Effect of OCR on sampling disturbance ofcohesive soils and evaluation of laboratoryreconsolidation procedures

Marika Santagata and John T. Germaine

Abstract: The paper presents the results of an experimental investigation of sampling disturbance in cohesive soilsthrough single-element triaxial tests on resedimented Boston blue clay (RBBC). The first part of the paper discussesthe effect of the overconsolidation ratio (OCR) (1–8) of the soil on postdisturbance compression and undrained shearbehavior. The results demonstrate that sensitivity to disturbance decreases markedly with OCR. It is also found that forthe medium-sensitivity soil tested, the estimate of the preconsolidation pressure is not significantly affected by OCR.The second part of the paper discusses laboratory reconsolidation procedures. For OCR1 RBBC, the recompressionmethod is not effective in recovering the stress–strain behavior of the soil and, for greater disturbance, provides anincreasingly unsafe estimate of the strength. For OCR4, provided the reconsolidation path reproduces the path thatoccurred in the field, this procedure succeeds in recovering the intact stress–strain–strength behavior of the soil.SHANSEP reconsolidation was investigated for normally consolidated RBBC only. For modest levels of disturbance,this is an effective means of evaluating both the stress–strain and the strength behavior of the soil. For greater levels ofdisturbance, the stress–strain behavior is not fully recovered, but the method continues to provide conservative esti-mates of the undrained strength.

Key words: sampling disturbance, clays, overconsolidation ratio, undrained strength, recompression, SHANSEP.

Résumé : Cet article présente les résultats d’une étude expérimentale sur le remaniement des échantillons de solcohérent au moyen d’essais triaxiaux sur un élément simple d’argile bleue de Boston resédimentée (ABBR). Dans lapremière partie de l’article, on discute de l’effet de l’OCR du sol (1–8) sur le comportement en compression et cisail-lement après remaniement. Les résultats démontrent que la sensibilité au remaniement diminue de façon marquée avecl’OCR. On trouve aussi que pour le sol de sensibilité moyenne testé, l’estimation de la pression de préconsolidationn’est pas affectée de façon significative par l’OCR. Dans la deuxième partie de l’article, on discute des procédures dereconsolidation en laboratoire. Pour un OCR de 1 de l’ABBR, la méthode de recompression n’est pas efficace pour re-couvrer le comportement contrainte-déformation du sol, et pour un remaniement plus grand, elle fournit une estimationde moins en moins fiable de la résistance. Pour un OCR de 4, pourvu que le cheminement de reconsolidation repro-duise le cheminement qui s’est produit en nature, cette procédure réussit à recouvrer le comportement en contrainte–déformation du sol intact. On a étudié la reconsolidation SHANSEP pour l’ABBR normalement consolidée (NC)seulement. Pour des niveaux modérés de remaniement, c’est un moyen efficace d’évaluer le comportement contrainte–déformation de même que la résistance du sol. Pour des niveaux plus importants de remaniement, le comportementcontrainte–déformation n’est pas complètement retrouvé, mais la méthode continue à fournir des estimations fiables dela résistance au cisaillement non drainé.

Mots clés : remaniement dû à l’échantillonnage, argiles, OCR, résistance non drainée, recompression, SHANSEP.

[Traduit par la Rédaction] Santagata and Germaine 474

Introduction

Geotechnical engineers have long recognized that sam-pling techniques and procedures have a significant impact onsoil properties measured in the laboratory. As a result, muchwork has been performed to understand the effects of thesampling process on the engineering behavior of soils and to

develop improved techniques for laboratory reconsolidation.Research in this field finds its roots in the early work byHvorslev (1949) and the fundamental research conducted inthe 1960s at Imperial College (Skempton and Sowa 1963),the Massachusetts Institute of Technology (MIT) (e.g., Laddand Lambe 1963), and the University of California at Berke-ley (e.g., Noorany and Seed 1965). Over the past 15 years,

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Received 15 October 2003. Accepted 27 September 2004. Published on the NRC Research Press Web site at http://cgj.nrc.ca on11 May 2005.

M. Santagata.1 Purdue University, School of Civil Engineering, 550 Stadium Mall Drive, West Lafayette, IN 47907–2051, USA.J.T. Germaine. Massachusetts Institute of Technology, School of Civil and Environmental Engineering, 77 Massachusetts Avenue,Cambridge, MA 02139, USA.

1Corresponding author (e-mail: mks@purdue.edu).

extensive work has continued to be conducted on this topic,a great part of which (Clayton et al. 1992; Georgiannou andHight 1994; Hird and Hajj 1995; Siddique et al. 1999;Santagata and Germaine 2002) has been founded on the ana-lytical modeling of the tube sampling process and the con-cept of ideal sampling introduced by Baligh et al. (1987).

At MIT, an extensive research program was undertaken toisolate the fundamental aspects of sampling disturbance ofcohesive soils through triaxial element tests (Santagata 1994)and laboratory model tests (Sinfield 1994). The triaxial testexperimental program included a large number of tests onboth normally consolidated (NC) and overconsolidated (OC)resedimented Boston blue clay (RBBC), in which samplingdisturbance was simulated in accordance with the perfectsampling approach (PSA) (Ladd and Lambe 1963) and theideal sampling approach (ISA) (Baligh et al. 1987) to modelblock and tube sampling, respectively. The effects of distur-bance on compression and undrained shear behavior werethen quantified by comparison with the intact behavior ofthe soil. This work was intended to provide a framework to(i) quantify the effects of tube penetration and stress release;(ii) determine which engineering parameters are most af-fected by disturbance; (iii) assess the sensitivity of cohesivesoils to disturbance as a function of the overconsolidation ra-tio (OCR); and (iv) evaluate two widely used reconsolidationprocedures. The first two objectives were addressed forOCR1 by Santagata and Germaine (2002), who presentedand analyzed in detail the data for NC RBBC.

This paper presents results for OC RBBC with OCR rang-ing between 2 and 8 and compares them with those obtainedfor the NC soil. Specifically, the paper highlights the sensi-tivity to disturbance of various engineering properties(preconsolidation pressure, compressibility parameters, un-drained strength, strain at failure, etc.) as a function of theOCR.

