Relining Wag reservoir3 February 2014
The Wag Reservoir, part of Verbund's Hieflau hydropower project in Austria, loses just an espresso cupful of water a day thanks to modernization of the lining system by Walo UK.
The Wag Reservoir is an equalising reservoir, constructed as part of a hydropower plant over fifty years ago near the village of Hieflau in an extremely remote part of the Austrian region of Stryia, roughly equidistant from Salzburg, Vienna and Graz. The reservoir is operated by Verbund AG, one of Austria's leading electricity companies and one of the largest producers of hydroelectric power in Europe. Walo has a long standing relationship with Verbund, having carried out its very first Austrian project for the company back in the 1960s. Walo has also worked on other recent projects for Verbund, including carrying out the rehabilitation of a further reservoir near Salzburg and also undertaking remedial work of a headrace channel at the same Hieflau power plant.
The Hieflau power plant is the most powerful hydropower plant in Styria, with an annual generation of 364MWh. After operating for 50 years, a routine investigation by Verbund revealed severe deformation in the base of the reservoir, owing to erosion of the limestone in the substrate. In addition to this, technical requirements for large reservoirs in Austria have changed significantly since the original construction of the Wag reservoir so the drainage and leakage monitoring system also needed updating. As a result, Verbund decided that the reservoir required complete modernisation of the lining system, as well as a total renewal of the drainage and drainage monitoring system and the concrete structures.
The key to the new construction was to suit it to the limestone geology of the area and to be robust enough to last for generations to come. Verbund invited Walo to undertake the re-lining work, knowing that Walo's Dense Asphaltic Concrete (DAC) is particularly suitable for a limestone landscape like this, providing a strong, flexible, non-toxic lining, which is thin enough to flex under the extreme pressure exerted by thousands of tonnes of water without cracking, while still being totally safe for human, animal and marine life - making it ideal for water storage.
Undertaking the re-lining work at Wag Reservoir represented a very unique challenge on several fronts. To begin with, Hieflau is incredibly remote, being at least two hours drive from any major town, meaning that all construction equipment and material had to be brought a long distance, in some cases all the way from Germany, along narrow, rural roads.
There was also considerable time pressure to complete the modernisation work as quickly as possible before further deterioration of the old lining and the ground underneath could occur.
Furthermore, the notoriously wet and rapidly changing weather conditions in the area presented an additional hurdle, with rain falling in Hieflau on over half the days of the year, making carrying out any kind of construction project especially challenging. DAC in particular ideally needs dry conditions for optimum laying performance.
To make sure the works could be delivered with maximum efficiency and on time, Walo addressed the challenges by approaching the project with a tailor made strategy.
In order to reduce the construction time dramatically - cutting it to six months over one summer season rather than taking two summers to complete - Walo elected to commence the works as early in the season as possible. In addition, Walo deployed two asphalt paving groups operating at the site at the same time for six days per week.
Walo also installed an asphalt mixing plant at the site. This plant (Amman Uniglobe 240) was installed and operated by Granit Bauunternehmung mbH on behalf of Walo and was capable of producing 24 tonnes of asphalt per hour - up to a daily maximum of 2000 tonnes. This greatly increased the efficiency of the works as no time was wasted transporting asphalt to the site from an off-site plant. It also meant that the team could be flexible and make quick adjustments to the schedule according to any changes in the very unpredictable weather patterns - although in the event the weather was not as bad as it could have been.
Given that the quality and durability of the asphalt was of paramount importance to the project, Walo installed its own laboratory and laboratory technicians on site. This was key in facilitating and monitoring quality control on site, ensuring the consistently high standards demanded by the project.
Scope of work carried out
When construction works started, a control gallery was installed and the existing single layer liner on a 24m long slope with an incline of 1:1.75 was upgraded into a controlled (double lined) sealing system. The slopes of the reservoir covered an area of 49,500m2, with a basal area of approximately 136,300m2. After Walo had milled off the old lining, the re-lining work was carried out. This consisted of an 80mm thick DAC layer of some 42,700 tonnes, a 60mm thick binder layer of some 30,100 tonnes and a 9100 tonnes drainage layer of varying thickness. The narrow leakage tunnel which had provided access to the reservoir was replaced by two ramps.
