Excessive leakage through the Vojmsjon Dam on the Angermanalven River in Sweden led owner Vattenreglerings-foretagen 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 steel pile cutoff wall with an interlock system. Since this work was completed in May 2013, leakage through the dam has been cut in half.
|Down-the-hole water-powered drilling was used to install a 170 meter-long drilled steel pile cutoff wall at Vojmsjon Dam in Sweden, to minimize the risk of disturbing the dam and subsoil.|
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.
Vojmsjon Dam is classified as a high-risk dam due to the major damage that would occur in the event of a dam breach. The dam is needed to control the flow of water from snowmelt in the surrounding mountains. It is filled in the spring and summer and releases water in the winter to power 13 hydropower stations located downstream. At peak flow, the dam is filled in two weeks.
The dam has leaked since it was completed in 1950, and leakage is collected in a small dam on the downstream side, where it is measured continuously in a Thomson weir. Leakage through the dam has increased over the past few years, rising to a rate of 20 liters/sec when the reservoir behind the dam is full.
In 2011, Vattenregleringsforetagen assigned Sweco of Stockholm to investigate options for reinforcing the dam.
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 steel 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.
The dam has undergone sealing, performed by Lemminkainen Oyj of Finland under a €2.3 million (US$3 million) contract awarded in November 2012. The time frame for the dam rehabilitation was only two months, so the dam would be available to impound water during the spring flood. The work took place from March to May 2013.
Performing the work
A 170-meter-long drilled steel pile cutoff wall is being installed with an interlock system to assure watertightness. The largest drill depth is 13.5 meters.
The drilling method chosen was a key factor in minimizing the risk of disturbing the existing dam and subsoil. Vattenregleringsforetagen asked Sweco to choose the best technology for the situation, and they recommended down-the-hole water-powered drilling by Wassara in Stockholm. Compared to the other methods, a water-powered hammer copes easier with drilling through boulder-rich formations (identified during test drilling) and has a comparatively lower total cost. In general, water-powered drilling requires less energy, as the compressing of air requires more energy. There is also a cost saving when compared to core drilling, as water-powered drilling has a rate of penetration that is up to five times higher.
A water-powered DTH hammer W200 was used, in temperatures down to -20 degrees Celsius. This equipment provides a hammer length of 2,055 mm and a standard hole size of 216 to 254 mm, with a maximum operating pressure of 150 bar and water consumption at maximum pressure of 460 to 670 liters/min. The interlocked RD piles were 323 mm wide and drilled down. Water needed for the drilling was taken from the lake, using a dewatering pump supplied by Halliburton that was mounted on a barge.
The casing system for the work was a SR/SF Robit Pile wall 324, and the steel 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.
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.
– By Ola Hammarberg, hydrolo- gist and water resources planner, Vattenregleringsforetagen