ABSTRACT

The underground powerhouse cavern of Tala Hydro Electric Project (THP), Bhutan had many incidents of roof falls and instabilities during construction. The cavern is located very close to the high stress thrust zone known as Main Central Thrust (MCT) of Great Himalayan region. After the project was commissioned there were a number of incidents of rock bolt failures in a violent manner from the side walls of the machine hall cavern. The failures observed in the upstream wall were more compared to the downstream wall. There were about 168 rock bolt failures reported prior to the microseismic monitoring. Hence the need for long-term monitoring of the powerhouse cavern stability was felt necessary in view of safety of the workmen and machinery. Microseismics/ Acoustic Emission technique was found to be the best solution for remote and real time global monitoring of this large underground cavern. This technique can be used without disturbing the power generation and in view of no scope for expansion of the existing conventional instrumentation. In order to assess the applicability, suitability and capability of microseismic monitoring technique for the permanent cavern strata monitoring, an advanced 24 bit digital microseismic instrumentation with twelve geophone stations was installed and monitored on a trial basis. This attempt of microseismic monitoring in an underground hydroelectric powerhouse cavern is the first time in the history of a commissioned hydro electric power project. Six borehole geophones and six surface geophones were installed on the upstream wall as well as in the downstream wall. Approximately 250,000 microseismic counts were recorded during the monitoring period. Many clear signatures of brittle rock fracturing signals within the walls were recorded. Four prominent microseismic events that were large enough to compute the source parameters were recorded during the monitoring period, including many events detected by only two to three or four sensors. Three of these prominent events were located on the upstream wall, and the fourth event was located in the downstream side. The system responded well for the stress resulted rock cracks in strata throughout the monitoring period. The induced microseismic noise from the cavern walls indicates stress redistribution phenomena in the structure even five years after its commissioning. Micro-cracking was observed very close to the sensors, indicating high seismic attenuation characteristics of the rock mass. Incidentally there was a rock bolt failure on the downstream wall side during the monitoring period, which has been correlated well with the high stress zone identified from the recorded data. This paper discusses the successful demonstration of the microseismic monitoring technique in a highly stressed strata around an underground cavern based on the results obtained during experimentation.

INTRODUCTION

The Tala Hydro Electric Project underground powerhouse (IC 1020 MW) is located on the right bank of Wangchu River, in Bhutan. The underground powerhouse is located very close to a major tectonic plane, the Main Central Thrust (MCT) of Himalayan region , which makes the tectonic boundary between rocks of Thimphu formation (crystalline) and Schumer formation (youngest) has been over ridden by Thimphu formation (older).

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