Water Power and Dam Construction. Issue August 2010
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12 AUGUST 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
DAM SAFETY
U
NTIL now no people have died from the failure or
damage of a large water storage dam due to earthquake.
Earthquakes have always been a signicant aspect of the
design and safety of dams.
A large storage dam consists of a concrete or ll dam with a height
exceeding 15m, a grout curtain or cut-off to minimise leakage of
water through the dam foundation, a spillway for the safe release
of oods, a bottom outlet for lowering the reservoir in emergencies,
and a water intake structure to take the water from the reservoir for
commercial use. Depending on the use of the reservoir there are other
components such as a power intake, penstock, powerhouse, device
for control of environmental ow, sh ladder, etc.
During the Richter magnitude 8 Wenchuan earthquake of 12 May
2008, 1803 concrete and embankment dams and reservoirs and 403
hydropower plants were damaged. Likewise, during the 27 February
2010 Maule earthquake in Chile of Richter magnitude 8.8, several
dams were damaged. However, no large dams failed due to either of
these two very large earthquakes.
WHAT EARTHQUAKE ACTION DOES A
DAM HAVE TO WITHSTAND?
In order to prevent the uncontrolled rapid release of water from the
reservoir of a storage dam during a strong earthquake, the dam must
be able to withstand the strong ground shaking from even an extreme
earthquake, which is referred to as the Safety Evaluation Earthquake
(SEE) or the Maximum Credible Earthquake (MCE). Large storage
dams are generally considered safe if they can survive an event with
a return period of 10,000 years, i.e. having a one percent chance
of being exceeded in 100 years. It is very difcult to predict what
can happen during such a rare event as very few earthquakes of this
size have actually affected dams. Therefore it is important to refer to
the few such observations that are available. The main lessons learnt
from the large Wenchuan and Chile earthquakes will have an impact
on the seismic safety assessment of existing dams and the design of
new dams in the future.
There is a basic difference between the load bearing behaviour of
buildings and bridges on the one side, and dams. Under normal con-
ditions buildings and bridges have to carry mainly vertical loads due
to the dead load of the structures and some secondary live loads. In
the case of dams the main load is the water load, which in the case
of concrete dams with a vertical upstream face acts in the horizontal
direction. In the case of embankment dams the water load acts normal
to the impervious core or the upstream facing. Earthquake damage
of buildings and bridges is mainly due to the horizontal earthquake
component. Concrete and embankment dams are much better suited
to carry horizontal loads than buildings and bridges. Large dams are
required to be able to withstand an earthquake with a return period
of about 10,000 years, whereas buildings and bridges are usually
designed for an earthquake with a return period of 475 years. This
is the typical building code requirement, which means the event has
a 10% chance of being exceeded in 50 years. Depending on the risk
category of buildings and bridges, importance factors are specied in
earthquake codes, which translate into longer return periods, but they
do not reach those used for large dams.
Moreover, most of the existing buildings and bridges have not been
designed against earthquakes using modern concepts, whereas dams
have been designed to resist against earthquakes since the 1930s.
Although the design criteria and analyses concepts used in the design
of dams built before the 1990s are considered as obsolete today, the
reassessment of the earthquake safety of conservatively designed
dams shows that in general these dams comply with today’s design
and performance criteria and are safe. In many parts of the world the
earthquake safety of existing dams is reassessed based on recommen-
dations and guidelines documented in bulletins of the International
Commission on Large Dams (ICOLD).
SEISMIC HAZARD IS A MULTI-HAZARD
Earthquakes represent multiple hazards with the following features
in the case of a storage dam:
r Ground shaking causes vibrations and structural distortions in dams,
appurtenant structures and equipment, and their foundations;
r Fault movements in the dam foundation or discontinuities in dam
foundation near major faults can be activated, causing structural
distortions;
IWP&DC presents a position paper of the International Commission on Large Dams
(ICOLD), prepared by the Committee on Seismic Aspects of Dam Design
Dam safety and earthquakes
Left: Maigrauge gravity dam built in 1872 in Switzerland. After a recent rehabili-
tation its service life has been extended by another 50 years. Photo shows the
main elements of a storage dam for power production: spillway, reservoir, con-
crete dam body, power intake and powerhouse (left) ; Below from left to right:
Downstream view of the 106 m high Sefid Rud buttress dam in Iran damaged
by the magnitude 7.5 Manjil earthquake of June 21, 1990. Bottom irriga-
tion outlets were opened after the earthquake to lower the reservoir (left).
