ARTICLE

Effect of particle fabric on the coefficient of lateral earth pressureobserved during one-dimensional compression of sandSheri Northcutt and Dharma Wijewickreme

Abstract: The effect of initial particle fabric on the one-dimensional compression response of Fraser River sandwas investigated.One-dimensional compression with lateral stress measurement was carried out on reconstituted Fraser River sand specimensusing an instrumented oedometer. Laboratory specimens were reconstituted by air pluviation, tamping, and vibration and wereprepared with an initial relative density ranging from medium loose to very dense. For Fraser River sand in one-dimensionalcompression, air-pluviated specimens yielded the highest values for the coefficient of lateral earth pressure at rest (Ko), tampedspecimens produced the lowest Ko values, and vibrated specimens produced intermediate Ko values. The results from the presentstudy demonstrate that specimens resulting from different laboratory reconstitution methods (i.e., different initial particlefabrics) exhibit different one-dimensional compression responses and produce different Ko values. A “fabric factor” was intro-duced to account for the effect of the initial particle fabric on themeasured coefficient of lateral earth pressure at rest. Using thefabric factor, the constant volume friction angle, and the specimen relative density, a new empirical equation defining thecoefficient of lateral earth pressure at rest during normally consolidated loading is proposed.

Key words: sand, one-dimensional compression, particle fabric, coefficient of lateral earth pressure at rest.

Résumé : L’effet de la structure initiale des particules de sable de la rivière Fraser sur leur comportement en compressionunidimensionnelle a été étudié. Desmesures de compression unidimensionnelle avec contraintes latérales ont été réalisées avecun odomètre instrumenté sur des échantillons reconstitués de sable de la rivière Fraser. Les échantillons de laboratoire ont étéreconstitués par pluviation a l’air, par pilonnage et par vibration, et ont été préparés avec une densité relative initiale variant demoyennement lâche a très dense. Pour les échantillons de sable de la rivière Fraser en compression unidimensionnelle, leséchantillons préparés par pluviation a l’air ont donné les valeurs les plus élevées du coefficient de pression latérale des terres aurepos (Ko), les échantillons pilonnés ont produit les valeurs de Ko les plus faibles, et les échantillons vibrés ont généré des valeursde Ko intermédiaires. Les résultats de cette étude démontrent que les échantillons produits par différentes méthodes dereconstitution de laboratoire (c’est-a-dire différentes structures initiales des particules) présentent des comportements encompression unidimensionnelle différents, et produisent des valeurs de Ko différentes. Un facteur de « structure des particules »a été introduit pour tenir compte de l’effet de la structure initiale des particules sur le coefficient de pression latérale des terresau repos mesuré. À partir du facteur de structure des particules, de l’angle de friction a volume constant et de la densité relativede l’échantillon, une nouvelle équation empirique qui défini le coefficient de pression latérale des terres au repos durant lechargement normalement consolidé est proposée. [Traduit par la Rédaction]

Mots-clés : sable, compression unidimensionnelle, structure des particules, coefficient de pression latérale des terres au repos.

IntroductionThe coefficient of lateral earth pressure at rest, Ko, is used to

define the stress state in soil where no lateral deformation occurs.Ko is expressed as the ratio of horizontal effective stress to verticaleffective stress. Different approaches have been proposed toquantify Ko, primarily in terms of the soil effective friction angle(� ′) and overconsolidation ratio (OCR) (Jaky 1948; Schmidt 1967;Mayne and Kulhawy 1982; Mesri and Hayat 1993). However, thesetwo parameters alone may not be sufficient for defining the “at-rest” condition of soil, specifically granular soils.

Research has shown that the mechanical response of soil isrelated to its particle fabric. For reconstituted granular speci-mens, the initial particle fabric prior to loading is a function of thecharacteristic grain shape and specimen reconstitution method(Oda 1972). Limited studies have been published investigating theeffects of fabric in one-dimensional compression of sand(Mahmood et al. 1976; Okochi and Tatsuoka 1984; Wanatowskiand Chu 2007, 2008).

