Water Power and Dam Construction. Issue May 2010

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20 MAY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

SAFETY

EW South Wales in Australia has a booming mining

industry, with both coal and metal mines prominent in

the state’s economy. Over 100 large dams overseen by

the New South Wales Dam Safety Committee (DSC)

are located on or near mine sites, or are affected by mining activi-

ties. NSW is notable for having extensive coal deposits – and the

associated mine workings – adjacent to several large water supply

dams which supply a large proportion of the water for Sydney,

Australia’s largest city with a population of 4M. The state govern-

ment moved to protect dams from the impacts of mining in 1978,

creating a system with a high level of regulation. The DSC man-

ages this system, regulating actions by both the mining companies

and dam owners.

Mining could have serious impacts on dams and the community,

ranging from potential environmental damage – release of toxic or

unsightly tailings, or loss of saline mine water, through to loss of life

in dam failures, and damage to major water supply infrastructure,

resulting in a loss of drinking water for large populations.

Table 1 is a list of potential risks to dams resulting from mining

activities, and typical management procedures used to deal with the

risks. Although the DSC’s main aim is to ensure dam safety is not

compromised, it is also aware that available coal resources should be

efciently utilised where possible. The consequence of not allowing

mining would be the loss of a valuable resource to the state, and of

the economic activity generated by exploiting mineral resources near

The dam safety minefield

Eight percent of coal production in New South Wales’ booming mining industry was mined

near dams and reservoirs last year. David Hilyard explains the balancing act which has to be

maintained between dam safety and the efficient utilisation of coal resources in Australia

Wollongong

Figure 1 – Major water supply dams in the Southern Coalﬁeld, south of

Sydney, are surrounded by extensive coal deposits, presenting a challenge to

dam safety and security of water supplies

< Sydney 40km

Nepean

Dam

Cataract Dam

Cordeaux Dam

Avon Dam

10km

TABLE 1: Risks to dams resulting from mining activities

Risk Mechanisms Management

Mine subsidence from

underground mining

Ground movements cause damage to dam and

possible dam failure:

Cracking

Instability

Piping

Loss of connement

Movement on geological structures

Loss of freeboard

Loss of ood capacity

Components (such as gates and conduits) fail

Minimise ground movements by varying mine layouts, and by keeping a

barrier between mining and dam

Dam surveillance

Blast, mainly from open cut

mining

Mine blast vibrations cause damage to dam and

possible dam failure:

Cracking

Instability

Piping

Loss of connement

Liquefaction

Manage blasts to limit vibrations by varying blasting practices, and by

keeping a barrier between mining and dam

Dam surveillance

Mine workings (usually open

cut, but may include portals

to underground workings)

downstream of dam

Dam failure causes inundation of pit occupied by

mine workers. Factors include:

Short warning times

Pit wall failure destabilises dam

Adequate pit wall design

Dam surveillance

Pit wall surveillance

Subsidence monitoring

Blast monitoring

Loss of storage resulting from

mining activities

Mining results in ow path between reservoir and

mine workings

Maintain barrier between reservoir and mine workings — in both

horizontal and vertical directions

Understand hydrogeology of mine

Piezometric monitoring

Inspection of mine workings — identify seepage into mine

Mine water balance

Water source ngerprinting

WWW.WATERPOWERMAGAZINE.COM MAY 2010 21

SAFETY

dams. In 2008/09, 13.5M tonnes of coal were mined near dams and

reservoirs, which was about 8% of total coal production.

POTENTIAL PROBLEMS

Mine subsidence is caused by underground mining, particularly

longwall mining where staged mine roof collapse is expected. Blast

impacts mainly result from open cut mining. The coalelds south of

Sydney, surrounded by drinking water infrastructure (see Figure 1),

are worked by underground mining, including large modern longwall

mines. These are extremely efcient but, if not managed properly,

may result in extensive mine subsidence impacts to water supply and

other infrastructure.

The DSC oversees the safety of all prescribed dams in NSW,

including water dams (public drinking water supply, irrigation,

mining, and power station dams), mine tailings and power station

ash dams, and dry detention basins. The DSC has powers under

NSW state legislation, mainly the Mining Act (1992) and Dams

Safety Act (1978). These powers allow the DSC to dene areas

around dams where mining activities can be overseen, and to amend

mining leases so that mining companies are required to comply with

the DSC’s requirements.