Rockfall damage near left abutment (middle) and right abutment (right).
WWW.WATERPOWERMAGAZINE.COM AUGUST 2010 13
DAM SAFETY
r Fault displacement in the reservoir bottom may cause water waves
in the reservoir or loss of freeboard;
r Rockfalls and landslides may cause damage to gates, spillway piers
(cracks), retaining walls (overturning), surface powerhouses (crack-
ing and puncturing and distortions), electro-mechanical equipment,
penstocks, masts of transmission lines, etc.
r Mass movements into the reservoir may cause impulse waves in
the reservoir;
r Mass movements blocking rivers and forming landslide dams and
lakes whose failure may lead to overtopping of run-of-river power
plants or the inundation of powerhouses with equipment, and
damage downstream;
r Ground movements and settlements due to liquefaction, densica-
tion of soil and rockll, causing distortions in dams; and
r Abutment movements causing sliding of and distortions in the dam.
Effects such as water waves and reservoir oscillations (seiches) are of
lesser importance for the earthquake safety of a dam. Usually the main
hazard, which is addressed in codes and regulations, is the earthquake
ground shaking. It causes stresses, deformations, cracking, sliding,
overturning, etc. However, some of the other hazards, which are nor-
mally not covered by codes and regulations, are also important.
Therefore, in the earthquake design of dams all seismic hazard
aspects must be considered and depend on the local conditions of a
storage dam project.
WHAT COULD HAPPEN IN STRONG EARTHQUAKES?
The experience with the seismic behaviour of large dams is still lim-
ited. Four dams – two concrete, an earth core rockll and a con-
crete face rockll dam – with heights exceeding 100m were damaged
during the Wenchuan earthquake. Elsewhere, there are another ve
concrete dams over 100m in height, which were damaged due to
strong ground shaking. In concrete dams the damage was mainly in
the form of cracks but also joints can open up leading to the release of
water from the reservoir. In modern embankment dams the damage is
mainly by deformations and cracks along the crest that can eventually
lead to internal erosion and piping through the dam.
However, we have to be aware that each dam is a prototype located
at a site with special site conditions and hazards. Therefore, based on
the observation of the earthquake behaviour of other dams it is still
very difcult to make a prediction of the damage that could occur in a
particular dam. At this time we are still in a learning phase as very few
large modern dams have been exposed to strong earthquakes. In the
case of the Wenchuan earthquake, a large number of rockfalls took
place, which caused signicant damage to dams and appurtenant struc-
tures. Surface powerhouses were particularly vulnerable to rockfalls in
the steep valleys in the epicentral region of the Wenchuan earthquake.
COULD RESERVOIRS BE LOWERED IN CASE OF SUC-
CESSFUL EARTHQUAKE PREDICTIONS?
If earthquakes could be predicted, one could attempt to lower the
reservoir prior to the occurrence of a large earthquake. There are two
problem areas related to this concept. First, despite some 40 years of
research on earthquake prediction, it is still not possible to predict
the time, location and size of a large earthquake reliably. Small earth-
quakes may be predicted but not large ones. The prediction is usually
given in terms of the probability of occurrence, e.g. there is a 50%
probability that a magnitude 7 earthquake occurs in a certain region
within a period of 30 years. Such predictions are basically useless for
warning purposes and for lowering a reservoir.
Even if a large earthquake could be predicted reliably, there would
not be sufcient time to lower large reservoirs. Lowering of a reser-
voir would have to happen by low level outlets (bottom outlets) or
the power waterways if the intake is at low elevation. Unfortunately,
bottom outlets are not available everywhere. Therefore the lowering
of a reservoir by say 50% may take weeks or months, and in some
cases it may not be possible at all.
As a conclusion, earthquake prediction, which is a slowly develop-
ing science, is not a viable option to improve the earthquake safety
of dams. The only real option is to have a dam which can withstand
the strongest earthquake effects to be expected at the dam site. This
is the current practice in dam design.
The greatest hazard of a dam is the water in the reservoir.
