Dam, It’s Sensitive: The Case Of Melo Hydroelectric Powerplant In Finland

Mincon’s Geotechnical Centre in Ylöjärvi, Finland has a very special work site in its backyard: the Melo hydroelectric powerplant and its adjacent embankment dam, which produces clean and renewable energy in the city of Nokia. It is also a place where Mincon’s expertise was used for renovating the dam, to help it remain in excellent condition for the generations to come. The powerplant is owned by PVO-Vesivoima Oy, a part of Pohjolan Voima Group, which is one of the biggest energy companies in Finland. Originally built in 1971, the 140-metre-long dam has recently been found to have water slightly seeping through. This issue was first repaired by injecting bentonite and concrete into the cavities of the dam.

Following this – to ensure the dam’s integrity into the future – engineers proposed that the dam be reinforced with a watertight, steel pipe-pile wall. KFS Finland, a specialty ground engineering company, was selected as the constructor. The most obvious challenge during the project planning was to avoid disturbing the soil masses of the dam during the renovation: conventional drilling would risk discharging high-pressure air into the surrounding ground and possibly weaken the dam.

Another challenge was posed by the topology of the area: the granite bedrock under the Nokianvirta river forms a deep V-shaped ravine and this meant that any drilling would have to be performed on a very steeply inclined rockface. Without precise drilling using the best tools it would be difficult to maintain the hole straightness required for the success of the project and where Mincon’s drilling expertise in sensitive and complex ground conditions came into play. The team at the companys Geotechnical Centre was asked to offer an interlocked retaining wall solution that would ensure the safety of the dam while still being cost-effective and quick to deploy. The solution chosen by engineers at KFS and PVO turned out to be Mincon’s integrated M-Wall ring bit casing system, which is purposely designed for building pipe-pile walls safely and quickly – even in technically demanding ground conditions like those in Melo.

The company’s Spiral Flush technology was a natural fit for the project, as it allows air control for high-pressure DTH drilling in highly sensitive ground conditions with the unique design ensuring that air and cuttings flush into the casing without escaping into the soil.

During project planning, engineers determined that drilling 508 mm pipe piles would be the sweet spot: a size that allowed for optimised steel usage, which was more practical and kept build time to a minimum. It was also an excellent opportunity for the new MP150-MC hammer to be used, as it was designed to deliver unbeatable drilling speed in rocky ground and bedrock. And speed, it turned out, was a key factor in the success of this project. Since the deepest piles reach 67 m into the ground – totalling 7,867 metres of drilled piles – every second saved per drilled metre added up in the end. Equally important was the robustness of the solution: drilling had to proceed no matter what was encountered beneath the surface.

Another benefit of the MP150-MC hammer was in its design: it supports drill bits that don’t have a foot valve, which eliminates the risk of expensive downtime should the foot valve fail. The delivered hammers were already coupled with Mincon’s SA15 shock absorbers at the factory, offering further protection for the drill rigs that drive these large hammers. And, of course, Mincon drill rods were supplied to round off the package for drillers at Melo.

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