A comprehensive instrumentation scheme was adopted for the monitoring of the underground powerhouse cavity of Koyna Hydroelectric Stage IV Project. The instrumentation involved multi-point borehole extensometers, closure studs for convergence monitoring using tape extensometer and rock bolt load cells. Central Mining Research Institute (CMRI) has undertaken the job and some of the interesting results are presented in the paper.
The underground powerhouse cavity is one of the main features of the Koyna Hydroelectric Stage IV Project located in the Sahyadri hill ranges in the state of Maharashtra, India. The D-Shaped powerhouse cavern is 23m wide, 50m high and 144m long. The transformer hall cavern, which is also D-Shaped, is 20m wide, 25m high and 173m long and is in close proximity to powerhouse cavern. The arch portions of both the caverns were excavated by first excavating a central drift and then by widening this drift on both the sides. The sequence of excavation of arch portion for the powerhouse cavern has been shown in Figure 1. After completion of the arch portion, lowering of the bench was taken up. Considering the importance of ensuring the stability of underground structures, it was decided to continuously monitor the behaviour of the rock mass and the support system as the excavation progresses. The monitoring work was started in January 1991 and remains continued for over a period of five years. The roof monitoring was carried out till the cavern roof is excavated to its full width and the bottom of cavern has reached to 150.0m level. In the paper mainly the results of roof monitoring are presented. The underground cavity is excavated through the igneous basaltic rock mass. For better appreciation, a brief geology of the area with the rock mass quality Q value is given below.
The Koyna valley and all the principal elements of the project are located in the Deccan traps of cretaceous eocene age. These rock masses consist primarily of a succession of basaltic lava flows covering 500,000 sq km or more. The Deccan traps lie in nearly horizontal flows showing a slightly easterly dip of about 30 minutes. The thickness of individual flow varies between 8m and 80m. There are no faults visible in the region. The cavern is excavated through a variety of rocks with diverse engineering characteristics. These rocks are the compact or non-vesicular basalts and the amygdaloidal basalts with gas cavities filled with secondary minerals which give them the spotted appearance. Common cavity fillings are zeolites and chlorophites. The compact basalts have occasional and random joints. Apart from the compact basalts and the amygdaloidal basalts, the volcanic breccias are also found at the lower level (between RL 130 and 122m). The volcanic breccias with random joints are weak in strength. In general, the rock masses except the volcanic breccia are considered good for underground construction. The rock mas quality Q values for compact basalt, vesicular basalt and volcanic breccia were 13.2, 3.21 and 1.5 respectively.