecording the overconsolidation ratio resulted fromthe observation of changes in strength parameters ofnon-lithified deposits, depending on the state of stressfound in the subsoil. An example illustrating thisdependence is an increase in shear strength of adeposit homogenous in terms of its physico-chemicalproperties with an increase in depth. Under such con-ditions determined strength parameters of a givendeposit, higher than expected, were thus considered to

be connected with a different state of stress in the sub-soil than that generated as a result of normal consoli-dation of the deposit. It needs to be emphasized thatconsolidation is considered to be normal if it resultsfrom a successively increasing load, affecting thedeposited sediment at a specific depth. A good exam-ple illustrating the process of normal consolidation isfree sedimentation of glacilacustrine deposits, notundergoing erosion processes. The overconsolidationratio OCR was defined by Casagrande as a ratio ofmaximum effective value of the vertical component ofgeostatic stress, found at any time in a given subsoilpoint, to the present effective value of the verticalcomponent of geostatic stress.

ANALYSIS OF CHANGES IN OVERCONSOLIDATION RATIO IN SELECTEDPROFILES OF NON-LITHIFIED DEPOSITS

Jędrzej WIERZBICKI *

* Dr; Department of Geotechnics, University of Life Sciences, Poznań; Piątkowska 94, 61-691, Poznań, PolandE-mail address: jw@au.poznan.pl

Received: 21.01.2009; Revised: 01.06.09; Accepted: 20.06.09

A b s t r a c tThe changes in overconsolidation ratio OCR under different geological conditions are analyzed in the paper. The applica-tion of modern subsoil testing methods, dilatometer tests (DMT) and cone penetration test (CPTU), made possible almostcontinuous observation of changes in overconsolidation ratio with depth. This fact allowed observing regularities and irreg-ularities in stress history of investigated deposits. Some of these changes in stress were interpreted as consecutive stages ofsedimentation and erosion, which made possible to find different stages of sedimentation and stratigraphic gaps along test-ed profiles. The paper points out the potential application of DMT and CPTU tests in geological analyses, particularly thoseconnected with the analysis of stratigraphy of Cainozoic sediments.

S t r e s z c z e n i eW artykule przedstawiono analizę zmian wartości wskaźnika prekonsolidacji (OCR) gruntu, w zależności od geologicznychwarunków występowania. Zastosowanie w badaniach nowoczesnych technik in-situ, sondowania statycznego CPTU i bada-nia dylatometrem płaskim (DMT), umożliwiło niemal ciągłą identyfikację wartości OCR wzdłuż analizowanych profili.Możliwość ta pozwoliła na obserwację pewnych nieregularności w przebiegu historii obciążenia podłoża. Niektóre z tychnieregularności zostały zinterpretowane jako przejaw następujących po sobie procesów sedymentacji i erozji. Takiespostrzeżenie pozwoliło na bardziej pełne odtworzenie stratygrafii badanych osadów. W pracy podkreślono możliwości jakiedaje zastosowanie nowoczesnych badań in-situ (CPTU i DMT) w analizie historii geologicznej podłoża zbudowanego z keno-zoicznych gruntów nieskalistych.

K e y w o r d s : OCR (overconsolidation ratio); In situ test; Stress history.

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However, the OCR index does not always unambigu-ously reflect changes in strength parameters, generat-ed as a consequence of the overconsolidationprocess. This results from the effect of severalpostsedimentation geological processes on thebehavior of the subsoil [5]. In relation to theseprocesses we should rather talk about the effect ofoverconsolidation, which is a wider concept andrefers to all postsedimentation transformations in thedeposit [23].The paper presents several examples of interpreta-tion for results of geotechnical analyses, facilitatingthe assessment of the overconsolidation ratio. Theselected deposits include both those which may defi-nitely be considered normally consolidated in thestrict sense of the term and those which may havebeen subjected to historical overload, i.e. shouldshow distinct symptoms of overconsolidation. A com-bination of results of geotechnical analyses with avail-able geological knowledge in certain cases made itpossible to find traces of past geological processesand to attempt and recreate their consequences.

