ABSTRACT:

Measured displacements are analyzed for 5 large-scale underground powerhouse caverns in Southwest China. Their principal stresses lie between 18 and 35MPa, however their displacements demonstrate completely different characteristics. In order to clarify the effects of stress, a term of stress index ? is defined to describe their stress state; Then the displacement is correlated with the stress index; And finally displacement characteristics is compared at various stress state.

1. INTRODUCTION

Southwest has been the center for hydropower development in the past decades in China. Due to the limit of topography in river valleys, underground powerhouse caverns have to be adopted in most cases. The authors joined in the work of field monitoring or interpretation of monitored data of 5 large caverns with maximum initial stress of 18 to 35MPa, however their displacement show completely different characteristics and magnitude. This paper is meant to clarify the effect of initial stress on the displacement. Firstly a term of stress index, i.e. the ratio of mean uniaxial compression strength of rock specimen and the maximum initial stress, is defined to describe the stress state of the caverns; then the displacement is correlated with the index and finally displacement characteristics is depicted at various stress state.

2. DEFINITION OF STRESS STATE

By now, there is no a widely accepted strict definition of stress state, but core disking is generally regarded as a symbol of high stress.

3. BASIC DATA OF THE 5 CAVERNS

The 5 caverns bear some similarities. Firstly they have similar plan layout and are all horseshoe-shaped, Fig. 1 is their typical plan view and cross section; Secondly they are all excavated by benched drilling and blasting method; Thirdly they have similar support method and multipoint extensometers are the major displacement

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