2006_43_1042

Hydrogeotechnical characterization of ametallurgical waste

R. Rodrguez

Abstract: This paper presents the results of hydrogeotechnical characterization tests carried out on the metallurgicalwaste (MW) from a tailings impoundment located on the terraces of the Moa River, Cuba. Characterization of the MWincludes chemical and mineralogical analysis, oedometer tests, triaxial tests, tensile strength tests, determination of thewater retention curve, and shrinkage and permeability tests. The MW, which has a grain-size distribution similar to thatof a silt, mainly contains iron and heavy metals minerals and has low plasticity. Consolidated undrained triaxial testson remoulded samples indicate a dilative behaviour, with a decrease in pore-water pressure near failure. The material issusceptible to liquefaction when subjected to a cyclic load in the triaxial test. Hydraulic conductivity, soil stiffness, andcompressive and tensile strength of the MW have an important dependence on the degree of saturation and vary signif-icantly during the drying process. The results indicate that, during the drying process, cracks in the MW initiate inquasi-saturated conditions. The cracks increase the hydraulic conductivity by more than one order of magnitude com-pared with that of intact samples of MW. The main environmental risk with this MW is the possibility of liquefactionunder a cyclic load due to an earthquake and the increase in saturated hydraulic conductivity due to desiccation cracks.

Key words: desiccation cracks, hydrogeotechnical properties, liquefaction, metallurgical waste, characterization.

Rsum : Cet article prsente des rsultats dessais de caractrisation hydro-gotechnique effectus sur des dchets m-tallurgiques (MW) provenant du rservoir de rsidus minier situ sur les terrasses du fleuve Moa, Cuba. La caractrisa-tion des MW inclut des analyses chimiques et minralogiques, des essais oedomtriques, des essais triaxiaux, des essaisde rsistance la traction, la dtermination de la courbe de rtention deau ainsi que des essais de retrait et de perma-bilit. Les MW prsentent un comportement faiblement plastique. La taille des grains correspond celle dun silt et ilscontiennent des minraux ferriques et des mtaux lourds. Les essais triaxiaux consolids non-drains sur chantillonsremanis indiquent un comportement dilatant, avec une diminution de la pression interstitielle lapproche de la rup-ture. Lchantillon est toutefois susceptible de se liqufier sous charge triaxiale cyclique. La conductivit hydraulique,la rigidit du sol, la rsistance la traction et la compression des MW dpendent fortement du degr de saturation etpeut varier de faon significative pendant le schage. Les rsultats indiquent que les fissures dans les MW commencenten conditions quasi-satures. Les fissures augmentent la conductivit hydraulique de plus dun ordre de grandeur parrapport celle des chantillons intacts de MW. Le risque cologique caus par les MW est donc reli la possibilitde liqufaction en prsence dune charge cyclique cause par un sisme et lapparition de fissures de dessiccation quiaugmentent leur conductivit hydraulique.

Mots cls : fissures de dessication, proprits hydro-gotechniques, liqufaction, rsidus mtallurgiques, caracterisation.

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Introduction

The majority of the studies of mine and metallurgicalwastes (MW) have focused on the geochemical and mineral-ogical behaviour of the wastes (Swarbrick and Fell 1992;Ribet et al. 1995; Vick 1996). According to a literature re-view, the available information on the geotechnical proper-ties of MW is usually on wastes from hard-rock mines (e.g.,Markland and Eurenius 1976; Aubertin et al. 1994, 1996;Barrera and Lara 1998; Dawson et al. 1998; Chapuis andAubertin 2003). Limited data have been found on MW tail-

ings from Fe (oxy)hydroxy minerals from lateritic deposits(Heredia 1980; Rodrguez et al. 1998a, 1998b; Tibana andDe Campos 1998; Rodrguez 2002a, 2002b). The environ-mental impact on surface water and groundwater has beenanalysed by different authors (Szymanski and Macphie1994; Younger 1999; Rodrguez et al. 2000; Rodrguez andCandela 2004).

As a result of the metallurgical process, water-borne slur-ries are generated and deposited in a liquid form into a spe-cific area bounded by tailings impoundments. Thesematerials are initially saturated but desiccate when exposedto certain climatic conditions (temperature and wind). Desic-cation induces volume and stress changes in the MW, lead-ing to vertical fissures. Depending on the geological materialand subsequent metallurgical process, MW is typically char-acterized by a grain-size distribution between 0.01 and1 mm and a particle density between 1.50 and 4.00 g/cm3(Blight 1994; Vick 1996; Gipson 1998; Rodrguez et al.

Can. Geotech. J. 43: 10421060 (2006) doi:10.1139/T06-061 2006 NRC Canada

1042

Received 24 March 2003. Accepted 26 April 2006. Publishedon the NRC Research Press Web site at http://cgj.nrc.ca on1 November 2006.

R. Rodrguez. Department of Geotechnical Engineering andGeosciences, Technical University of Catalonia (UPC),Barcelona, Spain (e-mail: [email protected]).

1998a). The mineralogy and chemical composition of MWare highly variable, depending on the orebody type and met-allurgical process, but (Fe, Al) (oxy)hydroxide minerals, sul-phide minerals, quartz, and clay minerals are the mostprevalent. According to several authors (Markland andEurenius 1976; Ribet et al. 1995; Vick 1996; Tibana and DeCampos 1998; Rodrguez et al. 1998a, 1998b), the internalfriction angle may range between 27 and 41, and the voidratio, e, between 0.8 and 3.6.

In general, MW slimes show low strength, low permeabil-ity, and low compressibility, and low consolidation rateshave been also reported (Vick 1996). They exhibit little orno effective cohesion and can be susceptible to liquefactionunder seismic loading during earthquakes or cyclic triaxialtests (Troncoso 1988; Yasuhara et al. 1994; Vick 1996;Barrera and Lara 1998; Rodrguez 2002a).

