structural
challenge
Water-tight
dam design
The earthquake that struck China on 12 May
could have aftershocks that reverberate long
after the seismic activity has abated. What
can engineers learn about the design of ever
more resistant dams?
At 7.8 on the Richter scale, the earthquake that hit
China’s Sichuan province caused catastrophic damage to
infrastructure and resulted in more than 69,000 fatalities.
For days, residents of the town of Dujiangyan waited
for a second catastrophe to strike, as damage to the
156m concrete-faced, rockfill Zipingpu dam triggered
fears that it was on the verge of more extensive failure.
“Few large dams have been exposed to such severe
shaking and people will pore over the performance of
the dam for some time to come,” says chairman of the
British Dam Society Jonathan Hinks.
Read the signs
“Every incident is learned from and there are already
some early findings,” says Professor Andy Hughes,
Atkins’ director of dam engineering. “The reservoir was
only a third full, but there was cracking and obvious
leakage,” he says.
The dam holds 1.1 billion cubic metres of water, but
was only holding 320 million cubic metres at the time of
impact, greatly reducing the dynamic loading on the
structure. But, despite this, the shallow depth and
severity of the quake caused damage serious enough to
raise concerns that the structure could fail and unleash
torrents of water, further devastating areas downstream.
Such concrete-faced rockfill dams are designed as a
concrete slab, or series of concrete slabs, that sit on a
rubble transition zone above the mass of rock at the heart
of the dam. This creates a water-tight membrane. They
are generally expected to perform well in earthquakes
and considered to have less risk of failure than earth-filled
structures, as the rock medium allows some water
seepage. However, because of the enormous quantity of
materials required to build a dam, the design and the
type of fill is often dictated by what materials are available
locally, rather than by performance criteria alone.
“The approach has changed a lot over the years,”
says Hinks. “There’s now much greater awareness of the
dangers of seismic liquefaction in dams and foundations,
and consequently the avoidance of low density sands.
Hydraulic fill has fallen out of favour.”
Seismic liquefaction occurs when the fill material in
the dam loses strength and stiffness under earthquake
loading. It happens in water-logged (saturated) soils as
forces from the movement generated by the
earthquake cause spaces between the grains of soil or
sand to collapse, leading to the fill no longer being able
to sustain the pressure from overlying soil or structures.
This fill material then flows like a liquid and can cause
extensive damage.
Hinks also sits on the seismic committee of the
International Committee of Large Dams (ICOLD), a
forum of 88 countries that work together to improve
technical guidance and experience in dam engineering.
“We will next meet in Bulgaria in June.
Improvements in practice often follow a close study of
incidents and failures, both worldwide and in the UK,
where post-incident reporting is likely to be made
mandatory,” he says.
This goal is supported by Professor Hughes, who is
working with the Environment Agency on
recommendations for improving reservoir safety. Such
mandatory information, he says, would be an extremely
valuable resource for all designers. This research is
about to be submitted to the Department for the
Environment Food and Rural Affairs and identifies 49
areas that require further study.
New challenges
One area highlighted is that of climate change and its
effects on dam safety. “Issues include more frequent,
extreme floods, resulting in more frequent overtopping
of dams and perhaps the requirement for increased
spillway capacity to accommodate larger floods,” he
says. Such changes could require dams to be raised and
spillways to be reinforced as design criteria adapt to
allow for the increasing severity of storm events.
One dam that could have benefited from such
retrospective attention is the Ulley reservoir near
Rotherham, UK, which suffered from erosion of the
downstream face in June 2007 after storms. The
turbulent action of the water over steps that were built
in 1873 led to some stones being moved and
undermined the dam itself.
The age of assets is a problem, particularly in the
UK, where the average dam is 110 years old. This,
combined with new legislative requirements to produce
flood hazard maps along with risk management plans
(EU Floods Directive 2007), and requirements to
quantify individual dam risk and establish contingency
measures (Civil Contingencies Act 2004), is leading to
new responsibilities for engineers.
“The biggest challenge for the profession is
not technical – we can solve those issues – it is about
knowledge transfer and succession planning,”
says Hughes. The lack of major new dam construction
in the UK is considered to be one reason that few
young engineers are specialising in dams and hydraulic
engineering. In China and India, where a wealth of
major dams, mainly for hydroelectric power generation,
are under way, there is no such problem. But such
schemes are also contentious, with the anti-dam lobby
arguing that they should not go ahead.
“There is also a requirement for engineers to be
much broader in their knowledge and views and not
hide behind the technical design. You have to be an
environmentalist, an accountant and a politician,” says
Hughes. To this end, he is assisting the University of
Bristol in teaching undergraduate courses in dam
engineering and supporting research initiatives. “We are
promoting dams in a world where there is a desperate
need for water, power, food and irrigation,” he adds.
“Engineers must talk the same language as
environmentalists. Better training through a good centre
for learning seems a good place to start.”
dams could be raised and spillways reinforced as design criteria
adapt to allow for the increasing severity of storm events
Kurobe dam, Japan
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