The present study was undertaken to investigate the effect ofinitial particle fabric on the behaviour of sand in one-dimensionalcompression. Laboratory specimens of Fraser River sandwere pre-pared by three different laboratory reconstitutionmethods over arange of initial relative densities. One-dimensional compressionresponses and determined Ko values were compared for speci-mens prepared at the same relative density, but reconstituted bydissimilar methods (i.e., different initial particle fabrics).

Material testedTesting was performed on dredged sand from the Fraser River,

located immediately south of Vancouver, British Columbia, Can-ada. From grain size analyses, the Fraser River sand utilized in thisstudy was found to be amedium-sized sand with less than 1% finesand an average particle size (D50) of 0.28 mm (see Fig. 1). The sandwas uniformly graded, with a coefficient of uniformity (Cu) of 1.80.In preparation for this study, the dredged Fraser River sand wascarefully divided into individual samples for testing using a split-ter to ensure consistent gradations.

Received 1 May 2012. Accepted 12 February 2013.

S. Northcutt and D. Wijewickreme. Department of Civil Engineering, The University of British Columbia, 6250 Applied Science Lane, Vancouver, BC V6T 1Z4, Canada.

Corresponding author: Dharma Wijewickreme (e-mail: dharmaw@civil.ubc.ca).

457

Can. Geotech. J. 50: 457–466 (2013) dx.doi.org/10.1139/cgj-2012-0162 Published at www.nrcresearchpress.com/cgj on 8 March 2013.

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Fraser River sand has an average mineral composition of 40%quartz, quartzite, and chert; 11% feldspar; 45% unstable rock frag-ments; and 4% other minerals (Garrison et al. 1969). Optical micro-scopic inspection of the sand showed angular to subroundedparticles (see Fig. 2). A specific gravity of 2.71 was measured for thesand, and maximum and minimum void ratios (emax and emin) of0.962 and 0.620, respectively, were assumed based on Sivathayalanand Vaid (2002).

Experimental aspects

Testing programThe main testing work for this study was conducted using air-

dried, reconstituted Fraser River sand specimens prepared by air-pluviation, tamping, and vibration. To represent a range ofconditions that may be encountered in the field, specimens wereprepared at three initial relative densities (Dr) of 30% (mediumloose), 60% (dense), and 85% (very dense). In the main testing pro-gram, a total of 54 one-dimensional compression tests were per-formed, as follows: (i) 20 tests on specimens prepared by airpluviation, (ii) 17 tests on specimens prepared by tamping, and(iii) 17 tests on specimens prepared by vibration (Northcutt 2010).

One-dimensional compression testing was conducted inthree successive phases: axial loading to a vertical pressure of250 kPa, axial unloading to a vertical pressure of 0 kPa, andaxial reloading to a vertical pressure of 400 kPa. All loading andunloading phases were performed at a constant rate of axialdeformation of 0.015 mm/min.

Supplementary to the above main testing program, an addi-tional 15 one-dimensional compression tests were performed, asfollows: (ia) three tests on dense (Dr ≈ 60%) specimens prepared bydifferent reconstitution methods at an axial deformation rate of0.0018 mm/min (about 10 times slower than the standard rate),(ib) three tests on dense (Dr ≈ 60%) specimens prepared by differentreconstitutionmethods at an axial deformation rate 0.15mm/min(about 10 times faster than the standard rate); (ii) three tests ondense (Dr ≈ 60%) specimens prepared by different reconstitution

methods and saturated prior to testing; and (iii) six tests on spec-imens prepared by gentle “pouring” deposition at zero dropheight, which produced very loose (Dr ≈ 0%) specimens dissimilarto conditions typically encountered in the field.

Testing apparatusTest specimens were reconstituted and tested in a highly pol-

ished, rigid, stainless steel oedometer. The oedometer was cir-cumscribed by a series of strain gauges capable of detecting verysmall hoop strains produced upon perimetric loading (see Fig. 3for schematic diagram). This testing equipment was initially de-veloped and has been used successfully by previous researchers atThe University of British Columbia. Similar instrumented oedom-eters have also been used by other researchers for lateral stressmeasurement during one-dimensional compression testing(Mahmood and Mitchell 1974; Mahmood et al. 1976; Mesri andHayat 1993).

Fig. 1. Pre- and post-test grain-size distributions of Fraser River sand.