Theoretical and experimental analyses of desiccationcracks in cohesive and plastic soils have been carried out byseveral researchers (e.g., Blight 1971, 1997; Miller 1975;Snyder and Miller 1985; Towner 1987a, 1987b; Morris et al.1992; Swarbrick and Fell 1992; Abu-Hejleh and Znidarcic1995; Drumm et al. 1997; Konrad and Ayad 1997; Kodikaraet al. 2000; Yesiller et al. 2000); however, limited informa-tion has been found on this phenomenon in low-plasticity MW (Heredia 1980; Lloret et al. 1998).

The aim of the paper is to present the physical, mechani-cal, and hydrogeotechnical characteristics of a specific MWand the influence of desiccation cracks on its hydraulic con-ductivity. This study constitutes a necessary step in the anal-ysis of the influence of desiccation cracks on preferentialflow and heavy-metal transport to underlying aquifers andthe analysis of the stability of the tailings impoundment.

Site description

The Moa mining district in northeastern Cuba (Fig. 1) hasbecome one of the most important centres in the world forthe production of nickel and cobalt (oxidizedhydroxidizedspecies) from lateritic orebodies (Anthony and Flett 1997).The area is characterized by temperatures between 24 and27 C and high rainfall (average 1800 mm/year, exceeding2000 mm in some years). There is some seismic activity inthe area, with estimated recurrence periods of 6080 yearsfor earthquakes of magnitudes greater than 7 (Rodrguez etal. 1996; Cotilla 1998).

Mineral exploitation began near Nicaro in 1943, and20 years later in the Moa area (Fig. 1). Tailings are accumu-lated in a specific area bounded by five impoundments andcover an area of about 10 km2, with the thickness of the de-posit ranging between 6 and 20 m at Nicaro and between 4and 8 m at Moa. The solid material deposited accounts formore than 180 million tonnes.

The Moa mining district is the only region in the worldwhere acid leaching (SAL) and Caron-type (ACL) industrialprocesses for the extraction of Ni and Co from lateritic de-posits coexist. The Caron-type process refers to ammoniacalammonium carbonate leaching (ACL) with oil addition, andSAL refers to sulphuric acid leaching (Ponjuan andRodrguez 1981; Anthony and Flett 1997). The MW produc-tion rate from the two processes is 5200 t/day (UNI 1994),which is hydraulically transported by pipelines and depos-

ited in the storage area. The slurry of MW transported bypipelines is 30%40% solid and 70%60% liquid (seeFig. 2c).

The mining activity and the MW have had several impor-tant environmental impacts in the area, among them heavy-metal pollution of surface water (Rodrguez et al. 1998a,1998b; Rodrguez 2002b), groundwater pollution with heavymetals and sulphate (Rodrguez and Candela 1998), pollu-tion of sea water (Gonzlez and Ramrez 1995), and pollu-tion of a coral barrier (Martnez et al. 1993).

Material

In April 1996, ten 2 kg samples of MW were collectedand analysed for their physicochemical and mineralogicalproperties in tailings impoundments 3 and 5 (five samplesper impoundment, Fig. 1). The 10 samples for chemicalanalysis were sealed in plastic bags and kept under refrigera-tion (temperature 68 C) until they were tested in the labo-ratory. Five more MW samples (20 kg) were collected atdifferent locations on the tailings surface of the Moa im-poundment (tailings impoundment 3, Fig. 1) and used formechanical and hydrological analysis. These samples weresealed in plastic bags, but not kept under refrigeration sothat their physical properties were not changed.

Method

Physical, mineralogical, and chemical characterizationGrain-size distribution of samples was obtained by means

of a laser technique using Malvern Mastersize/E equipment(range 0.5600 m) (Rodrguez 2002a). The X-ray diffrac-tion technique was used to characterize the minerals presentin the tailings. X-ray diffraction tests were conducted in aSiemens D-5000 diffractometer with a Bragg-Brentano :2configuration, using a Cu target tube and a graphite mono-chromator, operating at 40 kV and 20 mA. Samples wereanalysed within the 460 range, taking data every 0.05with a step-time of 3 s. X-ray diffraction plots were studiedusing the Siemens automatic software for peak recognition,mineral identification, and peak intensity calculations.

The chemical composition of the samples was determinedby digestion of 2 g of MW with acid at different acid to soil ra-tios (1:3 HNO3; 1:5 HClO4; 1:3 HF). Nineteen elements (Fe,Cr, Ni, Mn, Zn, Cu, Ni, Co, Al, Mg, Ca, Ba, Sr, Pb, Cd, Ag,Hg, P, Au) were analysed from the final solution by inductivelycoupled argon plasma atomic emission spectrometry (ICPAES) and inductively coupled plasma mass spectrometry(ICPMS). Oxidizable organic matter (OM) was calculated bythe electrochemical volumetric method (Buurman et al. 1996).

The Gilman method for acid soil was applied to determinethe cation exchange capacity (CEC). For the pH measure-ment, deionized water was added to a waste sample at awaste to water ratio of 1:2.5 (Page 1986).

Mechanical characterizationA series of mechanical tests on remoulded samples was

carried out, including oedometer tests, unconfined compres-sion tests, extension tests, and triaxial tests (ASTM 1993).Cyclic triaxial tests were also performed to assess the lique-faction potential of the MW. The mechanical characterization

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was performed under saturated and unsaturated conditions todetermine the physical parameters that control water flowthrough the MW and the development of desiccation cracks.The properties of different samples of MW used in the me-chanical tests are listed in Table 1.

Undrained and cyclic triaxial tests were performed in aTriaxial GDS (Imperial College type stress path cell), withautomatic data acquisition (GDS Instruments Ltd., Hook,UK). The instrumentation used comprised an internal loadcell and pore pressure, cell pressure, external axial displace-ment (100 mm), and volume change transducers. Pore pres-sures were measured with transducers attached to the baseand top of the triaxial cell.

The tensile strength for different degrees of saturation wasmeasured for three different densities (1.32, 1.44, and1.53 g/cm3) using a device similar to those described byMikulitsch and Gudehus (1995), Lloret et al. (1998),Rodrguez (2002b), and Kim and Hwang (2003).

