This document compares the design differences between water dams and tailings dams. Some key differences discussed include: - Tailings dams must safely contain mine tailings and process water in perpetuity after closure, unlike water dams which typically have a 100 year design life. - Seepage control is more critical for tailings dams due to environmental regulations around containment of contaminants from tailings. - Tailings properties, such as higher specific gravity, can increase loading stresses on the dam compared to water. - Tailings can be used advantageously in the design to reduce hydraulic gradients and piping risk, allow use of geosynthetic filters, and provide a seepage barrier, whereas water dams rely

1. TAILINGS DAM VERSUS A WATER DAM, WHAT IS
THE DESIGN DIFFERENCE?
ICOLD Symposium on Major Challenges in Tailings Dams, June 15, 2003
Harvey McLeod, Vice President
Len Murray, Manager of Mining, Asia-Pacific
Klohn Crippen Berger Ltd., Canada
Summary
The technology of tailings dam design is based on the same geotechnical
principles as water dams, however the presence of saturated tailings solids as
the stored medium, versus water only, presents unique challenges and design
benefits. The gradation of mine tailings typically varies from silty medium fine
sand to clayey silt. Tailings dam design sections vary considerably. For example,
the dam can be made entirely out of unprocessed tailings with upstream
construction, or the dam may be made out of borrow material with little or no
reliance on the tailings. Impounded tailing solids have hydraulic conductivity and
shear strength properties that can be used to the advantage of the designer. On
the other hand, sulphide rich mine tailings have a potential to oxidize and leach
metals through acid rock drainage. Seepage control, for environmental – not
dam safety, becomes a critical design parameter which can lead to much lower
tolerances for seepage losses from the impoundment, compared to water dams.
Unlike water dams, tailing dams are closed at the end of mine life (typically 20
years) but the tailings cannot be removed for decommissioning. Therefore
design must allow for safe decommissioning and low or zero maintenance in
perpetuity. This paper presents an assessment of the differences in design
details and approach between water dams and tailings dam and illustrates these
points with case histories from major international projects. The main design
issues of piping, drainage, structural fill, seepage control and closure are
discussed.
Introduction
Design, construction, operation, and closure of tailing dams have some
fundamental differences when compared to conventional water storage dams.
Some of these differences work to the benefit of the dam designer, and some
increase the complexity and difficulty. A guiding note, irrespective of the type of
dam, however, is that all dams are engineered structures and that the “principles
of soil mechanics still apply”. A tailings dam must safely contain mine tailings
and process water not only for operations, but for perpetuity.
1

2. Table 1 Summary of Differences between Water Dams and Tailing Dams
Component Tailings Dam Water Dam
Stored material Tailings solids, process water
(various contaminant levels) &
runoff water
Water
Regulatory regime Ministry of Mines, Ministry of
Environment
Ministry of Public Works,
Regional Authorities, National
Dam Associations
Operating Life Finite operating life (5 to 40
years)
Typically designated as 100
years, but “as long as required
by society”.
Construction
Period
Staged over mine life of 2 to 25+
years.
Usually 1 to 3 years.
Closure Infinite closure period, try for
“walk away” design
Often not addressed, but
facility may be
decommissioned.
Engineering Medium to high level High level
Continuity of
engineering
Varies: Owner and engineer may
change frequently during the
construction life
Usually one engineering firm
for design and construction.
QA/QC Generally good for starter dam
and variable levels during
operations. Can be at a low level
for some mining companies.
High level
Consequence of
failure
Tailings debris flow resulting in
physical damage and
environmental contamination.
Water inundation damages.
Dam Section Can vary during the design life,
e.g. transition to centerline, or
downstream.
Usually a consistent section
The following sections discuss some of the differences between the design of
water dams and the design of tailing dams against a framework of the main
design considerations, namely piping, drainage, tailings as structural fill,
seepage control and closure.
Design for Piping
The design for control of piping in tailings dams needs to consider the stored
tailings and the location of the free water pond, as well as the zonation and
grading of materials in the dam. There is an opportunity to minimize the use of
costly processed filters in drains by controlling the hydraulic gradients.
The tailings can be used to form part of the seepage barrier to water flow and
thereby reduce the hydraulic gradients along the upstream face of the dam. By
reducing the hydraulic gradient, the risk of piping of tailings, or other fill zones,
through the dam can be reduced. This approach is commonly applied on an
empirical basis where it is often specified that the upstream tailings beach should
be a minimum width to keep the free water pond away from the dam. This
reduces both the hydraulic gradient and the quantity of seepage flow. An
example of maintaining a tailings beach to mitigate piping was observed with the
dam “failure” of the Phoenix tailings dam in southern British Columbia (Klohn,
2