Therefore, in the seismic design of dams, we have to ensure the safety
of the dam under full reservoir condition. Although an arch dam may
be more vulnerable to the effect of ground shaking when the reservoir
Above left to right: Different types of damage of embankment dams caused
by the magnitude 7.5 Bhuj earthquake in Gujarat province, India, January
26, 2001. Mainly small dams for irrigation and water supply were damaged.
Right: Damage of reinforced concrete buildings on top of intake towers of the
Zipingpu reservoir, May 12, 2008 Wenchuan earthquake
14 AUGUST 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
DAM SAFETY
is empty, this case is not critical for the safety of the people living
downstream of the dam but it is, of course, an important economical
issue for the dam owner if the dam should fail.
CAN EARTHQUAKES BE TRIGGERED BY STORAGE DAMS?
There are a number of cases where earthquakes were triggered by
the reservoir. The main prerequisites for reservoir-triggered seismicity
(RTS) are (i) the presence of an active fault in the reservoir region,
or (ii) existence of faults with high tectonic stresses close to failure.
The lling of the reservoir in a tectonically active region may merely
cause the triggering of an earthquake which in any case would occur
at a later date. The occurrence of RTS has been mainly observed in
reservoirs with a depth exceeding 100m. Six events with Richter mag-
nitudes of more than 5.7 have been observed up to now. The larg-
est magnitude was 6.3. In two cases, concrete dams were damaged,
i.e. Hsinfengkiang buttress dam in China and Koyna gravity dam in
India. Since records did not exist on the local seismicity prior to dam
construction, there are still doubts whether these large earthquakes
have actually been triggered by the reservoirs.
If a large dam has been designed according to the current state-of-
practice, which requires that the dam can safely withstand the ground
motions caused by an extreme earthquake, it can also withstand the
effects of the largest reservoir-triggered earthquake. However, RTS
may still be a problem for the buildings and structures in the vicin-
ity of the dam, because they would generally have a much lower
earthquake resistance than the dam. In the great majority of RTS, the
magnitudes are small and of no structural concern.
The issue of RTS is always discussed in connection with large
dams. RTS was also considered in connection with the Wenchuan
earthquake as the Zipingpu reservoir is located close to the ruptured
segment of the Longmenshan fault. However, there is no conclusive
evidence that the earthquake was triggered by the lling and the
operation of the reservoir.
WHAT ARE THE MAIN CONCERNS WITH RESPECT TO
THE EARTHQUAKE SAFETY OF LARGE STORAGE DAMS?
The main concerns are related to the existing dams, which either have
not been designed against earthquakes – this applies mainly to small
and old dams – and dams built using design criteria and methods of
analyses which are considered as outdated today. Therefore, it is not
clear if these dams satisfy today’s seismic safety criteria. There is a
need that the seismic safety of existing dams be checked and modern
methods of seismic hazard assessment be used such as the guidelines
given in ICOLD Bulletin 120. This will result in a dam which will per-
form well during a strong earthquake. To follow these guidelines is
more important than to perform any sophisticated dynamic analysis,
which is only a tool to help understand how a dam will perform.
THE ROLE OF ICOLD
Hoover Dam in the US built in the 1930s was the rst concrete dam
designed against earthquake where both the inertial effects of the
dam and the hydrodynamic pressure of the reservoir were taken into
account. Embankment dams were designed against earthquakes as
early as in the 1920s in Japan where the seismic action was taken into
account in the stability analyses.
ICOLD has discussed the effects of earthquakes on dams at several
Congresses and Annual Meetings. At the 5th ICOLD Congress in
Paris, France in 1955 the following subject was discussed: ‘Settlement
of earth dams due to compressibility of the dam materials or of the
foundation, effect of earthquakes on the design of dams’.
In June 1968 the ICOLD Committee on Earthquakes was estab-
lished. This committee now exists under the name: Committee on
Seismic Aspects of Dam Design. Thirty-one countries from all conti-
nents are represented in this important committee. In recent years the
following Bulletins have been published by the committee:
r Bulletin 62 (1988 revised 2008): Inspection of dams following
earthquakes – guidelines;
r Bulletin 72 (1989 revised 2010): Selecting seismic parameters for
large dams - guidelines;
r Bulletin 112 (1998): Neotectonics and dams - guidelines;
r Bulletin 113 (1999): Seismic observation of dams - guidelines;
r Bulletin 120 (2001): Design features of dams to effectively resist
seismic ground motion - guidelines;
r Bulletin 123 (2002): Earthquake design and evaluation of structures
appurtenant to dams - guidelines; and
r Bulletin 137 (2010): Reservoirs and seismicity – state of knowledge.
