The Mauranger Hydro-Electric Power Plant in Western Norway utilizes a 455 metre static head unlined pressure shaft. The paper describes the investigations performed and the criteria applied for the design of the shaft.
Die Wasserkraftanlage Mauranger in West-Norwegen nuetzt das Wasser mit einer statischen Druckhöhe von 455 m durch eine unverbaute Druckschacht aus. Es wird ueber die ausgefuehrten Untersuchungen berichtet zusammen mit den angewandten Kriterien fuer den Entwurf der Schacht.
Le complexe hydro-electrique de Mauranger dans la Norvège de l''ouest comporte un puits sans revêtement sous pressión hydrostatique de 455 mètres. Le present article decrit les reconnaissances effectuees et les critères de design pour le puits de pression.
The Folgefonni Hydro-Electric Power Project, developed and owned by the State Power Board, utilizes run-off from the great Folgefonn glacier in Western Norway. The Project comprises the Jukla Pumped- Storage Station (Bergh-Christensen 1982) and the Mauranger Power Station, as shown in Fig. 1.
The general layout of the Mauranger Power Plant is indicated in Figs. 2 and 3. It includes a 20 m2 unlined pressure shaft with a maximum static head of 455 metres.
(Figure in full paper)
The utilization of unlined tunnels and pressure shafts in hydro-electric power plants has long traditions in Norway. More than 50 unlined shafts or tunnels with heads higher than 150 metres have been put into operation (Broch and Selmer-Olsen 1982).
Detailed planning of the Mauranger project started in 1970. The failure of a 300 metre static-head unlined pressure shaft at Brokke Power Plant in 1968 (Selmer-Olsen 1970), and then the failure of a 200 metre static-head unlined pressure tunnel at Åskåra Power Plant in 1970 (Bergh- Christensen 1975), indicated the need for detailed geological investigations for this type of project, as well as the need for re-evaluation of the "rule of thumb" design criteria applied up to then for siting of unlined shafts and tunnels.
Acting as engineering geological consultant to the State Power Board, A/S GEOTEAM was responsible for all site investigations and rock mechanics design evaluations performed prior to and during the construction of the Mauranger pressure shaft.
As the first step in the field investigations, a geological survey of the site was performed, followed by detailed geological logging during excavation of tunnels and shafts. The rock is a Pre- Cambrian granitic gneiss. The head-race tunnel and the upper 70 metres of the pressure shaft are cut by a series of faults containing swelling clay gouge. The close proximity of a road tunnel called for extensive sealing and grouting works to be performed in these parts of the project in order to avoid excessive leakages. The main part of the unlined pressure shaft and pressure tunnel designed for a maximum 455 metre static head was, however, placed in moderately jointed rock.
(Figure in full paper)
For assessment of rock stress conditions, borehole in situ rock stress measurements were performed at two locations in the unlined high pressure tunnel. These measurements showed the rock stress to be highly anisotropic, with63 -values of only 5 to 12 MPa(kp/cm2); i s e, only a fraction of the intended water pressure.
In order to establish design criteria for the project a "state of the art" study was performed, including:
Review of proposed design guidelines for unlined pressure shafts.
Case studies of existing unlined high pressure tunnels and shafts, including known projects where total failure or severe leakages had occurred.
Theoretical study of potential failure modes, including induced "hydraulic fracturing".