Monitoring plays major role in every stage of the project from exploration, construction and operation of the underground cavern. Large cavern in Bhutan Himalayas is being monitored with geotechnical instruments since the construction period. The behavior of the cavern is being studied for last 18 years with the monitored data. Instrumentation program include measurement of convergence of high walls, load on the support systems with load cells and instrumented bolts. Selection of instruments were based on importanat paramaters required for the design. The data analysis was carried out at all stages of excavation and appropriate decisions were made on support design and method of excavation. Long term monitoring of large cavern in Himalayas for such a long period is helping in deciding about the stability of the cavern in a complicated geological setup.
Long term monitoring of large underground caverns and tunnels in complex geological conditions using geotechnical instruments is very crucial in evaluating the stability from time to time as their service life is very large. Construction of large caverns in Himalayas is quite challenging as Himalayan region is affected by a constant tectonic uplifting as well as down cutting effects by several river systems. In addition, the compressional tectonic stress regime in the Himalaya has resulted in intense deformation of the rock mass, making it highly folded, faulted, sheared, fractured and deeply weathered. This complex geological setting has caused considerable stability problems and is a great challenge for successful tunneling. Therefore, the behavior of the rock mass needs to be continuously monitored during the excavation of the caverns and be continued during its entire life. Although, instrumentation was done at the powerhouse complex, this paper discusses one such monitoring program implemented using geotechnical instruments at one of the larger caverns of Tala Hydroelectric project in Bhutan Himalayas.