Mining currently impacts water supply dams, tailings dams,

and ash dams, mainly in the Sydney Basin coalelds. When

mining companies apply to mine in dened areas, the DSC assess-

es applications using a risk-based approach. The requirements

for mining in a high-risk environment – say longwall mining

near a public water supply dam wall and reservoir – will be sub-

stantially higher than in a low risk situation, such as open cut

mining distant from a small dam retaining stabilised inert tailings

with no signicant infrastructure downstream. The application

process requires the company to present a convincing argument

that mining will not reduce safety of the dam or storage below

limits acceptable to the DSC. Good understanding of site geol-

ogy, hydrogeology, mining engineering, and mine subsidence are

critical. Site-specic investigations by independent experts typi-

cally form a major part of the application.

RISK ASSESSMENT

Risks are assessed from a dam engineering perspective, with gener-

ally lower tolerance of risk, over longer time frames, than may be

common in the mining industry. The review process includes an

opportunity for the dam owner to make comments on the applica-

tion. The DSC has developed a coherent risk assessment procedure

for assessing applications (Reid 2007). Acceptance criteria are based

on frequency and consequence of the hazard, and also reect a civil

engineering perspective of tolerable risk (see Figure 2). Risks to the

dam wall and of storage loss are reviewed using a structured proc-

ess. Risks falling outside tolerable criteria require further input from

the company to develop additional controls and to demonstrate that

the modied proposal meets the criteria. Modications may include

review of mine layouts and practices, increased monitoring, and trig-

ger-action-response plans (TARPs).

For assessing cases where loss of storage is a potential risk, a sys-

tematic set of possible ow paths and mechanisms was developed.

Tolerable storage losses are dened in risk terms, with very low levels

of tolerability for large losses. Modelling of ows along identied

ow paths can indicate whether losses are regarded as tolerable or not

(see Figure 3). Uncertainties in input parameters are handled using

Monte Carlo methods.

Approvals to mine usually include a programme of monitoring, tai-

lored to suit the application. Typical monitoring streams may include:

r Dam surveillance.

r Inspection of mine workings — to ensure that pillar and roof strata

are behaving according to predictions, and to note changes in seep-

age into the mine.

r Survey measurement of surface movements resulting from mine

subsidence.

r Mine blast vibration at the dam wall.

r Mine water balance.

r Mine water chemical ngerprinting to determine the source of

inow water.

r Groundwater monitoring.