The second part of the paper evaluates procedures torecover the intact behavior of the soil in the laboratory. Inparticular, the paper presents additional tests performed toevaluate the validity of the recompression (Bjerrum 1973)and the SHANSEP (Ladd and Foott 1974) procedures for as-sessing the undrained strength and undrained stress–strainbehavior of cohesive soils in the laboratory. Results of testsperformed on RBBC with an OCR of 1 (SHANSEP andrecompression tests) and an OCR of 4 (recompression testsonly) are presented, and the limitations of the two recon-solidation procedures are discussed.

Supporting technology

Resedimented Boston Blue ClayRBBC is a soil manufactured in the laboratory from natu-

ral Boston blue clay, an illitic low plasticity (CL) clay pres-ent throughout the Boston area. The resedimentation processinvolves mixing under vacuum the soil powder obtained bydrying and processing the natural soil with de-aired water toproduce a slurry with a water content of 100%. A bacterialgrowth inhibitor (phenol) and salt (16 g/L required to pro-duce flocculation and achieve a structure similar to that ofthe natural soil) are also added. The slurry is sprayed undervacuum into a large consolidometer (D = 30.5 cm), inwhich, in a process that lasts about 19 days, it is

incrementally consolidated to a maximum stress of 100 kPaand then unloaded to 25 kPa to produce an OCR of 4. In thisclose-to-hydrostatic condition, the soil cake is removed fromthe consolidometer and prepared for storage. Triaxial speci-mens (H = 8 cm, D = 3.6 cm) are prepared from this blockby means of a wire saw and a miter box.

RBBC is uniform and saturated, with a well-known stresshistory, and is available in a virtually infinite supply, requir-ing moderate incremental work to produce additionalbatches. In addition, there exists an extensive database of itsindex and engineering properties. Finally, and most impor-tantly, although RBBC is not a natural soil, it exhibits be-havior similar to that of the natural origin material and ofother low-sensitivity marine clays, including stress–strain–strength anisotropy, low to medium sensitivity, and signifi-cant strain rate dependency (e.g., see Sheahan 1991;Santagata 1998). The structure of RBBC, evaluated on thebasis of the position of the sedimentation compression curve(Skempton 1970; Cotecchia and Chandler 2000), is also con-sistent with that of natural soils (with sensitivity as great as10) characterized by a “sedimentation structure”, that is, inwhich chemical or aging processes (postsedimentation pro-cesses, according to Cotecchia and Chandler 2000) have notplayed a significant role.

For all these reasons, RBBC is an ideal material for theinvestigation of fundamental aspects of soil behavior withoutthe variability of natural soils having to be taken into ac-count. The average index properties for the five RBBCbatches used to perform the tests presented in this paper aresummarized in Table 1.

Triaxial apparatusFigure 1 shows a schematic of the computer-controlled

stress-path triaxial testing apparatus used in the MITgeotechnical laboratory for testing cohesive soils (Sheahanand Germaine 1992). The apparatus includes six basic com-ponents:(i) the triaxial chamber, characterized by internal posts, a

fixed top cap, a loading piston riding through a low-friction linear bearing, a rolling diaphragm seal, and sil-icon oil as chamber fluid;

(ii) the system for load application, comprising the pressure–volume controllers (PVCs) used to regulate back andcell pressure, and the 9 kN capacity bench-topWykeham Farrance (Slough, UK) screw-driven loadframe with adjustable gear ratios used for axial loading;

(iii) the motors driving the two PVCs and the load frame, themotor controllers, and drivers;

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Natural water content, wN (%) 39.9±1.1Liquid limit, wL (%) 44.1±3.0Plastic limit, wP (%) 21.7±0.8Plasticity index, IP (%) 22.4±2.9Liquidity index, IL 0.83±0.14Specific gravity, Gs 2.78±0.005% passing No. 200 sieve >93Clay fraction 52.1±4.1

Table 1. Selected index properties of RBBC (seriesIII).

(iv) the instrumentation package, including a power supply;a shear beam load cell, which measures the axial load;two high-performance diaphragm-type pressure trans-ducers mounted at the base of the cell to monitor celland back (pore water) pressures; and two linear variabledifferential transformers (LVDTs) mounted on the load-ing piston and the back-pressure PVC piston to measurethe external axial strain and the volumetric strain, re-spectively;

(v) the personal computer (PC) responsible for automatedcontrol during all phases of the test (initial pressure up,saturation, consolidation along any stress path or K0consolidation, and shear in extension or in compres-sion); and

(vi) the central data acquisition system (not shown in thefigure), which is based on a PC interfaced with aHewlett Packard (Palo Alto, Calif.) HP3497A dataacquisition unit capable of analog-to-digital conversionand storage of data at a trigger rate of up to 1 Hz.

The triaxial chamber, the load frame, and the twopressure–volume controllers are housed in a temperature-regulated enclosure maintained at 25 °C ± 0.2 °C by an air-circulating unit with a heat source activated by a thermostat.

Research methodology

As mentioned above, the experimental program made useof the frameworks provided by PSA (Ladd and Lambe 1963)and ISA (Baligh et al. 1987) to simulate block and tube sam-pling, respectively, in the triaxial cell. The following sec-tions provide a brief overview of the two approaches anddescribe the test program in detail.

Perfect and ideal sampling approachesThe concept of perfect sampling was first introduced in

the 1960s by Ladd and Lambe (1963, p. 343) to denote the“[sampling] process where no disturbance has been given tothe specimen other than that involved with the release of thein situ shear stress”. Although subsequent work (see below)has identified additional and more significant contributions

to disturbance in the case of tube sampling, perfect samplingrepresents an exhaustive model of minimum sampling dis-turbance for the case of block sampling.

In more recent years, Baligh et al. (1987) have made useof the strain path method (Baligh 1985) to analyze soil ele-ment straining due to deep penetration of a simple sampler(S sampler), which is characterized by a curved cutting edgeand a slight reduction of the inner diameter (Fig. 2). Theiranalysis, which has had considerable impact on the study ofsampling disturbance, has shown that soil elements locatedinside a sampling tube undergo a complex strain history, in-volving both shear and normal strains. Recognizing that withtube sampling the minimum unavoidable disturbance wentbeyond the simple release of the in situ shear stresses de-scribed by the PSA, they introduced the concept of idealsampling. In recognition of the fact that the normal axialstrain (εzz) is the dominant strain component in the inner halfof the sample, the ISA proposes that the strain undergone bya soil element located at the centerline of the sampler beused to describe the effects of tube sampling. As shown inFig. 2, such soil elements undergo three distinct phases ofundrained triaxial shearing: a compression phase, ahead ofthe sampler, until εzz(max) is reached at about 0.35B below thetip; an extension phase, during which εzz decreases rapidlyand becomes zero at the tip of the sampler and εzz(min) (equalto –εzz(max)) is reached at about 0.35B inside the sampler; anda last phase, in which the soil element is once again sub-jected to compression, until εzz approaches zero. As shownin Fig. 2, the magnitude of the strains is a function of thegeometry of the tube, and the straining increases with therelative thickness of the sampler.

