a numerical study of soil cover perfomance.yanful, mousavi and de souza.2006

7/21/2019 A Numerical Study of Soil Cover Perfomance.yanful, Mousavi and de Souza.2006

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Journal of Environmental Management 81 (2006) 7292

A numerical study of soil cover performance

Ernest K. Yanful, S. Morteza Mousavi, Lin-Pei De Souza

Geotechnical Research Centre, Department of Civil and Environmental Engineering, The University of Western Ontario, London, ON, Canada N6A 5B9

Received 17 June 2003; received in revised form 8 October 2005; accepted 11 October 2005

Available online 23 March 2006

Abstract

In the investigation of soil cover design options for final decommissioning of reactive mine waste, it is often necessary to analyze or

predict the anticipated cover performance as a function of the cost of implementation, which is governed by the type, number andthickness of the layers in the cover system. An example of such investigation is presented in this study where one-dimensional

evaporation from hypothetical moisture-retaining cover systems is simulated to assess the influence of several cover properties and

hydrogeologic parameters on performance. The commercially available transient flow model, SoilCover, was used to compute suction

and water content profiles for different cover design scenarios. The predicted water content profile and porosity of layers were then used

to estimate the oxygen diffusion coefficients of the various layers. The oxygen diffusion coefficients were used to estimate oxygen flux

through the cover systems. The oxygen flux was, in turn, related to the maximum acid flux.

The studied cover and hydrogeologic parameters included soil type, thickness of barriers, and water table elevation. Two types of

infiltration and oxygen barrier and two types of capillary layer with different thicknesses were studied. The water table was either kept

constant at the base of the waste (tailings) or dropped by 0.5, 1, 2, and 3 m over 120 days. The results showed that the relationship

between water table depression and the thickness of capillary layers, on one hand, and desaturation of the infiltration and oxygen barrier,

on the other, is not linear. Relationships between oxygen flux and barrier thickness and between cost increase and performance

improvement of the studied cover systems are presented. Finally, a method that outlines steps for site-specific and economically feasible

design of multi-layer cover systems is introduced.r 2006 Elsevier Ltd. All rights reserved.

Keywords: Soil cover; Sand-bentonite; Optimum design; Infiltration barrier; Capillary layer; Oxygen diffusion; Water level; Environmental management

1. Introduction

1.1. Layered soil cover systems

The contaminated effluent generated when reactive

sulfide-bearing mine waste comes into contact with water

and oxygen is called acid rock drainage (ARD). Ifpreventative and control measures are not taken, ARD can

contaminate surface and ground water in communities

around a mine site. Limiting accessibility of water and

oxygen to the waste can reduce ARD production.

Covering waste (for example, tailings) with a soil layer

that has a low hydraulic conductivity limits accessibility of

water to the tailings. In addition, when the soil cover is

placed close to saturation and maintained at this high

water content, oxygen accessibility is also restricted. Thus,

a fine-grained soil cover with minimum hydraulic con-

ductivity but maximum degree of saturation is the best soil

cover to reduce oxygen ingress in a net-infiltration

environment.

The concept of soil cover is not new. Brown (1970),Nicholson et al. (1989),Aubertin et al. (1995a, b),Khire et

al. (1997),Chapuis (2002), andMbonimpa et al. (2003)are

among many researchers that have studied soil covers.

The challenge to implementing a soil cover above the

water table for a reactive mine waste is maintaining a high

degree of saturation in the cover over a long period. To do

this, both downward drainage of water from the cover

towards the waste and upward evaporation from the cover

towards the atmosphere, especially during dry periods,

must be prevented or at least minimized. Nicholson et al.

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doi:10.1016/j.jenvman.2005.10.006

Corresponding author.

E-mail address: [email protected] (E.K. Yanful).
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(1991)suggested that the selection of cover materials must

be site specific and recommended further study of the

hydraulic processes.

Yanful (1993) observed that a moisture-retaining multi-

layer soil cover over acid-generating tailings was able to

reduce the oxygen flux by 98%. This cover consisted of a

middle layer of fine-grained material that served asmoisture-retaining barrier layer and two protective

coarse-grained layers at the top and bottom of the barrier.

In the present study, the barrier layer is called infiltration

and oxygen barrier, while the two protective coarse-

grained layers at the top and bottom of the infiltration and

oxygen barrier are referred to as capillary layers.

Placing coarse-grained soil layers, or capillary layers,

above and below the infiltration and oxygen barrier

generally helps preserve the degree of saturation of the

latter under a wide range of climatic conditions (Rasmuson

and Eriksson, 1986;Collin, 1987;Yanful and Aube , 1993;

Benson et al., 1994; Shackelford et al., 1994; Choo and

Yanful, 2000). The role of capillary layers will be discussed

further in Section 2.1.

Layered soil covers have also been studied in a number

of field investigations in the last 15 years. Two- and three-

layer soil covers were constructed over acid-generating

waste rock at Rum Jungle Mine in Australia (Harries and

Ritchie, 1987), Heath Steele Mine site in New Brunswick

(Yanful et al., 1993a, b), Equity Silver Mine site in British

Columbia (OKane et al., 1998) and Bersbo mine in

Sweden (Lundgren, 1997). Test plots of a three-layer soil

cover were constructed on acid-generating tailings at the

decommissioned Waite Amulet site near Rouyn-Noranda,

Que bec in 1990 and monitored over a 3-year period(Yanful and St-Arnaud, 1991; Yanful et al., 1994). The

three layers consisted of an uppermost fine sand layer

underlain sequentially by a compacted silty clay and a

coarse sand. A blanket of gravel was placed over the fine

sand to prevent erosion. Monitoring data indicated that the

silty clay (infiltration and oxygen barrier), compacted close

to 2% wet of the optimum water content, maintained its

placement degree of saturation of approximately 95% and

hydraulic conductivity of 1 107 cm/s during the 3

years. As a result, the cover reduced oxygen diffusion and

water percolation significantly and allowed only 4% of

precipitation to percolate into the underlying tailings

(Woyshner and Yanful, 1995).

In all the soil cover systems described above, the nearly

saturated infiltration and oxygen barrier with low perme-

ability performed well because of the presence of protective

coarse-grained layers, which functioned as capillary break

layers. Other mine waste cover systems that do not use the

capillary layer concept have also been investigated. For

example, OKane et al. (2000), Wels et al. (2002), and

OKane and Waters (2003) have demonstrated the perfor-

mance of a store and release cover for reducing acid

generation in arid climates.

In spite of the good performance reported in the above

cases, soil covers have not been widely used in mine waste

decommissioning projects in temperate climates. Two main

reasons account for this trend. First, the potentially high

upfront cost of a soil cover: for example, for a 20-ha

tailings site, the cost of installation of a three-layer soil

cover is estimated to be $100,000$250,000/ha (site

specific), while the cost of implementation of a water cover

for the same site is $120,000/ha. Although these estimatesare for construction costs only and do not consider site-

specific details and maintenance, they show that, generally,

the upfront cost of implementation of a soil cover is more

than that of a water cover. The second disadvantage of soil

cover is uncertainty about long-term performance. Very

little information is available on long-term performance of

soil covers.

Significant long-term maintenance costs and detrimental

environmental impacts may result if a soil cover is not

properly designed and constructed. The primary motiva-

tion for the present work was to examine how an optimized

soil cover design can improve performance and lead to

possible cost reductions.

1.2. The present study

This numerical study attempts to correlate cover

performance with cost to assist the designer in selecting

case- and project-dependent optimum designs. Also, the

study examines covers under critical long-term scenarios

where the covers are allowed to evaporate during a 4-

month period with no rain. This is likely the worst-case

scenario and is similar to the study period used bySwanson

et al. (2003), who examined soil cover performance over a

153-day period that included an extreme dry summer. Theresults can help achieve cost-effective designs and signifi-

cantly reduce the number of scenarios that must be

examined in a pilot or test plot study. In decommissioning

projects where time is a limitation and test plots cannot be

constructed and monitored over a long period of time prior

to implementation, the method described in the present

study can be used along with sound engineering judgment

to select the final cover.

A number of researchers have highlighted the important

role of numerical modeling in the analysis of layered soil

covers. McMullen et al. (1997) observed that because the

hydraulic behavior of a layered cover system is complex,

computerized calculations based on numerical models may

be used to establish optimum configurations. Bussie` re et al.

(1995) analyzed covers with different material types, and

showed how numerical methods can be used to evaluate the

performance of capillary layers. Mbonimpa et al. (2003)

elucidated the benefits of using numerical simulations for

predicting soil cover performance. Also, Swanson et al.