Adequate dam surveillance is a fundamental monitoring tool in

these management plans. It should be robust, high-frequency, and

Figure 2a – Example of risk assessment of a mining proposal – longwall

mining below a tailings dam in the Hunter Valley, north of Sydney. In this

initial assessment a number of risks, including cracking and movement of the

wall and foundation, are unacceptable, based on information provided in the

application. The applicant was required to develop additional controls aimed

at managing the unacceptable risk cases

Negligible Monitor Minor Signiﬁcant Failure

Adequate safety demonstrated

Adequate safety

not demonstrated

Loss of freeboard

interface cracking

groundwater pressure

trate

Wall

cracking

Wall

movement

Foundation

cracking

Foundation

sliding

Impact on dam wall

1.E+00

1.E-01

1.E-02

1.E-03

1.E-04

1.E-05

1.E-06

1.E-07

1.E+00

1.E-01

1.E-02

1.E-03

1.E-04

1.E-05

1.E-06

1.E-07

Adequate safety

not demonstrated

Wall cracking

Wall movement

Foundation cracking

Foundation sliding

Loss of freeboard

interface cracking

groundwater pressure

Negligible Monitor Minor Signiﬁcant Failure

Impact on dam wall

Adequate safety demonstrated

Limit of tolerable loss

Fault 1

Fault 2

Fault 3

Sill margin

Aquifer region 1

Aquifer region 2

Aquifer region 3

-1

-2

-3

-4

-5

-6

Log probability, per panel

0.01 0.1 1

Sustained loss, ML/day

10 100

Tolerable loss

Loss intolerable

Figure 2b – After additional review and development of adequate monitoring,

risks were demonstrated to be tolerable. Mining proceeded without incident

Negligible Monitor Minor Signiﬁcant Failure

Adequate safety demonstrated

Adequate safety

not demonstrated

Loss of freeboard

interface cracking

groundwater pressure

trate

Wall

cracking

Wall

movement

Foundation

cracking

Foundation

sliding

Impact on dam wall

1.E+00

1.E-01

1.E-02

1.E-03

1.E-04

1.E-05

1.E-06

1.E-07

1.E+00

1.E-01

1.E-02

1.E-03

1.E-04

1.E-05

1.E-06

1.E-07

Adequate safety

not demonstrated

Wall cracking

Wall movement

Foundation cracking

Foundation sliding

Loss of freeboard

interface cracking

groundwater pressure

Negligible Monitor Minor Signiﬁcant Failure

Impact on dam wall

Adequate safety demonstrated

Limit of tolerable loss

Fault 1

Fault 2

Fault 3

Sill margin

Aquifer region 1

Aquifer region 2

Aquifer region 3

-1

-2

-3

-4

-5

-6

Log probability, per panel

0.01 0.1 1

Sustained loss, ML/day

10 100

Tolerable loss

Loss intolerable

22 MAY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

SAFETY

multi-stream. Visual inspections of the dam, crest movement surveys,

seepage, piezometers, and crack monitoring may all be useful. During

mining activities, frequencies of inspection and monitoring are usually

increased to intervals much shorter than those for conventional dam

surveillance programmes. Drawing down supply levels whilst mining

is nearby may also be a useful strategy, if this is feasible. Monitoring

is carried out by the mining company and dam owner, with regular

oversight and inspections by DSC staff.

In addition to monitoring programmes, for high-risk mining

proposals, the DSC requires contingency and mine closure plans,

and may make recommendations on security deposits to the state

mining agency.

In recent years, successful mining projects have included:

r Repeated subsidence of a Hunter Valley coal tailings embankment

by successive longwalls. The tailings in this 40m high homogenous

earth and rockll dam were not fully consolidated. An open cut and

an underground mine entrance were located immediately down-

stream, increasing the consequence of dam failure by placing ve

to ten identiable lives at risk. The embankment was subjected to

open cut mine blasts at the same time as longwall mining at depths

of 150-170m subsided sections of the dam by up to 1.5m. A com-

prehensive programme of dam wall inspections, crest subsidence

surveys, and monitoring of blast vibrations, piezometers, and exten-

someters was carried out. The result was successful mining of four

longwalls under the dam, and open cut mining downstream, with

no signicant impact on its stability.

r Longwall mining under a major Sydney water supply reservoir. The

hazard of leakage of reservoir waters into mine workings posed

a threat to a reservoir forming part of Sydney’s drinking water

supply. The coal seam was 300-350m deep, and in the area of the

reservoir, longwall mining was modied by reducing the amount

of coal extracted by using narrower longwall panels and wider pil-

lars. The revised mine layout reduced mine subsidence impacts on

the reservoir. Longwall mining was not permitted near the dam

wall, a 56m high rigid concrete gravity dam. Extensive monitoring

included observing stability of mine workings, geological mapping,

mine water ows, subsidence, strain, and dam wall movements.

The result was, again, successful mining of 15 longwalls from a coal

resource that otherwise would have been sterilised.

r Open cut mining of a pit located immediately downstream of a

mine water dam north of Newcastle. A crew of ve to ten workers

in the pit were subject to the risk of immediate inundation in the

case of catastrophic dam failure, as the pit was about 50m down-

stream of the toe of an 18m high zoned earth and rockll dam. The

dam was threatened by mine blast vibrations, and failure of the

pit wall destabilising the embankment. A pit wall stability review

was required. Strict blasting limits were imposed, and monitoring

included blast vibration measurements, regular dam inspections,

crest surveys, pit wall inspections, and piezometer readings. Mining

was completed successfully below the dam, without any signicant

impact on the stability of the structure.

