Case Studies
Enlargement of the Tierfehd Reservoir, Switzerland

The pumped storage hydro power station at Glarus is being expanded. One element of the project involves enlarging the capacity of the equalising reservoir in the valley.

To achieve this, WALO is working on providing a second equalising reservoir to the north of the current one. A channel linking them will allow water to move between the two so their levels correspond. The existing terrain forms the east side of the reservoir while a dam is being raised on the west side of the new reservoir. The crest of the dam will be 813 metres above sea level while the base of the reservoir will be 11 metres lower. The new reservoir is located within a groundwater protection zone which means the work has to meet a number of special requirements. Once the profile of the new had been prepared by the excavation contractor, it was covered with a layer of filter ballast. A binding layer was then applied followed by a sealing layer and a coating of mastic which protects the reservoir lining against the elements and UV radiation. An asphalted road on the crest of the dam has also been built to allow access for operators and maintenance staff during the service life of the facility.

Cottonmount Landfill, Northern Ireland

The Cottonmount Landfill Site is located in Mallusk, County Antrim in Northern Ireland, just outside Belfast. The site is located within Cottonmount Quarry, a working hard rock quarry, which receives municipal waste from Belfast City. WALO was appointed to construct three landfill cells using its Dense Asphaltic Concrete (DAC).

Given the proximity of the site to residential areas and to underground aquifers, it was of paramount importance that the landfill cells were constructed with an extremely strong lining that would remain impermeable to leachate or methane from the waste. The works had to be undertaken during extremely challenging weather conditions in the depths of winter. WALO’s DAC is highly robust and impermeable so is ideal for complete containment of all materials that need to be impounded such as landfill. It is also very quick to lay, making it the perfect solution for a project like this which had to be carried out within a limited time frame in very wintry conditions. WALO constructed the cells as part of a composite lining system of some 17,800 m² using DAC formulated in the company’s Zurich laboratory to a design mix unique to the project. The lining system comprised a 300 mm mineral attenuation layer; 200 mm sub-base material layer; 60 mm asphaltic binder layer, 80 mm dense asphaltic concrete layer and a mastic seal coat to the base, side slopes and bund areas. The works involved the placing of asphalt by paver and winch on a 40 m slope. Once the lining was completed, a 300 mm thick layer of leachate drainage stone was applied to the whole area.

Avče pump storage scheme, Slovenia

The Avče pump storage scheme has a turbine capacity of 185 MW. The lower reservoir is formed by the river Soča, with an associated upper reservoir 520 m higher, sited in a natural depression. The ground under the reservoir is calcareous, formed of karst formations, so the reservoir had to have an impermeable lining system.

After the initial earthworks and concrete structures were constructed, WALO commenced the lining works. The works consisted of an Asphaltic Binder Layer (ABL) and bituminous Dense Asphaltic Concrete (DAC) layer, the first project of its kind in Slovenia. Like every WALO project, the team applied DAC formulated to a unique design mix using specialised laying and compaction techniques. To comply with the rigorous required Quality Assurance regime, WALO installed its own asphalt laboratory at the mixing plant to control the quality of the asphalt. During the lining works the WALO team alternated the placing of the different materials throughout the project, dependent upon the prevailing weather conditions. This degree of flexibility enabled the optimization of the scheduling of the works. All works – including joint construction, ramps and access roads and mastic coating, a total area of 140,000 m² – were completed after only four months.

Soelden Snow Reservoir, Austria

The Soelden Snow reservoir in the ski resort of Soelden, Tyrol in Austria was built to store water to supply snow cannons for the production of artificial snow. At an altitude of approx 2,930 m above sea level, close to a glacier, it has a usable storage volume of some 400,000 m³. To prevent the loss of water through leaks and damage due to weathering in extreme temperatures, the most impermeable, strongest, yet still flexible liner possible was required and so WALO was commissioned to line the reservoir with its specially formulated Dense Asphaltic Concrete. 

