Abstract
Large underground powerhouse caverns in the mighty Himalayan region during construction and their operation after their completion experiencing instabilities, which are need to be monitored for life time using a real time regional monitoring systems for the safety of the structure, installed machinery and workmen. These structures experienced many incidents of strata instabilities during construction period including large roof falls and even during its operation in the form of large number of rockbolt violent failures etc. Since the conventional instrumentation that was installed during construction was not able to provide adequate information about the regional stability. A high resolution sixty channel real time, remote monitoring microseismic system was established in one of the project site with the objective to provide information related to the regional strata behaviour in real time and to address the regional structural and strata stability around the powerhouse caverns. The microseismic data is continuously recorded, auto processed and displayed in near real time at the central data recording laboratory situated inside the underground powerhouse and at regional data processing centers located far away from the site.
The microseismic data acquired were analyzed and mapped on the 3D plan of the caverns. Statistical data analysis using source parameters of microseismic events were carried out to correlate the events with rock bolt failures in order to study the signatures of the precursory signals. Few microseismic events occurred in clusters at some regions and majority of the microseismic events have the local magnitudes in the range of at about -2.5. The range of recorded events local magnitudes indicates that the cavern was stable during the monitoring period, there is no development of stress pockets or migration (changes in stress) of stress regime. The changes in the stress have been very low indicating the stability of the strata. The recorded low values show the efficiency of the monitoring system in response to minor changes of strata behaviour.
This paper presents the real time microseismic monitoring of regional strata behaviour of the underground hydro powerhouse caverns for about a period of one and half year after the commissioning of real time monitoring system in September 2013 and correlation of microseismic activity to explain the rock bolt failures and the caverns regional strata stability.
