HRW. Issue: September 2009
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The Alvkarleby hydroelectric plant has been in service since 1915. The intake canal wall in which the anchor
failures occurred is behind the bridge.
http://hrw.hotims.com RS #21
were wrapped in a corrosion-protecting
band that would allow some movement
inside the concrete. This would enable Vat-
tenfall’s dam safety engineers to measure
the actual force in each anchor in the fu-
ture. Finally, the hole was grouted and the
top of the anchor was treated for corrosion
protection and capped.
Discovery and initial
investigations of failures
After installation, Vattenfall established
a program to sample the anchors’ per-
formance at ve-year intervals. Tests
consisted of measuring the force needed
to just release the nut. In 2002, the test
revealed seven broken anchors. At the
same time, the engineers realized that the
anchors had been installed with plastic
caps, which needed to be replaced with
contacted the design consultant, contractor
who delivered and installed the anchors, and
supplier. The installation contractor sent
two of the failed anchors to an independent
metallurgical laboratory, where they were
examined microscopically and subjected
to strength tests. The fracture surfaces
were corroded, making visual assessment
steel. They also noted that the top cor-
rosion protection seemed to be of poor
quality. Over the following two years,
three more anchors ruptured; two dur-
ing a test. All of the fractures developed
less than 2 meters from the anchor’s top,
and most less than 1 meter from it.
After the initial discovery, Vattenfall
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__________________
30 HRW / September 2009 www.hydroworld.com
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dif cult. The laboratory reported that the
fracture surface appeared both ductile and
trans-crystalline and that the fracture ap-
peared to have been instantaneous. On one
surface, a crescent-shaped initiation point
was evident. The yield point and rupture
strength in the material were 3 to 5 percent
suggested a sudden water level increase
as a possible cause, along with ice load-
ing, bending of the anchor at the plate,
and material weakness in the anchor bar.
Finally, the anchor supplier mentioned
that welding often leads to ruptures in
this type of high-strength material.
Pursuing a better explanation
After reviewing the initial reports, Vatten-
fall’s dam safety engineers decided a more
detailed investigation was needed in order
to prevent similar incidents in the future.
T he team of fou r eng ineers assi gned to t he
investigation brought expertise in metal-
lurgy, concrete, design, and dam safety.
In conducting the investigation, they used
Vattenfall’s metallurgical and concrete
laboratories in Alvkarleby, as well as other
laboratories. The investigations included
chemical and metallographic analyses,
magnetic particle testing, penetrant test-
ing, ultrasonic inspection, impact testing,
and grease analysis.
Chemical and metallographic analyses
Ruptured anchors were subjected to a
chemical analysis using optical emission
spectroscopy. The analysis indicated that
the constituent elements in the steel fell
within published industry standards, with
the exception of a slightly lower-than-
standard manganese content. The micro-
structure of one of the anchors also was
studied under an optical microscope and
was consistent with the chemical analysis.
Vattenfall’s laboratory staff also per-
formed nondestructive testing on the
ruptured anchors, including ultrasonic
testing of the end of the bars and dye
penetration tests and magnetic particle
inspection of the surface of the ruptured
anchors. None of these tests revealed
cracks in the anchors. At the same time,
laboratory staff performed a separate
test of the ultrasonic method on an in-
tentionally cracked bar. The test showed
lower than the European standard values of
1,080 megapascals and 1,230 megapascals,
respectively. The laboratory report indicat-
ed that the probable cause of failure was an
overload, such as a sudden high water level.
The design consultant, who was not
aware of the laboratory’s ndings, also
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www.hydroworld.com September 2009 / HRW 31
http://hrw.hotims.com RS #23
that the method did not have the sensi-
tivity to detect aws of the size believed
to have initiated the ruptures.
To evaluate the brittleness of the steel
and the transition temperature, the labora-
tory also performed a notched-bar impact
test on samples from one of the anchors at
temperatures from -10 degrees Celsius (C)
to 25 degrees C. The resulting test values
were 2 to 3 Joules, much lower than the
expected value of at least 30 Joules for com-
mon construction steel alloys. The low test
values indicate a high degree of brittleness.
Grease analysis
During installation, the anchors were
covered by a special anti-corrosive grease
and wrapping, in addition to the corrosion
protection provided by the grout. Oxi-
dation of the grease would increase the
fractures. On at least one of the samples,
a crescent-shaped initiation zone, similar
to the in situ fractures, was visible.
Investigating a fracture
surface in detail
The anchor having the fracture surface
least affected by corrosion was chosen for
detailed microscopic investigation. A ste-
reomicroscope photograph clearly showed
the crescent-shaped. 3.2-millimeter by
0.8-millimeter initiation zone, as well as
rust stains. The initiation zone showed
signs of oxidation and was sharply delin-
eated from the remainder of the fracture
surface. The sharp difference between the
initiation zone and the remaining area al-
lowed the investigators to rule out fatigue,
which would have resulted in crack propa-
gation lines. Failure due to overloading
amount of free acid present. Grease sam-
ples from three failed anchors were ana-
lyzed and found to be more acidic than a
sample of unused grease provided by the
supplier. However, the test values were
not consistent enough or high enough to
support the theory of embrittlement due
to deterioration of the grease.
