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Shoring up Chardara dam

Flooding

Working out precisely what was wrong with the Chardara dam has been 'like a detective story', says Meldrum.

'There was a vibration problem with the spillway gates, which forced the operator to restrict opening to about 40% for fear of causing them to fail, ' Meldrum says. The expectation was that replacing the ageing gates would solve the problem, but it gradually became evident there were more serious problems with the structure.

'The gates were vibrating at a low frequency and very violently.' Limited spillway capacity meant that 'the reservoir was regularly being operated to its ood design level and above', says Meldrum.

There was a pressing need to get the spillway outlets back up to capacity.

Resolving the problem of the vibrating spillway gates requires some fairly simple modications. Arriving at the solution was far from easy, though.

Understanding what was causing the vibration involved monitoring the structure, carrying out diving surveys and hunting down old project drawings in the former Russian design institute in Tashkent.

'What they revealed was that the hydraulics of the structure were wrong. Water was rebounding from bafe piers in the spillway's two downstream stilling basins and hitting its roof and sides, stopping air flow. This was creating pressure waves behind the gates, causing the vibration, ' says Meldrum.

The afe piers were intended to remove energy from water as it passed through the spillway, preventing downstream erosion.

But they were oversized and badly proled for the flow volume and velocity.

Spillway inspections also revealed cavitation on the glacees, or slopes, from the gates to the stilling basin.

'They were the wrong shape - water was creating low pressure as it flowed over them and they had become seriously damaged.' The glacees were reproled, and smaller bafe piers designed. China Geo was contracted to carry out physical modelling to ensure performance before construction.

'To undertake its work, China Geo had to dewater the stilling basins, one side at a time.

'There's no way to isolate them individually, so we designed a caisson gate that could be oated into position and then sunk, to create a cut off barrier, ' says Meldrum.

This was by far the most complicated part of the project.

China Geo fabricated the vast steel caisson in sections in China and assembled it next to the river early last year.

'It was manoeuvred into place like a pontoon using cable winches, and they then let air out of the internal buoyancy tanks, very precisely, so that it tipped vertically and then sank in position to cut off the first pair of spillway gates.' As water was pumped from the stilling basin, props were installed to hold the walls apart, helping them withstand hydrostatic pressure from surrounding river water.

'We didn't know exactly how strong the structure's walls were - whether they'd hold up under external water pressure, ' Meldrum says.

Replacing the bafe piers and reproling the glacees involved simple demolition and insitu reinforced concrete construction.

The operation will be repeated to eliminate vibration in the other pair of spillway gates this year, enabling both sets to be fully opened.

But that is not the end of the story. The sand lled core of the Chardara Dam has suffered piping into a large 200m 3/second capacity culvert running through the dam, serving local irrigation schemes.

As a result, two sink holes had opened up in the dam's crest. These were being annually topped up by the operator.

Investigation of the culvert revealed that at least one of the joints between its 20m long concrete sections had fractured and opened up.

Phase one of the remedial works involved China Geo injecting 500m 3 grout under the culvert. This was followed by cone penetration testing last year to locate areas where material had been lost from the dam body into the culvert.

This revealed cavities under the concrete facing slabs on the dam's upstream face. 'It's a wonder that the embankment did not give way, ' Meldrum marvels.

This spring, high pressure compensation grouting will be carried out to 'densify' the embankment fill, he says.

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