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0.01 0.1 1 10Grain diameter (mm)

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Pre-test gradation: Fraser River sandPost-test gradation: very dense, air-pluviated specimenPost-test gradation: very dense, tamped specimenPost-test gradation: very dense, vibrated specimen

Fig. 2. Optical microscope photograph of Fraser River sand.

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Calibration of the instrumented oedometer was achieved bycapping the open ends of the oedometer ring and pressurizing theresulting chamber using a regulated compressed air supply. Themaximum pressure of the air supply is about 700 kPa. The preci-sion of the instrumented oedometer is ±0.04 kPa, and its stiffnessis approximately 2 × 10−5% radial strain per 100 kPa lateral pres-sure.

Specimen reconstitution methodsAs indicated in the previous section, the main test specimens

for this study were prepared by air-pluviation, tamping, and vi-bration. The specimen reconstitution procedures used are out-lined below. Further details are available in Northcutt (2010).

Air pluviation methodAir-pluviated specimens were reconstituted at different relative

densities by controlling the depositional intensity and particledrop height (Vaid and Negussey 1988). All air-pluviated specimenswere reconstituted in two separate lifts, each approximately12 mm thick. After each lift was deposited, a siphon connected toa vacuum was used to level the specimen surface.

For medium loose specimens (Dr ≈ 30%), a small funnel with a7 mm spout opening was used to pluviate sand into the specimencontainer from a drop height of about 22 cm. To ensure that thefunnel remained at a constant elevation during pluviation, a cy-lindrical frame was mounted onto the specimen container andthe funnel was traversed in a circular motion over the top of theframe. For dense specimens (Dr ≈ 60%), the funnel spout was con-stricted to 3.6 mm to reduce the depositional intensity and thesame drop height (as the medium loose specimens) was used. Forvery dense specimens (Dr ≈ 85%), the spout diameter was restrictedto 2.0 mm and the same drop height was used.

Tamping methodThe tamping method utilized in this study was based on the

moist tamping technique of “undercompaction,” which has beenshown to produce homogeneous laboratory specimens (Ladd1978). A custom tamping apparatus was devised for use with thespecimen container with a compactive foot weighing near 415 gand a diameter equal to that of the specimen. The compactive footwas connected to a tamping rod that was allowed to slide freely inthe vertical direction within the apparatus frame. To impart atamping blow, the rod was simply raised to a desired height andthen released under gravity.

For all specimens prepared with tamping, dry lifts of soil wereinitially deposited by funnel at a near zero drop height, a tech-nique that may be compared to a very gentle “pouring” deposi-tion. A small funnel with a 7 mm spout opening was traversedover the specimen container in a circular motion. As the level ofsand in the specimen container rose, the funnel was elevated to

maintain a consistent near-zero deposition height throughout thesand placement process. A tamping technique, as described be-low, was then applied to each lift to achieve the desired relativedensity and the lift was re-levelled with the siphon and vacuum.Two lifts were used for each specimen.

For medium loose, tamped specimens (Dr ≈ 30%), the dropheight of the tamping foot was 25 mm, and two blows for eachlayer were applied to reach the desired relative density. For dense,tamped specimens (Dr ≈ 60%), the drop height used was also25 mm, and the bottom layer received eight tamping blows andthe top layer received nine blows. For very dense, tamped speci-mens (Dr ≈ 85%), the tamping foot drop height was raised to35mm, and 38 blows were applied to the bottom lift and 45 blowswere applied to the top lift.

Vibration methodA vibrating table (Model CN-166, manufactured by Soiltest)

equipped with a cushioned steel vibrating deck and an electro-magnetic vibrator was used in this study (Soiltest 1990). The tableoperated at a frequency of 60 Hz and had a double-amplitudedisplacement range of 0.05 to 0.38 mm. For all specimens pre-pared with vibration, the method of initial sand placement in thespecimen container was the same as the gentle “pouring” deposi-tion technique used in the preparation of tamped specimens.

Medium loose specimens (Dr ≈ 30%) were prepared in two liftsand a single instantaneous pulse of vibratory compaction with adouble-amplitude of 0.08mm (as indicated by the vibrator controlknob) was applied over the entire specimen. Dense specimens(Dr ≈ 60%) were prepared in two lifts and then two vibratory pulseswith a double-amplitude of 0.10 mm were applied over the entirespecimen. For very dense specimens (Dr ≈ 85%), approximately 1 sof vibration was delivered to each individual lift with a double-amplitude of 0.10 mm. Subsequent to vibration, all specimenswere again re-levelled with the siphon and vacuum.