CONCLUSIONS
The technology is available for building dams and appurtenant struc-
tures that can safely resist the effects of strong ground shaking.
Storage dams that have been designed properly to resist static loads
prove to also have signicant inherent resistance to earthquake action.
Many small storage dams have suffered damage during strong earth-
quakes. However, no large dams have failed due to earthquake shaking.
Earthquakes create multiple hazards at a dam that all need to be
accounted for. There are still uncertainties about the behaviour of
dams under very strong ground shaking, and every effort should be
made to collect, analyze and interpret eld observations of dam per-
formance during earthquakes.
For further information, contact: Martin Wieland,
Chairman, ICOLD Committee on Seismic Aspects of
Dam Design, Poyry Energy Ltd., Hardturmstrasse 161,
CH-8037 Zurich, Switzerland. martin.wieland@poyry.com
IWP& DC
Above left: Damage of the Shapei powerhouse caused by rockfall during the May 12, 2008 Wenchuan earthquake; Above right: Sheared off pier of Futan weir
at the Minjiang river due to the impact of large rocks, May 12, 2008 Wenchuan earthquake
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16 AUGUST 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
AUSTRALASIA
W
YARALONG dam site is located in south-east
Queensland, Australia, near the township of
Beaudesert. The project is part of the south-east
Queensland water grid, providing water supply for
this region of Australia. When completed, the dam will have the
capacity to store approximately 103,000 ML of water.
The Wyaralong Dam Project is being delivered as an Alliance, where
the Client, Designer and Contractor work together in a contractual
arrangement which shares the risks and the rewards associated with deliv-
ering the project. Following the completion of the preliminary design,
a value engineering process occurred during the months of February
and March 2009 as part of the bidding process chosen by the client,
Queensland Water Infrastructure Pty Ltd, to select their Alliance partner.
The selected Alliance partners consisted of Macmahon, Wagners, ASI
Constructors, Hydro Tasmania Consulting, SMEC and Paul C. Rizzo
Associates – which combined with Queensland Water Infrastructure
to form the Wyaralong Dam Alliance. The design consultancy services
are being provided by Hydro Tasmania Consulting, SMEC and Paul C.
Rizzo Associates, while construction is being undertaken by the contract-
ing partners Macmahon, Wagners and ASI Contractors.
Detailed design of the roller compacted concrete gravity dam com-
menced in May 2009, with initial early works construction starting
on site in October 2009 in parallel with the design. Construction
is planned to be completed during early 2011. This paper provides
some of the details of the dam, along with some reasons behind the
nal design adopted at Wyaralong.
DAM - GENERAL
Wyaralong Dam is a 47m high (above lowest foundation level), 490m
long, roller compacted concrete (RCC) dam, with a centrally located
ungated primary spillway and a secondary spillway on the left abut-
ment. The total volume of RCC will be approximately 190,000m
3
.
The general arrangement is shown in Figure 1.
The dam has a wet intake tower attached to the upstream face of the
dam, located on the right abutment. This intake tower allows water
to be drawn from various levels within the reservoir. Water is then
discharged back in the river channel through the outlet works at the
base of the dam through two submerged, vertical discharge valves.
The dam is also designed to provide fish passage in both the
upstream and downstream direction. An innovative bi-directional
sh lift is being incorporated to provide this capability.
An aerial view of construction during June 2010 is shown in gure
2. In the photo, RCC placement has started in the spillway section.
The upstream side of the dam is on the left side of the photo.
FOUNDATION
The initial geotechnical investigation was carried out in 2006/07 and
in the rst half of 2009. This investigation consisted of boreholes
across the damsite and two dozer trenches were excavated, one on
each abutment of the dam. The dozer trenches involved removing soil
and overburden and mapping the top of the rock surface. After the
initial investigation was complete, 14 additional borings were drilled
and the dozer trenches were extended. The geotechnical investigation
also included water pressure testing, unconned compressive strength
testing, direct shear testing, and acoustic televiewer surveys.
