16.3 The Waldeck II pumped storage power-station
16.3.7 The design of the underground works (fig. 16.11)
The first designs for the Waldeck II machine hall were on conventional lines, with rigid concrete arch and sub-vertical walls (fig. 16.14). Conventional excavation methods were considered. Then the possibility of a design based on
surface, at 18 m and at 30 m from the excavation boundary, as calculated by the finite element method (according to Zienkiewicz).
Fig. 16.14 Waldeck II; successive designs (after Abraham, 1974 and Siemens).
(a) Conventional design with concrete arch and nearly vertical walls; (b) anchored cables supporting rock vault. (Figure 16.11 shows the final design as adopted by designer.) Cavern cross-sections of 33.40 m width and 46 m height.
Waldeck II 457 the new Austrian tunnelling method (NATM) was investigated. Such a design consisted in supporting the rock vault and walls with deep anchors. It was then decided to use the 'observational method' advocated by R. B. Peck (1969). The design would be based on 'average' rock quality. Alternative designs would be worked out in detail for more severe rock conditions.
Systematic measurements would check rock stresses and strains, rock de- formations and, if there were signs of excessive strains, the design would be altered. Agreements on this method were reached by all concerned: the Preussische Elektrizitat, the designers and the main contractor.
Few examples of very large excavations supported with cables and ancho- rages (NATM) were available, Veytaux (Switzerland) was a recent one and information from this site was very useful. For Waldeck II the concrete arch over the machine hall was abandoned, the rock vault had to be given a shorter radius and a nearly circular shape to decrease stresses. Another impotant problem concerned the tensile stresses along the vertical side walls of the excavation, which were apparent on the finite element method analysis. They could have been balanced by increasing the horizontal pressure on the walls with a higher density of anchorage cables. The designers found it more convenient to give an ovalized shape to the whole excavation (fig. 16.11).
The shape of the cavern had to be adapted to the mechanical equipment of a medium-head pumped storage plant, located in the centre of a large electric distribution net, and having to face sudden and extremely rapid change-overs from generating to pumping conditions and vice-versa. Such conditions called for two independent runners for the turbine and for the pump, located on the same very high vertical shaft. (Veytaux, with a much higher head has the runners arranged on a horizontal axis, which basically modifies the design of the cavern.) The problem was the location of the pump valves. A flat bottom of the machine hall causes high tensile stresses to develop under the machine sets. Some designers suggest a balancing of these stresses with the weight of the machines and the foundation concrete. For Waldeck II the pump valve was located under the pump, which allowed a better shaping of the cavern, as shown on fig. 16.11.
A possible arrangement for the transformers consists in locating them in an excavation parallel to the machine hall (Ruacana, on the Cunene River, South West Africa). Electrical connections between generating set and trans- former are reduced in length. But the finite element method shows how the two large excavations react one on the other. For Waldeck II it was decided to locate the transformers in a cavern in line with the machine hall.
The whole design required a careful balance between hydraulic and mechanical requirements and the conditions imposed by the rock mass structure.
Similarly the whole excavation programme was checked on the computer.
It was found that sharp corners caused very dangerous stress concentrations in excess of the rock crushing strength. Figure 16.12 shows the excavation
programme which was finally worked out to satisfy stress-strain conditions and the possibility of anchoring cables in deep boreholes.
The pilot gallery, at the top of the cavern excavation was 6-50 m high and 17 m wide. Blasting occurred every 3 m. Immediately after blasting 2-m-deep small anchors were grouted using epoxy resins in small boreholes to support locally unstable blocks of rock. A 3 to 8-cm-thick shotcrete layer was con- solidated with a wire mesh. Then 12-tonne, 6-m-deep anchors were grouted in a regular 1-33 x 1-50 m pattern. These anchors were part of the final rock support scheme and had to be protected against rusting in the same manner as the heavy anchors. These 170-tonne anchors, 19 m to 28-50 m deep could be cemented in the 116 mm boreholes and tensioned only after a length of 40 to 50 m pilot gallery had been excavated. The boreholes were drilled on a regular pattern 4-0 m x 3-0 m, but the directions of the boreholes were decided on the spot, according to the jointing and structure of the rock. Figure 16.15
24 mm
in 116 m m -
anchor for adherence (ZR 377)
precast - concrete head
backfilling77 Fig. 16.15 Detail of prestressing anchor for Waldeck II (Abraham, 1974).
shows details of an anchorage. The possibility of requiring intermediate anchors to be fixed had been foreseen from the beginning, but there was no need for any local reinforcement. Table 16.3 gives the number of heavy 170 tonne and 130-tonne anchors which were required for supporting the main machine hall.