Apparatus for Constant Rate-of-Strain Consolidation … for Constant Rate-of-Strain Consolidation of...

Kristin M. Sample1 and Charles D. Shackelford2

Apparatus for Constant Rate-of-Strain Consolidationof Slurry Mixed Soils

ABSTRACT: A constant rate-of-strain (CRS) consolidation apparatus was developed to allow testing of slurry mixed soils prepared within102-mm (4-in) inner diameter acrylic columns with varying lengths using a laboratory mixing apparatus. The laboratory mixing apparatus waspreviously developed to mimic the mixing process associated with the use of slurries containing bentonite and granular zero-valent iron (ZVI) forin situ remediation at sites contaminated with chlorinated solvents. Example consolidation results are provided for specimens of dry sand mixedwith bentonite-ZVI slurry with lengths of 305 mm (12 in), 457 mm (18 in), or 610 mm (24 in). After mixing, all of the specimens except for the610-mm specimen were consolidated directly within the same columns to minimize sampling disturbance. The 610-mm specimen was sectionedinto thirds before consolidation testing to evaluate effects of vertical heterogeneity. As expected, all specimens were highly compressible, withmaximum (large) strains ranging from 10.7 % to 48.0 %. For specimens prepared and tested within the same columns, the effects of differences incolumn length and strain rate were minor. The CRS testing apparatus offered a convenient, rapid (< 3 day), and economical approach for evaluat-ing the consolidation behavior of the bentonite-ZVI slurry mixed sand.

KEYWORDS: bentonite, consolidation, slurry, soil mixing, zero-valent iron

Introduction

The zero-valent iron (ZVI)-clay technology refers to in situ reme-diation of subsurface source zones contaminated with chlorinatedsolvents (e.g., tetrachloroethylene, trichloroethylene, carbon tetra-chloride) by injecting and mixing granular ZVI suspended inwater-based clay slurry directly into the contaminated soil. Theclay, typically either kaolin or bentonite, serves to suspend thegranular ZVI in the slurry, and the ZVI serves as the reactivemedia that reductively dechlorinates (i.e., treats) the chlorinatedsolvents (Gillham and O’Hannesin 1994, Wadley et al. 2005).

Injection of the water-based clay slurry results in surface heavingand an increase in soil compressibility and decrease in soil shearstrength. As a result, redevelopment of sites treated using this tech-nology may be hindered by the post-mixing condition of the treatedzone. In this regard, knowledge of the consolidation behavior of themixed soil is crucial to understanding the potential for future devel-opment of the treated site. However, the unique composition andproperties of such soil-ZVI mixtures present several issues that typi-cally are not encountered in routine testing of more uniform founda-tion soils. For example, the injection of ZVI-clay slurry duringmixing converts what typically is essentially an undisturbed, coarse-grained parent soil of minimal compressibility (e.g., contaminatedsand and/or gravel) into a remolded, highly compressible mixture. In

addition, the composition and properties of the mixed materials varysignificantly with depth due to changes in overburden stress and thehigh level of disturbance caused by the mixing auger.

A laboratory mixing apparatus was previously developed tomimic the mixing process associated with injection of ZVI-clayslurry into a coarse-grained parent soil. The mixing apparatus isdescribed in detail in Castelbaum et al. (2011). As a result of thenon-uniform property distributions of the mixtures, relatively largeconsolidation specimens were preferred in order to encompass thebehavior of as much of the mixed soil column as possible. As aresult, an apparatus was developed to perform constant rate-of-strain(CRS) consolidation testing of entire soil mixed specimens, whichcould be transferred directly from the mixing machine to the loadframe without removing the specimens from the mixing columns,thereby minimizing disturbance. The CRS method also allowed forshorter test durations than standard, incremental load tests, as wellas easier, automated data collection (e.g., Wissa et al. 1971, Armourand Drnevich 1986, Olson 1986, Sheahan and Watters 1997).

A standard has been developed for CRS testing (ASTMD4186-06), which is a method that has been used for consolida-tion testing and described in the literature since 1959 (e.g., Hamil-ton and Crawford 1959, Crawford 1964, Smith and Wahls 1969,Wissa et al. 1971, Deen et al. 1978, Gorman et al. 1978, Armourand Drnevich 1986). Generally, specimens are contained within aconsolidation cell and subjected to a constant rate of displacement.The rate of displacement is selected to limit excess pore pressuresgenerated during the test. Appropriate criteria for selecting a dis-placement rate (or strain rate for a particular specimen) have beendiscussed extensively in the literature (e.g., Smith and Wahls1969, Wissa et al. 1971, Armour and Drnevich 1986).

Standard triaxial load frames often are used to apply a constantrate of displacement. Therefore, specimen size is usually limited

Manuscript received January 26, 2011; accepted for publication October 26,2011; published online March 2012.

