New flood retention reservoir for Haslach-Ost

Euro 545 million for flood protection in Baden-Württemberg

Since 2004, the state of Baden-Württemberg has significantly increased its investments in flood protection and the expansion and renovation of dams; according to the Baden-Württemberg Ministry for the Environment, Climate and Energy, the investments total Euro 545 million, 47 million for the years 2013/2014 alone.

The state also uses these funds to subsidize the construction of two flood retention reservoirs in Hofstetten and Haslach-Ost to the tune of 5.6 million Euro (70 % of construction costs).

The LOGO.3 formwork system by PASCHAL is always there.

Overview of the almost 60 m wide construction site from the dam, which will soon reach the walls. Mühlbach creek is flowing from left to right. Here, the placing formwork is 10.20 m high.

Haslach (Ortenau district, Baden)

Repeated flooding had forced the town of Haslach and the neighboring community of Hofstetten to take flood protection measures. Even 5 - and 10-year floods represented a serious threat. The most practical and economic solution for protection from a hundred year flood is to build a smaller reservoir in Hofstetten on the Hofstetter Talbach (mountain stream) and a larger one near Haslach-Ost; other local extensions are also to be added. Construction of these facilities began in 2012.

The most interesting construction project from a structural engineering point of view is the one at Haslach-Ost, which completely blocks off the Mühlbachtal with a 360 m long earth dam of up to 8.8 m in height. In its rear, a flood retention area of 370,000 m³ is created, which covers an area of 12 hectares at maximum water levels.

Concrete structure and dam

The earthworks on the 35,000 m³ dam started in June of 2012. The fill was advanced to stop short of the culvert construction site and didn’t advance further for several months, because the dam may not be joined to the concrete structure and be completed until all work on the culvert has been completed.

Two dozen holes were drilled to set up a water control system that would keep the ground water level 2.2 m below the top level of the construction floor.

As lead construction company, Früh Ingenieurbau GmbH & Co. KG (Achern) began with the structural work on the two 1 m thick floor slabs made of C35/45 SM 31 of 243 m³ (with 32 tons of reinforcement) and 530 m³ (40 tons ) in July 2012. The culvert, which will be right in the center of the dam, consists of a 21.10 m long horizontal front section and a 32.40 m long sloping section split longitudinally into two 5.55 m wide parts, with one of the parts being one level lower and functioning as a spillway. The slope is 45 cm.

The two retaining walls of the culvert are sitting on the sloping part of the foundation and taper in thickness from 1.30 m at the base of the central area to 0.70 m at the top; the wings on the outer sides are also only 0.50 m thick. The conical walls are angled inwards at 2.86 degrees at the outer surfaces.

A small ring road is to run on top of the dam; a small bridge will be built a few meters downstream.

Closing formwork: Foreman Walter Lauber explains a job on the reinforcement to his people. The closing formwork is just being erected. Several formwork elements, put together into larger combinations, can be put up with a single crane.

Concrete and formwork

To keep the costs for the concrete formwork to a minimum and to optimally bring the amount of formwork in line with job site conditions, eight formwork cycles were chosen with an additional half a cycle of formwork modules being provided to work ahead. This method with only 442 m² of formwork promised the highest efficiency.

The LOGO.3 formwork system by PASCHAL was used, which was particularly well suited for this purpose – which is also borne out by the fact that PASCHAL has delivered formwork for many similar projects and no project teams had any problems with this solution whatsoever.

The majority of the formwork area was covered by large elements 3.40 m in height and 2.40 m in width. To provide the best possible formwork on a slope and to allow for surface treatment afterwards, elements of nine different dimensions were used overall, the smallest of which measured only 50 cm x 75 cm. The biggest pouring height of 11.90 m was achieved by using three 3.40-m elements and one 1.35-m element (each 2.40 m in width), the latter was in addition topped off with a 1.35 m wide and 75 cm high horizontal element at the sides.

Part of the formwork plan: cycles 2 and 3 have been poured, cycles 7 and 8 are encased. Forward planning of the use of all elements increases quality  and efficiency while reducing costs at the same time.

Supplier and consultant for concrete formwork is the PASCHAL-Werk G. Maier GmbH (Steinach).
PASCHAL formwork drawings are accurate to 1 cm. And the formwork can be adjusted to within 1 cm of the required architecture, including the tie points, whose counterparts must always lie exactly opposite for reasons of safety, force absorption and construction. With conically sloping walls, however, which taper both upwards and laterally by more than half the thickness, this can no longer be guaranteed. However, in practice the differences were so small that the tie points or safety requirements presented no problems.

The plywood sheeting was covered with rough-sawn boards.

Conclusion

Foreman Walter Lauber of Früh Ingenieurbau is delighted by the progress of work and especially praises the services of the responsible professional adviser. Working with the formwork was easy, it was very flexible, he said. And the means for connecting the formwork elements, the LOGO wedge clamp, is better than that of other providers.

Conical walls: The outer walls of the culvert, which will be in the middle of the dam, are angled inwards at 2.86 degrees. The side wings (in the rear of the picture) are strongly tapered. The base plate also has a noticeable slope. This presented no problems for the LOGO.3 formwork system and Früh Ingenieurbau.