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Waterproofing of the earth dam of the Melo power plant

June 02, 2023 6 min read

RD Pile wall drilling in Nokia Finland

Case details

Product RD® pile walls
Country Finland
Company KFS Finland Oy
Industry Building structures and infrastructure

The Kokemäenjoki river is one of the most important hydropower production areas in Finland. The Melo hydroelectric power plant in Nokia was built in the Kokemäenjoki river basin at the turn of the 1970s. The fall height of the power plant is 20 meters. In total, the dam is 123 meters long. The power plant has a capacity of 68 megawatts and an annual production of 200 GWh.

Case details

Product RD® pile walls
Country Finland
Company KFS Finland Oy
Industry Building structures and infrastructure
RD Pilewall Melon pato

Problems with the section of the earth dam were noticed in 2005 and it was repaired by injection and a steel sheet pile wall. However, the improvement measures did not guarantee a sufficient level of water tightness and in 2022 the earth dam of the Melo power plant was waterproofed using the SSAB RD® pile wall. The sealing of the Melo dam was carried out with 193 piles, of which the longest pile was 67 meters long. The piles are connected to each other by connector profiles, which are welded to the piles at the mill. Connectors are an important part of the pile wall, they provide a waterproof solution. In groove of connector is a factory-installed bitumen-based sealing material, which both makes the structure watertight and reduces friction at the installation stage.

The RD pile wall used in the improvement of the Melo dam is a permanent solution. The piles used in the structure are embedded and grouted into the bedrock. The total area of the pile wall was about 4500 m2 and the total length of piles was just under 8000 m. The lengths of piles in the RD pile wall of the Melo Dam are the longest piles ever used in RD pile wall and at the same time it is also the largest dam ever repaired by this method. There are several watertight RD pile wall structures and one of the largest can be found in the shopping center Mall of Tripla, to which SSAB delivered the piles between the years 2015-2016.

Over the years, the earth dam has been repaired by injecting first cement mixture and then bentonite clay into it, but the end result has not been as watertight as hoped. Other repair methods, on the other hand, would have required the reservoir to be emptied, and this was not possible at this site. By using the RD pile wall developed by SSAB for sealing, the repair work did not have an impact on the water level of Lake Pyhäjärvi above the dam or on the production of the power plant. The repair of the dam with SSAB RD pile wall produced a very good result.

The contractor at the Melo dam site was KFS Finland Oy. The project's construction and supervision consultant was Welado Oy. The project went very well thanks to the skilled operators and good cooperation. The project was started by widening the dam ridge to provide sufficient width for the equipment needed to install the pile wall. Drilling work on the pile wall began from the longest pile. The piles were extended to the required length from the elements of 18 meters, and the splicing was made by welding.

Welding of pilewall in Melon pato

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Melon pato RD pile wall drilling

At Melo dam, the topography of the rock is steep and it complicated the process of drilling the pile wall. When drilling the RD pile wall, a down-the-hole hammer was used, which operated on compressed air. Beside generating the percussion power, compressed air also lifts the drill cuttings up inside the pile pipe. The connectors attached to the piles have an injection channel, through which the annulus between the pile wall and the bedrock was filled with cement grout. In this way, the joint between the pile toe and the bedrock was made watertight. Finally, the dam was restored by cutting the piles about 0.5 meters below the ground level. The upper ends of the piles were protected by welding a 10 mm steel plate to them, after which the ends of the piles were covered with soil. The dam will serve as a pedestrian and bicycle lane once the repair work is completed.

The RD pile wall, which is installed by drilling from SSAB RD piles, is especially suitable for use in retaining wall structures in demanding soil conditions. Sites, where the structure is subjected not only to horizontal loads but also to vertical loads, are ideally suited for RD pile walls. The pile pipes are made from SSAB's own steel by spiral welding. The piles are available in several steel grades. By utilizing high-strength steel, the weight of the steel transported to the construction site can be significantly reduced. The same resistance is achieved with a smaller amount of steel.