1P.E., Ph.D. Candidate, Dept. of Civil and Environmental Engineering,1372 Campus Delivery, Colorado State Univ., Fort Collins, CO 80523-1372.

2P.E., Professor, Dept. of Civil and Environmental Engineering, 1372Campus Delivery, Colorado State Univ., Fort Collins, CO 80523-1372(Corresponding author), e-mail: [email protected]

Copyright VC 2012 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. 409

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by the triaxial frame size (Kassim and Clarke 1999), and previousstudies typically have involved relatively homogeneous specimenswith pre-consolidation thicknesses (heights) ranging from 10 mmto 30 mm (e.g., Crawford 1964, Armour and Drnevich 1986,Larsson and Sallfors 1986, Silvestri et al. 1986, Kassim andClarke 1999). Thicker specimens (60-mm-thick on average) weretested by Sheahan and Watters (1997), who used a modifiedRowe cell to consolidate Boston blue clay.

To capture the vertical heterogeneity of the ZVI-clay slurrymixed specimens used in this study, significantly thicker speci-mens than those used in previous CRS studies were required. Inaddition, to minimize disturbance of the mixed specimens, con-solidation testing within the same column as used for specimenpreparation was desired. Therefore, the relatively large columnsrequired to prepare ZVI-clay slurry specimens with the mixingmachine also had to serve as CRS consolidation cells.

The bentonite-ZVI slurry mixed specimens were prepareddirectly within relatively large acrylic columns, with an innerdiameter of 102 mm (4 in) and lengths of 305 mm (12 in), 457 mm(18 in), or 610 mm (24 in), using the laboratory mixing apparatus.Acrylic columns and boxes have been used for several decades toprepare and consolidate soil samples at high water contents (e.g.,Been and Sills 1981, Warren and Gabr 2005, Yeung et al. 2008,Meric et al. 2010). The 610-mm column was comprised of severalinterlocked 20-mm (0.8-in)-long segments, while the rest of thecolumns were single, solid acrylic tubes. Following mixing, thespecimens were consolidated directly within the mixing columnsusing a triaxial load frame, with the exception of the specimenmixed in the 610-mm (24-in)-long column, which was separatedinto approximate thirds and placed in 152-mm (6-in)-long col-umns prior to consolidation testing to evaluate possible effects ofvertical heterogeneity on the consolidation behavior.

Materials and Experimental Methods

Materials

The two constituent soils used in the study were fine, Ottawa sand(Grade F-58 foundry sand, U.S. Silica Company, Ottawa, IL), andpowdered, polymer modified, air-float bentonite (Hydrogel, WYO-BEN, Inc., Grey Bull, WY). The measured specific gravity of solids(ASTM D854), Gs, of the sand and bentonite were 2.67 and 2.75,respectively. The sand classified as poorly graded sand (SP) and thebentonite classified as high plasticity clay (CH) based on the UnifiedSoil Classification System (ASTM D2487). The granular ZVI usedin this study was obtained from Peerless Metal Powders & AbrasiveCo., Inc., Detroit, MI. The measured Gs for the granular ZVI filingswas 6.67, and the particle sizes ranged from 0.15 to 0.30 mm.

Deionized water (DIW), with an electrical conductivity (EC)less than 1.0 mS/m at 25�C was used during specimen prepara-tion. The DIW was de-aired for saturation of the pore-waterpressure line and preparation of saturated sand specimens.

Preparation of Injection Slurry

The injection slurry (i.e., bentonite plus ZVI suspended in water)was prepared by adding the appropriate mass of granular ZVI to

a 5 % bentonite-water slurry. The bentonite-water slurry was pre-pared by adding 5 g of bentonite (dry mass basis) per 95 g ofwater. The slurry was mixed with high-speed colloidal mixers andallowed to hydrate for a minimum of 16 h before adding the ZVI.The viscosity of the bentonite-water slurry measured using aMarsh Funnel ranged from 40 to 45 s, which is typical of benton-ite slurries used in soil-bentonite vertical cutoff walls (Evans1993). Further details on preparation of the injection slurry areprovided by Castelbaum (2007), Sample (2007), and Castelbaumet al. (2011).

The dry mass of sand in the column was used as the basis fordetermining the minimum required amount of the bentonite-ZVIslurry to be injected into the column. The required mass ofhydrated bentonite slurry was determined from the percentage ofbentonite in the slurry and the target bentonite content of the post-mixed column. The required mass of ZVI was determined simi-larly by the target iron content of the post-mixed column.