David Hilyard is a geologist currently employed as a

senior consultant in mining and dams safety by Aurecon

in Sydney, Australia. From 2003-10 he worked for the

NSW Dams Safety Committee, and was Manager of

Mining Impacts. Email: hilyardd@ap.aurecongroup.com

IWP& DC

Figure 3 – Example of risk assessment of potential loss of storage. The heavy

blue line deﬁnes a tolerability criterion in probability/consequence (loss of

storage) space. Risks falling above the curve are regarded as intolerable. The

coloured lines are modelled ﬂows for a variety of ﬂow paths identiﬁed during

review of the application, such as aquifers and geological structures. Some

ﬂow paths clearly fall into the tolerable ﬁeld, and some appear intolerable

without further management

Negligible Monitor Minor Signiﬁcant Failure

Adequate safety demonstrated

Adequate safety

not demonstrated

Loss of freeboard

interface cracking

groundwater pressure

trate

Wall

cracking

Wall

movement

Foundation

cracking

Foundation

sliding

Impact on dam wall

1.E+00

1.E-01

1.E-02

1.E-03

1.E-04

1.E-05

1.E-06

1.E-07

1.E+00

1.E-01

1.E-02

1.E-03

1.E-04

1.E-05

1.E-06

1.E-07

Adequate safety

not demonstrated

Wall cracking

Wall movement

Foundation cracking

Foundation sliding

Loss of freeboard

interface cracking

groundwater pressure

Negligible Monitor Minor Signiﬁcant Failure

Impact on dam wall

Adequate safety demonstrated

Limit of tolerable loss

Fault 1

Fault 2

Fault 3

Sill margin

Aquifer region 1

Aquifer region 2

Aquifer region 3

-1

-2

-3

-4

-5

-6

Log probability, per panel

0.01 0.1 1

Sustained loss, ML/day

10 100

Tolerable loss

Loss intolerable

Reference

Reid, P. R., 2007. Risk assessment of applications to mine near dams and

stored waters. Proceedings of the Mine Subsidence Technological Society

7th Triennial Conference, University of Wollongong, NSW, 165-176.

The author is grateful for discussion and critical review of this paper by Bill

Zeigler, Heather Middleton, Peter Reid, Ian Forster, and Paul Heinrichs.

Below, left to right: Open cut downstream of an ash dam; open cut downstream of a mine water dam; tailings dam with open cut blast preparation

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24 MAY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

SAFETY

HE Swedish mining company Boliden is at present con-

structing dams for a new tailings storage facility close to

the town of Boliden in northern Sweden. Planning for

the facility – named Hötjärnsmagasinet – started in 2002.

Sweco was rst involved in the beginning of 2003 with a conceptual

design and site investigations. Once complete,

the dams will have some unique characteristics,

especially in terms of the required lifetime of

the structures, as presented below.

Mining in the Boliden area started in 1924

and until today the company has operated 29

mines and four mills in the area. There are pres-

ently two mines and one central mill in opera-

tion. Figure 1 shows a map of Sweden with the

Boliden area marked and enlarged.

The Boliden central mine was in operation

until 1967. After the closure of the mine, the mill

still remained in operation processing ores from

the company’s other mines in the area – which is

still the case today.

Since 1953 the tailings from the mill have been

deposited in a tailings pond that was originally

a natural lake (Gillervattnet). Over time, dams

have been built in the lower parts and the water surface is now several

meters above the original lake. The deposit is expected to reach its

full capacity by 2011.

The ore reserves in the Boliden area are mainly complex sulphide

ores with a rather high content of pyrite.

LONG-TERM ASPECTS

A tailings deposit will normally go through several stages like con-

struction, operation, closure, rehabilitation and long-term control.

Due to the high content of pyrite in the ore and thereby in the tailings,

oxidation will take place if oxygen is present. The oxidation process

results in a low pH-value which in turn mobilises

metals in the tailings. Due to the environmen-

tal risk of some of the more hazardous metals

mobilising into the environment, the oxidation

process should be avoided.

When a tailings deposit is taken out of service

it needs some kind of rehabilitation where issues

such as preventing the availability of oxygen,

dusting and landscaping are considered.

The conventional way of rehabilitation is a dry

cover of the tailings. The cover is normally built

up by several layers including a low permeability

layer and protection layers with one or several

transition layers. An alternative to a dry cover is

to have a permanent water cover.

Boliden decided early on in the project that

an underwater deposit with a permanent water

cover as rehabilitation method would be a desir-

able solution for a new tailings facility. The main advantages of such

a deposit is:

r The tailings are kept away from exposure to oxygen during deposi-

tion phase and into the long term phase.