3. 1979). The tailings dam was constructed by the upstream method, with a decant
water pipe located in the dam. Twenty-five years after closure piping occurred
between an elevated water pond and the deteriorated decant pipe. The solution
to stop the piping was to distribute the surface tailings and “push” the pond away
from the dam crest. This eliminated subsequent piping development. The
authors are also aware of a number of reported cases where sinkholes have
been mitigated, or seepage controlled, by moving the pond away from the dam.
Seepage analysis programs are readily available to allow modeling of the
seepage gradients in response to percolation, spigot location or climatic
variations. However filter criteria for low head conditions are not as readily
available. Sherard does provide insight into how low head filter criteria can differ
from conventional criteria, which has been developed for high gradient
conditions. The seepage analysis of the dam, therefore, should include the
tailings mass, and the sensitivity of the pond location should be assessed to
optimize both the seepage control and piping control design for the dam.
The tailing dam designer should use caution in the placement of pervious rip rap
zones on the upstream face of an impervious dam, and with the use of upstream
rockfill shell zones. These zones have the disadvantage of introducing the full
hydraulic head on the upstream face or core of the dam and negating the
advantages of the tailings beach.
An additional piping concern has been emerging with water storage dams that
use glacial till core zones, with filters, to control seepage and piping. Long-term
internal erosion, such as has occurred at the Bennett Dam in northern British
Columbia raises concerns that the conventional filter criteria may not provide
long term security in design of seepage barriers and piping controls. The tailings
dam designer has the opportunity to consider the potential use of tailings to
minimize long-term reliance on impervious zones and filters. This can
significantly reduce the risk of the long term piping potential and internal erosion.
The Omai tailings dam failure also illustrates the case where a tailings beach
may have prevented a major dam failure. Failure of the dam occurred by piping
of the saprolite core zone into the rockfill shell zone, with subsequent piping of
tailings and failure of the dam. In this case the full hydraulic gradients were
acting across the core zone, and water was available for continued transport of
piped material.
Design for Filters and Drainage
The ability of tailings to reduce hydraulic gradients in the dam can also be used
to the advantage of the tailings dam designer by allowing the use of geosynthetic
filter fabrics for filters and drains. It is commonly debated that geosynthetics may
not last the hundreds to a thousand years that closure criteria may impose and
therefore they should not be used for permanent structures for closure. However,
in cases where the closure condition will be a dry cover (e.g. no free water
pond), or where the final water pond will be hundreds of meters upstream of the
dam, the resulting hydraulic gradients into the drain may not be high enough to
cause piping in the long term, thus relying on the geosynthetics only during
3

4. operations. The authors have used this analysis to show that a geotextile/gravel
drain could be used for a centerline cyclone sand dam recently designed in Peru.
In concrete faced rockfill tailing dams there is a risk of piping of tailings through
“damaged” concrete (e.g. cracks). The standard designs for concrete faced
rockfil dams (CFRD) includes a relatively coarse bedding-transition zones
downstream of the concrete face, which, while acceptable for water leakage,
these coarse transition zones encourage piping of tailings. This has been
considered to be a low risk because of the potential size of the cracks and the
likelihood that the hydraulic gradient, through the tailings, will be low enough to
control piping. However, in cases where the water pond is near the concrete
facing, piping through damaged concrete could occur. In most cases it is
probably prudent to place a geotextile filter fabric between the concrete and the
bedding layer to provide additional assurance.
The potential for chemical degradation of drains needs to be considered for
tailings dams. For example, the INCO R-4 tailings area (Plewes & MacDonald,
1996) stores tailings with elevated sulphide levels and mineral precipitates from
oxidation have plugged the granular drain system. The design solution for this
case was to design inverted saturated drainage systems where oxidation would
be minimized. Chemical precipitates can also cause filters and drains to become
cemented and thereby prone to cracking with a consequent reduction in piping
protection and potential for loss of fines.
Many tailing dam designs include pipes, either as underdrains or near the dam
face, to reduce water pressure and allow capture of seepage water for treatment.
While these drains can be effective in lowering the phreatic surface, they also
introduce a potential piping pathway and increase the risk of piping failure. There
are a number of case histories of tailings piping within PVC pipes installed in the
dam.
4

