In September 2002, when construction of the Siah Bishe pumped storage project resumed after 10 years, designers had to model the existing grouted diversion tunnel in the light of a new earthquake spectrum, as Amir M Horr explains
THE Siah Bishe pumped storage project is named for the village of Siah Bishe, which is located some 50km northwest of Tehran, capital of Iran. The project is close to the Khandavan tunnel, which is part of the pass road that crosses the Alborz mountains in the north-south direction and connects Tehran with Chalus city at the Caspian Sea.
The project is sited at elevations that range from about 1900m asl to 2400m asl. The upper and lower reservoir sites are situated in the Chalus Valley, which flows in the north direction.
Both reservoirs are to be filled by the discharge from the Chalus river and its tributaries. The catchment area of the upper reservoir is about 19km2 and that of the lower reservoir about 98km2. Both these catchment areas are of irregular and strongly mountainous character. The hill slopes are generally steep to very steep with no vegetation. Only small scattered lots in the river valleys are used for agriculture. Precipitation within the catchment areas varies widely, generally increasing with altitude. For the project, with its upper and lower dams, extreme precipitation and runoff are both very important.
To divert the water of the Chalus river during the construction period, diversion works consisting of cofferdams (diversion dykes) and diversion tunnels have been provided for both the upper and lower dams. In the final stage of the construction works, the tunnels will be transformed into bottom outlets for the upper and lower reservoirs.
The upper dam diversion tunnel, already built, is located in the right abutment with an inner diameter of 2.95m and the total length is about 605m. The lower dam diversion tunnel, also located at the right abutment, has an inner diameter of 4m with a total length of 700m. The discharge capacity of the lower diversion tunnel is about 209m3/sec, and that for the upper tunnel is 169m3/sec.
Both tunnels were designed and built ten years ago, based on the design loading conditions of the time. But since that time several moderate to sever earthquakes have occurred in the Alborz region and the design spectrum has to be updated, based on the new data.
Bottom outlet systems have been designed for both upper and lower dams to enable the discharge of downstream water in the case of a total shutdown of the power plant. They were also designed to draw down the reservoir for inspection and maintenance of underwater structures and in case of emergency. Design for the outlet systems – the intake structures, vertical shafts and outlets – has been completed. The water level in both reservoirs can be controlled to draw down the reservoir water in an emergency situation or control the reservoir filling rate at the time of initial impounding. The level can also be controlled to facilitate grouting operations, if they are required in future. In that case the reservoir level would be lowered to its minimum level to reduce seepage in the foundation where grouting is to be carried out.
The bottom outlet in the tender design satisfied US Bureau of Reclamation criteria for all practical purposes. The filling rate at the initial impounding should allow for monitoring of dam and reservoir performance.
To simulate the effects of the changed seismic wave propagation characteristic around the diversion tunnel liners, an advanced thermal-stress analysis based on the Laplace theory was used.
In the first stage, a steady-state thermal analysis was performed to simulate the distribution of high-pressure grouting. In the next step, a stress analysis using results from the thermal analysis and the structural two-dimensional plain strain elements was performed to obtain the pre-stress pressures induced by high-pressure grouting.
For the Siah Bishe project, a three-layered rock media with a depth of 250m was modelled (for material data see the table above right) using a fine-meshed damped finite-element model.
An impulsive wave was applied at the bottom surface of the media and wave propagation through the layers and the grouted areas around the tunnel liners was investigated. The symmetric boundary conditions were applied to both side faces of the model. A traction-free surface boundary condition was assumed for the top surface.
Plain strain structural solid elements were used to model the layered rock media and tunnel liner. The complex damping model used for viscoplastic materials was used to account for the dispersivity of the impulsive wave. Although it is almost impossible to model the cracks and joints in the rock media, a macro model with viscoplastic material characteristics was used to obtain seismic wave propagation around grouted tunnels. The accuracy of results for both the lower and upper dam’s diversion tunnels depends on the accuracy of the finite element models.