Saturation methodThe method used for saturating select reconstituted specimens

was modeled after Wijewickreme et al. (2005). In saturated speci-men preparation, the specimen was first reconstituted by eitherair-pluviation, tamping or vibration, and then about 20 pore vol-umes of de-aired water were slowly percolated up through thebase of the specimen from a reservoir. Only a small differentialhead of water was permitted between the specimen and the res-ervoir, not greater than 5 cm, to prevent specimen disturbance. Aconstant head of water approximately 1 cm above the specimenwas retained throughout compression testing to ensure contin-ued saturation. No flow between the specimen and the reservoirwas allowed during testing. Additional details of saturated speci-men preparation may be found in Northcutt (2010).

Fig. 3. Schematic of instrumented oedometer.

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Very loose preparation methodIn addition to the medium loose, dense, and very dense speci-

mens, it was considered of interest to test specimens reconsti-tuted in a very loose (Dr ≈ 0%) initial condition. As the abovepreparation approacheswould not yield very loose specimens, thegentle “pouring” deposition technique used for the initial place-ment of sand in tamped and vibrated specimens was utilized toproduce specimens near zero relative density. The authors wish torecognize that zero relative density is dissimilar to conditionstypically encountered in the field. However, it was still considereduseful to examine the response of such specimens, as it pertains toone extreme of the density range of the tested soil.

Specimen qualityTo verify the uniformity of reconstituted specimens, dissections

were performed on specimens prepared by different reconstitu-tion methods at different degrees of relative density. The proce-dure utilized for specimen dissection was modeled after thatpresented by Wijewickreme et al. (2005). Further details are avail-able in Northcutt (2010).

From specimen dissections, air-pluviated specimen lifts exhib-ited ±1% to ±7% deviation from the average specimen relative den-sity. Tamped specimen lifts varied a maximum of ±2% deviation,and vibrated specimen lifts showed ±2% to ±4%. These variationsare comparable to specimen uniformity data available in litera-ture (Vaid and Sivathayalan 2000; Wijewickreme et al. 2005).

Test results

Typical one-dimensional stress–strain response:air-pluviated sand

A typical stress–strain response for sand in one-dimensionalcompression is described below using data from testing under-taken on a Fraser River sand specimen prepared at a selectedinitial relative density and by a selected specimen reconstitutionmethod. Similar data for Fraser River sand specimens prepared atvarious initial relative densities and by different specimen recon-stitution methods (i.e., different particle fabrics) are presented indetail in Northcutt (2010). However, for brevity they are not pre-sented herein.

The typical stress–strain response formedium loose, air-pluviatedFraser River sand in one-dimensional compression is shown in Fig. 4in terms of specimen void ratio versus logarithm of vertical effectivestress. As shown, theplotted series of concave-downcurves coincideswith the three distinct loading phases: virgin loading, unloading,and reloading. First, the specimen undergoes virgin loading to250 kPa vertical stress. Note that the majority of the void ratiochange or axial deformation experienced by the specimen occursduring this phase. Next, the unloading phase from 250 kPa to near0 kPa vertical stress exhibits a slightly flatter slope than observed invirgin loading. Lastly, the specimen is re-compressed to 400 kPa inthe reloading phase, beyond the 250 kPa maximum vertical stressthat was applied during the virgin loading case. For comparison,Fig. 5 presents the specimen void ratio versus logarithm of verticaleffective stress for select air-pluviated specimens at different initialrelative densities.

Figure 6 shows a typical horizontal effective stress versus verti-cal effective stress plot obtained from a medium loose, air-pluviated Fraser River sand specimen. Each loading phase isplotted separately, for clarity. A strong linear relation through theorigin is apparent during both the virgin loading and reloadingphases. For reloading, note that the slope does not appear to changeas the specimen is loaded in excess of the pre-consolidation pressure(�p

′ =250kPa).During theunloadingphase, amildly “S-shaped” curveis exhibited.