Hydrogeotechnical characterizationHydrogeotechnical characterization is very important for

future analyses of pollution transport in the deposition areasthemselves. This work focuses on the hydrogeotechnicalcharacterization of ACL MW in the Moa mining district,Cuba. Because of the lack of previous characterizationsof MW, standard hydrogeotechnical tests (liquid and plasticlimit, saturated and unsaturated hydraulic conductivity, satu-rated oedometer and collapse in homogeneous samples) andmodified hydrogeotechnical tests (e.g., saturated hydraulicconductivity in layered and cracked samples with differentthicknesses) were performed (ASTM 1993). Additional tests

related to the material parameters under unsaturated condi-tions were also carried out. A series of MW water retentioncurves were obtained using different procedures (transistorpsychrometer, oedometer test with suction control, salt solu-tion) according to the suction range considered. Thesecurves are described later in the paper.

A transistor psychrometer (Dimos 1991) was used for to-tal suctions between 0.1 and 10.0 MPa. In this case, samples(30 small cylinders, 15 mm in diameter, 12 mm in height,and with three different densities of 1.32, 1.44, and1.53 g/cm3) were compacted with an initial water content toachieve a given dry density. When analysing wetting pro-cesses, saturated samples were initially dried by applying asuction of 38 MPa using an NaCl salt solution for 90 days.Water was added drop by drop to each sample (Rodrguez2002b; Lloret et al. 2003). The suction was measured with a12-channel transistor psychrometer (Dimos 1991; Rodrguez2002a; Lloret et al. 2003).

The behaviour of samples for suctions lower than 1 MPawas studied using an oedometer test with suction control(Rodrguez 2002b; Lloret et al. 2003). The apparatus al-lowed evaluation of the changes in volume and water con-tent of the sample with changes in suction or vertical loadunder unsaturated conditions. The oedometer (Lloret andAlonso 1985) uses the axis-translation technique (Hilf 1956)to control the matrix suction of the MW. Vertical stress waskept constant at 0.03 MPa during the tests. Suction varia-tions were imposed applying an air pressure of 1 MPa to theupper end of the sample (50 mm in diameter, 20 mm inheight, and dry densities of 1.32, 1.44, and 1.53 g/cm3) andvarying the water pressure on the high air entry value (AEV)

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Fig. 1. Location of the Moa and Nicaro mining districts, Cuba, and the existing tailings impoundments. Impoundments 1, 2, and 3 cor-respond to an ammonium carbonate leaching process (ACL) with oil addition, and impoundments 4 and 5 to a sulphuric acid leaching(SAL) process.

porous stone at the base. Measurement of the amount of wa-ter entering or escaping the sample allows the water contentto be controlled. Performing this measurement over time, theunsaturated permeability can be estimated from the watercontent evolution for a given change in suction. This re-quires back-analysis with the simplified Richards model,taking into account the low permeability of the porous stonewith its high AEV (Kunze and Kirham 1962).

Desiccation cracks: sample preparationThe in situ desiccation process in tailings ponds induces

volume and stress changes in the MW, which generates ver-tical fissures 2080 cm deep and more than 5 cm wide

(Figs. 2a, 2b). Due to different deposition events and differ-ent dry cycles it is possible to observe the stratification anddesiccation cracks in the deposit (Fig. 2c). To observe crackgeneration due to desiccation processes, five drying testswere performed with MW covering a circular plate (225 mmin diameter). This type of test is similar to that described byLloret et al. (1998) and Kodikara et al. (2000). The plateswere grooved to a depth of 1.5 mm to avoid slippage be-tween the plate and the MW (Fig. 3a). The MW was depos-ited on the plates in different thicknesses (4, 8, and 16 mm)and with an initial gravimetric water content of 50%, an ini-tial porosity of 0.66, and a dry density of 1.33 g/cm3. Fourseries of tests were performed in hermetic containers (with

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Fig. 2. (a) Desiccation cracks at impoundment 5. (b) Desiccation cracks at impoundment 3. (c) Metallurgical wastes (MW) dischargeand beach deposition in tailing dam, observe the estratification in MW with different thicknesses. Area in each photograph is approxi-mately 1 m2, with notebook and pen for scale.

capacities of about 12 L, Fig. 3a) where the relative humid-ity of the air was imposed using saline solutions with so-dium chloride or sulphuric acid (Rodrguez 2002b; Lloret etal. 2003). In a fifth series, the plates remained open to thelaboratory atmosphere with a 60% relative humidity. The psy-chrometric law (Fredlund and Rahardjo 1993) relates the finalsuction of the water in MW to the relative humidity of the at-mosphere surrounding the sample, assuming equilibrium.Temperature was held constant at 22 2 C, and relative hu-midity and temperature were measured with a hygrometer(Fig. 3a). The shrinkage of the MW was measured with a dis-placement transducer (Fig. 3a). When the desiccation testswere complete, a picture was taken and analysed withAutoCAD (Autodesk Inc., San Rafael, Calif.). The perime-ter of the crack and the distance between two crack planes

were measured (Fig. 3b). The average distance between thedifferent crack planes was determined for all samples.

Effect of desiccation cracks on hydraulic conductivity:sample preparation

To analyse the effect of desiccation cracks and stratifica-tion on the flow and transport of contaminant, four col-umns were made in the laboratory. After mixing the wastematerial with distilled water (56% water and 44% solid ma-terial) to simulate the sedimentation process at the miningsite, it was placed in this slurry state in a large column(300 mm in diameter and 120 mm high). Four columnswith similar physical characteristic were tested. The firstthree columns were constructed step-by-step in a stratifiedway. The volume of material deposited in each column

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TestsDiameter(mm)

Height(mm)

Dry density(g/cm3)

Conventional oedometer tests 50 20 1.39Collapse oedometer tests 50 20 1.39Unconfined compression tests 38 76 1.53Tensile stress tests 38 20 1.53Triaxial tests 38 76 1.53Cyclic triaxial tests 38 76 1.53Suction measurement (psychrometric method) 15 12 1.58

15 12 1.4415 12 1.32

Oedometer with suction control 50 10 1.53Shrinkage limit tests 38 76 1.53Hydraulic conductivity tests, homogeneous samples 100 120 1.53Cracks samples with three types of layers, 10, 20, and 40 mm thick 100 120 1.53

Table 1. Principal characteristic of the sample of MW for different hydrogeotechnical tests made in this study.

