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

2. 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

3. 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.

4. Principal and Potential Advantages of Microseismics

6. 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.

7. Principle of Microseismics Monitoring

8. Established 60 Channel microseismic network in SEP Calibrated and put in to Real time monitoring operation in FEB 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

10. Fig: Microseismic signal

11. Microseismic Source Parameters
Source Location
Microseismic Source Parameters

12. Mapped microseismic activity
on cavern 3D Plan

13. Microseismic events source location

14. Activity Vs Log EI

15. Cumulative apparent volume Vs. Log energy index

16. Event clusters in 2016
Event clusters in 2015

17. Fig: Apparent stress contour

18. Fig: Displacement contour

19. Rock Bolt Failures recorded and correlated with Microseismic activity/events

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

21. Fig: Number of events Vs Magnitude

22. Fig: Rockbolt Failure Signal

24. 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.

25. Thank You