(2003)reported the results of similar simulations to predict

the performance of the soil cover system at Equity Silver

Mine, British Columbia, Canada.Mbonimpa et al. (2003)

presented sample calculations of oxygen flux in cover

systems and suggested that changing key cover properties

and design parameters, such as the degree of saturation of

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the barrier and thickness of the various layers, could be an

effective way of predicting cover performance.

In the present work, the effect of material type and

thickness of capillary layers and infiltration and oxygen

barrier were studied during a 4-month period. The study

involved the use of the flux boundary model SoilCover

(Geo-Analysis 2000 Ltd., 2000) and appropriate materialproperties to simulate several scenarios.

The inclusion of evaporation is one important aspect

that distinguishes the present study from previous studies

(e.g.Shackelford et al., 1994;Bussie` re et al., 1995;Aubertin

et al., 1996). The present work is also different from

previous studies in the sense that it considers the key

aspects of cover design and analysis (material selection,

flow modeling, oxygen transport modeling, and cost

analysis). This approach provides an integrated picture

that allows the designer or proponent to eliminate less cost

effective options without recourse to costly long-term

laboratory and test plot studies. The following section

highlights the key considerations in the study.

2. Key aspects of cover design

2.1. Considered mechanism

Using a transient numerical model, Nicholson et al.

(1991) showed that in a layered soil cover static non-

equilibrium conditions would prevail in the coarse-

grained lower capillary layer for prolonged periods of

time, such that an overlying fine-grained infiltration and

oxygen barrier would not drain. In the numerical analyses

presented in this study, transient evaporation was con-sidered as top boundary condition at the soil surface and

static non-equilibrium condition was assumed in the

coarse-grained lower capillary layer, in accordance with the

observations of Nicholson et al. (1991) andBarbour and

Yanful (1994).

2.2. Importance of evaporation

Isothermal models may be used to predict responses of a

soil cover system over the short term; however, for long-

term analyses under extended drying conditions, coupled

heat and water transport soilatmosphere modeling should

be used to analyze soil covers (Swanson et al., 2003).

Water content within a cover system is not only a

function of the soil water characteristic curves of the cover

materials but is also influenced by the water flux that

develops through the cover due to climatic factors. Water

flux across the cover system will develop as a result of

precipitation or evaporation, and the pressure within the

cover will respond to accommodate these fluxes. Therefore,

if this point is neglected, the pressure profiles and the

resulting water contents within the cover system may

deviate significantly from those postulated. Consequently,

the adequacy of the selected materials can be fully assumed

only when their soilwater characteristic curves and their

hydraulic conductivitysuction functions are considered in

the light of surface infiltration and evaporation (Barbour,

1990).

Although Nicholson et al. (1991) did not study

evaporation, they recognized that evaporation from the

surface of a cover system was the primary process of

moisture loss from the cover. They recommended furtherinvestigation of the hydraulic concepts articulated in their

paper to include more realistic scenarios, such as the effect

of evaporation. Mbonimpa et al. (2003) confirmed that

layered soil covers with capillary layers could be employed

to reduce vertical percolation of water into the underlying

waste.

Choo and Yanful (2000) demonstrated that vapor

transport exceeded liquid water flow a few days after

drying.Swanson et al. (2003)performed a soilatmosphere

modeling of an engineered soil cover on acid-generating

mine waste in a humid alpine climate. They confirmed that

vapor flow was the dominant flow mechanism near the

surface of the cover a few days after the start of drying.

Swanson et al. (2003) cautioned the danger of under-

estimating water loss from covers by ignoring evaporation.

These findings emphasize the need to examine coupled heat

and water transport in soil cover analysis and design.

Although evaporation was not incorporated in most of

the early studies on soil covers (includingAkindunni et al.,

1991; Nicholson et al., 1991; Shackelford et al., 1994;

Bussie` re et al., 1995; McMullen et al., 1997; Mbonimpa

et al., 2003) those studies all recognized its importance. In

the present work, the movement of liquid water and water

vapor due to evaporation within a soil cover is included in

the analysis.

2.3. Estimating oxygen-diffusive flux

A knowledge of soil moisture content and hence oxygen

flux is necessary for determining the final (optimal)

configuration of a soil cover (Mbonimpa et al., 2003). In

fact, the production of acid in sulfidic mine waste is

controlled by the availability of oxygen at the waste

surface. The oxygen flux can be related to the theoretical

maximum acid flux on the basis of the stoichiometry of the

overall sulfide oxidation reaction (Yanful, 1993):

FeS2s154

O2g 72

H2Ol ! FeOH3s 2H2SO4aq.

(1)

Eq. (1) indicates that 3.75 mol of gaseous oxygen (O2)

oxidize 1 mol of pyrite (FeS2) in the presence of water

(H2O) to produce 2 mol of sulfuric acid (H2SO4). Thus, the

oxygen flux may be correlated to the acid flux to give an

indication of cover performance (Yanful, 1993).

The primary mode of oxygen transport in tailings is

diffusion through pore spaces. In unsaturated fine-grained

soils, such as the soils used in infiltration and oxygen

barriers (i.e., silts and clays) oxygen transport is generally

controlled by molecular diffusion (Collin, 1987;Collin and

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Rasmuson, 1988; Mbonimpa et al., 2003). In porous

geologic materials the most important factor that controls

the magnitude of the oxygen diffusion coefficient is the

degree of saturation (Nicholson et al., 1991). Modeling the

moisture flow in a cover system is a necessary step to

determining the oxygen flux through the cover (Swanson et

al., 2003). Due to the relatively low solubility of oxygen inwater and the four orders of magnitude decrease in oxygen

diffusion coefficient from air to water, soil covers at or near

water saturation exhibit reduced oxygen fluxes (Nicholson

et al., 1989). This led to the common practice of using soil

materials with low oxygen diffusivities to reduce oxygen

transfer from the atmosphere to reactive sulfide mine

waste. Several studies (for example, Yanful, 1993; Yanful

et al., 1993b;Yanful et al., 1999) have confirmed that the

reduced oxygen transfer leads to a decrease in oxidation

and acid generation. Measurement of the effective diffusion

coefficient of oxygen is required to evaluate the flux for

optimum design of soil cover systems (Mbonimpa et al.,

2003).

In the present study, modeled moisture profiles and

measured or estimated porosity of various soil layers in a

cover system were used to calculate oxygen diffusion

coefficients using the method of Millington and Shearer

(1971). Collin and Rasmuson (1988) indicated that the

Millington and Shearer (1971) equations are reasonably

accurate over a wide range of water contents, from dryness

to saturation. The resulting oxygen diffusion coefficient

and selected thickness of the various layers were then used

to estimate oxygen flux across the cover system. The

relative efficiency of a cover can be evaluated by the total

oxygen flux through the cover, which governs the amountof acid generation.

2.4. Cost analysis

Unit cost assumptions for geologic materials and earth-

works (Rowe, 1993) were used to estimate the cost of the

considered soil cover scenarios. These assumptions were

based on 1993 costs; however, prices and final cost may be

easily adjusted for inflation to bring them to 2005 figures.

Because the analysis was based on relative cost estimate

and the results are presented in percent, this should not

affect the conclusion on cover performance. The unit costs

do not take into account minor components and unknown

variables, which generally cannot be accurately known

until at the stage of detailed design. The main considera-

tions in the cost computations are the type and thickness of

the various soil layers present in the cover. For illustration,

cost increase and performance improvement versus thick-

ness of capillary layers and infiltration and oxygen barriers

for a few cases are presented. Correlations between cost

increase and performance improvement are also high-

lighted.

Optimum cover design has been an issue of importance

to many engineers and researchers including Bussie` re et al.

(1995)andMbonimpa et al. (2003).McMullen et al. (1997)

noted that as the hydraulic behavior of a layered cover

system is fairly complex, computerized numerical simula-

tions might be used to establish the optimum cover

configuration. However, it is obvious that there cannot

be a unique optimum design for every soil cover applica-

tion. Selecting an optimum design is definitely a site-

specific and project-specific issue and case-dependentprocess that hinges on several factors including

(i) site hydrology and hydrogeology, climate, and materi-

al availability and type;

(ii) intended short- and long-term functions of the cover;

(iii) construction constraints and challenges;

(iv) available project budget.

In general, one operator may want to prevent ARD

production, while another operator may find that imple-

menting a cover that is less than 100% effective and

combining it with collection and treatment of the resulting

residual ARD may be more economical than trying to

achieve complete ARD elimination.