2. METHODOLOGY OF THE ANALYSISThe application of information related to the over-consolidation ratio is closely connected with themethodology of its determination. An extensivereview of research methods facilitating the assess-ment of OCR was presented in his studies byWierzbicki [23]. Generally the overconsolidationratio may be determined by means of both laborato-ry and in situ methods. The application of laboratorymethods requires the recreation of the state of stressfound in nature, but it also makes it possible to simu-late such conditions which historically could haveaffected the analyzed deposit [9]. In turn, in situmethods facilitate the determination of strength anddeformation parameters in the state of stress foundin situ. The main problem connected with laboratorytesting is the fact that it is difficult to collect a samplewith an undisturbed structure. Collecting a high qual-ity sample, especially at lower depths, is hardly feasi-ble [6]. In certain deposits even samples of high qual-ity lose the properties they had in the subsoil [8]. Inrecent years numerous in situ testing methods havebeen developed, facilitating e.g. the assessment ofOCR, among which special interpretation potential isascribed to cone penetration test [10] and dilatome-ter [11] [17]. The primary advantage of these tests isthe analysis of deposits found in the natural state. Incase of non-cohesive deposits, in which collecting a

sample with a satisfactory quality is practically impos-sible, OCR assessment until recently was preformedonly by analogy with the neighboring cohesivedeposits. The development of the dilatometer test inthe 1980’s for the first time in history made it possi-ble to assess OCR of such deposits [11]. In turn, theapplication of a more universal cone penetration testto assess the overconsolidation ratio was made possi-ble by interpretation proposals given by Mayne andKulhawy [14] and Mayne [13], supplemented byWierzbicki [23].For a correct interpretation of CPTU and DMT test-ing results it is also required to take into considera-tion basic grain size parameters and the state of theanalyzed deposit [23] [24]. Thus, for each testing sitesupplementary analyses of grain size compositionwere performed and parameters describing the stateof soil found in the subsoil were determined. Due tothe point wise character of laboratory measurements,not corresponding to the frequency of measurementstaken during DMT and CPTU testing, the value ofoverconsolidation ratio was given only for the depthsfrom which samples were collected or for the entireprofile, assuming results of laboratory analyses asrepresentative for individual layers.In the assessment of OCR on the basis of DMT themethodology proposed by Marchetti [11], andSchmertmann [19] [20] was applied. In case of bothcohesive and non-cohesive deposits these methodsare based on index KD established on the basis ofDMT results. When investigating non-cohesivedeposits also effective internal friction angle of soil isused to assess OCR.In turn, methods developed by Sully et al. [21],Mayne [12] as well as Powell et al. [18], including alsoremarks by Szymański et al. [22], were used for thedetermination of OCR in cohesive soils on the basisof CPTU tests.The method developed by Sully et al. [21] requiresthe performance of cone tests with possibility to mea-sure “double” pore pressure (CPTUU). Tests of thistype were performed only in one of the analyzed test-ing sites. As it was shown by Wierzbicki and Wołyński[26] and Wierzbicki [23], precise assessment of theoverconsolidation ratio in non-cohesive depositsrequires, among other things, also the application ofa special conical sleeve part unique for CPT, i.e. adilatocone (DCPT) (equation 1). Since this device isnot currently available, some analyses were also con-ducted on the basis of a formula, using the measure-ment of normalized cone resistance – Qt – as well asparameters of state and grain size characteristics of

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the analyzed soil (equation 2).

where: ID – relative density in decimal; �’ – drainedfriction angle; fPr - percentage content of coarse sandgrains; fPs – percentage content of medium sandgrains; Qt – normalized cone resistance; Qd – nor-malized dilato resistance; fs – sleeve friction resis-tance [MPa].At one of the testing sites in which undisturbed sam-ples were collected using the MOSTAP sampler,oedometer tests were also performed using the CRSmethod, defining OCR on their basis. Interpretationprocedures for compressibility curves proposed byBecker et al. [2] were used for this purpose.

