ABSTRACT:

The hydroelectric power plant in Karun III complex consists of an underground powerhouse cavern and a large gallery, about 20 m away from it, at upstream. A narrow pillar was formed by excavation of the caverns, which was suspected to be overstressed. Systematic extensometers showed significant rock movements within the pillar. A special instrumentation program and stability analysis of the pillar was performed. It was shown that some part of the pillar have undergone excessive strain. Installationon of more support is recommended in such places.

I.
INTRODUCTION

Karun 111 hydroelectric power plant, located in central Iran on Karun River, includes 8*250 MW turbines and is extended by another set of 4*250 MW turbines to produce a total of 3000 MW electricity. The underground powerhouse complex lies at Right Bank next to the first diversion tunnel, Figure I. The power house cavern (PHC) is 26m wide,40m high and 251m long. The corresponding dimensions of Transformer Cavern and Inlet Valve Gallery (IVG) are 21m*30mx234m and 10m*24m*244m respectively. Pillar width, left between PHC and IVG, was designed to be 19.9m and could not be duly increased owing to. geometrical limitations. However, in some locations the effective width is reduced to 19m due to unfavorable geological conditions and blasting overbreaks. Occurrence of a crock on the roof of IVG close to PHC, plus relatively rapid displacement in pillar necessitated a detailed study or the pillar stability. The present paper presents a summary of a special monitoring program and the respective analysis, proposed and undertaken by the authors.

2.
GEOLOGY OF THE SITE

The two main formations at the site are Asmari and Pabdeh, the former consisting mainly of limestone and marly limestone and the latter or marly limestone, marl and shale. Asmari formation generally includes competent but permeable rock while the rocks in other formation are weaker but almost impermeable. Six sets or joints and a bedding plane exist in the host rock or the complex. Orientation of layers is 230/89. Strike of the PHC is nearly NS resulting in an angle or 35 to 40 degrees with bedding. Some major geotechnical parameters are presented in next sections.

3.
DESIGN AND INSTALLATION OF INSTRUMENTS

The study aimed at the stability analysis of the narrow pillar (19m - 19.9m width) which was formed between two large excavations i.e. PHC (26m span) and IVG (10m span). It was decided to monitor the deformation along the whole pillar width as it was suspected that the movements would extend to the pillar core (zero - displacement zone). A special borehole extensometer with 14 anchor points was designed and manufactured. The extensometer head was fixed in PHC wall, while the last anchor end point was extended in to the IVG wall. The location of 14 anchor points were duly designed to facilitate the determination or the displacements close to the sides of the caverns as well as the central pan of the pillar. The pillar was obviously subjected to induced stresses resulted from the surrounding caverns.

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