Geological sections were developed based on the investigations
conducted and rock was classied based on the degree of weather-
ing on the sections. Weathering classications used were distinctly
weathered with seams, distinctly weathered without seams, and
slightly weathered to fresh. Specic weak layers were also identied
on these geologic sections. Based on the geologic sections, the design
foundation level for the dam was developed.
The design foundation level was selected to provide a surface that
balanced the amount of time and treatment required to prepare the
foundation for RCC placement with the amount of rock that would
need to be removed and replaced with RCC. The foundation level
was chosen so that rock that was classied as ‘distinctly weathered
with seams’ was removed from the dam foundation. It was judged
that it would take too much time and treatment to prepare this rock
to provide a competent base for the dam and it would be removed.
Distinctly weathered rock without seams was generally left in place
in the foundation because it provides a competent foundation for the
dam. The dam is generally founded on distinctly weathered sand-
stone in the abutments and on slightly weathered to fresh sandstone
in the primary spillway section. Photos of foundation preparation at
the right and left abutments are shown in gures 3 and 4.
Through the development of the geological sections, the orienta-
tion and dip of the typical bedding at the site was identied. The sec-
tions also showed that any weak layers identied at the site generally
followed bedding. The slope of the foundation at the left abutment
With construction work well underway on the Wyaralong Dam project in Queensland,
Australia, Richard Herweynen, Colleen Stratford and Jared Deible provide details on the
design of the roller compacted concrete gravity structure
Wyaralong Dam design and construction
Secondary spillway apron Primary spillway apron
Secondary spillway Primary spillway Outlet works and fishway
Below: Figure 1 – General arrangement; Right: Figure 2 – Aerial view of
construction (June 2010)
is generally consistent with the bedding dip and dip direction, and the
right abutment forms a stepped prole.
Using the geologic sections, it was possible to look for potential
interpolations of similar weak seams between boreholes in 2D (along
sections) and 3D (between sections). Potential planes of weakness
were identied between groups of boreholes and surface exposures.
A total of eight surfaces of potential weakness were identied at the
site, and these surfaces were named surface A-H. Their location rela-
tive to the dam is shown in Figure 5.
Stability analyses were undertaken along each of the surfaces iden-
tied. Surfaces A and C are located deep below the foundation and
were left in place. During construction it was found that surfaces D,
E, G, and H were identied at approximately the anticipated loca-
tions in the right abutment and spillway section. These surfaces were
treated in accordance with the plan developed during design. Surface
B was identied in the left abutment, and the foundation in this area
was initially blasted to the design level and the presence of Surface
B was conrmed. The geometry of Surface B was slightly different
from the geometry used in the design, so based on actual survey,
additional stability analyses were undertaken. Based on this analy-
sis it was determined that Surface B needed to be removed over the
majority of the footprint of the dam. An additional blast was done
to remove the rock above Surface B, gure 6 is a photo of the surface
after the additional rock removal.
The design also includes a grout curtain to seal potential seep-
age paths in the foundation. The double line grout curtain is being
installed using the GIN method, and grouting is generally being per-
formed from the approved RCC foundation level. In the abutment
sections the grout curtain is located in the footprint of the dam and
is being done in advance of RCC placement. In the primary spillway
section of the dam, grouting is being done from an overbuilt RCC
plinth upstream of the heel of the dam, allowing RCC placement to
proceed independently of grouting.
RIVER DIVER SION AND DEWATERING
Flows in the Teviot Brook are similar to those in many Australian
rivers and streams, where there are periods of low ows, punc-
tuated by high ow events. Taking this ood risk into account,
the nal diversion arrangement consisted of a 6m high cofferdam
which was designed, with reno mattresses, to be overtopped and
had a sheetpile cuttoff down to foundation rock through the more
permeable alluvial materials in the main river channel.
The river has been diverted into a buried steel conduit for the duration
of construction. The diversion pipe consists of a 2.4m diameter steel
pipe, which is concrete encased under the foot print of the dam. The
capacity of the diversion is approximately 25m
3
/sec. In the event the cof-
ferdam is overtopped during the rst year of construction, various design
measures have been adopted to control the ow into the dam excavation
area, minimising the clean up and ensuring rapid dewatering.