Specimen Preparation in Test Columns

Before mixing, specimens comprised of fine sand were preparedin clear, acrylic columns. The bottoms of the columns were6.35-mm (0.25-in)-thick, 114.3-mm (4.5-in) inner-diameter ringsthat inserted into an acrylic base. A small reservoir (25.4-mmdiameter, 6.35-mm height) and pore-water pressure line werelocated under the center of the column (within the acrylic base).An inset for a 6.35-mm (0.25-in)-thick O-ring under the walls ofthe column allows for a watertight seal at the connection of thebase and the column. The top ends of the columns were cut to pro-vide a 6.35-mm (0.25-in)-thick, 102-mm (4-in) inner-diameterring that inserts into an acrylic top plate to secure the column tofour 650-mm (26-in) threaded rods, which also attach to thebase plate.

Prior to placement of the sand, the pore-water pressure line inthe base of the column was saturated with de-aired water, and a6.35-mm (0.25-in)-thick, 102-mm (4-in)-diameter porous stonewas placed on the base and covered with two sheets of saturatedNo. 42 Whatman filter paper (Whatman Inc., Florham Park, NJ).The sand was placed using air pluviation (i.e., raining of sandthrough air), as described by Zlatovic and Ishihara (1997) andOkamoto and Fityus (2006), via a funnel and tamped in lifts toprovide a uniform pre-mix density of approximately 1590 kg/m3

(99 lb/ft3). The pre-mix specimen thicknesses (i.e., heights) for the305-mm (12-in), 457-mm (18-in), and 610-mm (24-in)-tall col-umns were 200 mm (7.9 in), 250 mm (9.8 in), and 310 mm (12 in),respectively.

Mixing and Consolidation Equipment

Specimens of bentonite-ZVI slurry mixed sand were preparedusing the laboratory mixing apparatus. Sand-filled columns wereattached to the base plate of the mixing machine, as shown inFig. 1. The laboratory mixing machine mimics field applications,using a hollow-stem auger to inject slurry as the auger rotates andmoves vertically through the soil (Shackelford et al. 2005). Thebentonite-ZVI slurry was stored in a stainless steel reservoir anddelivered via piston displacement. All specimens were mixed with

410 GEOTECHNICAL TESTING JOURNAL


two soil-mixing passes. After the auger was extracted from themixed specimen upon completion of the second mixing pass, thecolumn was removed from the mixing machine and the finalthickness and weight of the mixed specimen were recorded. Thepost-mix specimen thicknesses typically were 240 mm (9.4 in),310 mm (12 in), and 395 mm (16 in) for columns with heightsof 305 mm (12 in), 457 mm (18 in), and 610 mm (24 in), respec-tively. After mixing, the specimens were covered, sealed withplastic wrap (Glad Products Company, Oakland, CA), andallowed to undergo self-weight consolidation for 1 day beforeCRS testing. Significant settlement (> 20 mm) or settling of thebentonite from the slurry at the top of the specimen was notobserved during this duration of self-weight consolidation. Furtherdetails of the mixing procedure are described by Castelbaum(2007), Sample (2007), and Castelbaum et al. (2011).

An evaluation of the resulting distributions in the physicalproperties of numerous specimens prepared using the mixingapparatus indicated that the procedure results in non-uniformproperty distributions, as illustrated in Castelbaum et al. (2011).Such distributions typically are characterized by looser conditionsnearer the top of the column due to lesser confinement, and arebelieved to mimic in a general manner the nature of the conditionsthat exist in field applications of the ZVI-clay technology (e.g.,Shackelford et al. 2005). As a result of this non-uniform distribu-

tion in physical properties, an evaluation of the effect of the loca-tion within the mixed column on the resulting consolidationbehavior was desired. Therefore, the specimen prepared in the610-mm (24-in)-tall column was subsequently split into segmentsrepresenting approximately the top, middle, and bottom thirds ofthe mixed column. The 610-mm (24-in) column was comprised of20-mm (0.8-in) long, interlocking sections of clear acrylic tubing,which were stacked to provide the total length of 610 mm (24 in).This configuration is described in detail in Castelbaum et al.(2011). The use of several interlocking segments allowed for split-ting of the specimen at a variety of heights (and, therefore, wasunnecessary for the other soil columns which were not split intothirds). The typical mixed length of the specimens prepared in the610-mm-long column was 395 mm (16 in), which was partitionedinto thirds (by splitting at the appropriate segment), and placed in152-mm (6-in)-long, solid acrylic tubes for consolidation testing.Specimens were trimmed prior to consolidation testing, resultingin typical specimen heights of 80 mm (3.1 in).