Figure 7 shows a typical coefficient of lateral earth pressure atrest versus vertical effective stress plot obtained from the re-sponse observed for a medium loose, air-pluviated Fraser River

sand specimen. Each loading phase is plotted separately, for clar-ity. During the virgin loading phase, Ko levels off to a relativelyconstant value, in excess of about 25 kPa vertical effective stress. Aslight decrease in Ko may be noted with increasing vertical stress,although the change is not significant. In unloading, Ko increasesconsiderably, usually in excess of unity. During reloading, Ko onceagain levels off to a reasonably constant value, in excess of about125 kPa vertical effective stress.

Observations from main testing programFigures 8, 9, and 10, respectively, present Ko determined from

the results of one-dimensional compression tests performed onair-pluviated, tamped, and vibrated specimens versus relativedensity. Ko is presented at the end of virgin loading (at a verticalstress of 250 kPa) and at the end of reloading (at a vertical stressof 400 kPa), corresponding to a normally consolidated state(OCR = 1). Relative density was determined from the specimendimensions at the instance of lateral stress measurement.

The effect of the specimen reconstitution method (i.e., initialparticle fabric) is demonstrated in Figs. 8, 9, and 10. Air-pluviatedspecimens produced the highest Ko values, tamped specimensranked the lowest, and vibrated specimens produced intermedi-ate values. The scatter in Ko values appeared slightly more prom-inent for air-pluviated specimens.

Because particle fabric can be closely linked tomaterial gradation,itwas consideredof interest to verify that the grain-size distributionsof the tested specimens were comparable and that no significantparticle damagehadoccurredduring either specimen reconstitutionor compression testing. A series of grain size analyses performed onselected FraserRiver sand specimens subsequent toone-dimensionalcompression testing exhibited no significant changes in particle-sizedistribution (see Fig. 1 for comparison).

In addition to observations of the effect of initial particle fabric,the results presented in Figs. 8, 9, and 10 show a decreasing trendof Ko with increasing specimen relative density for specimensprepared using all reconstitution methods in the tested range of30% ≤ Dr ≤ 85%. It is important to note that the observed Ko versusDr trends include data from both the end of loading and reloadingphases, which occur at significantly different levels of verticaleffective stress. This suggests that Ko is relatively insensitive to thevertical effective stress.

Figures 11 and 12 display Ko during unloading and reloading atselected OCR values of 2, 5, and 10, determined for air-pluviatedspecimens only. Testing results for tamped and vibrated speci-mens, available in Northcutt (2010), yielded similar results, but forbrevity are not presented herein. From Figs. 11 and 12, Ko valuesduring both unloading and reloading tended to increase withincreasing OCR. Also, the observed scatter increased with increas-ing OCR. Interestingly, comparison of data from Figs. 11 and12 indicates that Ko values differ considerably for overconsoli-dated specimens at the same OCR, but with different loadinghistories. However, Figs. 8, 9, and 10 show that Ko values are com-parable for specimens that are normally consolidated (OCR = 1),despite different loading histories.

As noted previously, considerable scatter was observed in theoverconsolidated Ko values, specifically during the unloadingphase. Still, the degree of scatter is consistent with that reportedin literature. Mayne (2011) presents an extensive compilation of Ko

values from a number of researchers for overconsolidated clean,unaged, and uncemented quartz and silica sands, derived fromlarge calibration chamber tests, laboratory triaxial and oedom-eter tests, and field measurements. The scatter observed for Ko

during the testing presented herein is less significant than thescatter observed in the database compiled by Mayne (2011) for agiven material and a given type of test.

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Observations from supplemental testing programIn addition to the main testing program, a limited number of

supplemental one-dimensional compression tests were carried out.Supplemental testing was performed at varied rates of axial defor-

mation and showed that air-pluviated, tamped, and vibrated speci-men responses were reasonably independent of the testing axialdeformation rate. Also, supplemental one-dimensional compressiontests were performed on saturated specimens and showed that spec-

Fig. 4. Void ratio versus vertical effective stress for medium loose, air-pluviated Fraser River sand specimen (initial Dr ≈ 30%).

Fig. 5. Void ratio versus vertical effective stress for air-pluviated Fraser River sand specimens at different initial relative densities.