In addition, a series of dewatering wells are provided to dewater
the alluvial material and rock in the foundation area, so as to mini-
mise the ground water inow into the 15m deep excavation.
DAM SECTION AND DESIGN
Analyses performed for the dam included static and dynamic stabil-
ity analyses. Stability analyses were performed along RCC lift joints
within the dam, along the RCC/Rock interface, along typical bedding
planes in the foundation, and along specic defects identied in the
foundation. A nite element model was developed to estimate stress-
es in the dam during an earthquake, and seismic deformation analyses
were performed to estimate displacements during an earthquake.
The dam section was governed by stability analysis along bedding
planes and specic defects in the foundation. Shear strength prop-
erties for the RCC/Rock interface were developed using the Barton
criteria for rough surfaces. Shear strength properties for bedding
planes were developed using a combination of the Barton criteria
and measured roughness values. Shear strength properties for weak
layers in the foundation were developed using direct shear testing. A
section with a crest width of six meters and a downstream slope of
0.8H:1.0V was selected based on the analysis. A typical section for
the primary spillway shown in Figure 7.
Design features at the dam include a 400mm thick layer of conven-
tional concrete facing, a drainage gallery, crest to gallery drains, foun-
dation drains, and reinforced concrete crest sections. Typical monolith
joints in the RCC are located at 30m spacing, with intermediate joints
in the facing concrete at 7.5m spacing. The thermal analysis indicates
the monolith joint spacing can be adjusted, so the actual location of
monolith joints are being adjusted to suit foundation conditions.
SPILLWAY
The spillway at Wyaralong dam is required to pass the probable
maximum ood, which has a design inow of around 7680m
3
/sec.
Various spillway options were investigated during the value-man-
agement phase of the project, including combinations of primary,
secondary and tertiary spillways located both within the dam section
and remote from the main wall.
The adopted arrangement comprises a centrally located, un-gated
primary spillway with a smooth downstream face. A 25m wide still-
ing basin is located at the base of the primary spillway. The design
also incorporates a stepped faced secondary spillway on the left abut-
ment of the main wall, which is designed to operate at oods less
frequent than the 1 in 100 AEP event. Flows over the secondary
spillway are directed along the toe of the secondary spillway in a
concrete-lined apron channel to the main stilling basin. For ood
events greater than the 1 in 2000 AEP event, the capacity of the sec-
ondary spillway apron channel is exceeded and ows spill out across
the left abutment towards the river channel.
Extensive investigations were undertaken during the design phase
to assess the behaviour of the spillways, with particular attention
to the erosion potential of the left abutment for ood events which
exceed the secondary spillway apron channel capacity. Two physical
model studies were conducted, comprising:
r 1 in 80 scale model of the dam
r 1 in 30 scale model of the primary spillway, used to assess the
performance of the shway, outlet works and stilling basin under
low ow events.
WWW.WATERPOWERMAGAZINE.COM AUGUST 2010 17
AUSTRALASIA
Above: Figure 3 – Foundation preparation on left abutment;
Right: Figure 4 – Foundation preparation on right abutment
18 AUGUST 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
AUSTRALASIA
The hydraulic models indicated that the spillways and stilling basin
perform as intended up to the PMF event. The 1 in 80 scale model
is shown in Figure 8.
Results from the model studies for ow over the left abutment for
ood events greater than the 1 in 2000 AEP were used in an analysis
of the erosion potential of the left abutment. A decision model was
developed to assess the acceptability of the design with regard to ero-
sion potential. The decision model took into consideration scour
potential based on geological and hydraulic characteristics, likelihood
of occurrence, storm duration, and impacts on dam stability if a scour
hole develops at the toe of the dam. The conclusion of this assess-
ment was that erosion was judged to be acceptable and the dam will
remain stable should a deep scour hole develop.
In order to provide an additional level of protection against scour
holes undermining to toe of the dam, a grid of passive anchors were
adopted throughout the reinforced concrete apron and stilling basin
slabs to ‘stitch’ the concrete slab to the rock and provide additional
resistance across rock joints.