Fig. 3. (a) System used for desiccation tests with different boundary conditions. 1, suction control solution; 2, scale; 3, sample ofwaste placed on circular plate; 4, displacement transducer; 5, hygrometer. (b) Method used to measure distance between cracks, area,and perimeter of waste samples. All measurements in millimetres.

gave more or less homogeneous layers with different thick-nesses (approximately 10, 20, and 40 mm). Only when theprevious layer had cracked and consolidated was the fol-lowing layer placed. The drying process was produced byapplying similar climatic parameters from the study area tothe column surface for 1 week (in each layer). For thiscase, the climatic boundary conditions were a temperatureof 26 0.5 C and a relative humidity of 60 5%. Duringthe drying process in a stratified column, cracks were pro-duced on the top of the deposited layers. In each test, vol-

ume and porosity variations due to shrinkage were con-trolled. The shrinkage and change in thickness were mea-sured using a displacement transducer similar to that shownin Fig. 3a. A photograph was taken when the desiccationtest was finished, the area and volume of cracks were mea-sured, and the distribution of cracks was drawn usingAutoCAD (Fig. 3b).

In the second test, the MW material prepared with thesame initial water content was placed all at once, forming acolumn with the same height, and was then desiccated fol-lowing the same procedure as that for the layered column.When the previous four columns were desiccated and theirtotal heights were calculated, one sample (100 mm in diame-ter, 120 mm high) in each column was taken to measure thehydraulic conductivity.

The initial average dry density for stratified columnswith 10, 20, and 40 mm thick layers was different in eachsample (1.38, 1.32, and 1.24 g/cm3, respectively; and1.40 g/cm3 in homogeneous samples). Each sample was putin a triaxial cell. To avoid preferential flow through thewalls, the columns were covered with a rubber membrane.In the triaxial cell the sample was saturated for 48 h, with aconfining pressure of 5 kPa and a hydraulic gradient of 0.1.When the sample was totally saturated, the samples wereconsolidated to achieve a density of 1.53 g/cm3. Thechange in porosity of the sample in the saturated conditionwas controlled by measuring the amount of water that en-tered (wen) and escaped (wes) from the sample. The differ-ence between wen and wes represents the change in volumeof the sample. The dry density and porosity were calculatedusing this change in volume. When the difference betweenvolumes of water entering and escaping became zero, thehydraulic conductivity was measured for 24 h in each sam-ple of MW.

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MineralTailings impoundment1 (n = 10)

Tailings impoundment3 (n = 5)


Magnetite, (Fe)Fe2O4 0.51.3 3.08.0 0.61.2Hematite, Fe2O3 6270 6070 6975Quartz, SiO2 1.53.9 2.04.2 1.33.1Gibbsite, Al(OH)3 1.85.2 1.03.0 1.46.0Anatase, TiO2 0.050.06 0.030.06 0.040.05Alunite, H3OAl(SO4)2(OH)6 9.013.4 8.914.0Gypsum, CaSO4H2O 0.010.10 0.101.20 2.505.60Serpentine, Mg3Si2O5(OH)4 0.31.2 0.61.4Chrome spinel, MgCr2O4 3.13.8 2.04.0 2.12.8Magnesioferrite, (Mg)Fe2O4 0.13.0

Note: n, number of samples.

Table 2. Mineralogical composition of the wastes in percentage of total weight.

Elements (% of total weight; n = 5)

pHEh(mv)

CEC(mequiv./100 g)

OM (% oftotal weight) Ti Mn Al Fe Co Zn V Ni Ba Cr

SAL 4.1 422 8.5 4.5 0.57 0.42 4.94 43.58 0.03 0.01 0.03 0.18 0.0028 0.53ACL 6.5 325 8.0 0.6 0.06 0.72 4.80 49.19 0.10 0.05 0.03 0.60 0.0030 1.72

Note: n, number of samples; OM, organic mater.

Table 3. Concentration of selected elements in the waste.

Fig. 4. Grain-size distribution of ACL MW obtained by a lasertechnique.

Results and discussion

Mineralogical, chemical, and physical characteristicsThe mineralogical study indicates that the main compo-

nents of the waste are iron and (oxy)hydroxide minerals, es-pecially hematite (60%75%) and magnetite (1%8%).Secondary minerals are quartz (1%3%), gibbsite (1%6%),and magnesioferrite (0.1%3.0%). The results of the mineral-ogical analyses are summarized in Table 2. The chemicalcomposition is given in Table 3. In general, the waste ismainly composed of iron and heavy metals: Fe (4050 wt.%),Cr (13 wt.%), and Mn (12 wt.%).

The ACL MW presents an almost neutral pH (6.5), andthe SAL MW presents an acidic pH (4.1). A CEC was ob-tained of 8 mequiv./100 g of solid waste, which is consistentwith values reported in the literature for (oxy)hydroxides ofFe, Al, and Cr (Sparks 1995).

According to the grain-size distribution shown in Fig. 4,99% of the residue has a grain size finer than 0.2 mm, with amean grain size of 20 m. The material has an in situ den-

sity of about 2.30 g/cm3, a particle density between 3.80 and4.04, and a void ratio between 1.3 and 2.2. The liquid limitis between 40 and 44, and the plastic limit about 3640.The MW shows a low plasticity index between 4 and 6. Theabsence of clay minerals gives the MW low plasticity. Ac-cording to the grain-size distribution, MW can be catego-rized as a sandy silt (class ML following the Unified SoilClassification System). Table 4 summarizes some of themost relevant physical parameters of the MW from the in-vestigated tailings impoundment 3 and presents physicalcharacteristics obtained in situ by Heredia (1980) from MWin tailings impoundments 1 and 5 (Fig. 1). In situ density forthe three tailings impoundments oscillates between 1.78 and2.38 g/cm3, with an average value of 2.19 g/cm3, and gravi-metric water content is between 25.0% and 35.0%, with anaverage value of 32.8%. The void ratio varies between 1.1and 2.2 (Table 4). The MW reaches full saturation duringrainy periods, whereas in dry periods, below a superficialcrust, the degree of saturation varies according to theweather (Fig. 2).