5. Design for Tailings Loading
Tailings has a higher specific gravity than water and therefore imposes a higher
stress onto the dam. Under most design conditions, this is not a controlling
factor, however there are several cases where the loading from the tailings been
shown to be important. The Los Frailes dam failure, in Spain, was due in part to
the higher specific gravity tailings (in this case greater than 4.0), which increased
the static loading to the foundation (resulting in higher pore pressures) and the
static load on the dam. The other condition where it can be important is in the
dynamic loading case for centerline dams. The liquefied tailings mass acts as a
heavy fluid against the dam during the earthquake.
Figure 1 Los Frailes Dam Failure, Spain: High specific gravity of tailings
contributed to loading on dam and foundation.
Design for Seepage Control
Seepage control for tailing dams and the tailings impoundment is an important
environmental consideration in the design, where in some cases the allowable
seepage rate (for environmental protection) may be a fraction of a L/s. Seepage
control can be improved through the use of tailings as part of the seepage
barrier. For example, this could involve designing the depositon sequence to
provide a central free water pond with wide tailing beaches around the perimeter
of the impoundment. The permeability of the tailings can be further reduced by
thickening to produce non-segregating tailings. Minimizing segregation results in
more uniform, low permeability tailings. The permeability of the tailings can also
be reduced by promoting consolidation, either by surface drying, or by
5

6. underdrains in the tailings impoundment. Great care must be taken with
underdrains, however because as previously discussed, they can also provide a
pathway for piping of tailings. The permeability of tailings typically reduces by an
order of magnitude due to consolidation.
Design for Closure
The design of the tailings dam cannot be decoupled with the tailings
impoundment. The physical and chemical properties of the tailings must be
accounted for in the design. For example, high sulphide content tailings need to
be stored to minimize the potential for oxidation and acid drainage. This imposes
constraints on the dam design both for operations and closure. For example, on
closure the tailings may need to remain saturated for perpetuity and this will
require that the dam and liner system have a low enough permeability to allow
the natural surface inflow to maintain saturation of the tailings.
The selection of construction materials should be carried out in the context of
longevity and closure. For example, till cores, coarse drains and pipes should be
avoided (if possible) and replaced with tailings core zones and graded
filter/drains. This can provide more flexibility (and hence less risk) for long-term
internal erosion. The potential for cumulative seismic displacements can also be
reduced, by avoiding narrow width, zoned fill sections.
Closure of tailings facilities that contain a high percentage of very fine tailings
presents a problem with consolidation of very fine sludge tailings. This is one of
the most significant issues facing closure of the oil sands tailing impoundments.
Thickening of tailings, to produce a non-segregated tailings mix assists
consolidation and final closure of the tailings impoundment.
The closure requirements for tailing dams, where the objective is to construct a
“walk away” solution require integration of long term geologic processes to
simulate natural structures to resist erosion, seismic loadings, floods, landslides,
snow avalanches, etc. This leads to the desire to design tailing dams that
simulate natural landforms and are naturally resistant to the geological and
environmental conditions that they are constructed in.
Case Examples Illustrating the Use of Tailings as
Structural Fill
The slow rate of construction and the tailings material provides a unique
opportunity for optimization of a tailing dam, which is not the case for water
dams. The use of tailings as a hydraulic fill material has formed the basis for
spigotted and cycloned sand construction for tailings dams for over 50 years and
many papers are available that document good practice in the use and
placement of tailings as part of the dam structural fill. Typical examples of high
dams using tailings as a structural fill are summarized in Table 2, along with
other high tailing dams.
6