Placing the asphalt
The asphalt was placed in the slopes utilizing a vertical placement methodology. Each paving group consisted of a slope finisher (paver) and attendant main winch as well as slope winch(es) and slope roller(s). For a vertical lining method the paver places a strip (mat) from the slope toe upwards. The main winch is placed at the crest and supports the paver on the slope by means of wire winch ropes. When the paver reaches the crest it drives onto the main winch ramp, which in turn moves sideways along the crest by the width of one strip (mat) and then the paver is lowered to the toe of the embankment. The cycle is then repeated until the slope is finished.
The material supply for the vertical lining was done via the crest road with a feed hopper transporting and tipping the material directly into the paver body. The feed hopper was operated inline with the paver and was also secured by the main winch. On slopes of short length the material loading into the paver can be done directly from the base of the embankment slope (toe).
Depending upon the placing machinery used, bituminous layers of up to 150mm thick and mineral or drainage layers up to 200mm thick can be placed in one pass. With the equipment available to Walo it is possible to place material in very narrow transverse vertical curves or steep and long slopes with lengths of more than 150m.
Asphalt lining works to the base were carried out in a conventional way by paver and direct feeding by asphalt wagons.
As with the placing works in the slopes, special attention was needed for the joints and the lateral areas of placed strips. The asphalt had to be placed as soon as was practicable against the adjacent/previous strip to guarantee that the joints were made "hot-to-hot" to ensure that when compacted the adjacent joints fused together to form one homogeneous sealing. It was also vital to ensure that the compaction/rolling of the laid material was carried out correctly to ensure the in situ material met all requirements with regard to air voids and permeability. The rollers used were designed for the compaction of soft and flexible DAC layers.
Attention to detail when placing asphalt in the slope and in the base was very important. The formation of joints and the specialist skills and equipment needed to place the asphalt create the difference between conventional asphalt work and that required to achieve the required exacting standards for placing asphalt essential in hydraulic engineering works.
Making joints impermeable
Special attention always has to be paid to the treatment of the joints between the placed strips once the material has cooled down - so called 'day-joints'. In order to ensure the impermeability and homogeneity of the 'day-joints', these sections were chamfered, cleaned, dried and coated by bitumen. Once cooled down, the asphalt adjacent to the joint area was carefully re-heated by infra red heating devices and then re-compacted by manually operated vibro-tampers. The joint, in the form of a 'wedge', forms an overlap between the two strips, which when recompacted guarantees a joint which is totally impermeable and extremely durable. Extensive testing of such joints has consistently proven that there is no difference in the quality and permeability of the area forming the joint to that of areas placed by a paver.
The interconnectivity between the structural concrete elements of the reservoir structure and the DAC was equally important to ensure the impermeability and flexibility of construction joints adjacent to the structures. Specific techniques and joints that have been specifically developed for hydraulic engineering were used. Such joints provide excellent performance for many decades. Differential settlement and movement of structures in the 'contact zone' (where flexible asphalt material adjoins the inflexible concrete elements of structures) had to be allowed for by the construction of such special joints. Depending upon local conditions, the following joint systems can be installed: joint with expanding element (for larger movements); joints under pressure; and joints without movement.
The expanding element of the joint normally consists of a corrugated copper sheet with a pre-formed loop. The copper sheet/jointing material has to be securely connected/anchored to the structure and the asphalt layer and the transition zone strengthened by an asphalt wedge. Prior to the construction of the joint, the area underneath and close to the concrete structure has to be compacted carefully and the asphalt itself needs further compaction. These areas are treated/ recompacted manually. The joint here was formed using a very durable and elastic bituminous material that will not deform and remains stable even on slopes.
The performance of the Wag reservoir has been transformed since the re-lining work was carried out. Thanks to the installation of a control gallery, any leakage can be observed and carefully measured. The entire 200,000m2 area now only loses the equivalent of a single espresso cupful of water every day - an extraordinary achievement.
Walo is a specialist in the field of hydraulic asphalt engineering and has been in the business of providing hydraulic asphaltic barrier systems for the containment of water all over the world for more than 60 years. The company specialises in providing impermeable barrier systems for a large range of applications including dams, reservoirs, landfill sites, hydroelectric infrastructure and artificial lakes. Working all around the world, Walo provides lining systems to dams and reservoirs of all sizes from relatively small projects to major hydroelectric plants.