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Out of danger

Earthquakes Usoy natural dam

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 Bartang Valley in Tajikistan, situated in one of the world's most seismically active regions.

Since 1911, when the village of Usoy was buried under a 650m high, 2.2km 3rock 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 JacobsGibb, 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 Stucky technical director, Patrice Droz.

'First we must design and develop a monitoring and early warning system to reduce the potential human losses should the dam fail. And secondly we are undertaking ongoing 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 Pamir mountain range to the Afghan border, the dam stands 650m 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 Usoy dam, but the wildly mixed and sharply contrasting materials making up its exposed flanks suggest it is an unstable hotchpotch of carboniferous shale and sandstone from 100mm up to tens of metres in size. Also, Triassic material includes fragments from golf-ball to truck size, set in a matrix of sandy silt.

Large quantities of rock dust are contained in the dam make up, says Droz, and over the past nine decades this cementitious material has 'clogged up' voids in the earth structure, making it watertight and causing the level of the lake to rise.

The dam has a stable basic geometry, with an upstream slope gradient of 33%, and shallower 14% downstream, making the risk of slip on either slope minimal. 'In depth analysis has shown that the likelihood of a sudden total collapse is actually very low, ' reassures Droz.

Even under seismic loading, the dam would still have a high factor of safety, calculated at 1.0 for a peak ground acceleration of 0.4g.

The 1911 earthquake measured 7.4 on the Richter scale and is reckoned to have produced a peak ground acceleration of 0.213g, with a return period of over 130 years. Analysis of ground displacement that could be expected suggested that ground acceleration exceeding 0.4g would produce less than 100mm of movement in the dam. Such shifts are considered negligible in so large a structure.

Of more concern is a 0.9km 3landslip mass 4km upstream of the dam, perched precariously above the lake and inching forward at a rate of 20mm per year.

This is considered to be at high risk of slippage in the event of a major earthquake.

'In the past, scientists from Tajikistan and Russia feared that this could generate a large wave that would then overtop the dam, threatening its stability and the people downstream, ' says Droz.

'However, through detailed wave propagation modelling we have proved that the wave would not breach the dam's crest.'

Uncertainties remain over how the impact of a large wave might affect the internal stability of the dam, however.

In generating an accurate analytical model of the dam, engineers had a wealth of historical data to draw on.

'Unfortunately this was all recorded by hand and meant that over 130,000 pieces of information had to be transferred on to a specially prepared Excel program, ' Droz recalls.

The records showed that leakage from the dam only began when the lake level rose, he notes. 'At a lower level the lake is impermeable, with looser material above.' Water levels are now rising at an average rate of 185mm 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 collapse of the dam, 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 100m downstream of the dam crest.

With no borehole or other kind of physical investigation of the dam, 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 100m below the surface of the lake, ' explains Droz.

But the expense is unlikely to be justified in the 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.

INFOPLUS www. stucky. ch

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