3. TESTING SITESThe study presents results of tests performed in threesites characterized by different geological structure.Sands deposited at the mouth of the Drammen Riverin Norway, in literature sources referred to as theHolmen sands (from the name of the island which isformed of them) were selected as an example ofdeposits considered to be normally consolidated.Based on archive tests and available literature [4] [7],the geological profile may be presented (Fig. 1). Thedeposits between 22 and 28 meters are composed pri-marily of silty sands and fine and medium-grainedsands. It is assumed that these deposits represent thefirst stage of fluvial sediment deposition of theDrammen River. The highest part of the profile ofnatural deposits, between 2 and 22 m below groundlevel, is formed from medium- and coarse-grainedsands. It is thought that this part of the deposits con-stitute the second series of the alluvial cone, corre-sponding to the later stage of their formation.The profile, at which both normally consolidated andoverconsolidated sediments were expected, may bedeposits in Toporów in the Lubuskie province,Poland. The testing site was located within so-calledPliszka outwash plain. The area of the Pliszka out-wash plain was described in detail by Żynda [27].Outwash deposits are composed primarily of sandsand gravels, occasionally of loams. These deposits layon top of older overconsolidated sands, the so-called

inter-moraine sands. However, studies conducted byWierzbicki [22] showed that, in accordance with theobtained profile to the depth of 20 m (Fig. 1),between upper and lower sandy deposits the series ofvaried clays has been deposited.The third testing site selected for analysis was thearea around the town of Włocławek. According toarchival data [1], the analyzed area is found withinthe outcrop of fluvioglacial sands of the Pomeranianphase of the Vistulian (Würm, Wisły) glaciation.These deposits, of several meters in thickness, filldepressions eroded in older glacial or fluvioglacialsediments, genetically connected with Riss (Saale,Odry/Warty) glaciations. Younger fluvioglacialdeposits may occur both on moraine deposits anddirectly on older, overconsolidated fluvioglacialdeposits. Numerous, presently fixed dunes are foundon the surface of the ground in the vicinity of the test-ing site. Results of performed preliminary testing [15]showed that sands stratified with fine-grained withadmixtures of clayey sands at the testing site from thesurface medium-grained sands are found, (Fig. 1).These deposits lay on a layer of compact silty clay,with numerous silty laminae, which may be consid-ered glacilacustrine deposits. In turn, cohesivedeposits lie on a series of interbedding medium- andfine-grained sands. Following the adopted geologicalmodel, the lower series of non-cohesive deposits maybe considered deposits overconsolidated by the con-tinental ice-sheet.

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(2)

Figure 1.Subsoil profile of the test sites, along with basic static pene-tration characteristics (DR – relative density, LI – liquidityindex, qc – cone resistance, Rf – friction ratio)

c

(1)

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4. RESULTS AND DISCUSSIONThe determined overconsolidation ratio for theHolmen deposits is presented in Fig. 2. Apart fromthe OCR value, defined following the adoptedmethodology, a graph was also presented showingchanges with depth in the value of the hypotheticaloverconsolidation ratio. Hypothetical values wereestablished with the assumption that the effect ofoverconsolidation was caused by geostatic load, act-ing in such a way that at the depth of 2 m the over-consolidation ratio was approx. 3. Such an OCRvalue was assumed as rational, ascribed byresearchers to the Holmen sands [25]. For this reasonthe plot of hypothetical OCR values may serve as areference point during the analysis.When observing data presented in Fig. 2, we need tostress a similar trend of OCR changes with depth,observed irrespective of the adopted researchmethod. A decrease in the overconsolidation ratiowas comparable with the hypothetical plot to thedepth of approx. 16 m. A slightly more rapid drop invalues measured in the upper and lower part of theprofile may be explained by the imperfect characterof the adopted geostatic model of load and the onlypartially known effect of the decay of the verticalcomponent of stress on the present value of the hor-

izontal component of stress. High OCR values foundon the basis of DMT and DCPT results between 2and 4 m of the profile are markedly different fromthose determined on the basis of measured Qt