The construction schedule had to be very flexible, owing to the changing weather conditions at such high altitudes. Even in the summer, more than two metres of snow fell at the site. The nature of the site, located in an area popular with winter skiers and summer hikers meant that the reservoir needed to blend in with its environment as far as possible. The formation level of the slopes was covered by a drainage gravel layer placed by paver. The complete surface of the reservoir was then surfaced with a Asphaltic Binder Layer, Dense Asphaltic Concrete DAC layer and a mastic coat. The surrounding top five metres of the slope was covered with local stone chippings rolled into the mastic layer to help the reservoir blend into the natural environment and give it a ‘natural’ finish. The dedicated WALO team’s experience of working with highly specialised equipment at high altitudes in all kinds of weather, meant that the asphalt lining works finished on time after an intensive four month period where 55,000 m² of ABL and DAC were laid, despite the climatic challenges.

Sulby Dam, United Kingdom

Sulby Reservoir dam was originally designed to be constructed in two stages, with construction commencing on site in 1979. The first stage dam was to have been 35 metres in height and was to be completed before the summer of 1982. The second stage was to have involved raising the dam by 25 metres when water demand had outgrown the yield available from the first stage reservoir. In the event it was decided by the Manx Parliament, Tynwald to fund the construction of the full height dam immediately and to use the surplus water available in the early years to generate hydro-electric power. The dam was completed in 1982. The reservoir’s capacity is over 1,000 million gallons. WALO UK’s Dense Asphaltic Concrete DAC forms the impermeable membrane which seals the upper part of the dam. The original first stage of the dam was provided with a 750 mm thick central DAC core constructed by another specialist contractor. The core is connected to the DAC membrance which WALO constructed on the upstream face of the second stage of the dam.

The choice of a combination of asphalt core and membrance as the main waterproof membrane was greatly influenced by needing to construct the dam in two stages. It was also most satisfactory during construction, enabling the deepest section of the cutoff to be located in the soundest part of the rock foundation and allowed the placement of the main fill material of the upper part of the dam to be completed without the impediment of having to keep core and dam fill in step with each other. Loss of water through fissures in the rock foundation is limited by a concrete filled cutoff trench, excavated 2 to 3 metres deep into sound rock across the valley beneath the dam. The rock beneath was sealed by means of cement grout being injected into a closely spaced curtain of holes drilled to depths of up to 50 metres. The asphalt core and membrane are sealed onto the concrete cutoff. In 1992, at the first independent inspection of the reservoir the reservoir water level was lowered to allow the whole membrane to be inspected. There was no evidence of any significant deterioration. The reservoir was last independently inspected in 2007. The Inspecting Engineer was happy with the condition of the membrane, and subsequent supervising engineer inspections have identified no changes to give rise to any new concerns.

Mastic Renewal of Upper Reservoir at La Muela, Spain

The Pump Storage Scheme at La Muela in Valencia is the biggest of its kind in Spain. The upper reservoir has a circumference of 4,800 m and stores 23 million cubic metres of water, generating a similar amount of energy to that of a nuclear power plant. A major upgrade programme is currently underway, consisting of the construction of a new underground power house and tunnels and penstock. As a shutdown of the plant had to be scheduled to undertake the upgrade, WALO was commissioned to carry out a mastic renewal of the upper reservoir, which covers an area of 1,400,000 square metres. The particular challenge facing WALO was to undertake the work after only partly emptying the upper reservoir so the work had to be carried out above the water, avoiding any contamination of the water remaining in the reservoir.

This meant that the huge amount of sediment on the slopes, which was cleaned off by WALO had to be removed from the site by truck, without any of it being washed into the reservoir water below the work area. To add to the challenge, the only access to the site was via the crest road. This presented a huge logistical problem as that route could easily be blocked by just one works vehicle so the movement of materials and work teams was very restricted and had to be carefully planned. After a scheduled shutdown of the Pump Storage Scheme, the upper reservoir was partly emptied so only 25% of the water was left and a new mastic coating was installed by WALO. The works consisted of cleaning the existing bituminous face by hydro-jet washing and removing the sediment on the slopes, spraying a new bitumen emulsion and applying a new mastic sealing to the 43 m long slopes which have an incline of 1:1.6.