Mixed columns were placed directly in the load frame for CRStesting, as illustrated in Figs. 2 and 3. The pore-water pressure lineat the base of the column was connected to the pore-pressure trans-ducer (ELE International model 27-1625, 1000 kPa capacity,Loveland, CO). Three No. 42 Whatman filter papers were placedcarefully on top of the mixed specimen, followed by a saturatedporous stone (trimmed to a diameter slightly less than 102 mm),three more No. 42 filter papers, and the top platen (Fig. 2(b)). Thepurpose of the stone on top of the specimen was to evenly distrib-ute drainage over the bottom face of the platen. Several 3.85-mm(0.15-in)-diameter holes were drilled in the platen to provide drain-age (Fig. 2(c)). Two O-rings fit into insets around the perimeter ofthe platen to prevent sidewall drainage, and the O-rings were gen-erously coated with vacuum grease to reduce wall friction. A smallspherical groove was machined into the top of the platen to accom-modate the tip of an S-type load cell (ELE International model 27-1583, Loveland, CO), which was secured to the cross arm of theload frame (Geotest model S5760, Evanston, IL). The capacities ofthe load cell and load frame were 8.9 kN (2000 lb) and 10 kN(2248 lb), respectively, and the maximum pedestal travel was76 mm (3 in). A linear position transducer, or LPT (Geotac, Hous-ton, TX), with a measurement range of 150 mm (6 in) was attachedto the top plate and used to measure the relative movement of thetop platen to verify the deformation rate during loading.

Testing Conditions

Selection of Strain Rate—The maximum value for thestrain rate, SR, to be used in CRS testing according to ASTMD4186-89 is the value of SR that limits the ratio of the excesspore-water pressure, uh, to the applied vertical total stress, rv, tovalues from less than 3 % to 30 % (note that the testing reported inthis study was conducted prior to promulgation of ASTM D4186-06, which revised the upper limit on uh, to only 15 % of rv). Thus,expected values of uh/rv, also known as the pore-water pressureratio, PPR, generally are determined based on the typical coeffi-cient of consolidation, cv, for the type of soil to be tested. How-ever, because cv values of bentonite-ZVI slurry mixed sand were

FIG. 1—Laboratory-scale mixing apparatus for injecting and mixing benton-ite-zero-valent iron slurries into dry sand.

SAMPLE AND SHACKELFORD ON CONSTANT RATE-OF-STRAIN CONSOLIDATION APPARATUS 411


unknown, SRs for the testing program initially were selected basedon those from the literature, which according to Head (1992) typi-cally range from 1.67� 10�8 s�1 (i.e., 0.0001 % min�1) to6.67� 10�6 s�1 (i.e., 0.04 % min�1).

On the basis of the reported ranges in SR and recommendedvalues for PPR, SR values from 6.33� 10�7 s�1 to 5.00� 10�6 s�1

were chosen for the specimens of bentonite-ZVI slurry mixedsand tested in this study. In addition, the deformation rate limits(minimum 1.67� 10�5 mm s�1) of the load frame prevented theuse of SRs lower than 2.08� 10�7 s�1, 6.94� 10�8 s�1, and5.38� 10�8 s�1 for specimens contained within testingcolumns with lengths of 152 mm (6 in), 305 mm (12 in), and457 mm (18 in), respectively. Strain rates faster than 2.50� 10�6 s�1

for specimens contained within longer, 305 mm (12 in) and 457 mm(18 in) test columns resulted in PPR values> 30 % and, therefore,were not used.

Evaluation of Sidewall Friction

Two possible sources of sidewall friction were considered, viz.,(1) friction between the O-rings in the top platen and the innerwall of the column, and (2) friction between the mixed specimensand the inner column wall. With respect to (1), several steps weretaken to account for friction between the watertight top platen and

the inner column wall, including (a) greasing the sides of the topplaten generously with vacuum grease, (b) performing severalfriction tests at the beginning to determine which set of conditions(e.g., strain rate, O-rings) would provide the lowest friction, and(c) performing separate friction calibration tests conducted with-out specimens to generate friction correction curves for eachacrylic column to allow for correcting the data for the frictioncomponent and ensure that the friction was a very small to negligi-ble component of the measured forces. In the case of (c), the fric-tion between the top platen O-rings and the inner column wallincreased with increasing strain rate (Sample 2007).

In terms of the second source of friction, the friction betweentest specimens and the inner walls of test cells tends to increase asthe height-to-diameter ratio of the specimens increases. Thus,given that the heights of the specimens tested in this study weregreater than the diameter of the specimens, there was some con-cern about the influence of the friction between the slurry mixedspecimens and the inner column wall during testing. In an attemptto reduce this concern, the columns were constructed from new,smooth, clean acrylic tubing (e.g., Gniel and Bouazza 2008),although the inside of the column was not coated or greased likethe top platens, primarily because the grease likely would nothave withstood the mixing of the specimens within the columns.Also, the friction between the top platen O-rings and the inner

FIG. 2—(a) Cross-sectional schematic of CRS testing apparatus, and pictorial views showing (b) filter paper and porous stone setup for the test specimens and (c)load cell placement and drainage through the top platen.



column wall likely was substantially greater than that between thespecimens and the inner column wall. Nonetheless, this issue isdifficult to quantify or avoid easily (as with most one-dimensionalconsolidation tests). The composition and properties of the mixedsoil specimens already vary significantly with depth, and addi-tional variations to the soil structure may be introduced by consol-idating the relatively long samples within the acrylic tubing,further complicating the evaluation of friction. Therefore, theresults of the study were evaluated with an understanding thatsuch sidewall friction may have influenced the data (e.g., Wonget al. 2008).