7. Table 2 Partial Summary of Large Tailings Dams (Greater than 80 m high)
DAM HEIGHT
PROJECT LOCATION
CURRENT DESIGN
DAM TYPE MINERAL
Highland Valley Canada 130 150 CS Copper
Gibraltar Canada 100 120 CS Copper
Kemess South Canada 145 165 CS/ECRD Copper
Brenda Canada 120 150 CS Copper
Kennecott USA 30 80 CS Copper
Fort Knox USA NA 115 ECD
Thompson USA NA USS Copper
Montana Tunnels USA NA 250 Modified
Centerline
Newmont Mill 2/5 USA NA 100 ECD
North Block USA NA 137
Antamina Peru 120 232 CFRD Zinc-copper-lead
Southern Peru Peru 70 110 DSS Copper
Candelaria Chile NA 163 FRD
Disputada Chile NA 120 Copper
Los Leones Chile 160 160
Chuquicamata Chile NA NA DSS Copper
Foskor Selati South Africa 45 140 CS Phosphate/Coppe
r
Anglogold Ergo South Africa 84 90 DSS/USS Gold
Impala Platinum South Africa 40 120 USS Platinum
El Teniente Chile NA NA DSS Copper
Alumbrera Argentina NA 120 Modified
Centerline/Rockfill
Copper
Dexing China NA 210 CSS Copper
Hongjiadu China NA 183 ECRD
Ok Tedi Interim
Dam
Papua New
Guinea
130 130 ECRD/CS Copper , Gold
Medet Bulgaria 105 105 USS Copper, Gold
Elatsite 1 Bulgaria 145 145 DSS Copper, Gold
Elatsite 2 Bulgaria 117 160 DSS Copper, Gold
Assarel Bulgaria 125 211 USS Copper, Gold
CFRD Concrete faced rockfill ECRD Earthcore rockfill
FRD Filter rockfill CS Centerline cycloned sand
USS Upstream cycloned sand DSS Downstream cycloned sand
EFD Earthfill dam
7

8. Centerline Construction
Centerline construction relies on the deposited tailings to form the main
upstream support for the tailings dam. Additional local support to the dam “raise”
is provided with either spigotted tailings, cycloned tailings, or borrow material.
The downstream zone may be constructed of conventional borrow materials or
cycloned sand. This method can typically be applied to almost all tailing dams,
although it can be limited in cases where large volumes of water are required to
be impounded, resulting in a relatively “high” dam section above the tailings
beach. In these cases the stability of the upstream slope (into the impoundment)
becomes the critical design condition. Examples of major centerline dams
include Highland Valley Copper (140 m high), Canada and Kennecott Utah
Copper (140 m high), United States.
Figure 2 Highland Valley Copper Tailings Dam, British Columbia, Canada:
Centerline cycloned sand with glacial till core.
8

9. Upstream Construction (Conventional)
Upstream construction, with limited drainage provision, can be used in non-
seismic areas. The slope of the dam and the rate of rise need to be controlled to
allow pore pressures to dissipate to prevent static liquefaction. A case history of
static liquefaction (Davies et al, 1998) is the Sullivan mine in southern British
Columbia. In this case the dam failed by static liquefaction caused by placement
of the fill material for the next stage of dam raising.
Upstream construction methods can be enhanced to provide additional drainage
through the use of upstream drain blankets & finger underdrains, or directional
drilling drains. The drainage systems, in some cases, are relied upon to lower
the phreatic level to provide seismic and static liquefaction protection. However,
in many cases drains can depressurize but not desaturate the tailings and the
use of drains alone to prevent seismic liquefaction is questionable.
Figure 3 Sullivan Mine, British Columbia, Upstream Dam : Static Liquefaction
Failure
Upstream Construction (Compacted, Consolidated and/ or “Air
Dried”)
9