An ogee prole was adopted for the crest of the primary spillway
with an elliptical curve on the approach to the crest. The secondary
spillway crest has been designed as a broad-crested prole, however
top forms used in construction of the primary spillway will be reused
on the secondary spillway to provide an elliptical curve approach
to the crest. Smaller step heights and lengths beyond the second-
ary spillway have been adopted to provide a transition into the large
1.8m high steps. This arrangement will ensure that ows over the
secondary spillway do not pull away from the steps, thus providing
improvement in energy dissipation over the steps.
RCC MIX DESIGN & PLACEMENT
The proposed mix design is an 85:85kg (cement:y ash) mix, using
sandstone aggregate quarried on site. With specialist advice from Dr
Ernie Schrader, this mix was selected as the result of extensive labo-
ratory testing during the design phase of the project. The sandstone
aggregate has delivered improved performance due to its stiffness
properties, despite presenting some initial challenges for the design
team. Early petrographic analyses and tests on the sandstone indi-
cated that it was cemented with clays, potentially of a swelling nature,
providing some concerns with the long-term durability and suitabil-
ity of the aggregate for RCC. Additional petrographic analyses and
numerous durability tests were undertaken on cores of sandstone and
RCC samples, including wet-dry cycles, soak tests in ethylene glycol,
heating and cooling cycles, breakdown tests and abrasion resistance
tests. Results indicated that the RCC mix performs remarkably well,
Above: Figure 5 – Surfaces A – H; Top right: Figure 6 – Surface B after
additional excavation
WWW.WATERPOWERMAGAZINE.COM AUGUST 2010 19
AUSTRALASIA
with the exception that erosion testing on the samples suggested that
a more erosion resistant aggregate is required for the faces of the
spillways. Basalt aggregate is being locally imported for use in the
conventional concrete facing to address this aspect.
The bulk of the RCC is being placed during night shift over the
winter months. Mass gradient and surface gradient thermal analy-
ses indicated that even without forced cooling, the use of sandstone
aggregate will minimise the risk of cracking. The main contributing
factor to the excellent thermal properties of the RCC is its high tensile
strain capacity, which allows the mix to ‘stretch’ more than most con-
cretes without cracking. The mix also has a low modulus of elasticity
(~9GPa). The high tensile strain capacity and the low modulus are a
direct function of the use of sandstone aggregate.
The RCC is being mixed with an Aran pug mill, capable of produc-
ing 400m
3
per hour. The RCC is being delivered to the dam site with
40t articulated trucks to enable it to be delivered directly down the left
abutment foundation. These trucks have ejector bodies with a spreader
box tted to their rear. The specially designed tail gate sections of the
ejector trucks prevent RCC segregation. The delivery from the pug
mill to the dam is via a concrete lined ramp, following the secondary
spillway apron prole which is at a slope of 1 in 6.
Two trial sections have been constructed for the project, one was
constructed with the 85kg:85kg mix and the second test pad was
constructed with a 75kg:75kg mix. The rst trial section was built to
evaluate the water tightness of lift joints and to serve as a practice and
training area. The second trial section was constructed to evaluate
the RCC mix with less cementitious content.
At the time of writing RCC was being placed in the spillway section
of the dam and progressing upward into the abutments. Figure 9 is
a photo of the placement area.
INTAKE WORKS AND OUTLET WORKS
The outlet works are located on the right side of the primary spillway
and comprises a wet well rectangular intake tower on the upstream face
of the dam with selective withdrawal capacity, a DN 1750mm MSCL
main outlet conduit that passes through the base of the dam, and outlet
works valve pit at the toe of the dam. Two vertical submerged discharge
valves of DN 600mm and DN 1200mm are located in the outlet works
valve pit, and discharge directly into the spillway stilling basin. Buttery
guard valves are provided upstream of both discharge valves.
The outlet works and shway are capable of making regulated
releases in accordance with the water resource plan. The largest regu-
lated release ow required is 2.33m
3
/sec. The outlet and shway
combined has been designed to release 165ML/day with a reservoir
level 2.0m above dead storage level. The outlet capacity has also been
designed to allow the storage to be drawn down from full supply
level to 10% of the storage over a maximum period of 100 days with
median inows.
The 2.8m by 6.6m intake tower will ultimately be attached to
the upstream face of the dam with passive anchor bars, however it
has been designed to be stable as a free-standing tower during the
construction phase to enable its construction to precede the RCC
placement. Detaching the construction of the intake tower from the
RCC placement has allowed increased exibility in the construction
program.