More recently, Clayton et al. (1998) used a finite elementapproach to investigate the effects of specific features of thesampling tube geometry (inside clearance ratio, cutting edgetaper angle, etc.) on the strain history undergone by the soilduring sampling. In particular, their results show that (i) thestrain path followed by a soil element located at the center-line of the sampler is not generally as symmetric as thatshown in Fig. 2 (e.g., for a sampler with high inside clear-ance ratio, the extension strain is greater than the maximum

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Fig. 1. Schematic of automated stress path triaxial cell (connections between transducers and PC not shown). DC, direct current.

compressive strain experienced by the soil ahead of the sam-pler); (ii) details of the sampler geometry play a significantrole (e.g., the greater the taper angle of the outside cuttingedge, the greater the maximum compressive strain); and(iii) the strains generated by a sharp-edge sampler aresmaller than those predicted by Baligh et al. (1987) for the Ssampler.

Disturbance simulation and evaluation ofpostdisturbance behavior

Different types of tests were performed to quantify the ef-fects of disturbance on the compression and the undrainedshear behavior of RBBC. Every test involved three or fourmain phases: a consolidation phase, to recover the intact be-havior of the soil; a disturbance phase, to simulate eitherblock or tube sampling; and one or two postdisturbancephases, to evaluate the effects of the previous disturbancephase on the behavior of the soil or to evaluate a recon-solidation procedure (see details below).

In the first phase, which was common to all tests, follow-ing back-pressure saturation, the specimen was K0 consoli-dated well into the virgin compression line (VCL) (to threeto four times the batch preconsolidation pressure of RBBC,which is equal to ~0.1 MPa; see the section on RBBC),maintained at constant stresses for 24 h to allow for somesecondary compression and then, if necessary, swelled to thedesired OCR. An additional 24 h period of constant stressesfollowed swelling. Unloading to the OC state occurredthrough stress path consolidation, targeting a value of K0given by the following equation: K0(OC) = K0(NC)(OCR)n,with average K0(NC) = 0.48; and n = 0.426 (= 1 – sin35°).The form of this equation (Schmidt 1966) has been found tobe valid for RBBC with the values of K0(NC) and n shown bySheahan (1991) and Santagata (1994).

RBBC exhibits normalized behavior, and consolidationpast 1.2–1.5σp′ erases the limited disturbance caused by theremoval of the soil cake from the consolidometer and by the

setup operations (O’Neill 1985). Therefore, at the end of theconsolidation or swelling phase, the soil specimen can beconsidered to be in a condition similar to that of an elementof NC or OC clay in the field prior to sampling, and thus itsbehavior will be referred to as intact.

The disturbance phase followed the secondary compres-sion phase at the end of consolidation (tests on NC RBBC)or swelling (OC RBBC). Depending on whether the distur-bance was simulated according to PSA or ISA, this phaseinvolved either the undrained release of the consolidationshear stress (PSA tests) or the undrained compression–extension–compression strain history shown in Fig. 2, fol-lowed by undrained unloading to the hydrostatic state (ISAtests). As indicated in Table 2, the experimental program in-cluded ISA tests performed with the nominal amplitude ofthe strain cycle (εc) varying between 0.5% and 8%. This wasdone to correlate the effects of disturbance to the geometryof the sampling tube (e.g., εc ≈ 1% and 2% for B/t ~ 40 and20, respectively) and to gain insight into the degradation ofthe engineering properties for soil elements not locatedalong the centerline of the tube. These elements, despite notbeing sheared in a purely triaxial mode during penetration ofthe sampler, are subjected to significantly higher verticalstrains (Baligh et al. 1987). As discussed above, recent workby Clayton et al. (1998) has shown that depending on thespecific cutting edge geometry of the sampler, the strain pathfollowed by an element located at the centerline of the tubeduring sampling may not be symmetric (i.e., εmax in com-pression not equal to �εmax� in extension). In this research,however, simulation of the centerline sampling disturbancestraining followed the framework provided by Baligh et al.(1987). Although it is recognized that if the presence of asharp cutting edge is neglected the disturbance undergoneby the soil may be somewhat overestimated, this approachlimited the number of tests performed. In addition, modeltesting of the effects of tube sampler geometry performedat MIT by Sinfield (1994) shows that for a given sampler

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Fig. 2. Strain path results for straining history of soil elements along centerline of S samplers, with B/t = 20, 40, and 50 (modifiedfrom Baligh et al. 1987). B/t, aspect ratio; ICR, inside clearance ratio; Be, minimum diameter.

geometry, the effects of sampling (as measured, for exam-ple, by the reduction in effective stresses and the decrease inundrained strength) are substantially greater than those pre-dicted by the triaxial element tests with strain cycle ampli-tude corresponding to that tube geometry. This is also thereason behind the large range in amplitude of the strain cy-cle explored in the testing program.

The last portion of the test differed, depending on whetherthe objective was (i) to evaluate the effects of the previousdisturbance phase on the undrained shear; (ii) to evaluate theeffects of disturbance on the compression behavior; (iii) toevaluate the SHANSEP reconsolidation procedure; or (iv) toevaluate the recompression reconsolidation procedure. In thefirst case, the specimen was sheared undrained (post-disturbance unconsolidated undrained (UU) tests with mea-surement of the excess pore pressure), whereas in the secondcase, the soil was K0 consolidated to at least 0.9 MPa(postdisturbance K0-consolidated tests). In the case of thepostdisturbance SHANSEP tests, the soil specimens wereconsolidated under K0 conditions to at least 0.9 MPa,allowed to creep for 24 h, and then sheared in undrainedcompression (SHANSEP tests were performed only for NCRBBC). Note that these tests are also used to study thepostdisturbance K0-consolidation behavior. Finally, in thelast case (postdisturbance recompression tests), followingdisturbance the soil specimens were reconsolidated to thepredisturbance effective stresses (σv′ and σh′ ), allowed tocreep for 24 h, and then sheared undrained to failure.

