A new underground powerhouse that combines a run-of river scheme with a pumped storage scheme is planned in course of the Renewal of Forbach HPP in Germany. The powerhouse cavern is designed to have a maximum span and height of 18.9 and 42 m, and a total length of 123 m. The rock mass conditions were found favourable in general. However, comprehensive investigations revealed weakness planes crossing the cavern. The support system was determined by joint evaluation of empirical methods, wedge analysis and the 2-D finite element analyses. Due to the complicated geometry of the cavern and the orientation of the weakness planes, the effect of weakness zones on the stability and deformations could not be evaluated by only 2-D analyses. Implementing an additional 3-D analysis and comparison of the results with the common 2-D approach allowed a more realistic assessment of the rock behaviour, especially under the existence of the weakness zones through the cavern. A very detailed 3-D finite element model for the whole cavern is still not practical, so a relatively larger mesh was adopted. Nevertheless, a reduced model for a limited part of the cavern was also analysed by using a finer mesh. Obtained results were similar indicating the suitability of the adopted numerical models for the rock support assessment. The deformations and the size of the plastic zones were identified as significantly larger for weakness zones passing through the cavern. For these intersection areas additional rock support measures including pre-stressed anchors were therefore planned.
The Rudolf-Fettweis Plant by Energie Baden-Württemberg AG (EnBW) in Forbach has great potential to increase storage capacity and thus contribute to the energy revolution. The run-of-river, storage, and pumped storage power plant consists of four individual power plants built between 1914 and 1926. The two major units will be replaced by the new power plant.
In course of the renewal and extension, a new underground powerhouse is planned. The new powerhouse cavern will have a length of 123 m, maximum span of 18.9 m and a maximum height of 42 m. The cavern will be located approximately 325 m below ground in crystalline rocks of the Forbach Granite. The expansion of the equalising reservoir in Forbach to include a cavern storage facility leads to an additional storage volume and thus a significant increase in possible turbine operation. The project is currently in the approval process. In case of approval and a positive investment decision by EnBW, construction could start in 2022/2023 at the earliest. The general layout of the project is shown in figure 1.