FISHWAY
Fish passage was required to be provided as part of the design of the
Wyaralong Dam project. The design criteria, options and evalua-
tion of options were developed through a series of workshops, which
included all the relevant stakeholders. Due to the extensive operation
range for both upstream and downstream sh movement, and due to
the fact that the sh density is relatively low in the river, the preferred
option adopted was a bi-directional sh lift. With this design a single
sh lift is used to provide sh movement in both the upstream and
downstream directions. This design has signicant operational ex-
ibility. It is envisaged that the sh hopper will be in the upstream
attracting position most of the time, attracting sh for downstream
sh movement. However, the same attraction ow used for attract-
ing sh in this upstream position will also be attracting sh into the
trapped area for moving sh upstream. Extensive hydraulic model-
ling was undertaken to ensure appropriate attraction ows during all
of the design conditions.
Jared Deible, Senior Project Engineer, Paul C. Rizzo
Associates, Inc., 500 Penn Center Boulevard, Suite 100,
Pittsburgh PA 15235 USA. Jared.Deible@rizzoassoc.com
Richard Herweynen, Principal Dam Consultant,
Hydro Tasmania Consulting, 89 Cambridge Park Drive,
Cambridge, Tasmania 7170.
Richard.Herweynen@hydro.com.au
Colleen Stratford, Dams Consultant, SMEC Australia, 71
Queens Road, Melbourne, VIC 3004.
Colleen.Stratford@smec.com
Conventional
concrete facing
Spillway apron
0.8
1.0
Crest to
gallery drains
Drainage
gallery
Foundation
drains
Rock surface
IWP& DC
Below, from top to bottom: Figure 7 – Spillway section;
Figure 8: Hydraulic model; Figure 9: RCC in spillway section
20 AUGUST 2010 INTERNATIONAL WATER POWER & DAM CONSTRUCTION
AUSTRALASIA
B
ENMORE hydropower station is located in the South
Island of New Zealand and is the sixth out of a chain of
eight power stations on the Waitaki river. At 540MW it is
the second largest hydro station in New Zealand in terms
of installed capacity and annual generation.
The station provides around 2200GWh annually which is 17%
of the energy delivered from Meridian Energy’s power portfolio.
Moreover, the station ensures hydrology exibility for Aviemore
and Waitaki power stations and provides essential support serv-
ices for the operation of the Transpower owned HVDC link.
Reduction in the station performance would not only decrease
energy output, but also the ability to transfer energy to and from
New Zealand’s North Island.
POWER SUPPLY
The six Francis turbines at Benmore are of an identical hydraulic
design and were commissioned between 1965-6. The six genera-
tors are each rated at 112.5 MVA. They are capable of running at
100MW at 0.9 pf but are limited to 90MW output due to opera-
tional constraints. Currently the total 540MW, 600MVA output of
Benmore is provided to Transpower’s 16kV bus which in turn can
supply either or both of Pole 1 of the HVDC, or the 220kV grid via
Transpower’s two 232MVA transformers (T2 and T5). The bus is
normally run split with three generators on each side to limit the fault
level. This conguration can be seen in Figure 1. Benmore is used to
provide voltage support for Pole 1 of the HVDC link.
SCOPE OF DEVELOPMENT
Several of the essential operating systems at Benmore are nearing the
end of their design/operating life. Signicant risks and opportunities
were identied in a series of risk management workshops during
2003. The ensuing investigations and remnant life assessments iden-
tied components of the generation plant to have either:
r Posed a signicant health and safety risk;
r Failed, in respect to no longer operating within the original design
performance parameters;
r R eached the end of their supportable economic life, or are due to
within the next few years;
r The potential to cause a substantial impact on revenue through
reduction in plant and HVDC availability.
Technical and commercial analysis of various refurbishment options
were completed to derive the optimum scope and timing of work to
maximise benets from the investment, and ensure an appropriate t
with Meridian’s long term corporate goals and strategies. This led to
a business case being presented to the board of directors.
The refurbishment of Benmore hydropower station in New Zealand is on track for
completion by January 2011. Johan Hendriks gives more details about the work taking place
The key link in the Waitaki chain