Figures 3–5 present examples of the complete stress pathsand e–logσv′ curves for the tests, and Table 2 summarizesthe experimental program. All consolidation and undrainedshear phases were conducted with axial strain rates of0.1%/h and 0.5%–1%/h, respectively.

Behavior of intact RBBC

Definition of the intact behavior of RBBC was based onresults from this testing program, as well as data from otherinvestigations. Specifically, the one-dimensional compres-sion behavior was characterized from data from the firstconsolidation phase of tests presented here, as well as fromconstant rate of strain (CRS) tests conducted to stress levelsas great as 0.8 MPa by Cauble (1993). For NC RBBC, thedisturbance phase (with the exception of the PSA tests) pro-vides most of the data for the undrained shear behavior, asthe soil fails at a strain of 0.1%–0.2%. In the case of OC

RBBC, however, the strain imposed during the first com-pression phase of the ISA disturbance cycle is almost alwaysnot large enough to cause failure of the OC soil. Thus, theintact undrained shear reference behavior was derived fromthe database provided by Sheahan (1991).

Compression behaviorFigure 6 presents a set of curves derived from the constant

rate of consolidation portion of direct simple shear tests per-formed by Cauble (1993) on soil obtained from one of thebatches used in this testing program. The curves, which ex-tend to a vertical stress of about 0.8 MPa, define the typicalone dimensional compression behavior of RBBC. The com-

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OCR1 OCR2 OCR4 OCR8

Test type UU K0 con/SH Rec UU K0 con/SH UU K0 con/SH Rec UU

PSA 5 — 1 1 — 1 — 1 —ISA±0.5 1 — — — — — — — —ISA±1 7 1 1 1 — 2 — 1 1ISA±1.5 1 — — — — — — — —ISA±2 4 1 1 1 1 1 1 1 1ISA±5 2 1 — — — — — 1 —ISA>5 — — — — — 1 — — —Subtotal 20 3 3 3 1 5 1 4 2

Total 26 4 10 2

Table 2. Overview of experimental program.

Fig. 3. Test conducted to investigate postdisturbance undrainedshear behavior (OC RBBC): (a) e–log σv′; and (b) p′–q space.

pression curves show a well-defined break corresponding tothe batch preconsolidation pressure and the approximatelyconstant slope of the VCL over the entire stress range inves-tigated (for some batches, a modest reduction in compres-sion ratio, CR (= ∆e/∆logσv′ ), of about 7%–9% is observedin the same stress range). Similar behavior is observed in theone-dimensional consolidation phases of the triaxial testsperformed for this testing program for all batches of RBBC,despite some limited batch-to-batch variability, and in gen-eral the compression behavior is quite repeatable.

As mentioned above, the coefficient of earth pressure atrest, K0, increases with increasing OCR following the empir-ical equation of the type proposed by Schmidt (1966): K0 =K0(NC) (OCR)n, with K0(NC) = 0.48 and n = 0.426 (Sheahan1991; Santagata 1998).

Undrained shear behaviorFigures 7a and 7b illustrate the typical undrained triaxial

compression behavior of RBBC (OCR1–8) sheared at a rateof 0.5%/h. The figures show that NC RBBC reaches peakstrength at a relatively small axial strain (~0.1%), followedby significant softening; and because of the development oflarge positive pore pressures, p′ decreases markedly. In-creasing OCR causes a decrease in the peak value of thestrength normalized to the maximum vertical effective stress(σvmax′ ), an increase in the axial strain at failure (εp~ 1%,3.5%, and 6% for OCR = 2, 4, and 8), and a reduction instrain softening. At larger strains, the effective stress paths

approach a common failure envelope. Associated with in-creasing OCR is also a decrease of the shear-induced (us =∆u – ∆σoct) pore pressure beyond 0.5%–1% axial strain, aswell as a reduced development of excess pore pressure (ue =∆u), which causes the transition from a fully contractive(OCR1) to an entirely dilatant (OCR8) undrained shear be-havior.

For all OCRs the soil is observed to exhibit normalizedbehavior, and the following SHANSEP parameters apply for

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Fig. 4. Test conducted to investigate postdisturbance K0-consolidation behavior and SHANSEP reconsolidation procedure(NC RBBC): (a) e–log σv′ ; and (b) p′–q space.

Fig. 5. Test conducted to investigate recompression recon-solidation procedure (NC RBBC): (a) e–log σv′ ; and (b) p′–qspace.

Fig. 6. One-dimensional intact compression behavior of RBBC(Cauble 1993). DSS, direct simple shear.

undrained triaxial compression (dε/dt = 0.5%/h): S = 0.33;and m = 0.7 (qf /σvc′ = S(OCR)m). The undrained strength ra-tio of NC RBBC is observed to decrease with increasingvalue of the lateral stress ratio, K, in a manner described bythe following relationship: qf /σvc′ = 0.54–0.45K (r2 = 0.79)(Fig. 8).

Effect of OCR on postdisturbance behavior

Preshear stress stateFigures 9a–9f present the stress–strain curves, stress

paths, and curves of excess and shear induced pore pressurefor the ISA ± 2 disturbance phase of tests conducted on NCand OCR4 RBBC. It is shown that for NC RBBC, basicallyall phases of sampling generate positive pore pressure, withthe first two phases playing the major role. This results in acontinuous decrease of the effective stresses (note that forlower disturbance, the second compression phase actuallyresults in a small increase in the stresses; Santagata andGermaine 2002).

The major difference with OC RBBC is that the soil di-lates during the initial compression to εa = εmax with conse-quent increase of the effective stresses. The positive porepressure produced during the subsequent extension phase re-sults in an overall reduction of the effective stresses, whichis more marked for decreasing values of the OCR. The lasttwo phases (second compression phase and shear stress

release) produce a further, less significant decrease of thestresses.