Mechanical characteristics

Oedometer testsFigure 5 shows the results of the oedometer tests on

remoulded samples of ACL MW at an initial water contentof 44% (tailings impoundment 3, Fig. 1) statically com-pacted to a dry density of 1.39 g/cm3. Under these condi-tions, the samples are saturated, the compression index (Cc)obtained is 0.26, and the swelling index (Cs) is 0.05. Thestiffness of these remoulded samples is lower than that mea-sured by Heredia (1980) for undisturbed samples. Somecompacted samples were air dried in the laboratory atmo-sphere, reaching an average water content of only 3%. Whena sample with that initial water content is loaded, its com-pressibility is lower than that under saturated conditions(Cc = 0.12). Nevertheless, when samples with that initial wa-ter content are saturated under a constant load, an importantreduction in volume (collapse) takes place and the samplesreach a final void ratio similar to that corresponding toloaded samples that were initially saturated. In Fig. 4 themagnitude of the collapse strains for different vertical con-stant loads can be observed. This behaviour is consistentwith that observed in natural soils with a grain-size distribu-tion and porosity similar to those of the MW (Alonso et al.1987).

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ParameterTailings impoundment1 (n = 22)a


Tailings impoundment5 (n = 20)a

Natural density (g/cm3) 2.151.73 2.382.29 2.382.23Dry density (g/cm3) 1.571.33 1.831.64 1.831.67Particle density (g/cm3) 4.113.52 3.804.04 3.993.77Water content (%) 30.925.0 35.0025.00 35.3029.40Liquid limit 40.435.3 44.040.0 25.023.0Plastic limit 36.830.0 40.036.0 24.021.0Plasticity index 7.43.1 6.04.0 11.06.0Void ratio 1.951.47 2.201.30 1.351.15

aData from Heredia (1980).

Table 4. Basic characteristics of the mine tailings materials (n = number of samples).

Fig. 5. Conventional saturated oedometer and collapse tests onstatically compacted waste. Collapse obtained when wetting un-saturated samples under constant loads. Vertical bars representthe consolidation of dry material when is saturated.

Unconfined compression testsUnconfined compression tests were performed on samples

statically compacted to three dry densities (1.32, 1.44, and1.53 g/cm3). Under these conditions, the samples were ini-tially saturated. The samples were dried in a laboratory-controlled atmosphere (relative humidity of 60 5% andtemperature of 22 2 C), reached different final water con-tents, and became unsaturated. Figures 6a and 6b (curve I),respectively, show the influence of the degree of saturationon the value of Youngs modulus and the unconfined com-pression strength measured in the tests. A clear increase instiffness can be observed when the material becomes drier.Apparently, the maximum value of unconfined compressionstrength corresponds to a degree of saturation between 80%and 90% in the water retention curve (see Fig. 9).

Tensile strengthThe tensile strength of the samples prepared under the

same initial moisture conditions as those for the samplesused in unconfined compression is measured using a devicesimilar to that described by Mikulitsch and Gudehus (1995).The measured tensile strength is plotted in Fig. 6b (curve II).The unconfined compression and tensile strength gave maxi-mum values for a degree of saturation of about 0.80.9. Theratio of the maximum compressive strength to the maximumtensile strength is about five and is generally the same forthree different dry densities (1.32, 1.44, and 1.53 g/cm3) be-cause the material has no cohesion and the internal tensilestrength depends on the optimal water content. The resultobtained for the saturation condition (Sr = 1) is in agreementwith the results of Nearing et al. (1991), who studied 33 sat-urated repacked soils and obtained tensile strength valuesbetween 1.00 and 3.23 kPa, which are similar to those ob-tained for MW (see Fig. 6b).

The results of the tests of tensile strength and uniaxialcompressive strength obtained from MW samples present asimilar trend for three different densities. They reach maxi-

mum values for degree of saturation between 0.8 and 0.9(Fig. 6b). The tensile strength of fine-grained soils dependson the interparticle attraction, which depends on the cohe-sion of the material and the water content. In this case, how-ever, the material has no cohesion. Once the degree ofsaturation reaches 90%, the tensile strength and unconfinedcompression decrease very rapidly (Fig. 6b).

Undrained triaxial testsUndrained triaxial compression tests (isotropically consol-

idated) were performed on the same type of specimens asthose used in the compression tests. The tests were carriedout on saturated samples for different vertical stress, 1 (650,700, 800, and 950 kPa), and initial effective confiningstresses, 3 (50, 100, 200, and 350 kPa). The results given inFig. 7 are obtained for Sr = 1 and show that there are largeincrements of pore-water pressure for small axial deforma-tions (smaller than 2%), whereas the material shows dilativebehaviour and pore pressures reduce when the strains arelarger than 2% (Fig. 7b). The angle of internal friction de-rived from these tests is 35.6 (Fig. 7c). This behaviour issimilar to that observed by Tibana and De Campos (1998)for loose iron tailings.

Cyclic triaxial testsStress-controlled cyclic triaxial tests were performed un-

der undrained conditions on saturated samples with an initialdry density of 1.53 g/cm3. Figure 8a presents the relation-ship between the number of cycles necessary to achieve liq-uefaction and the value of the cyclic stress ratio, d /(23),where d is the cyclic stress amplitude, and 3 is the initialeffective confining stress. Figure 8a also presents the num-ber of cycles necessary to reach different values of axialstrain for a given stress ratio and shows that, once liquefac-tion is reached, the deformations tend to grow rapidly. Fig-ure 8b shows that pore pressures (u) start to increase whenthe axial strain reaches values of about 0.1% and becomes

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Fig. 6. (a) Youngs modulus (E) for different degrees of saturation. (b) Uniaxial compressive strength (curve I) and tensile strength(curve II) for different degrees of saturation.

equal to the effective confining stress for axial strains ofabout 1% for all cases (irrespective of the confining stress).These results are consistent with those from Dobry presentedby the National Research Council Committee on EarthquakeEngineering (1985) from strain-controlled triaxial tests onsands of diverse origins and relative densities.

