Large dams |
Above: Overview of Trängslet dam Photo: Fortum
maintenance and dam safety measures From construction to operation,
– 1980s to today In the 1980s, the hydropower industry in Sweden entered a period of dam operation and maintenance that continues to this day. During this period, some deficiencies in construction and design were discovered, necessitating the reinforcement, modernisation, or reconstruction of many dams. The most significant of these deficiencies were discovered in the early 1980s, when it became apparent that the principles used to determine the design flood during the hydropower expansion were inadequate, resulting in dams with insufficient discharge capacities. This issue was investigated after high flows in 1983 and further highlighted when the Noppikoski dam failed in 1985. To address this problem, the first guidelines for determining design floods were issued in 1990, and a nationwide re-evaluation of design floods followed. As a result, most of Sweden’s large dams have been upgraded to withstand and pass larger design floods than their original design. The projects typically include the construction of new spillways or the reconstruction of existing ones in combination with the raising of the core of embankment dams to allow temporary surcharge during extreme floods. For embankment dams, the rapid pace of expansion during the 1970s meant that speed and economy sometimes had priority over best practices. As a result, many dams were constructed using faster, simpler, and cheaper methods and design solutions. In the 1980s, the combined effect of these shortcuts caused problems such as sinkholes and leakage incidents. Large investments have been made since the 1980s to increase the safety of embankment dams, with the dominant method being strengthening the dam by adding a rock fill toe berm along the downstream dam toe and part of the downstream slope. All three dams presented in this paper - Porjus, Storfinnforsen, and Trängslet - have a rock toe berm. The combination of relatively thin concrete dams and significant temperature variations between
24 | May 2023 |
www.waterpowermagazine.com
summer and winter in Sweden has resulted in temperature-induced cracks. Safety measures have been implemented to address this issue, including installing insulation walls on the downstream surface to limit crack propagation and post-tensioned tendons to increase the dams’ stability. A different set of challenges arose for the mining industry during this period. As production increased, so did the volume of tailings that had to be deposited, leading to larger construction volumes and higher costs associated with the raising of existing dams using the downstream method. Simultaneously, changes in legislation and environmental constraints limited the use of till and added requirements of staying within the same footprint. Therefore, in the 2000s, the main design principle shifted from constructing impermeable water dams to drained upstream construction. This led to the development of dams with an impermeable starter dam and upstream raises, sometimes founded on fine tailing and slimes that had settled underwater. In recent years, the global lessons learned from several catastrophic failures of tailings facilities have once again caused a change in the method for raising tailings dams in Sweden and led to the implementation of new measures to safeguard their stability.
Three examples of Swedish dams We have selected three dams, Porjus, Storfinnforsen,
and Trängslet, each representing a different era in the development of Swedish dams. Porjus hydropower dam in the river Stora Luleälv represents both the early and late eras. The dam was originally completed in 1914 as one of the first large embankment dams in Sweden. The dam was built with a “belt-and-braces approach” featuring double inpermable zones, both a dense core of puddled clay and a downstream placed 25cm thick concrete diaphragm of reinforced concrete. The dam functioned without problems until 1970, when it was heightened and partly replaced by a higher embankment dam of modern design built downstream to allow the raising of the reservoir. Upgrading measures were carried out in 2006-2008,
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53
