The Jukla Pumped-Storage Power Project in Western Norway includes a 6,200 m3 compressed air surge chamber designed for operational air pressures between 6 bar and 24 bar. The paper describes the site investigations performed as a basis for chamber design.
Das Pumpenspeicherwerk Jukla in West-Norwegen umfasst auch ein 6,200 m3 grosses Wasserschloss dass mit komprimierter Luft in Form eines Luftkissens arbeitet, mit einem Betriebsdruck zwischen 6 bar und 24 bar schwankend. Die ausgefuehrten Felduntersuchungen als Grundlage fuer den Entwurf der Kammer werden beschrieben.
Le projet pompage-emmagasinement de Jukla à I'ouest de la Norvege comporte une chambre à air comprime dimensionnee à 6,200 m3 pour des pressions operatives entre 6 et 24 bars. Le present article decrit les reconnaissances in situ effectuees en vue du design de cette chambre. The use of unlined, pressurized rock cavities has long traditions in Norway. More than 50 hydro electric power plants with unlined pressure shafts and tunnels have been constructed involving water pressures ranging from 150 to more than 500 metres static head. The traditional high pressure plant comprises a head race tunnel at near reservoir level, with surge shaft at the downstream end, and a pressure shaft down to the power station. Recent developments in power plant design has called for the use of deep-lying pres- sure tunnels. The conventional surge shaft is replaced by deep lying chamber containing a large bubble of compressed air, as indicated in Fig. 1. The air bubble or cushion functions as shock absorber to ensure hydraulic stability of the system. In 1973 and 1974, the world''s two first high pressure air cushion surge chambers were successfully put into operation, at the Driva and Jukla power plants in Norway, thus furnishing practical experience of Air cushion surge chamber Fig. 1. Utilization of compressed air cushion surge chamber, as compared with conventional shaft design. Fig. 2. The Folgefonni Hydro-Electric Power Development Project. large, unlined rock chambers for gas containment (Rahte 1975, Stokkebø 1972). The Jukla pumped storage power project, developed and owned by the State Power Board, utilizes run-off from the great Folgefonn glacier in western Norway. It forms part of the Folgefonni power development project as visualized in fig. 2. The Jukla project includes a 6200 m3 compressed air surge chamber, with operational air pressures varying between 6 bar (kg/cm2) and 24 bar. Acting as engineering geological consultant to the State Power Board, A/S GEOTEAM was responsible for site investigations and rock mechanics design evaluations performed prior to and during construction of this chamber.
The investigations were performed in three consecutive phases:
Geological mapping of tunnels.
Diamond core drilling, pore pressure measurements and air and water leakage tests, and (Equation in full paper)
Inspection and control during excavation.
Initially a detailed engineering geological survey in the already completed headrace pressure tunnel was performed. The rock here is Precambrian gneiss showing near horizontal foliation and moderate schistosity. The headrace tunnel crosses a few major swelling-clay gouges calling for local on-face concrete lining support. Except for these zones, the rock is moderately jointed. Based on the geological survey, a 100 m section of the headrace tunnel, situated 600 m upstream of the power station and with a rock overburden of 340 m, was provisionally selected for detailed investigations. The potential chamber location is indicated in figs. 3 and 4. Here three 40- 80 metres deep diamond core drillholes, as well as an 18 metres percussive drill hole were drilled to investigate the rock quality, as visualized in fig. 5. They revealed massive rock with RQD values better than 90 at the proposed chamber location. Decisive for a feasibility evaluation of an unlined air cushion surge chamber is the rate of air loss to be expected due to leakage through the rock. Theoretical studies indicate that such losses are governed by two main factors: - the permeability of the rock mass, and the ratio of chamber gas pressure to.pore water pressure in the surrrounding rock. Pore water pressure in the surrounding rock was continuously recorded during site investigations and the subsequent construction period, and showed a natural pore pressure in the surrounding rock of 17 bar (kg/cm2), fig. 5. For assessment of rock mass permeability, in situ air and water pumping tests were performed in two of the diamond core drillholes, employing special test equipment designed and operated by Norwegian Geotechnical Institute personnel. (DiBiagio and Myrvoll, 1972). Fig. 6. Pore pressures recorded at the chamber location. Water pressure tests were carried out at pressures up to 50 bar, using borehole test sections of 67 m and 46 m lengths. At peak pressure, water losses of 360 cm3 and 30 cm3 per minute respectively were recorded for the two test sections, corresponding to Lugeon values of 0.0015 and 0.0002. This extremely low rock mass permeability clearly indicated favourable conditions for surge chamber placement.