Hydrogeotechnical characterization

Water retention curvesHydrogeotechnical characterization is very important for fu-

ture analyses of pollution transport in the actual deposition ar-eas. Figure 9a shows the water retention curves (WRC)corresponding to the drying and wetting processes for sampleswith an initial void ratio (eo) of 1.75. The results show a veryimportant hysteresis in the retention curve, and the wetting pro-

cess shows a significant reduction in suction for an identicaldegree of saturation. Measurements performed with psychro-meters are consistent with those obtained in the oedometer test,which indicates that the total suction measured with psychro-meters is similar to the matric suction imposed with the oedo-meter. The retention curve shows that the residual degree ofsaturation is approximately between 8% and 12%.

Figure 9b shows the effect of the initial void ratio (eo) onthe retention curve for drying tests. A reduction in the initialvoid ratio implies a significant increase in the AEV of thematerial. In the WRC, the AEV is defined as the matric suc-tion at which air first enters the largest pores of the soil dur-ing a drying process (Brooks and Corey 1964, 1966).

Table 5 shows the estimated AEV for different dry testsaccording to the tangent method (Fredlund and Xing 1994;Aubertin et al. 2003; Yang et al. 2004). The AEV in MW is

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Fig. 7. (a) Stress paths of conventional triaxial compression tests. (b) Relationship between pore-water pressure and axial deformationfor different confining effective stresses. (c) Plot of the mean effective stress (p) versus deviator stress (q) relationship for largestrains. , angle of internal friction.

similar to that obtained by Aubertin et al. (1998) using tail-ings from hard-rock mines.

Figure 10 shows the change in volume and water contentexperienced by the MW in different suction steps. The vol-ume increases during the suction-controlled oedometer test.In the first case (Figs. 10a, 10c), with an increase in suctionof between 0.01 and 0.03 MPa, the sample, due to the low

value of the applied suction, remains practically saturatedand the change in total volume is very similar to the waterloss. When the suction increment is higher (from 0.4 to0.6 MPa, Figs. 10b and 10c), however, the sample has asmaller degree of saturation, and the change in total volumeof the sample is very small compared with the change of wa-ter volume, indicating that air replaces the water lost fromthe pores.

Shrinkage and desiccation cracks of metallurgical waste(MW)Shrinkage limited

Figure 11 presents the evolution of the void ratio and thedegree of saturation of the sample during a cycle of dryingwettingdrying performed under a constant vertical load of0.03 MPa in the suction-controlled oedometer. An importantirreversible deformation can be observed during the first cy-cle of drying, whereas in the subsequent cycles the strainsare smaller and practically reversible. It should be pointedout that most of the volumetric strains take place while thematerial is saturated (suctions smaller than 0.1 MPa), butvolume changes are already very small for suctions higherthan 0.2 MPa. The shrinkage limit can be estimated fromFig. 11b to be in the gravimetric water content range of37%39%.

Figure 12 shows the evolution of the volumetric strainwith volumetric water content for an initially saturated cylin-drical specimen (38 mm in diameter and 76 mm in height).The initial dry density was 1.33 g/cm3, and the sample wasexposed without confinement to an atmosphere with a rela-

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Fig. 9. (a) Water retention curves for drying and wetting paths(free volume conditions) for an initial void ratio of 1.75. (b) Waterretention curve for drying paths and different initial void ratios.

Fig. 8. (a) Cyclic stress ratio versus number of cycles to reach liquefaction for a given axial strain. (b) Ratio of pore-water pressure, u,and initial confining stress, 3, versus vertical strain.

Initial void ratio, eo1.50 1.75 2.00

Path Drying Drying DryingAir entry value, AEV (MPa) 0.389 0.144 0.041Degree of saturation, Sr 0.99 0.99 0.99

Table 5. Degree of saturation and air entry value for water reten-tion curves at different initial void ratios.

tive humidity of 60% and a temperature of 22 2 C. It isobserved that the relationship between the change of volume(measured as the change in the dimensions of the specimen)and the volumetric water content (obtained from weighing)is linear for high water contents. The shrinkage limit, wr, re-sulting from the final deformation is about 0.37. It should bepointed out that in the oedometer tests and in this shrinkagetest, the change in volume experienced by the MW in satu-rated conditions is large (Figs. 1012).

Desiccation cracksA series of drying tests with MW placed on circular plates

(225 mm in diameter) was performed to observe crack gen-eration due to desiccation processes. Table 6 summarizes themain characteristics of all the tests. Figure 13 shows threephotographs corresponding to tests carried out with threedifferent thicknesses of MW (4, 8, and 16 mm), with eachsample placed in hermetic containers imposing a relative hu-

midity of 75%. It should be pointed out that the thicker thesample, the longer the distance between fissures.

Some results corresponding to these tests and referring tocrack formation are presented in Fig. 14 and Table 6. Fig-ure 14a presents the measured relationship between samplethickness and distance between cracks. The average distancebetween cracks is determined by measuring the separationbetween two continuous planes of desiccation cracks. Theprocess was carried out for all samples, and the averagevalue was calculated (Fig. 3b). The width of the cracks isvariable (0.051.30 mm) and depends on sample thickness(Table 6). It should be noted that this relationship is almostlinear, and the imposed suction only plays a secondary role.