Example Results

Example results for the slurry-mixed sand are presented in whatfollows. The test conditions for the mixing stage and the CRSconsolidation stage are summarized in Table 1. These exampletests allowed for an evaluation of (1) the ability to preparebentonite-ZVI slurry mixed sand specimens with the mixingmachine and subsequently measure representative consolidationbehavior of the mixed material (as an entire column or in sec-tions), and (2) the effects of mixed specimen thickness and theapplied strain rate on measured consolidation behavior. As indi-cated in Table 1, specimens Nos. 1, 2, and 3 represented the top,middle, and bottom thirds, respectively, of a single mixed columnof bentonite-ZVI slurry mixed sand. Specimens Nos. 4 and 5 weremixed and consolidated at the same SR within columns with

lengths of 305 mm (12 in) and 457 (18 in) to facilitate an evalua-tion of the effect of specimen thickness (or column height) on themeasured consolidation behavior. Specimen No. 6 was testedwithin a 457-mm (18-in)-long column, but at a lower SR thanspecimen No. 5 to facilitate an evaluation of the effect of SR. Themixing stage for each test was completed within a couple of hours,while the consolidation stages were completed within periodsranging from 1 to 3 days.

The example test data were analyzed in accordance with thelinear and nonlinear small-strain theories published by Wissaet al. (1971) and the large-strain theory published by Lee (1981).Results from the linear and nonlinear theories were essentiallyidentical for all the specimens, indicating that the linear theorywas adequate for describing the consolidation behavior of thespecimens based on small strains. Therefore, the example resultspresented herein are based on linear small-strain theory andlarge-strain theory. The potential significance of self-weight con-solidation also was evaluated based on the large-strain theorypresented by Umehara and Zen (1980). Self-weight effects werefound to be negligible for the conditions of the tests conductedin this study. Further details on the analyses including all rele-vant equations can be found in the aforementioned referencesand in Sample (2007).

Pore-water pressure, load-cell, and linear-position transducerreadings were recorded at 1-min intervals with an automateddata acquisition system. Error in the load-cell readings wasapproximately 61 kPa (6 0.15 psi). In addition, significant scat-ter in the data was observed for all specimens for vertical

FIG. 3—Assembled test specimens for CRS consolidation testing with testing column lengths of (a) 152 mm (6 in), (b) 305 mm (12 in), and (c) 457 mm (18 in).



effective stress, rv0, less than about 30 kPa (4.4 psi) to 60 kPa

(8.7 psi), depending on the specimen. This scatter likely was dueto very low initial pore-water pressures, instrument accuracy,and/or potential frictional effects. As a result of these considera-tions, only the data corresponding to rv

0> 100 kPa (14.5 psi)were used subsequently for determining consolidation parame-ters, e.g., to limit the maximum error of the load cell to 1 %.

Consolidation Properties

Stress-strain curves for the six specimens described in Table 1

are shown in Fig. 4. The differences between the stress-strain

curves calculated using the small- versus large-strain theories are

relatively minor for all specimens except for specimen No. 1, rep-

resenting the top third of a mixed column (Fig. 4(a)). Overall, the

TABLE 1—Example test program for mixing and consolidation evaluation of specimens of sand mixed with bentonite and zero-valent iron slurries.

Mixing Column Characteristics Specimen Characteristics

No. Mixing ColumnLength [mm (in)]

BentoniteContenta (%)

No. Mixing ColumnLocation

Testing Column Length[mm (in)]

Initial SpecimenHeight [mm (in)]

Strain Rate(� 10�6 s�1)

Testing Duration(h)

1 610 (24) 2.27 1 Top 152 (6) 80 (3.1) 5.00 18.6

2 Middle 152 (6) 80 (3.1) 5.00 7.8

3 Bottom 152 (6) 80 (3.1) 5.00 6.7

2 305 (12) 2.58 4 Entire 305 (12) 240 (9.4) 2.50 17.4

3 457 (18) 2.33 5 Entire 457 (18) 310 (12.2) 2.50 17.7

4 457 (18) 2.33 6 Entire 457 (18) 310 (12.2) 0.633 67.5

aCalculated on the basis of the amount of slurry injected into the sand column; target bentonite content in all cases was 2.30%.