Figure 10 presents the change in mean effective stress(pm′ = (σv′ + 2σh′ )/3) normalized by the value at the end ofconsolidation (i.e., predisturbance value), pmc′ , caused bydisturbance versus the strain cycle amplitude, εc. For allOCRs, the effective stress decrease is more marked as theamplitude of the strain cycle increases. Note that in this fig-ure as throughout the paper, the results for the PSA tests areplotted at εc equal to zero. There is consistency in the resultsfor each level of straining and OCR, as well as significantlygreater effective stress loss with decreasing OCR, for a givenlevel of disturbance. With the exception of the PSA tests, thechanges in pm′ are substantial even for the higher OCRs.

Just as the stress change associated with swelling is quan-tified through the OCR, the undrained decrease in effectivestress caused by sampling disturbance can be expressedthrough a similar parameter, the apparent overconsolidationratio (AOCR), first introduced by Ladd and Lambe (1963)and here redefined as the ratio between σvmax′ , the maximumvertical consolidation stress, and σs′ , the effective stress atthe end of sampling. For OC RBBC, it is useful to distinguishbetween the stress decrease associated with the swellingphase (σvmax′ to σvc′ ) and that caused by sampling (σvc′ to σs′ ).This second contribution can be quantified through the in-duced overconsolidation ratio (IOCR) (= σvc′ /σs′ ; Santagataand Germaine 2002). From the definitions above, it is clearthat IOCR = AOCR/OCR and that for NC clay, IOCR =AOCR. In addition, for intact soils, IOCR = 1, and theAOCR is equal to the actual OCR.

Compression behavior

Compression curve and compressibility parametersFigures 11a–11c compare the intact and post-ISA ± 2 dis-

turbance compression curves for NC and OC RBBC withOCR equal to approximately 2 and 4. Each figure presentsthe regression line through the steepest portion of the com-pression phase (with slope Cc1 = ∆e/∆logσv′ ), which definesthe intact VCL, as well as the line (with slope indicated asCc2) through the postdisturbance compression curve at σv′ ~3σp′ (slightly lower value in the case of the OCR2 test). Thefigure indicates a less significant difference between Cc1 and

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Fig. 7. Intact undrained shear behavior of RBBC from K0

consolidated undrained compression (triaxial CK0UC) tests:(a) stress paths; (b) stress–strain curves.

Fig. 8. Relationship between lateral stress ratio and undrainedshear strength for NC RBBC.

Cc2, thus a reduced sensitivity to disturbance, for increasingvalues of the OCR. This is clearly shown in Fig. 12, inwhich the slope of the compression curve (∆e/∆logσv′ ) isnormalized by Cc1 and plotted versus σv′ /σvmax′ , the currenteffective stress normalized by the maximum vertical effec-tive stress. The figure demonstrates that in the OCR4 test

(and possibly also for OCR2 RBBC) reconsolidation to 3σp′is effective in recovering the intact compression curve (somedifference between Cc1 and Cc2 may derive from stress leveleffects). This is not the case for the NC soil. More substan-tial differences between the two slopes are observed forhigher degrees of disturbance (Santagata and Germaine2002).

Figure 12 also indicates no clear effect of the OCR on therecompression portion of the disturbed compression curve,as for σv′ /σp′ < 1 the three curves are basically overlapping;that is, for a given magnitude of disturbance, the recom-pression behavior of the soil is affected in a similar manner,independently of the OCR. This can be explained by observ-ing the limited variation in the AOCR at the end of distur-bance for the three tests (AOCR is equal to 10.3, 7.1, and11.7 for OCRs of 1, 2, and 4, respectively). In fact, the val-ues of the AOCR for the two OC tests fall within the rangeof data for NC RBBC subjected to ISA ± 2 disturbance(6.9–12.1). This indicates that for this level of disturbance(ISA ± 2), possibly because of the limited structure ofRBBC, disturbance does not affect the mechanisms of defor-mation in the recompression region.

Reconsolidation strainsFigure 13 plots the volumetric strains measured during

reconsolidation of the specimen to the in situ (i.e., pre-

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Fig. 9. ISA ± 2 disturbance of NC and OCR4 RBBC: (a), (b) stress–strain curves; (c), (d) stress paths; and (e), ( f ) excess (ue) andshear-induced (us) pore pressures normalized by maximum vertical stress.

Fig. 10. Decrease in mean effective stress due to disturbance(dashed lines indicate qualitative trends).

disturbance) vertical stress as a function of the level of dis-turbance for both NC and OC RBBC. The figure includesdata from the postdisturbance recompression tests (dis-cussed later in the paper), in which postdisturbance consol-idation was not performed one dimensionally. Overall, thedata are consistent and demonstrate that for a given OCR,the volumetric strain associated with reconsolidation to thein situ stresses is closely related to the degree of distur-bance undergone during sampling, as measured by thestrain cycle amplitude. For any level of disturbance, thereconsolidation strain decreases with increasing OCR. As aresult, any evaluation of the quality of a soil sample as afunction of the reconsolidation strain measured in the labo-ratory must be a function of the OCR of the soil. This is

recognized, for example, by Lunne et al. (1997), whoproposed two separate criteria for evaluating sample distur-bance in soft, low-plasticity soils, one for OCR1–2 and onefor OCR2–4. For example, in the case of RBBC, the crite-rion for an excellent sample proposed by Lunne et al.(1997) (which is expressed in terms of ∆e/e0) would corre-spond to a measured volumetric reconsolidation strainsmaller than 2% for OCR1–2 and less than approximately1.5% for OCR2–4.

Preconsolidation pressureFor NC RBBC, the effects of disturbance on the estima-

tion of the known in situ preconsolidation pressure wereevaluated by analyzing the one-dimensional compression be-

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Fig. 11. Effect of ISA ± 2 disturbance on the K0-compression behavior of (a) OCR1, (b) OCR2, and (c) OCR4 RBBC.

Fig. 12. Effect of ISA ± 2 disturbance on the compression ratio of RBBC.

havior of the soil following ISA ± 1, ISA ± 2, and ISA ± 5disturbance. The main conclusions from these tests, exten-sively discussed by Santagata and Germaine (2002), can besummarized as follows:(i) The uncertainty in the estimate of σp′ increases with dis-

turbance.(ii) Best estimates of σp′ have an accuracy of 10% ± 5%.(iii) The reloading behavior of the soil after disturbance ap-

pears to be concurrently influenced by the decrease ineffective stresses and the destructuring produced by dis-turbance, which would cause the preconsolidation pres-sure to be overestimated and underestimated, respectively.Which of these two mechanisms prevails depends on thetype of soil and the amount of disturbance.