10. Consolidation of tailings through drying or compaction, are also used for
protection against static and seismic liquefaction. Consolidation through air
drying, however, is limited to arid sites and where the rate of rise of the tailings
dam is low enough to allow complete drying of the placed layer. Typically, this
rate of rise may be 1 m per year (equivalent to 2 cm/week). The authors are not
aware of any case histories where consolidation, through drying, has been
sufficient to densify the tailings to prevent seismic liquefaction (assuming the
material is saturated). Machine compaction of upstream spigotted tailings is
carried out on some dams to provide seismic liquefaction resistance.
INCO R-4 Tailings Area showing Compacted Upstream Spigotted Beach
Hydraulic Fill
Cylconing of tailings to produce sand for dam construction is common,
particularly with large open pit copper mines. Recent advances in this technology
include the use of flotation circuits to remove sulphides, for acid drainage control,
such as at the Kemess gold-copper mine in northern British Columbia. The use
of cycloned sand in seismic areas requires careful control of compaction and/or
10

11. drainage. It is the author’s experience that the density required to preclude
seismic liquefaction in highly seismic areas (above PGA > 0.2g), assuming
saturation, can only be reliably achieved through mechanical compaction. The
use of underdrains, while promoting strong downward gradients and maintaining
a low saturation level, are not sufficient to densify the sand to preclude
liquefaction.
Thickened & Paste Tailings
The relatively recent introduction of high density thickeners is providing the
tailings dam designer with another tool to optimize the use of tailings as a
structural fill. Thickening of the tailings results in a steeper beach slope (1% to
5%), which can allow “stacking” of tailings above decant pond and dam
elevations. This can result in a lower height for the tailings dam, however it is
often accompanied by a larger impoundment footrprint. Steeper beach slopes
have been achieved for paste tailings (mix of thickened and dewatered tailings)
and for small tailing piles. Paste tailings, defined as low slump tailings which will
release little or no water, is being promoted by some designers as the solution to
tailings disposal (Newman et al, 2003). An important benefit of the paste tailings
is that the absence of a “free water” pond significantly reduces the risk of piping
and flow failures commonly associated with conventional tailings storage. As
with all tailings disposal methods and tailings dam designs, the designer must
balance the cost and risk of different tailings disposal technologies to select the
best available technology. (McLeod and Plewes, 2003).
Dewatered Tailings
Dry tailings disposal, where the tailings is mechanically dewatered, have been
used on a few sites. In this case the tailings dam can be formed by compaction
of dewatered tailings. If saturation and liquefaction are not a concern, the
disposal could consist of simply stacking the tailings, without compaction.
However, compaction could still be required to reduce permeability to limit
seepage through the tailings mass. Dewatered tailings disposal is currently being
carried out at several mines where a main requirement at the site is the
environmental concerns.
Conclusions
While tailing dams and water dams share a considerable amount of similarity in
design and construction they are two very different structures. The design of
tailing dams needs to consider the role of tailings, both as a part of the structural
component of the dam, as well as the environmental aspects of storing a mined
waste product. Tailing dams have additional considerations regarding piping,
filters, drainage, geochemistry, and structural support.
11

12. References
DAVIES, M. P., DAWSON, B.,TASAROFF, D. and CHIN,B., 1998. Static
liquefaction Slump of Mine Tailings – A Case History. Proceedings 51st
Canadian Geotechnical Conference, Edmonton.
KLOHN, E.J.,1979. Seepage Control for Tailings Dams, Proceedings of 1st
International Mine Drainage Symposium, Denver, Colorado.
MCLEOD, H.N. and PLEWES, H.D., 2003. Can Tailings Dams Be Socially
Acceptable?. International Congress of Large Dams (ICOLD), Symposium on
Major Challenges in Tailings Dams, Montreal.
PLEWES, H.D. and MACDONALD, T., 1996. Investigation of chemical clogging
of drains at Inco Central Area tailings dams. Tailings and Mine Waste ‘96’
Proceedings: 59:72, Rotterdam :Balkema
NEWMAN P., WHITE,R., and CADDEN, A., 2003. Paste – The Future of
Tailings Disposal?
12