FIG. 4—Stress-strain curves for CRS consolidation testing of bentonite-ZVI slurry mixed sand specimens for different testing column lengths and strain rates: (a)152 mm, 5.00� 10�6 s�1, top third; (b) 152 mm, 5.00� 10�6 s�1, middle third; (c) 152 mm, 5.00� 10�6 s�1, bottom third; (d) 305 mm, 2.50� 10�6 s�1; (e)457 mm, 2.50� 10�6 s�1; (f) 457 mm, 6.33� 10�7 s�1. [Note: SS¼ small-strain theory; LS¼ large-strain theory]



specimens were initially highly compressible, as expected, with

ultimate vertical strains, ev,max, reaching as high as 48.0 % (basedon large-strain theory). The observed compressibility is consistentwith the loose conditions of the specimens resulting from injectionof the bentonite-ZVI slurry (see Castelbaum et al. 2011).

All of the values of the pore-pressure ratio, PPR (¼ pore-waterpressure, uh, versus vertical total stress, rv, or uh/rv), were< 30 %for rv

0 � 100 kPa (14.5 psi) with the exception of the two speci-mens representing entire columns of bentonite-ZVI slurry mixedsands tested at the highest allowable strain rate of 2.50� 10�6 s�1

(i.e., specimen Nos. 4 and 5). For these two specimens, PPRwas> 30 % until rv

0 increased to 243 kPa (35.2 psi) and 700 kPa(102 psi), respectively. Gorman et al. (1978) recommended thatpore-water pressure values be greater than 7 kPa (1 psi), to avoidexcessively high, inaccurate values of the coefficient of consolida-tion, cv. The pore-water pressure typically was maintained above3 kPa (0.44 psi). The high sand contents of the middle and bottomthirds of the mixed columns (specimen Nos. 2 and 3) made gener-ation of pore-water pressures consistently> 7 kPa (1 psi) impracti-cal. For any given specimen, the value of PPR decreased with

increasing rv, indicating that the SR was sufficiently slow to allowsome dissipation of uh (uh increased at a slower rate than rv).

Values of cv were calculated based on large-strain (Lee 1981)and small-strain theories using time intervals corresponding to100-kPa (14.5-psi) increments in rv

0 (e.g., 100 kPa (14.5 psi),200 kPa (29 psi), etc.). The results for each specimen are providedin Fig. 5. To check the validity of the pore-water pressure distribu-tions assumed by the large-strain theory, cv values were calculatedat both the drained and undrained boundaries for each specimen.The similarity in the cv values calculated at each boundary sug-gests that the strain rates were sufficiently slow, such that theassumed pore-water pressure distributions in the large-straintheory were valid (e.g., see Lee 1981). For all test specimens, cv

increased with increasing effective stress regardless of the theoryused to calculate cv.

As expected, the calculated cv values for the bentonite-ZVIslurry mixed sand, which ranged from 1.9� 10�6 m2/s (Fig. 5(a),large-strain theory) to 5.0� 10�5 m2/s (Fig. 5(b), small-strain, lin-ear theory), are higher than those typically reported for pure clayspecimens, but lower than those typically reported for pure sand

FIG. 5—Coefficients of consolidation based on CRS testing of bentonite-ZVI slurry mixed sand specimens for different testing column lengths and strain rates: (a)152 mm, 5.00� 10�6 s�1, top third; (b) 152 mm, 5.00� 10�6 s�1, middle third; (c) 152 mm, 5.00� 10�6 s�1, bottom third; (d) 305 mm, 2.50� 10�6 s�1; (e)457 mm, 2.50� 10�6 s�1; (f) 457 mm, 6.33� 10�7 s�1. [Note: SS¼ small-strain theory; LSD, LSU¼ large-strain theory at drained and undrained boundaries,respectively]



specimens. For example, Holtz and Broms (1972) reported cv

values ranging from 4� 10�9 m2/s to 7� 10�9 m2/s for MexicoCity clay. In contrast, measured cv values for Ottawa sand mixedwith 15 % silt range from 4.4� 10�4 m2/s to 1.2� 10�3 m2/swhen consolidated in triaxial tests, and from 6.2� 10�4 m2/s to2.7� 10�3 m2/s when consolidated in oedometer tests (Carraroet al. 2003). The cv values for foundry sand mixed with 15 %nonplastic silt range from 3.5� 10�4 m2/s to 1.7� 10�3 m2/s(Thevanayagam et al. 2001).

