Hydroelectric Cavern’s Stability evaluation and rockbolts failure investigation using induced seismicity Sivakumar Cherukuri

Hydroelectric caverns in the Himalayas Most of the Himalayan region where hydropower projects constructed and other civil infrastructure projects in progress or planned are in seismic zone IV or V There is always a stability threat to most of these projects even during operation Tala hydropower plant Powerhouse has been facing a number of Rock Engineering stability issues since the start of it’s construction and in operation period A method for permanent stability monitoring is of high demand since the start of the construction to post excavation and during operation. Microseismic Monitoring is one such method established by NIRM and in use

Induced Seismicity/Microseismics Rock mass is subjected to stress Small-scale displacements/vibrations within the rock mass as result of Strain energy and released in to rock These seismic/ acoustic energy are known as induced seismic events /microseisms Detected with sensitive Geophysical equipment Microseismic events change of stress distribution Arising from man made action such as mining, excavation etc.

Principal and Potential Advantages of Microseismics

Objectives of Microseismic Method The purpose of microseismic monitoring is to obtain following information regarding seismic events in mines, tunnel, excavation, slope etc.: 1. When rock is breaking-Estimating if and when failure will occur 2. Where rock is breaking-Localizing and delineating affected areas, detecting stress areas or 3. How much rock is breaking (the size of the failure) - Estimating magnitude of stresses.

Principle of Microseismics Monitoring

Established 60 Channel microseismic network in SEP 2013 Calibrated and put in to Real time monitoring operation in FEB 2014 Uninterrupted continuous data till date Recorded Seismic / microseismic activity, Micro earthquakes Objectives : Evaluation of the Cavern stability and regional stability based on the variations of induced microseismic events source parameters Stress pocket formations and stress changes in the cavern for identifying potential instability areas using microseismic event clusters and contours Address the rock bolt failure problem by studying precursory signal information, Stress patterns / changes in stress, Strata movements of if any

Fig: Microseismic signal

Microseismic Source Parameters Source Location Microseismic Source Parameters

Mapped microseismic activity on cavern 3D Plan

Microseismic events source location

Activity Vs Log EI

Cumulative apparent volume Vs. Log energy index

Event clusters in 2016 Event clusters in 2015

Fig: Apparent stress contour

Fig: Displacement contour

Rock Bolt Failures recorded and correlated with Microseismic activity/events

Rockbolt Failure Details Date Time (Hr. Min) Ejected Rockbolt length (mm) Rockbolt location 04.01.2016 05.06 1000 IT store room EMU/AMU 20.04.2016 11.45 2500 Incoming bay III & IV, above EOT crane GIS 04.06.2016 03.30 1500 Between RD 164-175m upstream wall of the machine hall 17.07.2016 11.30 Near UAB panel unit no 01 at 509.m E.L 06.08.2016 16.11 6400 Above turbine pit unit no 02 at EL 523.5m,RD 97.3m 12.09.2016 17.10 3620 Upstream wall of machine hall at EL 531.95 above the crane rail 19.10.2016 10.59 670 Downstream wall of Machine hall EL 523m,RD 97.65m 31.12.2016 11.13 1400 Machine hall upstream wall EL 520m,RD 65.65m

Fig: Number of events Vs Magnitude

Fig: Rockbolt Failure Signal

Results and Conclusion Accumulation of microseismic events at few places but that has not been converted into rockmass failure and observed that the rate of events and number of microseismic events are not high. The strata of the powerhouse is stable with micro level variations of microseismic source parameters and no precursory rock fracture signals information observed/detected for the rock bolt failure. All the Rock bolt failure instances have been correlated and back analysis with microseismic data to study the precursory phenomenon and to identify seismic source parameter variations. There are some rock bolt failures preceded with signatures of metallic failure The study with very low level variations of seismic source parameters and micro-level changes in stress regimes indicate the stable strata of caverns and the rock bolt failures are not due to strata movements, it is the pre failed rock bolts shoot out because of binding material strength lose.

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