Construction challenge is another factor that can affect

the implementation of a cover system. For example,

working on a tailings surface with heavy construction

equipment can be difficult if not impossible. Trafficability

is an issue because high capillarity can result in an elevated

water table in the tailings, which can complicate the

placement of relatively thin cover layers with adequate

compaction and homogeneity. Thus, material placement

should be an important consideration in all design options

(McMullen et al., 1997). In some cases, winter construction

has been adopted to ensure that the tailings surface issufficiently frozen to support equipment.

Also, since homogeneity and free-drainage of the top

capillary layer are important for it to function as a good

capillary break, it is necessary to avoid pumping of the

underlying tailings into the overlying sand or gravel

capillary layer during placement and compaction. To

achieve this, the top capillary layer should be placed

during the coldest winter months when the tailings surface

is frozen. Available budget also has a critical role in the

selection of an optimum cover design. The proportion of

initial investment and maintenance cost to total cost is

dependent on many factors including the expected perfor-

mance of the cover and the length of the project. Therefore,

the concept of an optimum soil cover design is case-

dependent

2.5. Proposed approach

The approach proposed in the present work is the

development of site- and project-specific optimum designs

that take the above factors into consideration. It empha-

sizes the role of material type, layer thickness, water level

location, and involves the quantitative analysis of the

correlation between cost and performance for a given soil

cover scenario. Such an approach should help the designer

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select an optimum case-specific design for a cover project.

Fig. 1 presents the conceptual flowchart of the computa-

tional sequence used in the study.

A necessary requirement for maintaining a soil cover in a

high saturated state after prolonged drainage is that the

magnitude of the air-entry value (AEV) of infiltration and

oxygen barrier be greater than the sum of the thickness ofthe infiltration and oxygen barrier and the absolute value

of the suction at which the underlying coarse layer

approaches the residual moisture content (Akindunni

et al., 1991). The AEV is the suction required to overcome

the capillary forces exerted by the largest pore and initiate

drainage in the medium (Nicholson et al., 1989). Similarly,

the AEV of the infiltration and oxygen barrier should be

greater than the absolute value of the suction at which the

overlying coarse layer approaches the residual moisture

content. The thickness of the lower capillary layer should

be greater than the absolute value of its AEV to ensure that

drainage occurs to reach residual saturation (maximum

suction) and hence minimum hydraulic conductivity in the

lower capillary barrier, which would minimize water loss

from the infiltration and oxygen barrier (Bussie` re et al.,

1995).

A reliable and technically feasible cover design must

meet the above criteria. The design process for a particular

cover alternative would normally start with a considerationof site specifications and material type and availability. It

should also consider expected cover function, construction

challenges, and available budget. The design alternative is

then checked against the above criteria. The data needed

for these criteria include: AEV of infiltration and oxygen

barrier, thickness, hr (residual saturation head) of upper

capillary layer, and finally hr and AEV of lower capillary

layer. Although it is possible to obtain these parameters

from laboratory tests, it is useful to obtain first approx-

imation estimates from published empirical solutions.

Lambe and Whitman (1969) presented a range of

capillary heads data for soils draining from the bottom.

ARTICLE IN PRESS

NO YES

* Air entry value ** residual saturation head

Preliminary cost estimation

Transient flow numerical analysis using model and input data

Site-specific data (hydrology, hydrogeology, etc.)

Material type and availability

Expected cover function

Construction constraints and challenges

Available budget

Schematics of preliminary soil cover design

Profiles of: Suction, saturation degree,

volumetric water content

Diffusion modeling using volumetric water

content and porosity as input

Oxygen diffusion coefficient (De) of layers

Calculation of oxygen flux using De and

thickness of layers

Performance-Cost correlation analysis for soil cover system

Site specific Optimum soil cover design

Is thickness of infilt. & O 2barr.its| |AEV ?

Fig. 1. Conceptual flowchart of computational sequence.



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Nicholson et al. (1991) plotted the data on log-linear axes

and obtained the following relation:

Log d10 0:01AEV 0:421, (2)

whered10is the particle size (in mm) than which 10% of the

soil is finer, and AEV is the air entry value of the material

in cm of water. Nicholson et al. (1991) suggested thatEq. (2) may be used to estimate AEV of an infiltration and

oxygen barrier. However, it should be pointed out that the

data presented by Lambe and Whitman (1969) was for

cohesionless soils, and therefore the equation may not be

suitable for cohesive soils. Therefore, in the present study it

is suggested that Eq. (2) be used for cohesionless soils and

laboratory test be used for cohesive soils.

Lambe and Whitman (1969)also presented data on the

maximum height of rise of water in cohesionless soils

subjected to downward vertical drainage. This rise is

similar to residual water height (hr) in an upper capillary

barrier in a three-layer cover consisting of a middle lowpermeability layer. If the data are plotted on log-linear

axes, the log (d10) value (d10 in mm) versus the hr (cm

water) exhibits a linear trend over the range of sandy gravel

to silt (Fig. 2). Therefore, the d10 value presents an easily

measured parameter to provide an initial estimate of the hrof cohesionless materials (such as sand) used in upper

capillary layer.

The design steps integrating the above requirements for

the capillary barriers and infiltration and oxygen barrier in

a multi-layer cover system are outlined inFig. 1. The use of

Fig. 2and Eq. (2) at the first stage of the design process can

reduce the amount of subsequent laboratory testing. If the

answer to the three questions in the flowchart ofFig. 1isYES, then it is recommended that laboratory tests (soil

water characteristics tests) be run to obtain more reliable

values of AEV and hr.

The numerical analyses were performed using a satur-

atedunsaturated evaporation flux model, say SoilCover

(Geo-Analysis 2000 Ltd., 2000). Typical input include

climatic, hydrological and geotechnical data. Profiles of

suction, degree of saturation and volumetric water content

resulting from the simulation are used along with porosities

of the layers to calculate oxygen diffusion coefficients (De),

using the method of Millington and Shearer (1971). Next,

the oxygen flux through the cover system is estimated usinga steady-state oxygen flux model, De and thickness of

layers as input data. Finally, an analysis of cover

performancecost relation, similar to what is performed

in the present study for the considered cases, can be

performed to identify the optimum site- and project-

specific design. The flowchart in Fig. 1 summarizes these

steps.

3. Methods and materials

3.1. Numerical model

In a previous study,Yanful et al. (2002)presented results

that showed good agreement between experimental eva-

poration data and numerical simulations obtained using

the soilatmosphere model SoilCover (Geo-Analysis 2000

Ltd., 2000). AlsoSwanson et al. (2003)assessed the ability

of the same model to simulate field conditions and found

reasonable agreement between the model and field results.

Therefore, it was decided in the present study that

SoilCover could be used to predict evaporation and suction

and water content profiles in multi-layer soil covers.

SoilCover is a one-dimensional finite element package

that models transient liquid and water vapor flow, based ona theoretical model for predicting the rate of evaporation

from a soil surface (Wilson et al., 1994). The model

framework is a system of equations describing coupled heat

and mass transfer in a soil. The flow of water vapor and

liquid water are described on the basis of Ficks law and

Darcys law, respectively. Fouriers law is used to describe

conductive heat flow in the soil profile below the soil/

atmosphere boundary. Temperature is evaluated on the

basis of conductive and latent heat transfers. The model

calculates the vapor pressure in the soil using the relation-

ship provided by Edlefsen and Anderson (1943), which

calculates vapor pressure from the total suction in the

liquid phase.

Atmospheric coupling is achieved by calculating the soil

evaporative flux from environmental input data and soil

hydraulic properties. Evaporation is calculated using a

modified Penman formulation (Wilson, 1990), and eva-

porative flux can be calculated from either a saturated or

an unsaturated soil surface. Other user-defined soil

parameters used in SoilCover include specific gravity,

porosity, and saturated hydraulic conductivity. The var-

ious features of the software and the governing equations

are described in detail in the user manual (Geo-Analysis

2000 Ltd., 2000). In the present study the software was

used to evaluate hypothetical multi-layer cover systems

ARTICLE IN PRESS

Residual saturated value (hr) (cm water)

0 50 100 150 200 250 300 350 400

d(10)particlesize

(mm)

0.001

0.01

0.1

1

log(d10)=.00479(AEV)-0.579

r 2=0.97

Fig. 2. Relationship betweend10 grain size and residual saturation head

(hr) for dry, cohesionless soils undergoing bottom drainage (Data from

Lambe and Whitman, 1969).