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imens prepared by all reconstitutionmethods responded reasonablyindependent of specimen saturation (Northcutt 2010).

Further, supplemental testing was carried out to investigate thestress–strain response of very loose (Dr ≈ 0%) specimens preparedby gentle “pouring” deposition at zero drop height. The Ko valuesat the end of virgin loading observed for the very loose specimensgenerally ranged from 0.47 to 0.50. For comparison, Ko values formedium loose, air-pluviated specimens, whichwere considered tohave a similar reconstituted particle fabric, generally ranged from0.51 to 0.60. This comparison is in contrast to the Ko versus Drtrends observed over 30% ≤ Dr ≤ 85%.

Although the results for tests conducted on Dr ≈ 0% specimensappear anomalous, the authors are of the opinion that the veryloose reconstitution method produced an artificially loose, “col-lapsible” particle structure. This phenomenon has been observedin other laboratory sand specimens and may be attributed to un-stable grain contacts (Skopek et al. 1994). Similarly, punchingshear failure in very loose sand has been observed in practice,where vertical movement occurs upon loading, but no lateralmovement occurs (Vesic 1973).

DiscussionEffect of particle fabric on Ko

The present study has shown that the stress–strain response ofsand specimens is appreciably influenced by the reconstituted

particle fabric. While the particle arrangement was not directlyquantified in this study, some observations can be made withrespect to the general characteristics of granular particle fabricarising from different reconstitution methods.

Mahmood and Mitchell (1974) studied grain orientations in re-constituted granular specimens that had been impregnated withepoxy resin to preserve the reconstituted particle fabric. Pluviatedgranular specimens were shown to exhibit a strongly preferredgrain orientation in vertical sections, with the long particle axesnormal to the vertical plane. Tamped granular specimens exhib-ited a weakly bimodal preferential grain orientation in verticalsections, with the long particle axes aligned at about 45° fromhorizontal. For vibrated granular specimens, practically randomgrain orientations were observed (Mahmood and Mitchell 1974).

With respect to the present study, it is not unreasonable toexpect that the grain orientation would have some correlationwith the measured coefficient of lateral earth pressure at rest.Air-pluviated specimens consistently showed the greatest Ko val-ues, and as presented above, this would correspond to the casewhere the long particle axes are primarily oriented normal to thevertical plane. Vibrated specimens consistently showed interme-diate Ko values, with presumably random grain orientation.Tamped specimens consistently yielded the lowest Ko values, withthe long particle axes presumably tending to be oriented at 45°

Fig. 6. Horizontal effective stress versus vertical effective stress formedium loose, air-pluviated Fraser River sand specimen (initialDr ≈ 30%).

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from horizontal. It is possible that as grain orientations shift fromnormal positioning to more parallel orientation, with respect tothe vertical plane, the coefficient of lateral earth pressure at restdecreases.

Fabric factor to account for particle fabric in estimating KoTraditionally, Ko has been estimated using the Jaky (1948) sim-

plified formula. Jaky's formula is intended for the virgin loadingphase only and should be used with the effective friction angle(� ′) of the soil (Mayne and Kulhawy 1982). Estimating the effectivefriction angle of soil for use in this equation can be a complextask. Not only is it a function of relative density, but it is also afunction of confining pressure. Moreover, the value of the frictionangle measured in the laboratory can be affected by the mode ofloading (i.e., specific testing apparatus utilized) and also likely bythe particle fabric itself (Bolton 1986; Lee and Seed 1967).

Even if you assume that � ′ can be readily estimated, Jaky'sformula stillmay not provide a suitable determination of Ko. Fromthe results of the present study, Ko was shown to decrease withincreasing specimen relative density. As � ′ has been shown toincrease with relative density (Cornforth 1973), Jaky's formulawould be in agreement with these observations. In contrast, theresults of the present study indicate that Ko is reasonably indepen-

dent of the vertical effective stress. As � ′ is dependent on confiningstress (Lee and Seed 1967), Jaky's formula would be inconsistent. AsMichalowski (2005) and Wanatowski and Chu (2007) have also indi-cated, it is the authors' opinion that the use of Jaky's equation doesnot necessarily a provide a reliable estimation of Ko.

