1. A.J.Hyett1 B.J. Forbes1 A.J.S. Spearing2
Enlightening Bolts Using Distributed Optical Sensing to Measure the Strain Profile along Fully Grouted Rock Bolts
A.J.Hyett1 B.J. Forbes1 A.J.S. Spearing2

2. Rock Bolt Progression Rock Bolting Optimization Cycle
Verify
Design
Model
Constraints
Safety & Costs
Capacity
Demand
Implement
Communication
Installation
Quality Control
Rock Bolting
Optimization
Cycle
Observation
Instrumentation
Feedback
Modified after Diederichs and Hutchinson (1993)
AIMS 2012 Rock Bolting and Rock Mechanics in Mining

3. NIOSH – Short Base-Length Resistive Foil Strain Gauges
Coal
0.9m
1.4m
1.3m
1.5m
1.0m
Mudstone
rock 15 31 83
106 68
101 8 26
107
100 7 11 38 98 56
129 6 75 99 85 67
116 5
AXIAL LOAD, kN
1.8 m 2 -8
-17
-21 53 -2
-12 -5
-13
-18
-170 -1 4 -4
-57 1
-32 78 37
BENDING LOAD, N-m
Signer SD, Cox D and Johnson J. A method for the selection of rock support based on loading measurements In: Proceedings of the 16th International
Conference on Ground Control in Mining. Morgantown (WV); p. 183–90.

4. Long Base-length Inductive Strain Gauges
Typical base-length of mm
Discrete “zones”
Capable of monitoring load on any section of the rebar

5. Long Base-length – Strain Contour Mapping
Four Instrumented Bolts at the Mid Pillar of a Room and Pillar mine
Strain localize towards center of mid pillar heading
Visualize “stretch arch”
με scale
1500 με = 100kN (or 10 tons)

6. Long Base-length – Rebar Arrays

7. Long Base-length – Rebar Arrays
Readings taken:
09/07/ :00
Readings taken:
09/07/ :00
με scale
Steel rebar
Steel rebar
1500 με = 100kN (or 10 tons)

8. Long Base-length - Limitations
1. Not Intrinsically Safe (IS approved)
2. Limited spatial resolution along the bolt
3. Not designed to measure shear
Does a technology exist that can overcome these limitations?

9. Objective
Validate the use of fiber-optic technology for rock bolt instrumentation
Develop a superior marketable product for monitoring and safety services

10. Testing Developing a prototype
Diametrically opposed grooves along the length of a Rebar Bolt
Run fiber- optic instrumentation along the grooves in Rebar

11. Testing Point Load Bending (Symmetric and Cantilever)
Axial Pull-Test (Short Embedded Length)
Double Shear Configuration

12. Symmetric Point Load

13. Symmetric Point Load
Experiment
Theory

14. Cantilever Load Direction of applied load
0.2m Embedment length in concrete block (held in place)

15. Cantilever Load
Experiment
Theory

16. Pull-Test

17. Pull-Test
Full Length
Embedment Length

18. Pull-Test

19. Double Shear Configuration
Direction of applied force

20. Double Shear Configuration

21. Shear Couplet

22. Summary of Tests Fiber-Optic instrumentation is fundamentally viable
Output data from experiments compare within ± 5% of theory
The shape of experiment and theory plots are essential identical

23. Comparison of Methods Foil Strain Gauge Long Base-length Inductive
Foil Strain Gauge
Long Base-length Inductive
Distribute Optical
Cost/Instrument
~$2800+
~$1000
~$800
Cost/Strain Point
140+
160+
<$1
Cost/Readout Unit
$1000+
$300
$50,000 (was $250,000 in 2005)
Data-Logging
Yes
Yes: but only two instruments/readout
Data-Logging Frequency
1/s
0.1/sec
10/sec
Intrinsically Safe
Requires Investment
Yes: Analyzer 120ft away in clean air
Type
Monitoring
Inspection
AIMS 2012 Rock Bolting and Rock Mechanics in Mining

24. Major Conclusions
Fiber-Optic instrumentation is the future of ground monitoring:
- Higher Resolution and increased Accuracy
- Cheaper and less Difficult to manufacture
Improved empirical correlations:
- More accurate modeling = increased productivity
- More accurate monitoring = Workplace Safety

25. Thank You
Questions?