Steeped in tradition4 June 2008
Flexible and robust construction was a necessity for the Leibis-Lichte dam in Germany. Michael Heiland, Lars Schaarschmidt and Thomas Roos explain why traditional block-type construction proved to be an attractive alternative to more modern RCC methods
Various dam construction methods were investigated prior to construction of the Leibis-Lichte dam in Germany. Investigations focused on the fast paced RCC technology which could facilitate cost savings of between 20-40% in contrast to conventional block casting. Although at that time, in 1994, 112 RCC dams had been built worldwide and 27 more were under construction: none had been built in Germany.
Successful RCC dam construction centres upon a sequence of highly mechanised activities. The key to a fast, economic and quality project is a simple RCC design which facilitates a smooth construction process without any interruption or even downtime. The effective scheduling of construction sequences involved with the RCC process – such as the inbound delivery of materials, concrete production, transfer to the casting positions, spreading and compaction – helps to facilitate continuous progress. Successful RCC dam sites should also be accessible and have large working areas.
Upon further analysis it became clear that Leibis-Lichte did not fulfil the essential criteria required for construction of a good quality RCC dam. Primarily, stringent safety requirements in Germany stipulated the provision of extensive built-in components to be included in most dam wall blocks. These allow for the installation of measuring and monitoring equipment to ensure sufficient surveillance of the structure. However, this places high demands on geometry and surface quality. At Leibis-Lichte such requirements included:
• 1300m of inspection galleries and connecting passages.
• 140 branch galleries and recesses to install various measuring devices.
• 1511m of vertical inspection shafts.
Furthermore, as the dam was located in the immediate vicinity of the tourist town of Unterweibach, pleasing aesthetics and a quality surface finish were paramount, and were other obstacles that RCC technology had to overcome.
The surface of a classic RCC dam is not smooth and a cascaded surface is built for each separate layer. In some countries even though the cascaded, porous and coarse surface leads to greater penetration of downstream surface water and accelerates weathering of the surface material, such a surface can be used for the spillway. However, this variation is not considered to be acceptable in Germany.
A good surface quality can be achieved through RCC by using additional measures and can be manufactured by separate formwork of concrete elements. Although RCC placing was already possible directly against formwork in the 1990s, enormous difficulties still existed when achieving complete compaction in the boundary areas. Bubbles visible at the surface required expensive treatment. The use of conventional form-vibrated concrete as a facing shell would have called for an expensive second concrete placing method. The procedures common at that time provided for the arrangement of vertical joints which, in turn, created bonding problems between the shell and core concrete.
Furthermore, there was no agreement on whether the construction joints needed treatment. It wasn’t clear whether any action had to be taken or if the application of a bedding course was required to improve bonding between every layer. Accordingly, there was considerable scepticism about the watertightness of the dam body, as well as concern about its performance in relation to frost protection for changes between freeze-thaw action.
Other barriers also had to be overcome in order to achieve a successful RCC dam. For example, smooth, uninterrupted RCC operations could not be guaranteed during construction. The dam is located in an environmentally protected tourist area and, under nature conservation laws, night time operations were prohibited. Another drawback was due to the topographical conditions of the dam site. The upstream face of the dam is truly vertical, while the downstream side presents an incline of 1:0.78. The relatively steep slopes would have led to considerable difficulties during construction and continuous adaptation of access roads would have been needed for RCC construction.
The main advantages of RCC technology compared with conventional construction methods include cost savings due to a shorter construction period, less expenditure on machinery and personnel, and savings on cement. However, the complex geometry at the dam site, a large number of built-in components, poor accessibility and the fact that the required aggregates had to be supplied by third parties over a long distance, contributed to less advantageous results. Cost savings were only expected to a lesser degree at <10% for Leibis-Lichte, in comparison to 20-40% for other RCC projects.
RCC technology might have shortened the construction period for Leibis-Lichte dam if the design had been simplified to meet RCC conditions. Such an approach was not possible due to strict safety requirements and scepticism about a construction method which had not been used in Germany. With this in mind, the only solution available to keep up efficiency and flexibility was conventional block type construction.
It is the transportation, placing and compaction methods that differentiate RCC dams from those built by conventional concrete placing methods. Typically, the latter dams are divided into dam wall sections which are separated by joints, and dewatered by inspection shafts. The dams are constructed section by section in blocks covering several working levels and several positions at a time. While formwork is placed at some blocks, built-in components are installed and specific formwork is mounted for inspection galleries and recesses for measuring equipment elsewhere. Depending on the capacity of the crane installations, concrete can be poured in parallel in a third work step. Careful joint design between the dam wall sections is a key requirement for this technology.
The body of Leibis-Lichte dam is divided into 35 dam wall sections, with each one 15m wide at the valley bottom and 10m wide in the slope area. Block length is between 10-30m, with a casting segment height limited to 2.5m each. To tighten the joints between the wall sections, 10km of expansion joint sealing strips have been placed at the upstream and downstream faces and at the dam’s foundation.
In every dam wall section, construction joints are dewatered by vertical inspection shafts extending over the entire wall height. The maximum shaft length is 90.85m with the overall length of all shafts totalling 1511m. Inspection galleries running parallel to the slope over the three levels ensure surveillance of the structure and allow the installation of various measuring and monitoring facilities. These inspection galleries were also used for performing underground grouting works, and are available for later grouting if required.