(CPTU) and from the course of the hypotheticalcurve. The observed difference may be affected bythe effects of earth work performed during the rede-velopment of the island surface or changes in depositstructure, which to a different degree affects resultsobtained by means of individual methods [16]. Thiszone constitutes at the same time the zone of groundwater table, subjected to natural changes, related tofluctuations in the sea level. Thus it seems likely thathigh OCR values in this part of the profile are theresults of changes in ground water level and relatedchanges in run-off pressure. These changes may leadto generation of the effect of quasi-overconsolidationand increase in the value of vertical component ofgeostatic stress [16]. In relation to the discussed partof the profile deposits lying at a depth below 16 m aremarkedly different. In this part of the profile anincrease is observed in the overconsolidation ratio,whose value exceeds 2. Although the quantitativechange is not considerable and still it is difficult toclassify these deposits as definitely overconsolidated,results of analyses indicate unambiguously a changein subsoil geotechnical parameters in this zone.Based on the conducted investigations a hypothesis

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Figure 2.Changes in overconsolidation ratio (OCR) along the Holmensands profile (NC – normally consolidated, OC – overconsol-idated)

Figure 3.Changes in overconsolidation ratio OCR recorded inToporow (NC – normally consolidated, OC – overconsoli-dated)

ANALYS I S OF CHANGES IN OVERCONSOL IDAT ION RAT IO IN SE L ECT ED PROF I L ES OF NON- L I TH I F I ED DEPOS I TS

may be proposed that deposits found in the analyzedprofile represent two stages of deposition, i.e. theolder, whose deposits are consolidated to a higherdegree and the younger, exhibiting in the surfacezone definite symptoms of overconsolidation. Theobserved effect of overconsolidation in youngerdeposits might be caused primarily by the erosion ofa layer of sediments of several meters in thickness,resulting from the isostatic relative seal level fall. Inturn, a local increase in OCR recorded in the zoneadjacent to the ground water table results most prob-ably only from the contemporary fluctuations in thesea level.Analysis of testing results in Toporów requires a jointpresentation of values of the overconsolidation ratiodetermined in cohesive and non-cohesive deposits(Fig. 3). When analyzing OCR changes in the profile,its distinct division into two parts is clearly seen: theupper part to the bottom of cohesive deposits foundat the depth of approx. 15 m and the lower one,including non-cohesive deposits found below thisdepth. In the upper part a characteristic drop isobserved in OCR values with depth. For comparisoncalculated values were presented in Fig. 3 togetherwith hypothetical values calculated on a similar basisas in case of the Holmen sands. In view of the hypo-thetical values it may be observed that to a certaindegree mean OCR values, calculated by means of dif-

ferent methods, reflect the expected trend forchanges in the overconsolidation ratio. Such a findingsuggests a thesis on uniform mechanical overload, asa cause for the effect of overconsolidation in this partof the profile. Rather rapid increments in OCR,observed in the roof parts of the series of deposits –both cohesive and non-cohesive, may be in turnexplained by the effect of drying out in the surfaceparts of these deposits. In case of the non-cohesiveseries it is connected with contemporary changes inthe level of ground waters. In turn, in case of thecohesive series it may be assumed that the observedeffect results from the stratigraphic discontinuity andchanges in environmental conditions between thedeposition of the cohesive series and the beginning ofthe accumulation of non-cohesive deposits. Thelower part, composed solely of non-cohesivedeposits, differs markedly from the upper part of theprofile, discussed above. Values of the overconsolida-tion ratio determined in this part of the subsoil arevery high and show unambiguously that depositsfound there are strongly overconsolidated. This con-clusion is confirmed by all three applied testing meth-ods. A characteristic decrease in OCR value withdepth is also observed in this case, which confirms theassumption on the mechanical overconsolidation ofthe deposit. Determined values of the overconsolida-tion ratio, on average approx. 30, may not beexplained by changes in the ground water level, dry-ing out or ageing of the deposit [5]. The only twoprocesses, which may lead to such a marked overcon-solidation effect, are overload and cementation.When analyzing the undisturbed sample collectedfrom that depth no effect of cementation was found,which would considerably increase the cohesion andinternal friction angle of the analyzed sediment mate-rial [23]. Thus mechanical overload – found on alarge scale – remains an option in this respect.Results obtained in the area of Włocławek exhibiteda certain similarity to the profile in Toporów. At thattesting site subsoil may also be divided into two parts(Fig. 4). In the upper part of the profile, composedboth of non-cohesive and cohesive deposits, amarked overconsolidation effect is observed with itscourse homogenous for both types of deposits. Alsoin this case the distribution of calculated hypotheticalvalues of OCR suggests the adoption of the effect ofmechanical overload of the ground surface as thesource of overconsolidation of the profile. The lowerpart of the profile differs markedly from its upperpart. Deposits lying below the bottom of cohesivesoils exhibit a higher degree of overconsolidation,

