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Installing a sheet pile cutoff wall to slow leakage through a dam

Excessive leakage through the Vojmsjon Dam on the Angermanalven River in Sweden led owner to consider a renovation. The earth embankment dam has leaked since it was completed in 1950, but leakage has increased over the past few years, with 20 liters/sec of water leaking through the dam when the reservoir is full. To deal with this situation, Vattenregleringsforetagen chose to install a sheet pile cutoff wall with an interlock system.

Understanding the situation

Vattenregleringsforetagen is responsible for regulating water for power generation in the northern part of Sweden. In six rivers, the company manages water control at 130 reservoirs with a total volume of 20 billion cubic meters. This regulation provides the ability to produce 16.7 TWh of energy annually.

Options considered included installing secant poles, using jet grouting, a combination solution consisting of a rammed steel sheet pile wall and jet grouting, and the installation of a drilled sheet pile cutoff wall with an interlock system. The use of secant poles was considered too expensive. A major drawback with jet grouting is the shadow effect that can occur if large boulders are present. This would result in insufficient sealing of the dam.

Performing the work

A 170-meter-long drilled sheet pile cutoff wall is being installed with an interlock system to assure watertightness. The largest drill depth is 13.5 meters.

The casing system for the work was a SR/SF Robit Pile wall 324, and the sheet pile wall was a Ruukki interlock system. The piles were drilled and emptied, then filled with a mix of cement slurry/concrete and bentonite. The purpose of this slurry was to both seal off underneath the piles and to protect the piles from developing corrosion.

Some unexpected situations encountered during the repair work included the fact that drilling cuttings 2 to 3 cm in size were not brought up together with the return water coming up from the piles. The volume of this material was about 100 liters for a 13.5-meter drill hole. Because it was important that the piles were emptied completely, this material had to be evacuated by suctioning afterwards.

After the sealing work was completed, the leakage seems to have stabilized at about half the level experienced before the sealing operation. However, a longer period of time is needed to be certain of the results of the rehabilitation.

Lessons learned

It is important to perform thorough geological measurements/surveys of the dam body itself before the real rehabilitation work starts. This is needed to avoid underestimating the wear on the drilling equipment. In addition, because the consequences of not complying with the time plan in this kind of reservoir/dam rehabilitation project could be severe, ensuring spare parts availability and estimating spare parts consumption for drilling equipment is crucial.

This article comes from hydroworld edit released