Example plots of the coefficient of volume compressibility, mv

(¼ Dev/Drv0), versus rv

0 are shown for each specimen in Fig. 6.Unlike cv, only one value of mv was calculated based on the large-strain theory, because the value of Dev used to determine mv isbased on the difference in Eulerian (large) strain between drainedand undrained boundaries (Lee 1981). For all test specimens, mv

decreased with increasing rv0, indicating that the bentonite-ZVI

slurry mixed sand becomes less compressible with increasingeffective stress. The measured values of mv for the bentonite-ZVIslurry mixed sand ranged from 0.013 m2/MN (Figs. 6(e) and 6(f),small-strain theory) to 0.241 m2/MN (Fig. 6(a), large-straintheory). On the basis of the classification by Tomlinson (1986),the bentonite-ZVI slurry mixed sand specimens ranged from verylow compressibility (mv� 0.30 m2/MN) to only medium compres-

sibility (0.10 m2/MN�mv� 0.30 m2/MN), which is in contrast tothe significant strains experienced by some of the specimens (seeFig. 4). The lack of higher values of mv can be attributed to thefact that the majority of the measured strain in the specimensoccurred at rv

0 � 100 kPa (14.5 psi), whereas the data used todetermine mv were restricted to rv

0> 100 kPa (14.5 psi) due to theaforementioned problem associated with the difficulty of deter-mining accurate values of rv

0 less than about 30 kPa (4.4 psi) to60 kPa (8.7 psi). Thus, in this case, the calculated mv values do nottruly reflect the overall compressibility of the specimens.

Values for the hydraulic conductivity, k, calculated as the prod-uct of the measured cv and mv values and the unit weight of water,cw, or k¼ cvmvcw, are plotted versus rv

0 for each specimen ofbentonite-ZVI slurry mixed sand in Fig. 7. As shown, differencesbetween the calculated k values resulting from the different theoriesor location of the specimen boundary are virtually imperceptible.This closeness in the calculated k values is due to the compensatingeffect of the increase in cv with increasing rv

0 (Fig. 5) versus thedecrease in mv with increasing rv

0 (Fig. 6). The calculated k valuesfor all six specimens ranged from 3.4� 10�9 m/s (Fig. 7(a)) to1.9� 10�8 m/s (Fig. 7(b)). As expected, this range in k values ishigher than those typically reported for clay specimens, but lowerthan those typically reported for sand specimens. Castelbaum and

FIG. 6—Coefficients of volume compressibility based on CRS testing of bentonite-ZVI slurry mixed sand specimens for different testing column lengths and strainrates: (a) 152 mm, 5.00� 10�6 s�1, top third; (b) 152 mm, 5.00� 10�6 s�1, middle third; (c) 152 mm, 5.00� 10�6 s�1, bottom third; (d) 305 mm, 2.50� 10�6 s�1;(e) 457 mm, 2.50� 10�6 s�1; (f) 457 mm, 6.33� 10�7 s�1. [Note: SS¼ small-strain theory; LS¼ large-strain theory]



Shackelford (2009) measured k of bentonite slurry mixed sandspecimens using the same mixing apparatus as used in the presentstudy to prepare the specimens, and found that k ranged from2.4� 10�9 m/s to 6.8� 10�6 m/s, depending primarily on theamount of bentonite in the specimen and the void ratio of the ben-tonite in the specimen. Thus, the range in k values calculated forthe example tests presented here is at the lower end of the range ofvalues measured by Castelbaum and Shackelford (2009).

Effect of Testing Column Length

The testing column length essentially reflects the initial height orthickness of the specimen, ho, prior to consolidation (Table 1). Inthis regard, two testing columns, Nos. 2 and 3, contained intactspecimens (Nos. 4 and 5) that were prepared in an identical man-ner and consolidated at the same strain rate (i.e., 2.50� 10�6 s�1),such that the primary difference between the specimens was thelength of the testing columns (i.e., 305 mm (12 in) for No. 4 and457 mm (18 in) for No. 5) or specimen thickness (i.e., 240 mm(9.4 in) for No. 4 and 310 mm (12 in) for No. 5). Thus, a compari-

son of the results from test Nos. 4 and 5 allows an evaluation ofthe effect of testing column length or specimen thickness on theresults, although the slight difference in the bentonite contents inthese two specimens (2.33 % versus 2.58 %; see Table 1) alsoshould be considered.

The resulting stress-strain behaviors for the two specimenswere similar (see Figs. 4(d) and 4(e)). For example, based onsmall-strain theory, the maximum vertical strains experienced bythe two specimens were 14.4 % for No. 4 and 14.8 % for No. 5.The similarity in the results suggests that the stress-behaviors ofthe two specimens were essentially independent of the specimenthickness. As expected, the values of uh and PPR for the speci-mens increased as the specimen thickness increased, due to theincrease in drainage distance. The values of cv for the thicker spec-imen (No. 5) tended to be lower than those for the thinner speci-men (No. 4) at any given value of rv

0. This difference can beattributed directly to the difference in the drainage distances forthe two specimens. For values of mv, the test specimen in the lon-ger column was less than a factor of two more compressible thanthe test specimen in the shorter column at low values of rv

0, and

FIG. 7—Calculated hydraulic conductivity based on CRS testing of bentonite-ZVI slurry mixed sand specimens for different testing column lengths and strain rates:(a) 152 mm, 5.00� 10�6 s�1, top third; (b) 152 mm, 5.00� 10�6 s�1, middle third; (c) 152 mm, 5.00� 10�6 s�1, bottom third; (d) 305 mm, 2.50� 10�6 s�1; (e)457 mm, 2.50� 10�6 s�1; (f) 457 mm, 6.33� 10�7 s�1. [Note: SS¼ small-strain theory; LS¼ large-strain theory]



this difference in compressibility decreased with increasing rv0.

