Article reformatted from
NEW CIVIL ENGINEER 7 NOVEMBER 2002
SPECIAL ISSUE: EARTHQUAKES
Pages ii - iii
t Formed by an earthquake triggered landslide mer 90 years
ago, the Usoy Dam is the tallest
natural dam in the world.
Out of Danger
Perched above a Tajikistani valley
that is home to 15,000 people is an
unstable natural dam of uncertain
geology. Alan Sparks finds out how
engineers are weighing up the risk.
The constant threat of
devastation hangs over the
people of the Bar-tang Valley
in Tajikistan, situated in one
of the worlds most seismically
active regions.
Since l911, when the village
of Usoy was buried under a
65Om high, 2.2km3 rock and
sandstone landslide, the Usoy
natural dam has loomed above
the Murgab River creating
Lake Sarez, which today
stretches
back
60km.
Scientists in the former Soviet
Union propagated fears that
the dam was on the verge of a
sudden failure.
Over the past two years, a
team of European engineers
has been analysing Just what
threat the dam does pose to the
15,000 people who live
downstream. Engineers from
UK firm Jacobs Gibb, Swiss
consultant Stucky, Swiss
geologist Norbert and local
specialists form the heart of
this team, which is financed
by a Swiss trust fund managed
by the World Bank
“There are two phases to
our work on the project,” says
Stocky technical director,
Patrice Droz.
“First we must design and
develop a monitoring and
early system to reduce the
potential human losses should
he dam fall. And secondly
we are under-taking on going
studies aimed at finding long
term solutions to stabilising
the earth and rock structure.”
Stretching 3.75km across
the Murgab River, which
snakes through the Parnir
mountain range to the Afghan
border, the dam stands 65Om
tall. On one side of the valley
there is 370m between the
crest of the dam and the lake
waters, and on the other,
where the landslip originated,
there is 50m freeboard.
Little is known about the
internal composition of the
Wave propagation modelling dismissed historical fears that a
landslide into the lake would generate a massive surge, over-topping
the dam and devastating the valley below.
0.4g would produce less than
Usoy dam, but the wildly 100mm of movement in the
mixed and sharply contrasting dam.
Such
shifts
are
materials making up its considered negligible in so
exposed flanks suggest it is an large a structure.
unstable
hotchpotch
of
Of more concern is a
carboniferous
shale
and 0.9km3 landslip mass 4km
sandstone from 100mm up to upstream of the dam, perched
tens of metres in size. Also, precariously above the lake
Triassic material includes and inching forward at a rate
fragments from golf-ball to of 20mm per year. This is
truck size, set in a matrix of considered to be at high risk of
sandy silt.
slippage in the event of a
Large quantities of rock major earthquake.
dust are contained in the dam
“In the past, scientists from
make up, says Droz, and over Tajikistan and Russia feared
the past nine decades this that this could generate a large
cementitious
material
wavehistorical
that would
then
overtop
Wave
propagation
modellinghas
dismissed
fears
that
a
“cloggedInto
up”
in the
the a massive
dam, surge,
threatening its
landslide
th, voids
lake would
generate
earth structure,
maldng
it stability
the people
overtopping
and devastating
the valleyand
below
watertight the
anddam
causing
the downstream,”
says Droz.
level of the lake to rise.
“However, through detailed
The dam has a stable basic wave propagation modelling
geometry, with an upstream we have proved that the wave
slope gradient of 33%, and would not breach the dam’s
shallower 14% downstream, crest.” Uncertainties remain
making the risk of slip on over how the impact of a large
either slope minimal. “In wave might affect the internal
depth analysis has shown that stability of the dam, however.
the likelihood of a sudden
In generating an accurate
total collapse is actually very analytical model of the dam,
low,” reassures Droz.
engineers had a wealth of
Even under seismic loading,
historical data to draw on.
“Unfortunately this was all
the dam would still have a
high factor of safety, calculated recorded by hand and meant
that over 130,000 pieces of
at 1.0 for a peak ground
information had to be transacceleration of 0.4g.
The 1911 earthquake mea- ferred onto a specially
sured 7.4 on the Richter scale prepared Excel program,”
and is reckoned to have pro- Droz recalls.
The records showed that
duced a peak ground acceleration of 0.213g, with a return leakage from the dam only
period of over 130 years. began when the lake level
Analysis of ground dis- rose, he notes. “At a lower
placement that could be level the lake is impermeable,
expected
suggested
that with looser material above.”
ground acceleration exceeding Water levels are now rising at
an average rate of lS5nun per
year, but with the rate filtering
out through the dam wall also
increasing, Droz predicts the
lake level will ultimately
stabilise.
Another fear was that internal
erosion could cause partial cob
lapse of the darn, raising flow
rate by a factor of 20 and
leading to full collapse in
which the lake waters would
be released in an annihilating
torrent. But the model found
flow rate would be stable and
people downstream would
suffer only mild flooding,
claims Droz. The threatened
population will have ample
evacuation time.
Nonetheless, “the biggest
threat to the dam’s long term
stability is the changing
patterns
of
filtration,
characteristic of an immature
earth structure,” Droz says.
The lake’s discharge permeates through the dam and
springs into a canyon l00m
downstream of the dam crest.
With no borehole or other
kind of physical investigation
of the darn, its exact geophysical composition remains
unknown. “We have no idea
what the exact filtration
processes are or what they
might be in the future,” says
Droz. Unpredictable erosion
and potential pressure build up
within the dam could be
problems lying in wait, he
warns.
As part of the risk mitigation task a monitoring and
early warning system is
currently being put out to
tender.
For long term stabilisation,
the team of consultants is
investigating the merits of a
tunnel cut through one of the
dam’s abutments to reduce the
water level in the lake. “This
throws up a whole host of
added engineering considerations because the tunnel
opening would be 50m to
l00m below the surface of the
lake,” explains Droz.
But the expense is unlikely
to be justified in the present
cost-benefit analysis unless it
can be used to deliver hydroelectricity as well. Tajikistan
already has an advanced
hydroelectric power infrastructure, and there may be
potential to export power to
neighbouring states.
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