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overlying mine waste (tailings). Evaporation was consid-

ered as top boundary condition at soil surface, and suction

as bottom boundary condition at the covertailings inter-

face. Temperature, relative humidity, radiation, and wind

speed were used as climatic input data. Other input soil

properties included saturated hydraulic conductivity, por-

osity, soil water characteristic curve and hydraulic con-ductivitysuction function. Profiles of suction, degree of

saturation and water content obtained as output were used

to calculate oxygen diffusion coefficient and then oxygen

flux and, finally, to evaluate the performance of the cover

systems.

3.2. Oxygen diffusion coefficient and flux calculation

procedure

Millington (1959) proposed the following equation for

diffusion of gases in non-aggregated porous media:

DD0

1 Sw2PT y2x, (3)

whereDis the diffusivity of a gas in the porous medium, D0is the diffusivity of the same gas in air, and D/D0 denotes

the normalized diffusivity. Also, Sw defines the degree of

saturation, y the volumetric water content, andPTdenotes

the total porosity of the medium, while x is obtained from

the following relation (Millington and Quirk, 1961):

P2xT 1 1 PTx (4)

i.e., the minimum or effective pore area, P2xT cm2 per cm2, is

associated with a maximum area occupied by solid,

(1PT)x

cm2

per cm2

.In the present study, Eqs. (3) and (4) were used to

calculate the diffusion coefficient of each soil layer in the

cover systems, following the method of Millington and

Shearer (1971). Diffusion coefficient and thickness of the

layers were then used to compute oxygen diffusion flux as

described in the sections that follow.

The computation of steady-state diffusion through a

multi-layered sequence is based on the steady-state diffu-

sion through a finite layer (Ficks first law):

F DeqC

qx, (5)

whereFis the mass flux [M/L2

T] and other parameters areas previously defined. The concentration profile through

the sequence of layers is calculated by equating the flux

from each consecutive pair of layers at the interface

between them. The flux can be equated at the interface for

any two adjacent layers with the resulting general equation:

Di

Li

Ci1

Di1

Li1

Di

Li

Ci

Di1

Li1

Ci1 0, (6)

where the subscript is notation for any layer. This provides

a system of equations (equal to the number of layers minus

one), which can be solved simultaneously. The boundaryconditions are represented by a constant concentration at

surface and a concentration of zero at the base of the

deepest layer.

The needed input data are the number of layers, the

thickness (m) and diffusion coefficient (m2/s) for each layer

starting at the top of the sequence. A computer program

based on the above equations was developed by Gillham

and Nicholson (1990) for analyzing transient and steady-

state diffusion through a single-layer cover, and steady-

state diffusion through a multi-layer cover system. This

program was adopted for the present study. The analysis

focused on steady-state oxygen flux that is more dealt with

in long-term analyses.

3.3. Modeled soil cover scenarios

The conceptual cover system used in the present study

typically consisted of an infiltration and oxygen barrier

placed between upper and lower capillary layers. The two

soil types used as infiltration barrier were sandbentonite

(Cases A1A5), and silt (Cases C1C10). Tailings and

Waite Amulet fine sand were used to texturally represent

mine waste, and Waite Amulet fine sand and coarse sand

were used as upper and lower capillary layers, respectively

(Choo and Yanful, 2000). Porosities of fine sand, sand

bentonite, coarse sand, silt, and tailings were 0.40, 0.39,

0.39, 0.45, and 0.45, respectively. Saturated hydraulic

conductivities, soilwater characteristic curves, and un-

saturated hydraulic conductivitysuction functions of the

studied soils are presented in Table 1 and Fig. 3. The

unsaturated hydraulic conductivitysuction functions for

all soils were determined using the method of van

Genuchten (1980). The properties of silty tailings given

by Gonzalez and Adams (1980) were used to represent

those of the mine waste and the alternate silt infiltration

and oxygen barrier.

A schematic of each modeled soil cover system and watertable location are shown in Fig. 4 and beyond. Plots of

suction and degree of saturation profiles are also presented

for some cases. The suction profile is presented to facilitate

the discussion of cover performance using the soilwater

ARTICLE IN PRESS

Table 1

Laboratory-measured saturated hydraulic conductivities of study soils

Soil type WA coarse sand WA fine sand sand-bentonite Tailings/silt

Ksat(cm/s) 5.3102 6.1103 5108 5.8106

Taken fromGonzalez and Adams (1980).



8/21

characteristic curve and the unsaturated hydraulic con-

ductivitysuction function. The volumetric water content

profiles and the porosities of the layers were used to

estimate oxygen diffusion coefficients using the procedure

proposed byMillington and Shearer (1971). The estimated

diffusion coefficients and the selected thickness of the

layers were then used to estimate oxygen flux through the

cover systems. This flux is a maximum value (worst-case

scenario) and in reality the flux would be less because of the

lower flux during transient state. The results are presented

in terms of cases related to the type of infiltration barrier

(sand-bentonite, or silt) implemented in the cover system. It

should also be noted, however, that oxygen fluxes

calculated using the average Sr are only approximate

and, in some cases, may not necessarily represent the actual

flux through the cover.

3.4. Initial and boundary conditions

A perched water table condition was simulated in the

uppermost fine sand to ensure that it was initially

saturated. As noted by Gardner and Fireman (1958),

Wilson et al. (1997) andYang and Yanful (2001), a high

rate of evaporation cannot exist when the water table is

deep. Hence for the cases where the water table was deep,

the uppermost fine sand layer had to be initially saturated

to promote evaporation. This would represent the situation

where a brief rainfall period follows prolonged drying that

leads to water table depression.

Environmental fluctuations such as infiltration andevaporation influence the performance of a cover. How-

ever, infiltration would generally contribute to maintaining

a high degree of saturation in the infiltration and oxygen

barrier and may not be a critical performance factor for

moisture retaining covers. Thus, evaporation, and not

water infiltration, was applied as a top boundary condition.

This condition would be analogous to the most critical field

situation where, following a period of precipitation,

evaporation occurs on a soil surface for a long time before

the next rain. Air temperature of 1825 1C, relative

humidity of 1651%, and potential (pan) evaporation rate

of 18.3 mm/day were used as climatic input data. This

selected potential evaporation was a conservative value

derived from previous laboratory column studies (Choo

and Yanful, 2000).

Both constant and changing water table conditions were

considered for the bottom boundary conditions during the

simulations. Therefore, the bottom boundary conditions

included either a constant or time-dependent pressure

head. The datum was located at the base of the tailings in

Cases A and at the interface of the tailings and the cover

system in Cases C. In some of the modeled cases, the water

table was placed at the surface of the tailings, that is, at the

interface of soil cover and tailings. In these cases, the water

table was gradually lowered to 1 m below the surface of thetailings during the simulation period. This drop in water

table was found to be typical of some tailings deposits in

Canadian temperate climates, such as the Waite Amulet

site (Yanful et al., 1990;Woyshner and Yanful, 1993). To

evaluate the effects of different water table conditions,

water table drops of 0.5, 1, 2, 3, and 4 m below the surface

of the tailings were also examined. Other cases involving

constant water level during the analysis period were also

investigated.

4. Analysis of soil cover scenarios

4.1. Sand-bentonite as infiltration barrier

Sand-bentonite was used as an infiltration barrier

because of its generally low hydraulic conductivity, ability

to maintain a high degree of saturation (Yanful and

Shikatani, 1993) and apparent resistance to the influence of

freezing and thawing (Wong and Haug, 1991; Chapuis,

2002). Furthermore, the soilwater characteristic curve

(Fig. 3) shows that sand-bentonite can maintain signifi-

cantly high water content even at high suctions. Thus this

type of material may be used as infiltration and oxygen

barrier when the water table is deep.

ARTICLE IN PRESS

Suction (kPa)

0.1 1 10 100 1000

Volumetricwatercontent

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

WA coarse sand

WA fine sand

tailings and silt

sand bentonite

0.1 1 10 100

Hydraulicconductivity(m

/s)

10-2010-1910-1810-1710-1610-1510-1410-1310-1210-1110-1010-910-810-710-610-510-4

WA coarse sand

WA fine sand

tailings

sand bentonite

Suction (kPa)

Fig. 3. Soilwater characteristic curves and suctionhydraulic conductiv-

ity functions for the soils used in soil cover scenarios.



9/21

In Cases A1A5 a 60-cm thick sand-bentonite layer was

used as infiltration barrier. In Cases A1A3, the water

table, initially located at the surface of the tailings, was

decreased to 1 m below the surface of the tailings over 120

days. However, in Cases A4 and A5, the water table was

kept constant at 1 m below the surface of the tailings at all

times.