Design of a tailings dam

for permanent water cover

Dag Ygland and Nils Isaksson of Sweco report on the design and site investigations

for new dams to be constructed at a tailings storage facility in Boliden, Sweden

Figure 1 – Map

of Sweden with

the vicinity of the

site marked

Figure 2 – Excavation of moraine

WWW.WATERPOWERMAGAZINE.COM MAY 2010 25

SAFETY

r Dusting is avoided at all times.

r A lake is an esthetical form of landscaping.

The main disadvantage is that the dams surrounding the facility must

withstand a water pressure for a very long period of time, several

hundreds – or even thousands – of years.

Dams for other purposes like hydro power can, at least theoreti-

cally, be dismantled once past its service life. For dams surrounding

tailings deposits this is not possible unless the tailings are removed,

which is normally unrealistic.

Apart from requiring stable dams, a permanent water cover must

have a suitable catchment area. The catchment must be big enough

to allow a minimum inow of water to safeguard the water cover, i.e.

there must be no risk of the cover drying out. On the other hand the

catchment must be small enough so that ood events can be handled.

This implies that a long term permanent water cover is only possi-

ble where the hydrological conditions are suitable. In this aspect the

north of Sweden is quite favourable.

The outlet also has to be adapted to last over the long term and it

has to be constructed in such a way that it will not need to be manu-

ally operated. It can not be built in concrete as the chemical stability

of concrete is not sufcient to full the structural requirements over

time. The outlet has to be constructed of “natural” materials, which

have proven to endure different chemical conditions, temperature dif-

ferences and water strain over time. Thus it will be constructed as a

free overow weir, entirely carved out of the high quality bedrock

available at site.

SUITABLE CONSTRUCTION MATERIALS

What possibilities are there to design and construct long-term stable

dams? A basic requirement is that the construction material should

be stable under the inuence of a hydraulic gradient over the long-

term. This will almost automatically rule out all manmade materials

which have had very little opportunity to prove themselves resistant

to nature for the time periods required.

In the mining industry the tailing itself is commonly used as con-

struction material for tailings dams. Although not an optimal con-

struction material due to its very ne grained nature, it is possible

to use tailings if proper design considerations are taken, for instance

gentle slopes. Another possibility is to use natural earth material if

available on site or within a reasonable distance.

At the site in Boliden, as in many places far north, the last ice age

has left huge deposits of a competent material -– moraine. Being a

material that has rst been crushed, mixed and transported by a

thousand meter thick ice cover and then left for ten thousand years,

it has proven itself to withstand the forces of nature. It also has the

advantage of being a well known material since it is used as core

material in most earth ll dams in Sweden, as well as in other north-

ern countries.

Since it is desirable for environmental reasons to prevent exposure

of the tailings to oxygen during operation, it is advantageous to build

the dams in advance and create a man made lake into which deposit

of tailings can be sent by the means of oating pipe lines. This can

easily be done with an external building material, unlike when using

tailings from the mining process.

During the early stage of the project an inventory of available

moraine was performed and it was concluded that there was enough

moraine with suitable quality available within or just outside the

planned impoundment. The moraine at site is a broad graded mate-

rial containing silt, sand, gravel and stones. Between 20 and 40% of

the material falls below the No 200 sieve (0.075mm). Figure 2 shows

excavation of moraine during construction of the embankments.

DESIGN

Since the embankment must be a long-term impermeable construc-

tion the design cannot be the same as for conventional hydro power

dams, with a core and lters. This is because the steep pore pressure

gradient through the core would make it vulnerable to internal ero-

sion over the long-term.

The embankment therefore has to be designed in a way that it can

full the requirement of low hydraulic conductivity without the tradi-

tional use of core and lters and be a long term stable construction.

These criteria and the availability of moraine in the vicinity of the

dam site has led to the decision to build the embankment mainly

out of moraine. By constructing the embankment as a more or less

homogenous structure, an impermeable embankment that it not sen-

sitive to internal erosion is achieved.

To achieve stability in the long-term, the slopes of the embank-

ment must be considered. The embankment downstream slope will

be constructed with a 1V:3H slope which gives a slope angle that is

half the angle of internal friction of the moraine. A slope as gentle as

this will remain stable even if fully saturated. This has therefore been

a major stability criterion for the long-term phase.

To protect the slope from rain erosion until proper vegetation has

been established in the rehabilitation phase, the lower part of the

downstream slope will be covered with a two layer erosion protec-

tion. It will be constructed with a 1V:3.3H slope which will also

increase the stability of the slope.