Lining Terroba Dam, Spain

The Terroba Dam impounds the winter rainfall to enable constant irrigation of the fertile valley during the drier spring and summer months. The initial plan for the dam included an embankment dam with a clay core. However, clay is in short supply in the surrounding area so sourcing and transporting clay to the site would have proved prohibitively expensive. An alternative technical solution was required and so to solve the problem, the design was changed to a bituminous upstream lining system. As the design had been so radically changed, the earthworks were completed much more quickly so far more economically. Also, by substituting clay with a more sophisticated option of a bituminous lining, the material costs for the project were considerably reduced. WALO, with its specialist Dense Asphaltic Concrete (DAC) capability, was selected to provide the lining system, given that DAC is ideal for projects like this, being highly robust yet flexible and quick to lay, even on very steep slopes.

The work also included a 425 m joint construction to the plinth and abutments and 305 m joint construction to the parapet wall at the crest. This meant the WALO team had to bond together securely two very different materials – the flexible DAC and the rigid concrete. This kind of highly specialised technical work can only be carried out successfully by exceptionally skilled and experienced craftspeople. The bituminous upstream lining system to the dam constructed by WALO covered a total surface area of 23,100 m², with a steep 94 m long slope with an incline of 1:1.8. The lining system consisted of 150-200 mm of crushed aggregates, 2 kg/m² of emulsion spray applied tack coat, an Asphaltic Binder Layer (ABL) of 70 mm and a DAC layer of 60 mm and 2 kg/m² mastic coating.

Rehabilitation of Ulmbach Dam, Germany

The Ulmbach Dam (built in 1964/5) is located in the Lahn-Dill area, north of Frankfurt, Germany. It provides flood protection and is also a tourist destination, attracting fishermen, walkers and campers. 

Although the dam showed no signs of leaking, the client inspected the installation regularly in order to keep it maintained according to current technical and safety requirements. An inspection identified some minor issues with the dam face – partial cracks in the asphalt lining and weathering damage to the mastic protection layer. The leakage detection system also needed updating. Dense asphaltic concrete is dense, durable, flexible and robust enough to resist mechanical, chemical or biological attack. It is also safe for humans, fauna or flora, making it ideal for dam lining. However, like any other structure, after decades of operation, maintenance and rehabilitation will be needed. Experience has shown that bituminous linings in basal areas or areas permanently under water do not show any signs of ageing. The ‘water exchange zone’ – the area where the water changes height – is the area which over time may become weathered. This was the situation at the Ulmbach Dam so just the upper part of the dam and the surface of the dam face needed rehabilitation.


The Challenge

The flood protection function of the dam had to be maintained throughout the project so the dam had to remain secure the whole time. The dam crest is 280 m long and the slope area to be rehabilitated was 37 m long and measured 6,400 m², with an incline of 1:1.8. The client also decided to improve the overall structure by reshaping the crest area, to reduce future maintenance costs.


The Project

All the water was removed for the modernisation and rehabilitation works, although the Ulmbach river continued to flow through the bottom outlets. WALO milled off 10 – 20 mm of the old asphalt seal, leaving 40 – 50 mm to keep the dam secure in case of a flood. WALO installed a new controlled lining, including integrating an innovative stainless steel tube drainage system, which divided the dam into four sections for easy monitoring. This means any leaks can be readily located and the amount of leakage easily measured. A new 80 mm asphalt binder layer and a new 60 mm sealing layer were installed using a slope paver. In addition, the joints to the concrete structures and crest were also renewed and a new wave breaker and guardrail were installed.

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