Load tests
To veri fy t he st reng t h tests made on sm all
specimens in the initial phase of the in-
vestigation, a laboratory was engaged to
perform uniaxial load tests on two of the
ruptured anchors. Rupture occurred at
1,159 and 1,136 kiloNewtons, or 7 and
10 percent lower than the European stan-
dard. The characteristics of the fracture
surfaces produced during the tests were
similar to those observed on the in situ
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__________
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http://hrw.hotims.com RS #24
would have been marked by deformation,
which was not detectable in the sample in-
vestigated. Finally, the absence of visible
grain boundaries in the area of the frac-
ture showed that coating by brittle phases
in the grain boundaries, such as cementite
or sulphide of iron, was not the cause.
precipitation from a liquid or gas phase.
These crystals could not have been
formed during the steel manufacture
process, because the subsequent hot-
rolling of the bar would destroy them.
The investigators concluded that the
crystals were the result of water entering
the crack, and therefore that the crack
formed after the anchor came into use.
Reviewing possible
sources of failure
The various professionals queried about
the failures raised several possible expla-
nations, including ice loading, transient
water pressures due to a plant trip, bend-
ing, and damage during welding. Vatten-
fall’s investigative team considered each
of these but did not nd a persuasive case
for any of them, based on the laboratory
results and other project data. Overloads,
due to either ice pressure or plant tripping,
should have left evidence in the form of
deformation in the fracture surfaces. Vat-
tenfall also had performed calculations for
ice loading and physical model and full-
scale tests for loading during a plant trip
and found that the resulting loads did not
A more detailed microscopic view
showed signi cant details of the bound-
ary between the initiation zone and the
remainder of the fracture surface. A
structure just inside the initial fracture
zone contained needle-shaped separated
crystals, such as would be formed by
32 HRW / September 2009 www.hydroworld.com
When the Alvkarleby plant was upgraded in the
late 1980s, post-tensioned anchors were installed
in the left intake wall. The anchors extended
5 meters into bedrock and were tensioned
to 720 kiloNewtons.
Figure 1 Post-Tensioned
Anchors at Alvkarleby
AnchorFree
length
Anchor
length
in rock
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____________________
www.hydroworld.com September 2009 / HRW 33
Power generating solutions.
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http://hrw.hotims.com RS #25
exceed the actual force of the tensioned
anchors. Regarding ice loads, at least
some of the ruptures occurred during
ice-free periods. Ice formation is normally
prevented in the intake canal, although no
records existed to con rm this for the pe-
riod of outage for refurbishment.
Anchor bars also may be subject to
bending, caused for example by an an-
chor plate that is installed at an incorrect
angle. However, the spherical surface
of the nuts used should have prevented
bending by this mechanism. In addition,
the ruptures were too far from the nuts
to support this explanation.
Welding damage also was considered
an unlikely explanation, since there was
no record of welding at the time of instal-
lation. The protective wrapping should
also have protected the anchors against
tally assisted cracking. Environmentally
assisted cracking could be brought about
by either hydrogen-induced brittleness or
stress corrosion. The hydrogen theory was
based on the fact that, over time, additives
to the concrete grout develop free hydro-
gen, which has a high rate of diffusion
into concrete and steel. Increased hydro-
gen content in a high-carbon steel could
cause the steel to rupture. The time to
rupture would depend on the kind of steel,
hydrogen content, and stresses. However,
the grease analysis had not shown sig-
ni cantly elevated levels of free hydrogen.
Therefore, the investigators believed that
the anchors had not undergone prolonged
exposure to free hydrogen. Furthermore,
hydrogen embrittlement would not have
been limited to the top 2 meters of the an-
chors, where all of the ruptures occurred.
welding sparks, and the fractured sur-
faces showed no sign of welding damage.
It was apparent that the ruptures had
occurred well after the installation and
tensioning of the anchors. Generally, ex-
planations for this type of failure could
be either fatigue or environmentally
induced cracking of the material under
stress. The investigative team rejected
the fatigue hypothesis because the loads
acting on the anchors did not vary signif-
icantly over time and because of the ab-
sence of stop lines on the crack surface.
