An adequate management of an aging underground powerhouse is recognized to be a mandatory issue through the experience of which significant damages were happened in a few support members of approximately50-year old cavern for the powerhouse. The studies have been made to verify the soundness of the cavern by arranging two diffused laser range finders at the generator sections for monitoring the convergence, in-situ stress measurements of major concrete structures of the cavern and lift-off tests of PS anchors. Based on these results, the present soundness of the cavern was confirmed with the interpretation by the structural calculation using the simple frame model of the underground powerhouse. The management criterion was updated by applying the future scenarios of the deterioration of the cavern supports to its present structural condition. These procedures were compiled as a new management method for aging underground powerhouses, referred to as the continuous management method, which was characterized by updating periodically its management criteria aided by monitoring and structural analysis.

Following updated management criteria at the above-mentioned underground powerhouse, five-year convergence data at two generator sections has been accumulated so far. While one is stable, however another has been shortened gradually, but does not touch the warning level. In this paper, the convergence behaviors are examined from the standpoint of the availability of the monitoring device and the structural response of the cavern.

1. Introduction

Since the 1940s, more than fifty underground caverns for hydropower plants have been constructed in Japan, almost all of which have been operated for the past few decades. Monitoring the deformation of the caverns and surrounding rock, as well as the forces acting on the supports, is vital to confirm the soundness of these caverns during the construction. Because the caverns are not deemed to be exposed to any additional forces, since they also incorporate reinforced concrete as inner supports, concerns over cavern stability rarely emerge after the completion of the construction.

In those one during the operation for forty-six years, several rock anchors were broken, and the stability of the cavern should be re-evaluated. For this purpose, additional monitoring and numerical study (Kashiwayanagi et. al., 2009, 2011 and 2013) were conducted. This paper summarizes field measurement results and their interpretation, using diffused laser range finder, new device of convergence measurement for the stability evolution of caverns for underground powerhouses under operation.

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