4.1.1. Cases A1 and A2: decreasing water level

4.1.1.1. Influence of lower capillary layer. In Case A1 the

soil cover scenario consisted of 30 cm of Waite Amulet fine

sand overlying 60 cm of sand-bentonite, which in turn

overlaid 30 cm of Waite Amulet coarse sand (Fig. 4). The

thickness of the sand layers (capillary layers) was based on

field design data presented by Yanful and St-Arnaud

(1991). The 60 cm thickness of sand-bentonite (infiltration

and oxygen barrier) was selected on the basis of both field

experiment and diffusive flux calculations performed by

Yanful (1993), which showed that this thickness reduced

the oxygen flux substantially. The suction profile in Case

A1 (Fig. 4) shows steadily increasing suction in the lower

capillary layer (coarse sand) over time, in accordance with

the gradual drop in the water level until the suction at the

residual water content is attained. Due to the low AEV of

the coarse sand (Fig. 3), it loses water rapidly by drainage,

resulting in a decrease in hydraulic conductivity and overall

flux of water and, hence, a high degree of saturation in the

infiltration barrier.

In Case A2 (Fig. 4) the Waite Amulet coarse sand layer

was removed from the cover system studied in Case A1, in

order to evaluate the feasibility of reducing costs by

removal of the lower capillary layer. Comparison of

suction profiles for Cases A1 and A2 in Fig. 4shows that

the suction at the base of the sand-bentonite layer increased

by 39%, from 6.9 kPa for Case A1 to 9.6 kPa for Case A2.

When a lower capillary layer is used, its steep hydraulic

conductivitysuction function results in a pressure profile

that starts at zero at the water table, and decreases to

negative pressure until near the residual suction value.

4.1.1.2. Influence of upper capillary layer. To investigate

the influence of the upper capillary layer in the perfor-

mance of a cover system with a 60-cm thick sand-bentonite

infiltration barrier, when the water level drops 1 m from the

ARTICLE IN PRESS

initial water level

tailings

Case A1

WA coarse sand

sand-bentonite

WA fine sand30 cm

60 cm

30 cm

100 cm

final water level

Case A1-Suction (kPa)

-10 -5 0 5 10 15

Elevation(m)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

day 1

day 15

day 30

day 60

day 90

day 120


-10 -5 0 5 10 15

Elevation(m)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

initial water level

60 cm

30 cm

100 cm

final water level

tailings

sand-bentonite

WA fine sand

Case A2

Fig. 4. Schematic diagrams of soil cover Cases A1 and A2 and profiles of suction.



10/21

top to the bottom of the tailings during the simulation

period, covers with 0, 15, 30 and 50cm thick upper

capillary layers were analyzed.Fig. 5shows that at the end

of 120 days, the average volumetric water content of the

infiltration barrier increased from 0.335 for a cover system

without upper capillary layer, to 0.369, 0.386, and 0.387 for

cover systems with 15, 30 and 50-cm-thick upper capillarylayers respectively. Accordingly, the average oxygen diffu-

sion coefficient of the infiltration and oxygen barrier also

decreased from 1.2 108 m2/s to 6.71010, 3.4 1011,

and 3.0 1011 m2/s respectively. Fig. 5 illustrates the

influence of the upper capillary layer, and also presents a

non-linear relation between the thickness of the upper

capillary layer and the performance of the soil cover system

in terms of the volumetric water content of the infiltration

and oxygen barrier.

Fig. 6shows that including a 15-cm-thick upper capillary

layer in a cover system increases cost by as much as 25%

but improved performance by only 3.7%. This apparently

marginal improvement in performance is an artifact of the

modeling boundary condition used for the case without a

top capillary barrier. In order to maintain the sand-

bentonite layer at the same boundary condition in the two

scenarios (with and without upper capillary layer), a

fictitious thin (1 mm) upper capillary barrier with a perched

water table was assumed to exist at the top of the sand-

bentonite barrier. The modeling results suggest that this 1-

mm-thick saturated layer apparently protected the sand-

bentonite against desiccation within the modeling period,although it is inconceivable that this would happen in

practice.

Fig. 6 also shows that increasing the thickness of the

upper capillary layer to 30 cm increases cost by 50% and

improved performance by 38%, which appeared to be cost

effective. However, a greater increase in the thickness of

the upper capillary layer from 30 to 50 cm increased cost

from 50% to 83%, but improved performance only

marginally, from 38% to 49%. Thus the relation between

the thickness of the capillary layer and cover performance

was not linear.

4.1.2. Case A3: influence of grain-size contrast between

layers

Impedance to drainage in a multi-layer cover is due to

the contrast in the hydraulic properties of two soil layers in

contact with each other (Nicholson et al., 1991). As shown

inFig. 7, Case A3 was similar to Case A2, except that in

Case A3 the importance of grain-size contrast between the

sand-bentonite layer and the underlying layer was eval-

uated by replacing the silt tailings with sand tailings

possessing the same hydraulic properties as the Waite

Amulet fine sand.

Since the AEV of the fine sand tailings was approxi-

mately only 2 kPa (Fig. 3), the 1-m drop in the water tablethat induced a suction of 10 kPa at the base of the tailings

was enough to initiate drainage in the tailings. The degree

of saturation which remained nearly constant in the silty

tailings in Case A2 (Fig. 7) now decreased rapidly over time

in the fine sandy tailings in Case A3 (Fig. 7) and reached its

residual saturation value. However, even in this case the

infiltration barrier (sand bentonite) did not lose saturation

(Fig. 7) because the suction in the fine sand at residual

saturation was about 10 kPa, which was still much less than

the AEV (100 kPa) of the sand bentonite (Fig. 3).

4.1.3. Case A4: fixed water level

Selective layering can be used to maintain near saturated

conditions in covers regardless of the depth of the water

table (Nicholson et al., 1991). The soil cover configuration

for Case A4 (Fig. 8) is exactly the same as in Case A1

(Fig. 4). However, in Case A4, unlike the previous case, the

water table was kept at the base of the tailings during the

simulation period. As the induced suction at the base of the

coarse sand is higher than the suction at its residual

saturation, the coarse sand would drain to residual

saturation. However, in this case residual saturation was

reached throughout the entire coarse sand profile in Case

A4 (Fig. 8) instead of just in the lower part in Case A1

(Fig. 4). The results show that the maximum suction at the

ARTICLE IN PRESS

0 10 20 30 40 50 60

Volum

etricwatercontent(%)

0.33

0.34

0.35

0.36

0.37

0.38

0.39

Oxygen

diffusioncoefficient(m/s)

Volumetric water content

Oxygen diffusion coefficient

0.0

4.0x10 -9

6.0x10 -9

8.0x10-9

1.0x10-8

1.2x10-8

1.4x10-8

2.0x10 -9

Thickness of top capillary layer (cm)

Fig. 5. Volumetric water content and oxygen diffusion coefficient in

infiltration barrier versus thickness of top capillary layer.

0 10 20 30 40 50 60

Performanceimprovemen

t(%)

0

20

40

60

80

100

Costincrease(%)

0

20

40

60

80

100

Cost increase

Performance improvement


Fig. 6. Percentage of cost increase and performance improvement versus

thickness of top capillary layer in a soil cover with sand-bentonite

infiltration barrier and without bottom capillary layer.



11/21

top of the coarse sand which was 6.9 kPa in Case A1 (Fig.

4) increased to about 10 kPa (45% more) in Case A4 (Fig.

8). The degree of saturation profiles for both Cases A1 and

A4 presented inFig. 8show that the infiltration barrier was

nearly 100% saturated in both cases, because the suctions

of 6.910.7 kPa induced at the contact between the coarse

sand and the infiltration and oxygen barrier was still

smaller than its AEV value, which is approximately

100 kPa (Fig. 3).

4.1.4. Case A5: influence of upper capillary layer with fixed

water level

AsFig. 9shows, Case A5 was similar to Case A4 in the

sense that the elevation of the water table was kept

constant at the base of the tailings. However, the thickness

of the upper capillary layer (Waite Amulet fine sand) was

reduced from 30 to 15 cm.Table 2shows the influence of

the thickness of upper capillary layer on the performance

of a cover with a 60-cm-thick infiltration barrier and a 30-

cm-thick lower capillary layer when the water table was

kept at the bottom of the tailings. Increasing the thickness

of upper capillary layer from 0 to 15 cm increased the cost

of the cover system by 16.6% but improved the perfor-

mance by only 3.5%. However, while doubling the

thickness of the upper capillary layer (from 15 to 30 cm)

increased cost by only 33.3%, it resulted in 38% improve-

ment in performance, again confirming a non-linear

relationship between the thickness of capillary layer and

performance, even when the water level was constant.