(iv) For a low- to medium-sensitivity soil such as RBBC andthe levels of disturbance considered, the first mechanismappears to control the behavior of the soil; as a result,disturbance may lead to an overestimate thepreconsolidation pressure.

As illustrated in Fig. 11, ISA ± 2 disturbance tests wereconducted on RBBC with nominal OCR of 1, 2, and 4 toevaluate the effect of the OCR. Table 3 summarizes somerelevant data for these tests. Comparison of these data shedsfurther light on the observations listed above. First, the im-pact of disturbance on the determination of the preconsolida-tion pressure does not appear to be affected by the OCR ofthe soil. This is highlighted by the very consistent values of(σp(max)′ – σp(min)′ )/σp′ , which quantifies the uncertainty asso-ciated with the estimate of the preconsolidation pressure.

Second, with the exception of the σp(min)′ data, the resultsindicate that disturbance leads to overestimation of thepreconsolidation stress. These two observations suggest thatthe behavior of the soil is indeed controlled by the loss in ef-fective stress associated with disturbance. In fact, as dis-cussed above, for all three tests the AOCR is approximatelythe same. This is consistent with the results for the com-pressibility data in the recompression portion of the curve.Although the varying effect of disturbance on the slope ofthe VCL should be reflected at least in the values of σp(min)′(the determination of which is independent of the initial por-tion of the curve), comparison of the results for OCR1 and

OCR4 (these two tests are characterized by the same valueof σp′ and were conducted on soil from the same batch, andreconsolidation was extended to 3σp′ ) suggests that forRBBC, this effect may be secondary. More data are neededbefore a conclusive statement on this point can be made.

Third, the data confirm that for the level of disturbance in-vestigated, it is possible (particularly in the case of a contin-uous compression curve, such as that provided by CRSconsolidation) to estimate the preconsolidation pressure withconsiderable confidence (the σp(be)′ data are within 10% ±5% of the true value).

Undrained shear behaviorThe UU postdisturbance tests described above (see Fig. 3)

were used to investigate the effects of PSA and ISA distur-bance on the undrained triaxial compression behavior ofRBBC.

Figures 14a and 14b show postdisturbance stress–straincurves for NC and OCR4 RBBC. For NC RBBC (Fig. 14a),as already discussed by Santagata and Germaine (2002), asdisturbance increases, the stress–strain curve loses all thecharacteristics typical of NC RBBC, and the behavior ismore like that observed for intact OC RBBC (see Fig. 7).Associated with increasing levels of disturbance (PSA toISA ± 5) are an increased reduction of the undrained strength;an increase in the strain at peak shear stress; a loss of strainsoftening; and an increased reduction of the soil stiffness.The stress paths presented by Santagata and Germaine(2002) for these same tests indicate a change from contrac-tive to increasingly dilatant behavior and an increase in max-imum obliquity.

Although qualitatively similar effects are observed in thecase of OCR4 RBBC (Fig. 14b), the results demonstrate asignificantly reduced sensitivity to disturbance. In particular,because the intact K0 value for OCR4 is close to 1, PSA dis-turbance has no effect on the stress–strain–strength behaviorof the clay, and the results for this test fall within the rangefor the intact material. The results of the ISA tests suggestmore significant effects of disturbance on the stress–strainbehavior than on the undrained strength.

Figure 15 compares the undrained strength data for NCand OC RBBC following disturbances of different magni-tude (PSA tests plotted at εc = 0). The data for the intact soilare also included. The range shown reflects some inherentvariability, as well as variations due to the value of thepreshear K0, and the results are quite consistent for eachlevel of straining and OCR. NC RBBC shows a well-definedtrend of decreasing strength with strain cycle amplitude(strength is reduced by as much as 45% in the ISA ± 5tests). As the OCR increases, the sensitivity to disturbancedecreases substantially; and for OCRs larger than 2, the un-drained strength appears essentially unaffected by the distur-bance cycle, at least for strain cycle amplitudes of ≤2%.This is most likely associated with the increase in the failurestrain with OCR: whereas NC and OCR2 RBBC fail at ap-proximately 0.1% and 1%, respectively, at 2% strain neitherOCR4 nor OCR8 RBBC has reached the peak strength.

Figure 16 highlights the effect of disturbance on theprefailure stiffness (data below 0.1% axial strain not shown,as measurements were performed with external LVDTs).Santagata and Germaine (2002) discussed how for NC

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468 Can. Geotech. J. Vol. 42, 2005

Fig. 13. Effect of disturbance on the volumetric reconsolidationstrains.

RBBC the effects of disturbance on the prefailure behaviorderived from three distinct influences: the effective stressdecrease and the destructuring process associated with sam-pling, which both cause the stiffness to decrease; and theload reversal occurring as a result of shear, which producesan increase of the stiffness in the prefailure nonlinear region.For NC RBBC, it was observed that for all ISA tests the firsttwo effects dominated and, further, that destructuring playeda significant role, as the decrease in the effective stress alonecould not account for the reduction in stiffness. In an analy-sis of the OC data, the direction of the preshear stress pathneed not be distinguished, as both the intact and disturbedshear are associated with load reversal. It is of interest, how-ever, in this case to understand to what degree the reduction

in stiffness derives from the effective stress decrease. Forthe ISA ± 1 and ISA ± 2 tests the difference in stiffness canbe explained simply by the reduction in effective stresscaused by sampling (i.e., the stiffness is approximately thesame as would be measured on a soil specimen consolidated

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OCR1 OCR2 OCR4

σp(true)′ (kPa)a 306 430 306

σp(min)′ (kPa)b 289 (–5.4%) 432 (+0.5%) 295 (–3.6%)

σp(be)′ (kPa)c 314 (+2.6%) 471 (+9.5%) 316 (+3.3%)

σp(max)′ (kPa)d 343 (+12.2%) 515 (+19.8%) 348 (+13.7%)

(σp(max)′ – σp(min)′ )/σp(true)′ (%) 18 19 17

Note: An example of the determination of the various σ p′ values is provided in Santagata and Germaine (2002).aKnown true σ p′, adjusted to account for the effects of secondary compression (Mesri and Godlewski 1977).bFrom strain energy procedure (Becker et al. 1987), assuming that the initial slope of curve is zero.cFrom strain energy procedure, enlarging the low stress portion of the curve for greater accuracy.dFrom strain energy procedure, choosing a higher initial slope, as would typically occur in the presence of only a

limited number of data points (e.g., incremental oedometer results).