Pinpointing the cause
As a result of the investigations, par-
ticularly the investigation of the fracture
surface under a scanning electron micro-
scope, the team concluded that the most
likely cause of the failure was environmen-
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34 HRW / September 2009 www.hydroworld.com
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http://hrw.hotims.com RS #26
The explanation for the ruptures -
nally adopted by the investigative team
was stress-induced corrosion, in combi-
nation with the high degree of brittleness
of the steel. The visible oxidation in the
initiation area pointed to corrosion, and
the brittleness and relatively low steel
strength explained the material’s low re-
sistance to stress corrosion. The anchors
initially had been installed with a plastic
cap to keep grease in and water out. How-
ever, the caps proved to provide inad-
equate corrosion protection. Inadequate
protection at the top of the anchors might
explain the location of the ruptures in the
top 2 meters of each anchor.
Lessons learned
from the investigations
As a result of the anchor failures and sub-
method could not detect some small but
potentially serious fractures. Vattenfall
has joined a research effort with CEATI
Dam Safety Interest Group to nd new
and better methods for anchor testing.
Correcting the situation
At the conclusion of the investigations,
Vattenfall’s dam safety engineers recog-
nized that the ruptured anchors should
not be replaced in kind and the remain-
ing anchors should not be relied upon for
stability. Three main alternatives were
considered for meeting safety criteria:
installing new anchors and strengthen-
ing the existing concrete; constructing
supporting walls; and complete replace-
ment of the dam. Although there was a
strong interest within the company in
alternatives that did not involve anchors,
sequent investigations, Vattenfall installed
steel cables to replace all the anchors at
Alvkarleby. The remaining anchors were
left in place but were assumed ineffective
in stability calculations. Vattenfall now
checks the few anchors of the same type
installed in its other dams by pulling with
a jack until the nut is just released.
The investigations also brought new
insights about methods of testing anchors
in dams. When the ruptures were discov-
ered, Vattenfall was testing anchors by
measuring the actual load in a sample of
anchors with a hydraulic jack. A solution
used at some other plants was to install
additional test anchors in the dam, which
were used for testing but not considered in
stability evaluations or design. Vattenfall
also experimented with ultrasonic testing
but found in the Alvkarleby tests that the
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www.hydroworld.com September 2009 / HRW 35
http://hrw.hotims.com RS #27
the rst option was clearly the most
economical. The supporting walls were
aesthetically undesirable, particularly
because the Alvkarleby plant is a tour-
ist attraction and is known as a facility
with few negative environmental effects.
There was also little space for such sup-
ports. The cost of a complete dam re-
placement was much higher than that of
the other alternatives, especially consid-
ering the cost of lost generation.
Concrete test results from the investi-
gation showed that the existing concrete
in the dam was in poor to very poor
condition. The situation provided an
opportunity to apply the results of one
of Vattenfall’s recent research efforts.
This research project had focused on us-
ing concrete grout injections to increase
the life of aging structures. The grout
were lled with grout and the tendons
lowered into the grout. After the grout
had cured, the anchors were tensioned
to 1,500 kiloNewtons.
The ruptured anchors were replaced
in 2004, and the remaining new cables
were installed in 2005. Vattenfall plans
to select a sample group of anchors for
testing at ve-year intervals. The ex-
pected life of the new anchors is about 50
years, and there is ample space to install
new ones when needed.
■
Reference
Cederstrom, Malte, Per-Erik Thorsall, Bengt
Hildenwall, and Stig-Bjorn Westberg, “In-
cident with Loss of Seven Post-Tensioned
72 Ton Anchors in a Dam,” Dam Safety
2005 Proceedings, Association of State Dam
Safety Of cials, Lexington, Ky., 2005.
composition and the speci c installation
te ch n i que s u se d at A lvk a rleby we re b as ed
on lessons learned through the research.
Grout was injected into holes drilled at
1-meter intervals along the dam. After
completing the grouting, the contractor
drilled holes for the new anchors and
the drill cores were saved for analysis.
The analysis of the cores showed that
the grout injection had substantially im-
proved the condition of the concrete.
The new anchors comprised 40 steel
tendons, each consisting of 12 12-milli-
meter-diameter wires. The wires were
split apart in the lowest 5 meters to im-
prove the anchoring. Above the anchor
zone, the wires were coated in protec-
tive grease and placed inside plastic
pipes that would allow the wire to move
freely when tensioned. The anchor holes
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___________________
36 HRW / September 2009 www.hydroworld.com
http://hrw.hotims.com RS #28
Tech Notes
ICOLD Forum: New committee
releases work plan
A new International
Commission on Large
Dams (ICOLD) com-
mittee has released its
work plan and intends
to publish a position
paper later in 2009. The ICOLD Com-
mittee on Engineering Activities Associ-
ated with the Planning Process for Wa-
ter Resources Projects was established in
2007; seven countries are represented on
the committee. Arthur H. Walz Jr., P.E.,
former ICOLD vice president, is chair-
man of the new committee.
The position paper has ve purposes,
Walz says.
