AFTER 20 years of planning, the foundation stone-laying ceremony for the gravity dam at Leibis-Lichte, around 50km south of Erfurt, Germany, finally took place in September 2002. Since then, excavators and bulldozers have been hard at work in the Thuringian Forest near the holiday region of Rennsteig. By mid-2004, the dam had already reached upto some 70m towards its planned crest level of 102.5m. By the beginning of 2005, the second-largest dam in Germany, for a drinking-water reservoir with a capacity of 39.2Mm3, will have been completed.

The project, which will be completed well within schedule, is being built by the Leibis-Lichte dam joint venture – Bilfinger Berger AG, Oevermann GmbH & Co. KG, Bickhardt Bau AG, and Max Bögl Bauunternehmung GmbH & Co. KG. wasserkraft-volk AG, one of the worlds leading manufacturers for small hydro plants up to 10MW, delivered one Crossflow turbine with a capacity of 216kW and one Francis Turbine with a capacity of 488kW, as well as governors, switchboard controls and accessories for the project.

The structure
The concrete gravity Leibis-Lichte dam will be 80.6m thick at the base, 9m at the crest, and a height of 102.5m. This results in a total volume of over 620,000m3. The dam has an inclination of 1:0.78 on the downstream side, and is perpendicular on the upstream side.

Joints running parallel and at right-angles to the axis of the wall divide the groundplan into 35 fields. The joints are sealed by joint-sealing tapes. Three inspection tunnels – all accessed from the downstream side – pass through the structure at different heights, parallel to the upstream side of the wall. In addition, there are also circular inspection shafts running in the vertical.

A jet grout curtain, installed after construction, lessens seepage beneath the structure and through its valley flanks. The undersluice dewatering conduit consists of three DN 1200 steel tubes. Flood discharge is routed through a ski-jump spillway on the downstream wall leading into the stilling basin. Raw water is withdrawn through five inlets located at various heights. The two hydro power plants connected to the raw-water outlet and the dewatering conduit, with a capacity of approximately 1MW, further enhance the dam’s overall economic viability.

Every day, an on-site mixing plant delivers up to 2000m3 of low-heat mass concrete the short distance by lorry to the loading station of the two cableway cranes. The concrete skips have a capacity of 6m3. Despite the enormous 19t load, including the weight of the skip, and the 630m length of the cableway, the special cranes bring the concrete exactly to the point of placement in cycles taking only five minutes. The locally crushed aggregates, with a maximum particle size of 125mm, present a challenge to the Doka formwork system in use at the dam. Mini-excavators and bulldozers distribute and compact the earth-moist concrete within the blocks, assisted by the 150mm diameter hydraulic vibration probes fixed to the excavator booms and bulldozer blades. Rapid distribution of up to 975m3 of concrete in each block is only possible because the structure is non-reinforced, meaning that there is no reinforcing steel to get in the way.

Formwork logistics
The origins of the formwork technology for this dam go back to the time of the bid submission in autumn 2000. The Doka Leipzig branch presented the firms now comprising the joint venture with a formwork concept, drawn up in collaboration with the team from Doka HQ in Maisach and specialists from the Technology and Special Projects department from the Doka central plant in Amstetten, Austria.

Doka received an order for over 375m3 of formwork for the stilling basin in October 2001. A precondition of this order was that this and all subsequent formwork had to be supplied to the site ready to use.

Owing to the huge volumes of equipment involved, and the short lead-times, Doka installed a complete pre-assembly service facility at the site. This includes a 2500m3 ‘site-within-a-site’, complete with a fully equipped assembly hangar with 250m3 of floor-space. Three cranes and a fork-lift truck are in use here solely for assembling and dismantling the formwork. As many as 175m3 of custom formworks have to be assembled and set up ready for the first time of use. Until the trial filling at the turn of the year 2004/2005, sufficient equipment must be commissioned for forming an area of approximately 125,000m3.

The stilling basin has long been completed, and the formwork elements adapted for other assignments. With this element of the project, Doka wall formwork FF20, combined with Top 50 and supporting construction frames for pour heights of up to 7.5m and a fresh-concrete pressure of 50kN/m2, had to be assembled in a very short space of time.

The clean concreting results may be attributed to the use of Doka 3-SO 3-ply sheets, made of selected spruce timbers. Used on all the facing formwork for the Leibis-Lichte dam, this high-grade sheet ensures superlative concreting results. In spite of the coarseness of the crushed aggregates, and the extreme concrete-placing conditions, the formwork sheet manages up to 15 repeat uses.

Doka developed and fabricated two unique circular shaft-formwork sets specifically for this project . Formworks with a diameter of 1.3m, each designed as a formwork-half, are erected between the construction joints. In this way, the joint venture is constructing a total of 1450 linear metres of trafficable inspection shafts inside the body of the wall. Another 300 linear metres of shafts, this time only 0.8m in diameter, will house vibrating wire pendulum apparatus to measure the movements and deflection taking place in the dam. The formworks for these shafts are smaller, consisting of one part only, and are stood in the upright in the middle of the block. They are secured against toppling over, and are aligned to the finished casting section below, as it is not possible to fix them at the top. Both sets of formwork are 2.5m high. After casting, they can be detached from the concrete by turning spindles from above, and then easily lifted out.

1575m of inspection tunnels – 1.5m wide, 2.2m high and topped by a three-centred arch – run through the inside of the dam in both the horizontal and (to interconnect the different levels) the diagonal. During pouring, each side of the tunnel formwork is stayed against the opposite side by bracing-spindles, which make it easy to strip and remove the formwork from previously cast sections.

All these custom solutions are largely based on series components from the Doka formwork systems. This makes them easier to plan and use, and helps keep costs down.

The main quantities of formwork for this project are accounted for by the dam formwork for the downstream and upstream sides, and by the stop-ends for the block and field joints. This alone has required the planning, assembling and commissioning of sufficient formwork to form an area of approximately 111,000m3. After each assignment, the formwork is dismantled by Doka, and the returned equipment is given a quality-control inspection.

The elements are made up of Doka large-area formwork Top 50, and Doka dam formwork units or ‘Variabel’ supporting construction frames. Doka dam formwork units have a huge load-bearing capacity and adapt readily to every shape of a structure, such as slopes or kinks in walls – and even to the arched shape at the outlet to the stilling basin. With their ample, 2m wide platforms, they offer maximum workplace safety for the site-crew operating the formwork.

Doka ‘Variabel’ supporting construction frames can easily be combined with standard steel walings and other accessories to make heavy-duty single-sided supports for the stop-ends of the block and field joints.

Most of the blocks are 2.5m in height, but have different lengths and widths. For the biggest blocks, measuring 15 x 26m, 975m3 of concrete is required. A mobile crane relocates the formwork to the next section, and is then itself lifted from one block to the next by cableway crane.

Formwork planning
There are as many as four project engineers at Doka’s Leipzig Branch working on the project-specific detailed final drawings for the formwork systems. The detailed planning and fine-tuning is done by Doka project co-ordinator Jörg Otte directly on the site: ‘On a site as big as this one, with such tight deadlines and constant changes in the construction sequence, comprehensive all-round service is an absolute must.’