Although doubling the thickness of the upper capillary

layer from 15 to 30 cm does not double the volumetric

water content of the infiltration barrier, it does improve

cover performance by more than 10 times. This is similar to

what was the case inFigs. 5 and 6. The reason probably is

that in a steady-state analysis of a layered soil cover, the

total oxygen flux is determined not only by the volumetric

water content of the infiltration barrier, but also by the

volumetric water content and oxygen diffusion coefficients

of other layers.

This emphasizes the importance of ensuring that

adequate thickness of the upper capillary layer is used,

taking into account the type of material used for the

infiltration and oxygen barrier (Fig. 10). Another reason

for using enough thickness for the top capillary layer is that

otherwise under dry conditions, cracks may form at the

surface of the infiltration and oxygen barrier, which may

facilitate evaporation, thereby resulting in desaturation of

the infiltration barrier to some extent.Swanson et al. (2003)

ARTICLE IN PRESS

Case A2-Degree of saturation (%)

0 20 40 60 80 100

Elevation(m)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

day 1

day 15

day 30

day 60

day 90

day 120


0 20 40 60 80 100

Elevation(m)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

initial water level

60 cm

30 cm

100 cm

final water level

tailings

sand-bentonite

WA fine sand

Case A2

Case A3

initial water level

60 cm

30 cm

100 cm

final water level

WA fine sand

sand-bentonite

WA fine sand

Fig. 7. Schematic diagrams of soil cover Cases A2 and A3 and profiles of degree of saturation.



12/21

reported differences between measured and predicted

cover responses and attributed them to desiccation

induced cracking at the cover surface. Most saturatedun-

saturated flow models, including that used by Swanson et

al. (2003) and the one used in the present study, consider

soil as a continuous material and ignore the presence of

any cracks.

4.2. Silt as infiltration barrier

Almost invariably, the selection of a soil type for a cover

system is governed by its availability at or near the project

site. At many Canadian mine sites where crystalline rocks

dominate, there is a limited quantity of fine grained, clay

type soil materials while silts may be present in sufficient

ARTICLE IN PRESS


-4 -2 0 2 4 6 8 10 12 14

Elevation(m)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

day 1

day 15

day 30

day 60

day 90

day 120

Case A4

tailings

WA coarse sand

sand-bentonite

WA fine sand30 cm

60 cm

30 cm

100 cm

constant water level


0 20 40 60 80 100

Elevation(m)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.82.0

2.2


0 20 40 60 80 100

Elevation(m)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.82.0

2.2

Fig. 8. Schematics and suction profile for Case A4 and profiles of degree of saturation for Cases A1 and A4.

tailings

WA coarse sand

sand-bentonite

WA fine sand15 cm

60 cm

30 cm

100 cm


Case A5

0 5 10 15 20 25 30 35

Volumetricwatercontent(%)

31

32

33

34

35

36

37

38

39

Oxygendiffusioncoefficient(m2/s)

0

5e-9

1e-8

2e-8

2e-8

2e-8

3e-8

Volum. water content

Oxygen diffusion coeff.

Thickness of top capillary barrier (cm)

Fig. 9. Schematics of Case A5 and volumetric water content and oxygen diffusion coefficient versus thickness of top capillary layer.



13/21

quantities to merit consideration in cover projects (Nichol-son et al., 1989). Thus in a number of cases, it may be

necessary to evaluate non-clayey materials, such as silt, as

potential barriers in multi-layer cover systems.Mbonimpa

et al. (2003) used artificial silt as infiltration and oxygen

barrier in a cover system. In Cases C1C10 of the present

study, silt was investigated as a potential alternative to a

clay-type barrier such as the sand-bentonite.

4.2.1. Cases C1, C2, C3, and C4: influence of water depth

A single-layer silt cover (60 cm thick) placed directly on

3 m of tailings (mine waste) was investigated as shown in

Fig. 11. The influence of water table depression on the

drainage behavior of the silt barrier was evaluated. The

elevation of the water table, initially at the silttailings

interface in all cases, was decreased to 0.5, 1, 2 and 3 m

below the interface in Cases C1, C2, C3, and C4,

respectively.

The water table was lowered by 0.5 m in Case C1. The

suction at the bottom of the silt infiltration and oxygen

barrier in Case C1 at Day 120 was 5 kPa (Fig. 11), which

was less than the AEV of the silty tailings (approximately

10kPa) (Fig. 3). As a result, the average degree of

saturation in the silt infiltration and oxygen barrier in

Case C1 at Day 120 was still 90.3% (Fig. 11).

The water table was lowered by 1.0, 2.0, and 3.0 m over120 days in Cases C2, C3, and C4, respectively, as indicated

in Fig. 12. Lowering the water table increased suction at

the base of the cover, promoted drainage and decreased the

degree of saturation. The average degrees of saturation of

the silt infiltration and oxygen barrier in Cases C2, C3 and

C4 (Fig. 12) at Day 120 were approximately 81.1%, 66.9%,

and 57.9% respectively. Fig. 13 shows the desaturation

that occurred in the silt cover in Cases C1C4.

The data are further highlighted in Fig. 14 as a plot of

the average degree of saturation in the single silt cover

versus water level drop below the base of the cover in Cases

C1C4.Fig. 14shows that at the end of the first day, the

degree of saturation was the same for all four cases of the

modeled single silt cover. The reason for this behavior

probably is that during the first day the loss of water by

evaporation for all cases was the same and equal to the

potential evaporation, which is controlled by environmen-

tal conditions and not by soil properties and/or the depth

of water level. At later times, the loss of water from covers

is controlled by the actual evaporation, which is governed

by soil properties and water level location. Significant

reductions in the degree of saturation occurred when the

water level was lowered deep enough to exert suctions that

exceeded the AEV of the silt (10 kPa or 1 m of head), as

shown inFig. 14. This shows that the elevation of the water

ARTICLE IN PRESS

Cost increase (%)

0 2 4 6 8 10 12 14 16 18

Performanceimprov

ement(%)

0

10

20

30

40

0 5 10 15 20 25 30 35

Costincreaseorpe

rformance

improvement(%)

0

10

20

30

40

Cost increase



Fig. 10. Percentage of cost increase and performance improvement versus thickness of top capillary layer, and correlation between cost increase and

performance improvement due to adding top capillary layer.

Table 2

The influence of thickness of top capillary layer on performance and cost of a soil cover system with a 60 cm thick sand-bentonite infiltration barrier and a

fixed water level

Soil cover

case

Thickness of top

capillary layer

(m)

Thickness of

bottom capillary

layer (m)

Average vwc in

infiltration

barrier (%)

Oxygen diffusion

coeff. of

infiltration

barrier (m2/s)

Oxygen flux

through soil

cover (kg/m2/s)

Performance

improvement

with thicker, top

capillary layer

(%)

Cost increase

with thicker, top

capillary layer

(%)

A0 0 0.30 32.06 2.4108 9.41012

A4 0.30 0.30 38.58 3.41011 5.81012 38.0 33.3

A5 0.15 0.30 36.89 6.91010 9.01012 3.5 16.6

Note: (i) vwc volumetric water content; (ii) A0, A4, and A5 modeled cover scenarios (Figs. 8 and 9).



14/21

table need not be very deep for a single layer soil cover with

a relatively low AEV, such as a silt cover, to desaturate.

4.2.2. Cases C5, C6, and C7: influence of thickness of lower

capillary layer

As mentioned before, Case C4 (Fig. 12) modeled a soil

cover consisting of only a silt infiltration and oxygen

barrier (without any capillary layer). The infiltration and

oxygen barrier desaturated when the water table was

lowered to a depth that caused suctions in the silt to exceed

its AEV. In Cases C5C7, the 60-cm-thick silt infiltration

and oxygen barrier was placed above different thicknesses

of Waite Amulet coarse sand lower capillary layer, as

shown in Fig. 15. Similar to Case C4, the covers were

subjected to a gradual 3-m drop in water level over a 120-

day period. The objective of studying these cases was to

evaluate the influence of different thicknesses of a lower

capillary layer on performance of silt as infiltration and

oxygen barrier. Three cases, C5, C6 and C7, with respective

lower capillary layers 15, 30 and 50cm thick were

considered. For a large site, the use of reduced thickness

of capillary layer can lead to significant cost savings, if

performance can be shown to be acceptable.