— Focus on the development and
management of water resources using
integrated water resources management
in the watershed;
— Summarize the current basic
planning process being used for water
resources projects;
— Articulate the need to improve the
process in order to plan realistic and
sustainable water resources projects with
accurate cost estimates;
— Present and discuss enhancements
needed to the process to ensure the se-
lection of better and more cost-effective
alternatives; and
— Present what ICOLD will recom-
mend as a new strategy and process,
which includes initial guidance from the
decision-makers, input from all techni-
cal disciplines, collaboration with stake-
holders and the public, and an external
review, if needed. This new strategy and
planning process, Comprehensive Vision
Based Planning, is based on a systematic
and holistic or watershed approach to
both comprehensive planning and inte-
grated water resources management.
The position paper is scheduled to
be published in the fall of 2009, and the
committee’s next step is to publish guide-
lines. Walz says the guidelines likely will
be available in 2010.
— ICOLD is a nongovernmental or-
ganization that provides a forum for the
exchange of knowledge and experience in
dam engineering. The organization leads
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www.hydroworld.com September 2009 / HRW 37
http://hrw.hotims.com RS #29
the profession in ensuring that dams are
built safety, efficiently, and economically,
and without detrimental effects on the en-
vironment. To learn more about ICOLD
activities, contact Michel De Vivo, Secre-
tary-General, ICOLD 151, Bd Haussman,
Paris 75008 France; (33) 1-40426824; E-
mail: secretaire.general@icold-cigb.org.
Voith Hydro updates
German laboratory
Voith Hydro recently updated its Brun-
nenmuehle laboratory in Heidenheim,
Germany. This work, which cost more
than 20 million euros (US$25.9 mil-
lion), included doubling the capacities of
the test rigs, installing super-computers
for more precise simulations, and con-
structing a new of ce building.
Friedrich Voith, son of company
founder Johann Matthaeus Voith,
founded this hydroelectric turbine test-
ing laboratory in 1908. The location
had been a mill. To power the turbines,
Friedrich Voith used a water storage tank
above the facility to construct the rst
pumped-storage plant in Germany.
The Bruennenmuehle laboratory
offers research and development ser-
vices for all Voith Hydro operating units
worldwide. In addition to being a model
testing facility, the laboratory is a devel-
opment center for complete hydro units,
including generators, excitation and
control systems, and components (such
as butter y and spherical valves).
EPRI seeks funding of
generator repair research
EPRI is seeking $180,000 in funding
from the hydroelectric industry to sup-
port research on how to restore safe op-
eration of a generator after failure of one
or more coils in the stator winding.
The institute seeks supporters who
will provide $30,000 each. Companies
that fund any EPRI program can take
advantage of tailored collaboration funds
for up to half of their contribution, says
Jan Stein, senior project manager.
After failure of a stator winding, proj-
ect personnel can isolate, or cut out, the
failed coil from the rest of the winding and
quickly return the units to service, Stein
says. In addition, one or more healthy coils
may need to be cut out from the electrical
circuit to ensure the electric current dis-
tribution in the remaining winding does
not cause thermal damage to the insula-
tion. Use of temporary repair procedures
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38 HRW / September 2009 www.hydroworld.com
such as this can keep a machine in service
until it can be permanently repaired or
replaced, providing substantial economic
bene t, Stein says.
This research project updates work
described in EPRI report EL-4983,
Synchronous Machine Operation with
Cutout Coils, published in 1987. Work on
this project, which is estimated to take 18
months, will begin as soon as all partici-
pants are identi ed.
The objectives of this project are to:
— Update and expand calculations
about how many and which coils can be
safely cut out;
— Provide guidance for performing
parallel circuit current measurements to
validate stator winding operation within
its thermal limits;
— Learn more about the effect of the
winding with cut-out coils on vertical
shaft run out; and
— Document worldwide experience
and practices.
— For more information about funding
this research, contact Jan Stein at (1) 650-
855-2390; E-mail: jstein@epri.com.
Brazil university begins study
of fi sh passage at dams
Research is needed to determine how
to preserve the about 4,500 species of
migratory sh in South America and
to build and monitor sh passage de-
vices at dams, says Dr. Paulo dos San-
tos Pompeu, department of biology at
the Universidade Federal de Lavras
in Brazil.
To provide more information on pro-
tecting sh species in South America,
researchers at the university are launch-
ing several studies. Dr. Pompeu says the
studies include:
— Baseline biological studies before
dams are built. These studies are vital to
provide information on migratory move-
ments and determine if sh passage is
necessary or would be bene cial.
— Determination of critical habitats
for each life stage of the various spe-
cies and the effects of fragmentation of
those habitats by dams and impound-
ments. This research is needed to de-
termine if a river section that will be
ooded by a reservoir associated with
a hydro project is a critical habitat for
certain sh species.
— Long-term monitoring to assess
the ef cacy of sh passage and its rela-
tive bene ts to biological communities
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