Table 3. Evaluation of the preconsolidation pressure.

Fig. 14. Postdisturbance stress–strain behavior (UU tests) of(a) NC and (b) OCR4 RBBC.

Fig. 15. Effect of disturbance on the undrained strength of RBBC.

Fig. 16. Effect of disturbance on the prefailure behavior ofOCR4 RBBC. Numbers in parentheses represent consolidationstress; average value for all tests is 0.097 MPa.

to σs′ with OCR = AOCR corresponding to ISA ± 1 orISA ± 2 disturbance). For the ISA ± 8 test, this is not thecase, and the rearrangement of the soil structure causes theadditional reduction in stiffness (~50%). Note that these as-sessments were made using the data and the interpretativeframework for the prefailure behavior of RBBC provided bySantagata (1998). These results substantiate the existence ofa critical threshold strain (see, for example, Georgiannouand Hight 1994), beyond which the soil undergoes majorfabric distortion (destructuring).

Recovering the intact behavior in thelaboratory

Recompression testsFigures 17a–17c compare the intact undrained stress–

strain behavior of NC RBBC with that observed in thepostdisturbance recompression tests for three levels of dis-turbance (PSA, ISA ± 1, and ISA ± 2). Note that for the ISAtests the first portion of the disturbance cycle is used to de-pict the intact behavior, whereas for the PSA test the datafrom another test with the same K0 were selected. For refer-ence, the curves derived from the UU tests performed afterthe same degree of disturbance are shown. Comparison ofthe curves for the UU and recompression tests indicates thatthis reconsolidation procedure does partially recover theintact behavior of the soil. The differences between the re-consolidated and the intact curves are, however, still quitemarked: the soil fails at a much larger strain (Fig. 18), andstrain softening is essentially eliminated in the ISA tests.Most important, the results show that recompression to thein situ stresses is consistently associated with an overesti-mate of the undrained strength of the NC soil (Fig. 19). Al-though the overestimation is negligible (<1%) in the case ofthe PSA test, it becomes very significant in the case of theISA tests, increasing from +8% for the ISA ± 1 test to al-most 20% in the ISA ± 2 test. This effect is expected to beeven more significant for higher degrees of disturbance.

Figure 20 presents a similar set of plots, as well as thecorresponding stress paths relative to ISA ± 2 disturbance ofOCR4 RBBC. In this case, the first compression phase ofthe ISA disturbance cycle shear does not reach failure.Therefore, the results from another test that had the sameOCR and that displayed similar stress–strain behavior up to2% axial strain (see both tests plotted with continuous linesin Fig. 20a) are used to represent the intact behavior of thesoil. The figure shows that in the case of OC RBBC,recompression to the presampling stresses yields a satisfac-tory estimate of the undrained strength (approximately 2%greater than the average value reported for OCR4 RBBC).Given the limited volumetric strains (see Fig. 13), the over-estimate in strength is limited to below 5%, even for ISA ± 5disturbance. Unfortunately, the effects of greater disturbancewere not quantified.

The figure also demonstrates that there remain significantdifferences in stress–strain behavior compared with that ofthe intact soil. To explain this observation, it should be notedthat by reconsolidating the soil specimen to the presamplingeffective stress through a continuous loading path, thereconsolidation procedure did not reproduce the loading his-tory that generated the OC conditions in the first place. Dur-

ing the initial consolidation phase the predisturbance fieldstresses were reached as a result of unloading from higherstresses, and as a result, the following shear involved a loadreversal. Researchers have pointed out that the direction ofthe preshear stress path affects the undrained stress–strainbehavior (e.g., Jardine et al. 1991), and Santagata (1998) hasshown that for OC RBBC this effect is very marked. Thissuggests that for a realistic representation of the intactstress–strain behavior of OC soils (that have reached thecurrent state of stress as a result of unloading), thereconsolidation process should involve reaching the targetstress conditions through an unloading path. This may be

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470 Can. Geotech. J. Vol. 42, 2005

Fig. 17. Undrained shear following recompression for (a) PSA,(b) ISA ± 1, and (c) ISA ± 2 disturbance of NC RBBC.

achieved by first loading the soil slightly beyond the targetvertical stress and then unloading. Although such a test wasnot performed in this experimental program, results areavailable for the stress–strain behavior (0%–1%) of intactOCR4 RBBC sheared following reloading from a higherOCR (Santagata 1998), that is, with a preshear consolidationpath similar to that followed in the recompression test.These data, which are shown as hollow symbols in Fig. 20a,fall exactly on the postrecompression curve. This indicatesthat for OCR4, at least for this level of disturbance, whendifferences in the preshear path are accounted for therecompression procedure is indeed effective in reconstruct-ing the intact stress–strain–strength behavior of the soil.

SHANSEP testsFigures 21a–21c compare the stress–strain curves for the

undrained shear phase following SHANSEP reconsolidationafter ISA ± 1, ISA ± 2, and ISA ± 5 disturbance with thecurves for intact NC RBBC determined during the first por-tion of the disturbance cycle (compression shear to εzz(max)).Again for reference, the figures report the data from thepostdisturbance UU tests. For comparing the intact behavior

of RBBC (derived from tests with σvc′ ~ 0.3 MPa) with thatfollowing SHANSEP reconsolidation (derived from testswith σvc′ ~ 0.9–1 MPa), the effect of stress level must betaken into account. On the basis of the results of direct sim-ple shear tests performed at stress levels of 0.1–1.2 MPa,Ahmed (1990) observed that for RBBC the effective stresslevel has a significant effect on the strain at peak (εpeak in-creased by about 100%–120% over the stress range exam-ined) but negligible influence on the undrained strength.Another aspect requiring consideration is the effect of thepreshear lateral stress ratio. It was discussed earlier in thepaper that the undrained strength and to a lesser degree thestrain at peak are influenced by the preshear value of K(Fig. 8). Therefore, some difference between the strengthand the strain at peak after the SHANSEP reconsolidationand the corresponding intact strain at peak may arise if the Kvalue after reconsolidation is different from the in situ value.The limited number of SHANSEP tests performed has notresolved whether, and to what degree, disturbance affectsK0: whereas in the ISA ± 1 and ISA ± 5 tests, the K0 at theend of reconsolidation was found to be greater than the insitu value, the two values were virtually identical (0.444 vs.0.445) in the ISA ± 2 test. As shown in Fig. 8, the results forthe SHANSEP tests fall within the K0–(qf /σvc′ ) band deter-mined for the intact soil and provide a reasonable and con-servative estimate of the undrained strength of NC RBBCfor the levels of disturbance investigated.