The upstream slope will be constructed with a 1V:2H slope which

is gentle enough during the operational phase. In the long-term phase

the upstream slope will be supported by tailings, hence the upstream

stability will not be an issue during this phase. The slope has been

provided with an erosion protection layer to avoid erosion due to

wave inuence during the operational phase.

A horizontal drainage is placed under the downstream part of the

embankment to help keep the pore pressure low during the opera-

tional phase and increase the leakage control (Figure 3).

The moraine used as the main dam ll material is divided into

two different material types, coarser moraine placed downstream and

ner moraine placed upstream in the cross section. The purpose of

this distinction is to increase the drainage of the downstream part

Foundation (Moraine)

Tailings

Fine moraine

Coarse moraine

Erosion protection

Horizontal drainage

Slope protection

Optical cable-leakage &

pore pressure surveillance

First construction phase

221.5

224.5

:3.3

Foundation (Moraine)

Tailings

Fine moraine

Coarse moraine

Erosion protection

Horizontal drainage

Slope protection

Pore pressure

First construction phase

221.5

Fs=2.2

Fs=2.1

224.5

100m beach

1V:2

:3.3

1V:3.

Operational phase

Long term phase

Foundation (Moraine)

Tailings

Fine moraine

Coarse moraine

Erosion protection

Horizontal drainage

Slope protection

Optical cable-leakage &

pore pressure surveillance

First construction phase

221.5

224.5

:3.3

Foundation (Moraine)

Tailings

Fine moraine

Coarse moraine

Erosion protection

Horizontal drainage

Slope protection

Pore pressure

First construction phase

221.5

Fs=2.2

Fs=2.1

224.5

100m beach

:3.3

1V:3.

Operational phase

Long term phase

Figure 3 – Overview of the cross section design

Figure 4 – Operational and long term phase, result of stability calculation of a fully

functioning embankment section during the operational and remediation phase

26 MAY 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

SAFETY

of the embankment thus lowering the pore pressure and increasing

the stability. Note that the difference in gradation and hydraulic

conductivity between the ne and coarse moraine is small and has

no denitive boundary. A massive material testing program during

the first construction phase showed that the conductivity of the

coarse moraine varies between 10

-7

–10

-6

m/sec and the ner moraine

between 10

-8

–10

-7

m/sec.

DIMENSIONING CRITERIA

Stability and pore pressure calculations have been performed in the

geostudio software environment – Seep/W and Slope/W.

Several calculations describing stability and pore pressure during

operational and the long-term phase have been carried out.

In the operational phase calculations, the horizontal drainage has

been assumed to be properly functioning and the differentiation of

ner and coarser moraine is assumed to be adequate, i.e. satisfying

difference in hydraulic conductivity has been achieved. The angle of

friction of the moraine has been set to 35˚.

Stability calculations of the operational phase with the assump-

tions mentioned above renders a factor of safety of approximately

2 for both shallow slip surfaces and for deeper slip surfaces. This

indicates that no stability issues are expected during the operational

phase (Figure 4).

During the operational phase, a beach will be constructed forcing

the free water surface away from the embankment, thus minimising

the risk of wave erosion and lowering the pore pressure through the

embankment. The beach will have a conventional dry cover in the

long term phase.

Stability calculations of the long term phase, describing a fully

functional embankment with a properly designed beach have also

been carried out. The calculations verify that the stability is adequate

in this case, with factors of safety of about 2 (Figure 4).

A major criteria in calculations of long-term stability has been that

the embankment must be stable even if the horizontal drainage has

deteriorated. This criteria is set due to the likeliness of a gradually

failing drainage effect caused by slow material transport, chemical or

biological processes during the long term phase.

In combination with this criterion the phreatic surface has then

been calculated for two different cases.

r The differentiation of ner and coarser moraine during the con-

struction is adequate, i.e. satisfying difference in hydraulic conduc-

tivity has been achieved and will remain.

r The differentiation of ner and coarser moraine is not achieved with

regards to hydraulic conductivity and the embankment is regarded

as homogeneous when calculating the phreatic surface.

Stability calculations with the assumptions mentioned above renders

factors of safety of approximately 1.7-2.0 for both shallow and

deeper slip surfaces. This indicates that no stability issues are expected

during the long-term phase (Figure 5).