Figure 14b shows the gravimetric water content measuredwhen cracks initiate. It can be seen that this water content isquite high for imposed suctions smaller than 60 MPa, whichimplies that samples are unsaturated. For higher suctions,however, the measured water content at the initiation of

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Fig. 10. Sample volume change and water exchange for first drying path: (a) suction increments from 0.01 to 0.03 MPa; (b) suctionincrements from 0.40 to 0.60 MPa; (c) change in solid and water volume for different suctions and times.

cracks is smaller than the shrinkage limit. This behaviourcan be explained by an increase in the tensile strength withan increase in suction.

Figure 14c shows the time required for cracks to initiatefor a given suction. In general, the higher the suction, theshorter the time required. The time to crack initiation alsoincreases with an increase in sample thickness. Solid sym-bols in Fig. 14 refer to samples open to the laboratory atmo-sphere that do not follow this general pattern. This could bedue to the larger air volume available for exchange com-pared with the volume available in the hermetic containers.The gravimetric water content in samples when cracks beginis very high and suction is very low. According to thegravimetric water content in all samples, the saturation de-gree is between 0.80 and 0.96; for this value, the suctionin MW is lower than 0.1 MPa considering the retentioncurve (Fig. 9). These results show that the samples were notin equilibrium with the salt solution when cracks appeared.Desiccation cracks appeared for a degree of saturation be-tween 0.80 and 0.96 (see Fig. 9). Vertical strain measuredwith the displacement transducer is more pronounced underlaboratory atmospheric conditions than in the hermetic con-tainers and increases when the prescribed suction and sam-ple thickness also increase (Fig. 14d).

Figure 15 shows the loss of water versus time for differenttests under laboratory atmosphere conditions (open system)

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Fig. 11. Evolution of the void ratio and degree of saturation of the sample during a cycle of dryingwettingdrying performed under aload of 0.03 MPa in the suction-controlled oedometer: (a) void ratio change versus prescribed suction; (b) shrinkage curve; (c) waterretention; (d) degree of saturation versus water content.

Fig. 12. Volumetric strain versus water content (compacted sam-ple with diameter 38 mm, height 76 mm, and dry density1.53 g/cm3). Shrinkage limit (wr) is around 0.37. V, change ofthe volume; Vo, initial volume.

and in hermetic containers (closed system). The water lossper unit area is higher in samples tested in the laboratorythan in those in the containers. According to this result, atthe time of the crack initiation, none of the samples attainedequilibrium. The linear relationship between water losses perunit area versus time in the initial drying process is in agree-ment with that from other works (Towner 1987a; Blight1997; Rodrguez 2002b).

The initial dry density, under saturated conditions, affects theshrinkage behaviour and water content when cracks appear(Fig. 16). The cracks do not appear for water contents lowerthan the retraction limit. When the sample is compacted to adegree of saturation between 0.90 and 0.99, and the gravimetricwater content is superior to the retraction limit, cracks appear,and the geometric characteristics and space distributions arevery different from those of the saturated condition. The desic-cation cracks appear at the periphery of the sample.

Hydraulic conductivitySaturated hydraulic conductivity, ksat, was measured by

imposing a constant-head gradient on a saturated specimenplaced in a permeameter cell. The results are plotted inFig. 17a, in which a clear dependence of ksat on the void ra-tio, e, is highlighted. This relation is defined by the follow-ing equation (see Aubertin et al. 1996):[1] k eysat =where y is the material parameter.

For the ksat values measured and shown in Fig. 17a, thevalue of y = 2.66, considering the material in solid andslurry form (e = 3.6); when the value of e is low, however,then y = 3.90.

Figure 17b presents the unsaturated hydraulic conductiv-ity, kunsat, versus the degree of saturation measured duringthe changes of suction imposed in the dryingwetting paths

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Fig. 13. Photographs of cracks for different material thicknesses: (a) 4 mm; (b) 8 mm; (c) 16 mm. Samples were confined in hermeticcontainers with a relative humidity of 75%; diameter of the container is 225 mm.

Imposedrelativehumidity (%)

Imposedsuction(MPa)

Initial drydensity(g/cm3)

Soilthickness(mm)

Time tocrackinitiation(days)

Gravimetricwater contentat crackinitiation (%)

Finalcrackopening(mm)

Distancebetweencracks(mm)

Verticalstrain atcrackinitiation (%)

Finalverticalstrain (%)

Hermetic container97.8 3.0 1.33 4 22.00 41.9 0.40 17 1.1 1.2

3.0 1.33 8 35.00 43.5 0.80 37 1.4 1.53.0 1.33 16 58.00 43.7 1.20 117 1.5 1.7

75.0 38.0 1.33 4 9.00 42.6 0.100.50 14 2.7 3.038.0 1.33 8 15.00 43.8 0.100.60 39 3.0 3.138.0 1.33 16 26.00 45.9 0.100.80 55 2.4 2.5

60.0 58.9 1.33 4 6.00 41.9 0.100.50 14 4.9 5.258.9 1.33 8 12.60 43.5 0.050.60 30 6.1 6.458.9 1.33 16 19.50 43.1 0.051.30 66 5.6 5.7

15.6 251.0 1.33 4 3.00 29.6 0.050.10 14 7.8 8.0251.0 1.33 8 10.00 30.1 0.050.50 36 7.2 7.5251.0 1.33 16 14.00 30.5 0.051.20 70 6.0 6.2

Laboratory atmosphere60.0 58.9 1.33 4 0.17 41.9 0.050.50 13 7.6 8.0

58.9 1.33 8 0.45 43.5 0.100.50 28 7.0 8.258.9 1.33 16 1.07 43.6 0.102.00 66 6.9 8.7

Table 6. Sample characteristics of desiccation tests in a controlled atmosphere for open and closed systems.

used to define the retention curve shown in Fig. 10c. As ex-pected, a large decrease in the permeability can be observedwhen the material is unsaturated.

Figure 17c presents hydraulic conductivity versus hydraulicgradients of between 3 and 25 for two different void ratios.The hydraulic conductivity is constant for different hydraulicgradients and different void ratios in homogeneous samples.