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Fig. 18. Strain at peak from undisturbed, UU, recompression,and SHANSEP tests on NC RBBC.

Fig. 19. Change in strength due to recompression of disturbedNC and OCR4 RBBC.

Fig. 20. Undrained shear following ISA ± 2 disturbance andrecompression reconsolidation of OCR4 RBBC: (a) stress–straincurves; and (b) stress paths.

In terms of stress–strain behavior, Figs. 21a and 21b indi-cate that for both ISA ± 1 and ISA ± 2 SHANSEP tests, thereconsolidation procedure proposed by Ladd and Foott(1974) yields a realistic description of the undrained behav-ior of NC RBBC, including the small value of the strain atpeak and some strain softening.

When the degree of disturbance is much greater (ISA ±5), it appears that the effects of disturbance are not erased,as the stress–strain curve shows a much larger strain at peakand the absence of any strain softening. The higher stresslevel and the variation in the lateral stress ratio cannot ex-plain the large difference in the undrained behavior of the

soil. In this case, it is likely that because of the high level ofdisturbance, either the structure of the soil is altered irre-versibly or consolidation to 3.2σp′ is not sufficient to bringthe soil back to the VCL (Santagata and Germaine 2002),and thus the final shear is not representative of a truly NCsoil.

Summary and conclusions

Triaxial element tests were conducted to investigate theeffect of sample disturbance on measured soil properties.The goal of the study was to investigate (i) the variation ofthe effects of disturbance with OCR; and (ii) the effective-ness of recompression and SHANSEP reconsolidation for re-covering the intact behavior of the soil.

Comparison of the behavior of NC and OC (OCR2–8)RBBC shows that for the same degree of disturbance (asmeasured by the magnitude of the strain cycle amplitude im-posed when disturbance is being simulated), as the OCR in-creases the effects of disturbance (effective stress loss,reduction in strength, increase in strain at peak, magnitudeof reconsolidation strains, etc.) are less significant.

The results for OC RBBC confirm previous observationsmade for NC RBBC (Santagata and Germaine 2002) that thedecrease in the effective stresses, and the destructuring ofthe soil are the fundamental mechanisms occurring as a re-sult of sampling. Unless the magnitude of the disturbancestraining is sufficient for the soil to fail, it appears that theeffects of disturbance on the undrained strength are modest.The stress–strain behavior is more significantly affected. Forlower degrees of disturbance, the observed behavior can beexplained by the effective stress decrease. The effects ofdestructuring of the soil appear to come into play when dis-turbance is associated with strain exceeding a critical thresh-old that depends on the stress history of the soil.

With regard to the postdisturbance K0-consolidation be-havior, the results substantiate the observation previouslymade for NC RBBC (Santagata and Germaine 2002) that thepreconsolidation stress is affected to a lesser degree by dis-turbance. Moreover, the data for OC RBBC confirm that atleast for the degrees of disturbance investigated, the K0-reloading behavior is primarily controlled by the effects ofeffective stress reduction. This, for the medium-sensitivitysoil tested, leads to an overestimate of the preconsolidationpressure as a result of disturbance.

The dependence of the magnitude of the reconsolidationstrain on the OCR of the soil limits the use of this parameteras a practical indicator of sample quality.

The investigation of the recompression reconsolidationprocedures substantiates statements by other researchers(e.g., Ladd and De Groot 2003) that for NC soils, thismethod provides a highly unsafe estimate of the undrainedstrength. For these soils, the SHANSEP method appears amore appropriate solution, as it always generates a safe esti-mate of the strength and for modest disturbance can providea reasonable stress–strain curve.

For OC soils, the recompression procedure is appropriatefor recovering the strength. If characterization of the stress–strain behavior is needed, the reconsolidation path must berepresentative of that which took place in the field.

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472 Can. Geotech. J. Vol. 42, 2005

Fig. 21. Undrained shear following SHANSEP reconsolidationfor (a) ISA ± 1, (b) ISA ± 2, and (c) ISA ± 5 disturbance ofNC RBBC.

Acknowledgements

The authors wish to thank Dr. Joseph Sinfield, who per-formed part of the tests presented in this paper while a grad-uate student at MIT. The support of the National ScienceFoundation, which provided funding (grant NO. 9114447-MSS) for this project, and that of the Italian National Re-search Council, which granted the first writer a fellowshipduring her first year at MIT, is also gratefully acknowledged.

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List of symbols

AOCR apparent overconsolidation ratioCc compression index

CR compression ratioe void ratio

Eu undrained secant stiffness

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IOCR induced overconsolidation ratioK lateral stress ratio

K0 coefficient of earth pressure at restm SHANSEP equation exponent

OCR overconsolidation ratiop′ average effective stress = (σv′ + σh′ )/2pm′ mean effective stress = (σv′ + 2σh′ )/3

pmc′ end of consolidation mean effective stress =(σvc′ + 2σhc′ )/3

q shear stress = (σv – σh)/2qf shear stress at failureRR recompression ratioS undrained strength ratio at OCR = 1

ue excess pore pressure = ∆uus shear-induced pore pressure = ∆u – ∆σoct

εa axial strainεc strain cycle amplitudeεp axial strain at failure (peak)εv volumetric strainεzz normal axial strain undergone by soil elements

during tube samplingσhc′ horizontal consolidation stressσp′ preconsolidation stress

σp[be] best estimate of preconsolidation stressσs′ sampling effective stressσv′ vertical effective stress

σvc′ vertical consolidation stressσvmax′ maximum vertical effective stress reached during

consolidationσoct octahedral stress = ( )/σ σ σ1 2 3 3+ +

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