These results can be expected by comparison with natural forma-

tions. Slopes of moraine have naturally come to a slope of about 1V

to 3H since the last ice age.

Sweden is situated in a geologically stable environment where the

common approach is not to consider earthquake induced loads in

embankment design, i.e. the seismic loads are considered to be lower

than allowable increase in stresses under extreme loads. However

due to the shear length of time these embankments are expected to

last, the occurrence of considerable earthquakes might still need to be

taken into account even in a geological stable area like this. During an

earthquake of high magnitude there is an imminent risk of liquefac-

tion of tailings inside the reservoir. It is therefore necessary to ensure

that the construction is stable even during this type of extreme load.

Control calculations have been executed with the assumption of full

liquefaction in the tailings. The result of the calculations implies that

the slip surfaces inuenced by the liqueed tailings has a factor of

safety well over what’s required in such an extreme event (Figure 6).

MONITORING

The main focus of the monitoring program is to measure seepage

through the embankments and their foundation, phreatic surface and

displacements of the crest.

Displacements are to be measured manually on concrete plinths

installed in wells on the dam crest. The plinths have to be founded

deep enough to ensure that measurements are not disturbed by freeze

and thaw actions since frost can reach some 3m into the ground

during winter.

The phreatic surface is, in a number of cross sections, measured by

means of stand pipe piezometers. In each cross section a number of

pipes are installed at different distances from the crest. In some of the

locations pipes are installed with lter tips at different depths of the

embankment and foundation. This is to ensure a correct measure-

ment even if the pore pressure is not hydrostatic.

Seepage is collected in a pipeline placed under the downstream toe

of the embankments. Measuring the amount of seepage will be done

continuously in a number of measuring wells along the embankments.

The distance between measuring points is between 100 and 300m.

Piezometers and seepage measurement can only be done

in specied cross sections and there will always be a distance

Figure 7 – Plan of part of embankment with chosen placing of ﬁbre optic cable

Figure 6 – Long term phase, result of stability calculation of a homogenous

embankment with fully deteriorated horizontal drainage in case of full lique-

faction of tailings

Figure 5 – Rehabilitation phase, result of stability and pore pressure calcula-

tions in case of a deteriorated drainage

WWW.WATERPOWERMAGAZINE.COM MAY 2010 27

SAFETY

between measuring points where conditions have to be assumed

to be similar to what is being observed. A possibility to indirectly

observe conditions continuously along embankments has arisen

in recent years by means of temperature measurements. By using

bre optic cables and a special laser instrument, temperature can

be measured with an accuracy of less than 0.1 degree and 1m

along the cable.

These types of temperature measurements will be used in the

actual embankments and can be described as a passive system

based on natural variation of temperature in the reservoir. In

summer time the temperature in the reservoir will be some 15 to

20 degrees centigrade and in winter close to zero. The temperature

below the phreatic surface within the embankment will follow the

seasonal variation like a sinus wave with an amplitude and phase

that depends mainly on a few parameters such as length of ow

path and permeability of materials.

The maximum temperature deep inside an embankment will nor-

mally be observed several months after the peak temperature of the

reservoir. However if there is a zone with higher seepage than aver-

age this will be noted both as a higher amplitude and that the peak

comes sooner.

In parts of an embankment above the phreatic surface (but still

far enough from the ground surface) the temperature variation is

much smaller than below. This can be used to detect the position

of the phreatic surface by placing a bre optic cable in such a

way that the expected phreatic surface is crossed as many times as

desired. Figure 7 shows a plan of the chosen placing with one bre

placed in a square pattern 20 by 20m. In this way the phreatic sur-

face can be observed every 20m as long as it is positioned within

plus or minus 10m from what is expected. The bre optic cable is

also marked on the cross section shown in Figure 2.

CONSTRUCTION PHASE

Figure 8 shows a picture of the ongoing construction. The embank-

ments are to be built in two stages. The rst construction stage start-

ed in 2008 and will be completed during 2010. The second stage is

planed to be built in approximately 10 years.

In the rst stage the embankments are being built up to an elevation

of 219m above sea level, with a maximum height of about 16m. In the

second construction stage the embankments will be raised an additional

5.5m reaching up to an elevation of 224.5m above sea level.

The rst construction stage has been designed with the second stage

in consideration, e.g. the foundation of stage two has already been

prepared in the rst construction stage.