Thus, based on both the similarity in stress-strain behaviors andthe trends in and magnitudes of the mv values for specimen Nos. 4and 5, the difference in specimen thicknesses had a relativelyminor effect on the compressibility of the two specimens.

Effect of Applied Strain Rate

Two testing columns, Nos. 3 and 4, of identical length (457 mm(18 in)) contained intact specimens (Nos. 5 and 6) that wereprepared in an identical manner, but specimen No. 6 was consoli-dated at a slower strain rate (6.33� 10�7 s�1) relative to specimenNo. 5 (2.50� 10�6 s�1). In addition, both of these specimenscontained the same amount of bentonite (Table 1). Therefore, acomparison of the results from these two tests allows for an evalu-ation of the effect of the applied strain rate during consolidationtesting on the results. The consolidation behavior of a soil speci-men is controlled by a unique vertical effective stress-verticalstrain-vertical strain rate relationship (Leroueil et al. 1985,Leroueil 1988). For specimen Nos. 5 and 6, at any given value ofev, the value of rv

0 increased as SR increased (Fig. 4). This trendof increasing rv

0 with increasing SR has been reported extensively(e.g., Kabbaj et al. 1986, Leroueil 1988, Silvestri et al. 1986). Interms of the maximum vertical strain, SR had little effect on theresults of specimen Nos. 5 and 6 (maximum difference< 0.3 %,regardless of definition of strain). Consequently, SR had virtuallyno effect on the mv values (compressibility) of the two specimens.

The excess pore-water pressures generated at the undrainedbase of a specimen, uh, and the calculated PPR values increasedas SR increased. These trends previously have been reported forCRS testing (e.g., Smith and Wahls 1969, Znidarcic et al. 1986,Leroueil et al. 1985, Armour and Drnevich 1986, Silvestri et al.1986, Wissa et al. 1971). The difference between the cv values forthe two specimens based on strain rate was minor, with a tendencytoward slightly decreasing cv with increasing SR.

Summary and Conclusions

A CRS consolidation apparatus was developed to allow testing ofslurry mixed soils prepared with a hollow-stem auger mixingmachine. Specimens of bentonite-ZVI slurry mixed sand wereprepared with the laboratory mixing apparatus and transferreddirectly to a consolidation load frame within the original mixingcolumns, thereby minimizing specimen disturbance after prepara-tion. Data were recorded continuously throughout the consolida-tion tests via an automatic data acquisition system. The requiredtest durations for the mixing stage and consolidation stagewere< 1 day and< 3 days, respectively.

Example results are provided for six specimens of bentonite-ZVI slurry mixed sand. As expected on the basis of the loose na-ture of the prepared specimens, all the specimens were highlycompressible, with maximum vertical strains, ev,max, ranging from9.63 % to 48.0 %. The unique consolidation behavior of thebentonite-ZVI slurry mixed sand resulted in two difficulties asso-ciated with the CRS testing. First, the use of slow strain rates andthe high sand content of the specimens resulted in the generation

of low values of excess pore-water pressures (< 3 kPa (0.4 psi)).Second, significant scatter in the stress-strain data occurred for rv

0

less than about 30 kPa (4.4 psi) to 60 kPa (8.7 psi), depending onthe specimen, and this scatter did not dissipate completely inall specimens until rv

0 was greater than about 100 kPa (14.5 psi).As a result of these difficulties, only the stress-strain data corre-sponding to rv

0 � 100 kPa (14.5 psi) could be used to determinethe relevant consolidation parameters (i.e., cv, mv, k). Thus, com-pressibility of the specimens at rv

0< 100 kPa (14.5 psi) may notbe reflected by the determined consolidation parameter values.

Comparison of the results based on two specimens strained atthe same strain rate (2.50� 10�6 s�1), but with different columnlengths (e.g., specimen thicknesses) indicated that the effect ofcolumn length was insignificant. Similarly, comparison of theresults based on two specimens of the same thickness but strainedat different rates (6.33� 10�7 s�1 and 2.50� 10�6 s�1) indicatedthat the effect of the applied strain rate was minor.

Ackowledgments

Financial support for this project was provided by DuPont. Theassistance of Tom Sale, Gustavo Rocha, and David Castelbaum atColorado State Univ. also is sincerely appreciated.

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