Five types of concrete were used to build the Leibis-Lichte dam structure. A compressive strength of B 20/90, high watertightness and a hardened concrete gross density of 2300kg/m3 were required for both core concrete and facing concrete.
The cement used was high-value CEM II/B-S 32.5 R NA made in Lafarge’s Karsdorf cement mill. The total quantities needed for the concrete amounted to 900,000t of split gravel and broken rock; 300,000t of sand; 100,000t of cement and 30,000t of fly ash. The water-cement ratios used were 0.64 for the core concrete and 0.59 for the facing concrete. A maximum grain size of 125mm and a cement content of 180kg/m3 (core concrete) and 240kg/m3 (facing concrete) were used in order to meet the requirements of mass concrete technology. Cement substitution by a maximum of 25% of hard coal fly ash was allowed in order to delay hardening and heat development.
In addition, fresh concrete temperature was limited to a maximum of 15°C. This was ensured by adding flaked ice, depending on the weather conditions. Additional heating or excessively fast cooling through the surface was prevented by placing blanket insulators.
In total, the concrete for the dam structure was cast in 1189 blocks over a period of 38 months. The concrete plant belonging to the site installations consisted of a double mixing plant comprising two compulsory double-shaft mixers. A concrete volume of 4.5m3 could be prepared in every mixing run, which took 60-90 seconds. A maximum workload of 2500m3 of concrete per working day was achieved.
The water for concrete mixing was obtained by a raw water pumping station on the Lichte brook, and stored in a feeder tank holding the volume required for a day’s production.
Concrete was carried by tippers with a carrying capacity of 6m3, a charging time of between two and five minutes, and a total transport time of five minutes to the bucket loading dock. Under optimum conditions, the concrete loading cycle between concrete preparation and placing could be reduced to a minimum of ten minutes.
The performance of the double mixing plant was rated at a quantity of 2 x 180=360m3/h in terms of fresh concrete output. After its transfer at the bucket loading dock, the concrete was carried to the casting positions and poured into the block formwork from a maximum height of 1m using 6m3 buckets and two radially mobile cableway cranes with a fixed point on the left and a spherically bent rail track on the right abutment. Track-type vehicles were used to distribute and compact the concrete. A CAT D4C type spreading Caterpillar with a track width of 2m and a blade width of 2.7m was employed for concrete distribution, while two CAT 307 hydraulic excavators of the Netter NVI 3 type ensured compaction using hydraulic compacting units.
A compacting unit consisted of a crossbeam holding three vibrating cylinders with a distance of 80cm between them. These cylinders had a diameter of 150mm, and were dipped into the concrete over a length of 0.9m. Their radius of action covered 1500mm.
Deutsche Doka Schalungstechnik GmbH supplied its proven D15/3 formwork system (5m element). Initially, the system comprised three consoles but it had to be reinforced to five consoles due to the high stresses existing on the downstream side. A difference was made essentially between downstream and upstream climbing forms, and section joint formwork. Special formwork segments were positioned in the corresponding blocks to create the recesses for inspection galleries and shafts. The use of flexible climbing formwork enabled work to progress following a chequerboard pattern, switching between the various sections according to their situation in order to make certain that a high degree of work could be performed in parallel. A concrete placing volume of between 7100-21,300m3 per month was reached depending on construction progress and weather conditions.
RCC technology provides an enormous competitive edge on an international level, particularly in global growth regions where the onus is on cost and construction schedules. In contrast, the block-type construction procedure is more adaptable and less susceptible to problems during construction. Nonetheless, at the Leibis-Lichte dam in Germany, the high capacity of the mixing plant and optimum coordination of crane and bucket delivery enabled a commendable placing rate using this traditional construction method. At times it was more than 30,000m3 per month in spite of a two-shift operation and elaborate built-in components.
At the time that the Leibis-Lichte dam was planned, this procedure was considered to be proven and fully developed in the central European region. It was also able to guarantee the high standards required for surface finish and dam safety monitoring.
The authors are Michael Heiland, CEO; Lars Schaarschmidt, Department Head of Hydraulic Design; Thomas Roos, Department Head of Construction Management, Hydroprojekt Ingenieurgesellschaft mbH, Rießnerstraße 18, 99427 Weimar, Germany
Leibis-Lichte is a gravity dam across the Lichte valley in the Thuringian Slate Mountains in Germany. It was constructed using traditional block-type methods between the spring of 2002 and the autumn of 2005. Its main purpose is to ensure drinking water supply, flood control and, to a lesser extent, to generate renewable energy. The plant has been equipped with two hydroelectric power units. A Francis turbine (510kW) is located in the raw water intake, and a direct flow turbine (310kW) has been installed in the bottom outlet bypass.
|Block-type construction has a long tradition in Germany. The following dams were constructed using this technology: Rappbode – 106m high (Germany’s highest dam in the Harz mountains, built in 1959). Pohl – 59m (Vogtland built1958-64). Wendefurt – 43.5m (Harz Moumtains, built 1957-66). Rauschenbach – 47.9m (Erzgebirge, built 1960-68). Dröda – 52.2m (Vogtland, built 1964-71). Gottleuba 65m (Erzgebirge mountains, built 1965-76). Eibenstock – 65.5m (Erzgebirge mountains, built 1974-1984).|