The total crest length of the embankments is 3600m and the ll

volume is 2Mm

. Of this about 1.2Mm

is lled in stage 1.

REHABILITATION AND CONTROL PHASE

During the rst 30 years after closure the behaviour of the embankments

will be monitored. Regular evaluations will be performed to ensure that

the design fulls requirements for long-term stability.

For more details contact: Sweco Infrastructure AB,

Gjörwellsgatan 22, P.O. Box 34044, SE-100 26

Stockholm, Sweden. Tel: +46 8 695 60 00. www.sweco.se

Figure 8 – Part of embankment with

upstream wave protection being placed

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WWW.WATERPOWERMAGAZINE.COM MAY 2010 29

RCC

HE Bassara project is located in northeastern Iraq, on the

Tawooq Chai river, one of three main tributaries of the

Adhaim river. The project site lies approximately 20km

from Sulaimaniya and 40km upstream of Qadir Karam.

Bassara Dam, which stands 67m high, and ist accompanying res-

ervoir with a capacity of 59.7Mm

were envisaged to provide stor-

age and regulation of the Tawooq Chai for irrigation of Bassara

Irrigation eld, situated some 11km downstream of the dam. The

regulation of the river and head difference created by the dam also

enable generation of hydroelectricity as a secondary benet of the

project. The dam was designed between 2005 to 2008.

SITE GEOLOGY

The Bassara region is mostly mountainous. The ridges of the hills of

the area are generally at and the slopes are steep, cut by deep gullies.

A characteristic of this area is the large Tawooq Chai-Bassara gorge,

which was foreseen as the Bassara dam location.

With regards to the geology of the reservoir basin, karstied lime-

stone are observed at high levels in the layers on the right side, and

heavily fractured marls and sandstones on the left side. The presence

of alternating marls, sandstone and limestone has also been noted.

The geological composition of the reservoir area is governed by the

fold form observed in the Tawooq Chai River cut through Jabal

Darbandi Bassara ridge. The upper level of Gercus formation pro-

vides the impervious barrier for possible seepage paths.

Extensive site investigations have been performed consisting

of core drilling and water pressure testing. The dam site and

prospective borrow area for ne grained materials have been

investigated by resistivity proling.

Ground water conditions are favorably inuenced by imperme-

ability of the rock units. Therefore groundwater may only develop

during the rainy season. No permanent springs have been observed in

the sandstone complex within the dam foundation area. Low bearing

capacity and small resistance to erosion of the rock is a characteristic

of the left river embankment at the dam site location, as conrmed

by results of geological investigations.

Regarding the seismicity of the North Iraq region, there are many

indications of neotectonic activities. Zones of Taurides and Zagros as

part of Alpine orogenic belt are well known as potentially seismically

active all as result of the contact of Euroasian and African plates.

DAM DE SIGN – PROPOSED TECHNICAL SOLUTIONS

On the basis of detailed geological site investigations a combina-

tion of roller compacted concrete (RCC) and ll dam type was pro-

posed. The right bank dam is designed as an RCC gravity dam. This

structure is founded in rock which consists of sandstones and sandy

siltstone complex. The RCC dam structure consists of an overow

(spillway) and non-overow part.

The spillway section, founded at elevation 653m, is the central part

of the RCC dam. The upstream vertical face and downstream (1:0.90)

outer slopes of the RCC dam are designed to provide standard safety

factors. It is divided into three blocks by expansion joints. Each block

has a spillway bay, 11.5m wide, 59.85m long and 53m high. The

thickness of the spillway lateral walls is 2m. A bridge 11.5m long

and 9.2m wide is located at the upstream part of the crest to allow

construction and other trafc to pass.

Vicko Letica, Vladislav Skoko and Ivana Grubljesic of IK Consulting Engineers report on

the design of the rst RCC dam in Iraq

Technical solutions

of Bassara RCC Dam

Excavation line

Original ground surface

Guard rail

Fill dam RCC dam

Reinforced concrete

Mass concrete

C.M.C.

R.C.C. Type -1

R.C.C. Type -3

Lower drainage gallery

R.C.C. Type -1

Stoplog

deposit

Upper drainage gallery

Compact

sandstone

Degraded

sandstone rock

Debris (clay,

fragments limestone)

Drainage curtain

Figure 1: Dam Longitudinal Section with geological proﬁle