Fig. 15presents the profile of degree of saturation in the

infiltration barrier for Case C5 when the cover system

included a 15-cm-thick lower capillary layer. The figure

shows that the lower capillary layer desaturated rapidly

and reduced desaturation of the infiltration barrier.

Schematics of Cases C6 and C7 (30- and 50-cm-thick,

respectively) are shown inFig. 15.

Including a 15-cm-thick lower capillary layer consider-

ably decreased the total oxygen flux through the cover

system; however, further increase in the thickness of this

layer did not result in measurable further improvements.

The reason for this trend is illustrated in Fig. 16showing

that the provision of a thicker lower capillary layer only

marginally increases the degree of saturation of the silt

cover from 70% to 71%. These results imply that a single

silt cover would not be a suitable oxygen barrier in

situations where the water table is deep enough to induce

suctions equal to or greater than the AEV of the silt.

Fig. 17 presents percent cost increase and performance

improvement versus thickness of the lower capillary layer

for Cases C4C7, while Fig. 18 shows a plot of percent

performance improvement against cost increase.

4.2.3. Cases C8, C9, and C10: influence of thickness of

upper capillary layer

These cases were studied to examine the influence

of the thickness of the upper capillary layer on cover

ARTICLE IN PRESS

60 cm

300 cmtailings

Case C1

siltinitial water level

final water level

50 cm

Degree of saturation (%)

0 20 40 60 80 100

Elevationofinfiltrationbarrier(m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

day 1

day 15

day 30

day 60

day 90

day 120

Suction (kPa)

-5 0 5 10 15 20 25

Elevationofinfiltrationbarrier(m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Fig. 11. Schematic of Case C1 and profiles of degree of saturation and suction for Case C1.



15/21

performance. First, an unprotected 60-cm-thick silt cover

was studied in Case C8 (Fig. 19). Then a 15-cm-thick fine

sand layer was placed on the silt cover in Case C9 and,

finally, the fine sand thickness was increased by 100% to

30cm in Case C10 (Fig. 19). The water level was kept at

1 m below the base of the cover during the simulation

period.

As the profiles of degree of saturation show (Fig. 20), the

unprotected silt cover (silt) in Case C8 lost quite a bit of

water because of evaporation. In fact, the average degree of

saturation of the infiltration barrier decreased to about

75.7% in 120 days. By adding 15 and 30 cm thick protective

layers in Cases C9 and C10 respectively, the average degree

of saturation of the silt cover increased to approximately

94% and 95% at the end of 120 days, as indicated in

Fig. 20. AsFig. 21demonstrates, the inclusion of a 15-cm-

thick upper capillary layer decreased the total oxygen flux

significantly. However, further increase in the thickness of

the upper capillary layer did not appear to decrease the

oxygen flux further.

Fig. 22 presents the computed percent cost increase

versus performance improvement with the addition of an

upper capillary layer to the single-layer silt cover. The

results suggest that, everything else being the same, the

ARTICLE IN PRESS

60 cm

300 cm tailings

Case C3

silt

initial water level

200 cm

final water level

60 cm

300 cm tailings

Case C4

silt

initial water level

final water level

60 cm

300 cm tailings

Case C2

siltinitial water level

100 cm

final water level

Fig. 12. Schematics of soil cover Cases C2C4.


20 40 60 80 100

Elevation(m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.5 m (Case C1)

1 m (Case C2)

2 m (Case C3)

3 m (Case C4)

Fig. 13. Profiles of degree of saturation versus decreasing water level

(Cases C1C4) in a single layer silt cover overlying mine tailings at day

120.



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provision of a 1530 cm thick upper capillary layer

increased the cost of the cover system by 2550%, but

improved performance by nearly 99%. Although the

results demonstrate the important role of the upper

capillary layer in preventing desaturation of the oxygen

barrier, they also show that increasing the thickness of the

upper capillary layer considerably more than an optimum

value would not be cost-effective, because the resulting

increase in the degree of saturation of the silt infiltration

and oxygen barrier would be only marginal.

ARTICLE IN PRESS


0 10 20 30 40 50 60 70 80 90 100

Elevationofcoversystem

(m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

day 1day 15

day 30

day 60

day 90

day 120

60 cm

300 cm

tailings

Case C7

silt

50 cm WA coarse sand

initial water level

final water level

60 cm

300 cm tailings

Case C5

silt

15 cm WA coarse sandinitial water level

final water level

60 cm

300 cmtailings

Case C6

silt

30 cm WA coarse sand

initial water level

final water level

Fig. 15. Schematic diagram of soil cover Cases C5C7 and profile of degree of saturation for Case C5.

Water level drop

Degreeofsaturation(%)

0

10

20

30

4050

60

70

80

90

100

day 1

day 15

day 30day 60

day 90

day 120

0.5 m(Case C1)

1 m(Case C2)

2 m(Case C3)

3 m(Case C4)

Fig. 14. Influence of water level drop on the degree of saturation of the infiltration barrier.



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5. Advantages and limitations of the proposed approach

Evaporation is an important factor that controls the

performance of covers designed to retain moisture and hence

reduce oxygen transport. The present work is different from

many previous studies of soil covers because it includes

evaporation as flux-boundary condition, and also considersall key aspects of cover design and analysis (material

selection, flow modeling, oxygen transport modeling, and

cost analysis). This integrated approach allows the designer

or proponent to eliminate less cost effective options without

the need for costly long-term laboratory and test plot studies.

The steps outlined in the conceptual flowchart of

computational sequence (Fig. 1) and the approach described

in this paper can be a helpful guide in the design of an

economically feasible case- and project-dependent soil cover.

It is based on known and proven theories and published

methods. Also, it is fairly easy to use and the needed input

data are generally routinely available from most field

measurements or standard laboratory testing. The results

of this study, when combined with in situ monitoring and

physical study of the effects of freeze-thaw, desiccation, and

erosion, can constitute a rigorous evaluation of the long-term

performance of a soil cover system. The use of the method

along with well-instrumented field test covers can eliminate

performance uncertainties in full-scale applications.

Despite these advantages, there are a number of

limitations with the study. The use of the one-dimensional

model provides only an approximate analysis of water flow

as it is recognized that a two-dimensional analysis, or even

a three-dimensional analysis, would be more appropriate.

Also, the numerical model used in the study considered soilas a continuum and did not consider processes such as

desiccation, cracking, and freeze-thaw. Hysteretic effects

caused by alternate cycles of wetting and drying conditions

were also not considered.

The oxygen flux through single-layer and multi-layer

cover cases was calculated using a steady-state analysis.

The steady-state flux provided a long-term assessment of

possible reductions in oxygen ingress and was considered

adequate. In a previous study, Yanful (1993) computed

both laboratory and field transient oxygen fluxes and

confirmed reductions in oxygen ingress by a multi-layer

cover. Finally, the present study dealt with the hydraulic

performance of soil cover systems and their impact on

oxygen transport, but did not include chemical transport

analysis. It was assumed that a reduction in oxygen flux

due to the presence of a cover would necessarily lead to

decreased mine waste oxidation and hence contaminant

release. This is a reasonable assumption and has been

confirmed by several field and laboratory studies (for

example, Yanful, 1993; Payant et al., 1995; Yanful et al.,

1999;Yanful and Orlandea, 2000). It should be noted that

although numerical modeling was used here to predict the

performance of different designs, it is recognized that field

data from constructed test covers may be used to confirm

or modify the results of the numerical analysis.

ARTICLE IN PRESS

0 5 10 15 20 25 30 35 40 45 50 5556

58

60

62

64

66

68

70

72

Averagedegre

eofsaturation

atinfiltrationbarrier(%)

Thickness of lower capillary layer (cm)

Fig. 16. Influence of thickness of lower capillary layer on degree of

saturation at the infiltration barrier.

Thickness of lower capillary barrier (cm)

0 10 20 30 40 50 60

Costincrease(%)

0

20

40

60

80

100

Performanceimprovement(%)

0

20

40

60

80

100

Cost increase


Fig. 17. Cost increase and performance improvement versus thickness of

lower capillary layer for a silty infiltration barrier (Comparison of Cases

C4C7).

Cost increase (%)

0 25 50 83

Performanceimprovem

ent(%)

0

5

10

15

20

25

Fig. 18. Performance improvement versus cost increase for a silty cover

with lower capillary barrier.



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6. Summary and application.