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Figure 4.Changes in overconsolidation ratio OCR recorded in thetesting site in the Włocławek site profile (NC – normally con-solidated, OC – overconsolidated)

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which undergoes only slight changes with depth. Inthis respect these deposits do not behave as typicalmechanically strongly overconsolidated deposits, orat least not those whose overconsolidation resultssolely from a change in load. Such an almost homoge-nous OCR distribution is characteristic for lower partof subsoil, being mechanically overconsolidated, orfor deposits subjected to the action of run-off pres-sure with a similar gradient along the profile orcemented deposits. Unfortunately, the scope of theconducted laboratory analyses did not make it possi-ble to find a possible effect of cementation. However,indirectly the values of the degree of consolidation,characterizing the analyzed deposit, do not prove itscementation. Sands with a high degree of consolida-tion and at the same time cemented should exhibitmuch bigger cone resistance values during cone pen-etration test than those recorded here. In turn, theassumption of consolidation by overload of theground surface requires the adoption of a thesis thatdeposits presently located at the depth of approx. 6 mwere in the past covered by deposits not only severaltimes thicker, but also strongly overconsolidated.

5. CONCLUDING REMARKSConducted analyses facilitated an assessment ofchanges in overconsolidation ratio OCR under dif-ferent geological conditions. The application of mod-ern subsoil testing methods, dilatometer tests andcone penetration test, made possible an almost con-tinuous observation of changes in overconsolidationratio with depth.Results obtained by means of different methods in allthe three testing sites discussed in the study were sim-ilar. A characteristic element of this assessment isthe fact that in spite of certain differences in absolutevalues of the overconsolidation ratio, obtained usingdifferent methods, the recorded course of OCRchanges along the profile was very similar in all cases.This shows that if OCR values themselves may differfrom the unknown actual values of the overconsoli-dation ratio, in the course of the investigations char-acteristics of changes in the degree of deposit over-consolidation are observed to be clearly consistent.When interpreting the testing results it may be use-

ful to apply the graph of hypothetical overconsolida-tion ratio, which indicates the distribution of OCRvalues with the assumption of influence of onlymechanical overload of subsoil on the effect of over-consolidation. A comparison of calculated valueswith hypothetical values showed that only in some

cases determined overconsolidation ratio may betreated as consistent with that resulting from subsoiloverload. Processes affecting deposit overconsolida-tion may overlap, enhance recorded effect, especiallyin surface parts of subsoil. These processes mayinclude changes in ground water level and run-offpressure they caused, cementation or reconstructionof structure as a result of deposit creep.An especially valuable piece of information is possi-bility to identify and indicate in subsoil successivephases of deposit accumulation, separated by strati-graphic gaps. This possibility results from the factthat deposit subjected to the action of different geo-logical processes records environmental changes,which is manifested by changes in specific geotechni-cal parameters. However, such an interpretation ofthe OCR profile requires considerations on possibil-ity of individual processes affecting the depositedsediment in the past.

ACKNOWLEDGEMENTSPreparation of this paper was possible thanks to thefinancial and professional commitment of numerousindividuals and institutions. The author would like toexpress his special thanks to Professor ZbigniewMłynarek, the Norwegian Geotechnical Institute inOslo and the following companies: Hebo – PoznańSp. z o.o. and A.P. v.d. Berg from Holland.This paper is supported by founds intended forscience in years 2005-2007 as research projectNo. 0562/P04/2005/29.

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