Therefore, while Ko is related to the effective friction angle, itcannot be accurately defined by this parameter alone. Based ex-clusively on results from the present study, the following relation-ship was constructed to describe the coefficient of lateral earthpressure at rest for normally consolidated sand specimens (Konc)as a function of the constant volume friction angle (�cv

′ ), the re-constituted particle fabric, and the specimen relative density:

[1] Konc � �F �Dr

2

4 �(1 � sin �cv′ )

where F is the fabric factor.Given the uncertainties associated with the estimation of � ′, it

was considered prudent to instead use �cv′ as the parameter rep-

resenting drained strength. The constant volume friction angle isgenerally accepted to be a unique function of mineral composi-

Fig. 7. Coefficient of lateral earth pressure at rest versus vertical effective stress for medium loose, air-pluviated Fraser River sand specimen(initial Dr ≈ 30%).

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tion, independent of relative density and confining stress(Negussey et al. 1988). Moreover, experimental studies on FraserRiver sand confirm that �cv

′ is also independent of the specimenreconstitution technique (i.e., initial particle fabric) (Vaid et al.1999). In accordance with constant volume friction angle valuesreported in literature for Fraser River sand, �cv

′ is assumed to beequal to 32° for the present study (Vaid and Thomas 1995).

To account for the effect of reconstituted particle fabric, a “fabricfactor” is introduced in eq. [1]. The constant value of the fabric factor

is selected according to the specimen reconstitution method. Basedon examination of data from the present study, F ranges from 1.02 to1.38 for air-pluviated specimens, from 0.94 to 1.18 for vibrated speci-mens, and from 0.82 to 1.04 for tamped specimens. As shown inFigs. 8, 9, and 10, the new fabric factor relation provides a reasonablefit for the normally consolidated reconstituted sand specimens fromthe present study. Although not attempted herein, the authors be-lieve that the idea of a “fabric factor” could also applied in determin-ing Ko of overconsolidated reconstituted sand specimens.

Fig. 8. Coefficient of lateral earth pressure at rest versus relative density for normally consolidated, air-pluviated Fraser River sandspecimens. F, fabric factor; Konc, coefficient of lateral earth pressure at rest for normally consolidated sand specimens.

Fig. 9. Coefficient of lateral earth pressure at rest versus relative density for normally consolidated, vibrated Fraser River sand specimens.

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ConclusionA study was conducted to examine the effect of initial particle

fabric on the one-dimensional compression response of sand spec-imens reconstituted using air pluviation, tamping, and vibration.From the present study, air-pluviated specimens yielded the high-est Ko values, tamped specimens produced the lowest Ko values,and vibrated specimens ranked intermediate.

As the currentmethods utilized in practice for determination ofKo do not account for particle fabric, a “fabric factor” (F) wasintroduced to express the effect of the initial particle fabric on the

coefficient of lateral earth pressure at rest for normally consoli-dated specimens (Konc). Using the fabric factor, the constant vol-ume friction angle, and the specimen relative density, a newequation defining Ko for normally consolidated specimens wasshown to fit well with results from the present study.

Further research should be pursued to more thoroughly investi-gate the behaviour of sand in one-dimensional compression. Specificattention should be paid to the effects of both initial particle fabricand evolving particle fabric during compression. The applicability ofamore general “fabric factor” for other sands andgranularmaterials

Fig. 10. Coefficient of lateral earth pressure at rest versus relative density for normally consolidated, tamped Fraser River sand specimens.

Fig. 11. Relative density versus coefficient of lateral earth pressure at rest for overconsolidated, air-pluviated Fraser River sand specimensduring unloading.

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should be considered. Also, the behaviour of Ko for different loadinghistories should be examined more thoroughly. Such research mayhelp to define themost appropriatemethods for determinationofKoin practice.

Overall, the present study should serve as a reminder that particlefabric in laboratory specimens and in situ deposits must be dulyacknowledged. Diverse particle fabrics can have a distinct effect on asoil's mechanical behaviour, as demonstrated herein, and must notbe overlooked in the laboratory or in practice.

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Fig. 12. Relative density versus coefficient of lateral earth pressure at rest for overconsolidated, air-pluviated Fraser River sand specimensduring reloading.

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