A report is presented on the determination of the rock mechanics for the concrete lining (prestressed by interface grouting) of the upper stage shaft (800 m head) and the steel lining of the lower stage shaft (1500 m head) as well as on the test chamber for a concrete lining with plastic sheeting.
Compte-rendu des recherches en-mecanique des roches pour le revêtement en beton (precontraint par injection) du puits de Kuehtai (Pmax = 800 m) et pour le blindage du puits force de Silz (Pmax= 1500 m) ainsi que des essais de pression pour un revêtement etanche en beton avec feuille de plastique.
Bericht ueber die Ermittlung der felsmechanischen Grundlagen för die durch Injektionen vorgespannte Betonauskleidung des Oberstufenschachtes (800 m Druck) und die Panzerung des Unterstufenschachtes (1500 m Druck), sowie ueber den Druckkammerversuch fuer eine Betonauskleidung mit PVC-Dichtfolie.
The 780-MW Sellrain-Silz Power Plant built by the Tiroler Wasserkraftwerke (TIWAG) makes use of a total head of 1678 m in two stages (Fig.1). The PSS Kuehtai in the upper stage powers the water from the 60 million-m3 Finstertal seasonal reservoir through a 1800-m-long pressure shaft without surge chamber to the 400-m-lower Langental equalizing reservoir (capacity 3 million m3). The two reversible units with pump turbines in the Kuehtai Shaft Power Plant are designed for a maximum capacity of 285 MW using turbines and 250 MW using pumps. The lower stage is a typical high head power plant, characterized by its total head of 1257m. From the Langental Reservoir, the water runs through a 4600-m-long headrace tunnel with throttled twin surge chamber and a 2400-m-long steel-lined pressure shaft to the Silz Power House and powers two generating sets with six- nozzle Pelton turbines having a total capacity of 495 MW. For both the execution of the prestressed con create lining with and without plastic sheeting as well as for the design of the steel lining required to share the internal pressure with the rock in the heavily loaded shafts of the two power stages, comprehensive investigations, examinations and rock mechanical test were necessary.
The pressure shaft for the PSS Kuehtai was constructed in variable geology including schistous gneiss and amphibolite of the Ötz Valley Cristalline Series and is situated so far underground that it was possible to apply a prestressed concrete lining in a large portion of the shaft (Fig.2). The schistous gneiss was partially disturbed in the lower elbow, for which reason a waterproof membrane of PVC sheeting was used along 400 m. The transition to the 270-m long steel-lined portion adjoining the Kuehtai Shaft Power Station is bridged by a 35-m section with thin-walled steel lining and a concrete inner lining. The particular operating conditions of the pump turbines result in an exceptionally large increase in dynamic pressure up to 60% of the static pressure in the shaft. This put extreme loads of up to max. 485 mlc static head and max. 740 mlc dynamic head on the concrete lining. In order to prevent the concrete lining from cracking under this internal pressure, it was prestressed by interface grouting at high pressure using the TIWAG procedure (LAUFFER 1968). The prestressed concrete lining was designed according to the graph shown in Fig.3. Due to the interface grouting, both the concrete lining and the rock mass are put under a radial load. The gap occurring as a result of deformation of the lining and the rock mass is filled with cement grout, thereby prestressing the lining against the rock mass. A permanent prestressing effect is guaranteed, if the primary field stresses are remobilized by prestress grouting under sufficient rock overburden in order to resist the internal pressure. For this reason it was attempted in the planning stage to measure the in-situ stresses and the load lines of the rock mass in an exploratory tunnel in the level section. The 1200-m-long section of the pressure shaft up to the valve chamber was driven mechanically by TBM (PIRCHER 1980).
In the course of exploration work, bore cores were taken from the surface and an exploratory tunnel was driven in the upper and lower level headrace tunnel sections, which were used for geologic and petrographic evaluation of the rock mass as well as for quantitative description of the discontinuities by the geologist. In the following, a description is given of the comprehensive rock mechanical measuring program performed to evaluate the rock mass properties, which provided the basis for selection of the shaft's lining (points 3.1 to 3.4). After excavation of the shaft, additional procedures were applied along its entire length for the purpose of relative measurement of the rock mass properties (points 3.4 to 3.7).