Effect of desiccation cracks on hydraulic conductivityIt has been observed that subsequent wetting and swelling

of the MW do not eliminate the discontinuities generated by

desiccation cracks, as occurs in clayey soils (Blight 1997).This could be of a great importance for in situ hydraulicconductivity under cyclic wetting and drying conditions. If acertain degree of discontinuity remains in the MW even af-ter sealing and mechanical confinement by the addition offresh MW, the hydraulic conductivity should reflect this ef-fect. Saturated hydraulic conductivity, ksat, was measured byimposing a constant-head gradient on a saturated specimenplaced in a triaxial cell under different confining stresses(from 7 to 1500 kPa). Measurements of saturated hydraulicconductivity were carried out on continuous samples and on

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Fig. 14. Results from desiccation tests: (a) distance between cracks versus waste thickness; (b) water content at crack initiation versusprescribed suction; (c) time to crack initiation versus prescribed suction; (d) vertical strain versus prescribed suction. Solid symbols re-fer to samples open to the laboratory atmosphere. h, material thickness.

layered and cracked samples (during 24 h). They are com-pared in Fig. 15a, in which all measurements are shown as afunction of the sample void ratio. The continuous sampleswere 120 mm in height and 100 mm in diameter, with aninitial dry density of 1.53 g/cm3. The layered samples wereprepared with layers of thicknesses H = 10 mm (12 layers),

20 mm (6 layers), and 40 mm (3 layers) with the same initialdry density. It should be pointed out that, according toFig. 15a, the effect of layering and crack formation in-creases hydraulic conductivity. The value of saturated hy-draulic conductivity depends on the initial void ratio and thethickness of the layers. It is possible to increase the value ofsaturated hydraulic conductivity between one to four ordersof magnitude in the samples with cracks compared with thatof the intact sample, depending on the initial dry density andthickness of the layer. In all cases the relation between ksatand the void ratio is defined by the equation ksat = ey. In thiscase, the y value for homogeneous samples is similar to thevalue obtained for hydraulic conductivity measured in thepermeameter test for a homogeneous sample with a similardry density (see Figs. 17, 18).

It is important to determine whether further addition ofmaterial or further mechanical actions may close the cracksand return the permeability to its original value (continuoussamples). Figure 15b shows that this is not the case forthese samples of MW, as isotropic stress confinementof MW samples does not eliminate the presence of the dis-continuities generated by previous desiccations. The resultswere obtained for a continuous sample and for layeredspecimens with different layer thicknesses (H), as previ-ously indicated.

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Fig. 15. Rate of loss of water versus time for three different samples during the desiccation process in hermetically closed containers(relative humidity = 65%, temperature T = 22 C) and in the laboratory atmosphere (relative humidity = 65%, T = 22 C).

Fig. 16. Relationship between initial gravimetric water contentand gravimetric water content at crack initiation and shrinkage.

The observed increase in hydraulic conductivity due tocracking is in agreement with results reported for clayeysoils (Bronswijk 1988; Scanlon 1992; Drumm et al. 1997;Jrgensen et al. 1998; Kelly and Pomes 1998). In particular,the work of Jrgensen et al. (1998) presents laboratory infil-tration tests where the effect of cracking on chloride andpesticide transport is studied in three columns of clayey soil.Jrgensen et al. observed that the hydraulic conductivity andtransport of contaminants are mainly controlled by fracturesthat become preferential flow paths. This could also be thecase in the deposits considered in this paper, and future workshould take this effect into account.

Conclusions

From the experimental study, it can be concluded that theammoniacal ammonium carbonate leaching (ACL) metallur-gical waste (MW) collected from a tailings impoundment inMoa, Cuba, has a grain-size distribution similar to that of asilt soil. It is formed by iron minerals and heavy metals. It

presents low plasticity, a null cohesion, and a high frictionangle (36). The material studied can be geotechnically clas-sified as low-plasticity silt (ML). Results of undrainedtriaxial tests on remoulded samples indicate a dilative be-haviour, with a decrease in pore-water pressure near fail-ure. MW is susceptible to liquefaction in the presence of acyclic load in triaxial tests. The void ratio decreases signifi-cantly during the consolidation test and the drying process.The relationship between the hydraulic conductivity andvoid ratio is ksat = ey.

Hydraulic conductivity, soil stiffness, and compressiveand tensile strength of the MW have an important depend-ence on the degree of saturation and vary significantly dur-ing the drying process. In the range of MW thicknesses andsuctions used in the tests, the distance between cracks seemsto be related to thickness and not to the imposed suction. Ingeneral, in a drying process applied to saturate MW, initia-tion of cracks occurs under quasi-saturated conditions andwith a small increase in horizontal tensile stresses due to thelow tensile stiffness of the MW close to saturation.

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Fig. 17. Relationship between (a) saturated hydraulic conductivity and waste void ratio, (b) degree of saturation and unsaturated hy-draulic conductivity for void ratios (e) between 1.70 and 1.56, and (c) saturated hydraulic conductivity and hydraulic gradient for twodifferent void ratios (1.75 and 1.45).

Unconfined compressive and tensile strengths gave maxi-mum values for degrees of saturation (Sr) of about 0.8 and0.9, respectively. These values correspond to suctions be-tween 0.05 and 0.40 MPa in the water retention curve. Ap-parently, desiccation cracks appear under similar conditions,for Sr values between 0.8 and 0.9.

It has been observed that after desiccation-induced surfacecracking, subsequent wetting or crack filling with fresh MWdoes not eliminate the original discontinuities. The hydraulicconductivity measured in samples with cracks is one orderof magnitude higher than the value measured for continuoushomogeneous samples of MW. From an environmental pointof view, the hydraulic conductivity results for the desiccationcracks have important implications in flow and transportanalyses on these MW deposits. These results may be usefulfor other cases involving similar and poorly character-ized MW.

Acknowledgements

This work was done in the framework of the Spanish Na-tional Project PPQ2001-2100-C04 and PB/44/FS/2002 fi-nanced by Fundacin Sneca, Murcia, Spain. The paper alsobenefited from the constructive comments of two anony-mous reviewers and the associated editor.

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