(1) Single cover: A single-layer soil cover consisting of only

an infiltration or oxygen barrier could be used if

sufficient quantity of the material is available at or

within economic hauling distance of the mine site.

However, the cover must be protected with an over-

lying soil layer against erosion, freeze-thaw, desiccation

and cracking. A single silt cover cannot be used as an

infiltration and oxygen barrier in situations where the

ARTICLE IN PRESS

Case C8-Degree of saturation (%)

0 10 20 30 40 50 60 70 80 90 100

Co

verelevation(m)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

day 1

day 15

day 30

day 60

day 90

day 120

60 cm

100 cm

tailings

Case C8

silt


60 cm

15 cm

100 cm


tailings

silt

WA fine sand

Case C9

60 cm

30 cm

100 cm


tailings

silt

WA fine sand

Case C10

Fig. 19. Schematic diagram of soil cover Cases C8C10 and profile of degree of saturation for Case C8.

Thickness of upper capillary barrier (cm)

Degreeofsaturation(%)

0

10

20

30

40

50

60

70

80

90

100

day 1

day 15

day 30

day 60

day 90

day 120

0 15 30

Fig. 20. Influence of the thickness of upper capillary layer on degree of saturation of silt cover.



19/21

water table is deep enough to induce suctions equal to

or greater than the AEV of the silt.

(2) Layered soil cover: The largest component of the cost of

a soil cover is likely the construction of the infiltration

and oxygen barrier. The effectiveness of a material as

an infiltration and oxygen barrier is dependent on its

compaction and water retention characteristics includ-

ing the magnitude of its AEV. Generally, this barrier

need not be thick (typically, 0.51.0 m) if protective

upper and lower capillary layers are provided to

prevent its desaturation. A layered soil cover is

generally the preferred cover system for acid generating

mine waste. Infiltration and oxygen barrier may consist

of clay and a sand-bentonite mixture.

In this study, a sand-bentonite barrier with a saturated

hydraulic conductivity of 1 107 cm/s was observed to

retain its moisture. The movement of water in the capillary

layers caused by upward evaporation and downward

drainage did not produce significant water flow within

the sand-bentonite. In fact, a large change in suction

resulted in only a small change in the water content or

degree of saturation. The hydraulic conductivity of

1107

cm/s was realistic as it is similar to typical field

measured values for well-compacted, clayey soils (Elsbury

et al., 1990).

At many Canadian mine sites there is a limited quantity

of fine grained, clay type soil materials while silts may be

present in sufficient quantities to merit consideration in

cover projects. In the cases considered in the present study,

where silt was the infiltration and oxygen barrier, a lowercapillary barrier effectively stabilized the degree of satura-

tion of the silt. A contrast in grain-size distribution would

promote rapid drainage in lower capillary layer and

maintain a high degree of saturation in the infiltration

and oxygen barrier.

Silt is more susceptible to frost heaving than clay.

Therefore, when it is used as an infiltration and oxygen

barrier in wet and cold areas, it may be necessary to place it

below the zone of frost penetration. This could translate

into using upper capillary and erosion protection layers of

at least 1.2 m thick in many parts of Canada where frost

can penetrate that deep. In such an application though, the

primary role of the silt would be to function as an oxygen

barrier, although it could also reduce water infiltration to

some extent. Besides, non-plastic silt and/or silt with low

plasticity have a self-healing potential that allows them to

recover their hydro-geotechnical properties after freeze-

thaw cycles (Eigenbrod, 2003).

From the results of the present study, the relation

between the thickness of a capillary layer and the degree of

saturation of the infiltration and oxygen barrier was not

linear implying that increasing the thickness of a capillary

layer considerably more than an optimum amount only

marginally increases the degree of saturation of the

infiltration and oxygen barrier.It is obvious that the capillary layers and infiltration and

oxygen barrier did not perform independently; rather, they

had mutual hydraulic interactions that influence cover

performance. For example, it was found that simply

doubling the thickness of the capillary layer did not

produce a corresponding doubling in performance im-

provement.

(3) Minimum design criteria and proposed approach: The

properties of interest in the selection of candidate soil

cover materials include the pressure head at which the

lower capillary barrier (coarse sand in this study)

approaches its AEV and residual saturation head (hr),

the AEV of the infiltration and oxygen barrier, and hrof the upper capillary barrier.

7. Conclusions

An investigation of the influence of a number of soil

cover properties and hydrogeologic parameters on perfor-

mance has been presented. The investigation simulated

one-dimensional evaporation from hypothetical moisture-

retaining cover systems using the commercially available

transient flow model, SoilCover, to compute suction and

ARTICLE IN PRESS

Thickness of upper capillary barrier (cm)

0 5 10 15 20 25 30 35

Costincrease(%)

0

20

40

60

80

100

Performanceimprovement(%)

0

20

40

60

80

100

Cost increase


Fig. 22. Percentage of cost increase and performance improvement versus

thickness of upper capillary layer (Comparison of Cases C8C10).

Time (day)

0 20 40 60 80 100 120

TotalOxygenFlux(kg/m2/s)

0 cm Case C8

15 cm Case C9

30 cm Case C10

Thickness of upper capillary layer

0.5x10-9

1.0x10-9

1.5x10-8

2.0x10-8

2.5x10-8

3.0x10-8

3.5x10-8

0.0

Fig. 21. Total oxygen flux through soil cover with top capillary layers of

0, 15, and 30 cm thick.



20/21

water content profiles. Water content was used as input to

an oxygen diffusion model to calculate oxygen flux for the

different design scenarios. A cost analysis was then

performed for the various scenarios to obtain a relation-

ship between performance and cost. Based on the results,

the following is concluded:

(1) The key properties in the selection of candidate soil

cover materials are the saturated hydraulic conductiv-

ity, compaction characteristics, oxygen diffusion coeffi-

cient, hydraulic conductivitysuction function, AEV,

and the pressure head at which the various layers (for

example, lower and upper capillary barriers) ap-

proaches their AEVs.

(2) A single-layer silt cover would not perform well as

infiltration and oxygen barrier in situations where the

water table is deep enough to desaturate the cover. Silt

may be a good oxygen barrier if it is protected against

drainage with an underlying coarse layer and against

erosion, freeze-thaw, desiccation and cracking with an

overlying coarse soil.

(3) A multi-layer soil cover consisting of an infiltration and

oxygen barrier, drainage barrier and an upper protec-

tive layer performs better than a single-layer cover.

(4) Although increasing the thickness of the upper

capillary barrier in a multi-layer cover improves cover

performance significantly, it only does so to a certain

point. A greater increase in thickness, say from 30 to

50cm, increases cost significantly (by 33%), but

improves performance marginally (by 11%). Thus the

relationship between the thickness of the upper

capillary barrier and cover performance is not linear.(5) Sand-bentonite can serve as an excellent infiltration and

oxygen barrier, if the economics would allow. A sand-

bentonite layer with a saturated hydraulic conductivity

of 1107 cm/s can retain its moisture, even if it is

subjected to a large change in suction. The change in

suction results in only a small change in the water

content or degree of saturation.

(6) The step-by-step method of analysis proposed in this

paper would be a useful tool for making technical and

economic decisions on site-specific cover designs prior

to, or in combination with, pilot scale testing. Such an

approach can lead to a cost-effective and technicallyfeasible design of multi-layer cover systems.

References

Akindunni, F.F., Gillham, R.W., Nicholson, R.V., 1991. Numerical

simulations to investigate moisture-retention characteristics in the

design of oxygen-limiting covers for reactive mine tailings. Canadian

Geotechnical Journal 28, 446451.

Aubertin, M., Chapuis, R.P., Aachib, M., Bussie` re, B., Ricard, J.-F.,

Tremblay, L., 1995a. E valuation en laboratoire de barriers seches

construites a` partir de re sidus miniers. Ecole Polytechnique, CDT

P1622. Final Report, Mine Environment Neutral Drainage Program

Programme de Neutralisation des Eaux de Drainage dans lEnvir-

onnement Minier (MEND/NEDEM).

Aubertin, M., Ricard, J.-F., Chapuis, R.P., 1995b. A study of capillary

properties of mine tailings: measurements and modeling. In: Proceed-

ings, 48th Canadian Geotechnical Conference, Vancouver, BC, pp.

1724.

Aubertin, M., Bussie` re, B., Aachib, M., Chapuis, R.P., Crespo, J.R., 1996.

Une mode lisation nume rique des e coulements non sature s dans des

couvertures multicouches

a numerical study of soil cover perfomance.yanful, mousavi and de souza.2006

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