Enlightening BoltsUsing Distributed Optical Sensing to Measure the

Strain Profile along Fully Grouted Rock Bolts

A.J.Hyett1 B.J. Forbes1A.J.S. Spearing2

AIMS 2012 Rock Bolting and Rock Mechanics in Mining

Verify

DesignModel

ConstraintsSafety & Costs

Capa

cityDem

andImplement

Communication

Inst

allat

i

on

Quality Control

Rock BoltingOptimization

CycleObs

erva

tionInstrum

entation

Feedback

Modified after Diederichs and Hutchinson (1993)

Rock Bolt Progression

NIOSH – Short Base-Length Resistive Foil Strain Gauges

Coal

0.9m 1.4m 1.3m 1.5m 1.0m

Mudstone rock

1531

83

10668

101

82631

83

10731

100

7

1138

98

10656

129

6

1575 99

85 67

116

5AXIAL LOAD, kN

1.8

m

Coal

0.9m 1.4m 1.3m 1.5m 1.0m

Mudstone rock

2

-8-17

-2153

-2

8

-12-5-13

-18-170

-1

7

4-4

0

0-57

1

6

15

0

-32 78

37

5BENDING LOAD, N-m

1.8

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 InternationalConference on Ground Control in Mining. Morgantown (WV); 1997. p. 183–90.

Long Base-length Inductive Strain Gauges

a. Stacked Configuration

Sensor 1Sensor 3Sensor 5

Sensor 2Sensor 4Sensor 6

Side A

Side B

b. Staggered Configuration

Sensor 1Sensor 3Sensor 5

Sensor 2Sensor 4Sensor 6

Side A

Side B

- Typical base-length of 200-500mm

- Discrete “zones”

- Capable of monitoring load on any section of the rebar

Long Base-length – Strain Contour Mapping

- Strain localize towards center of mid pillar heading

- Visualize “stretch arch”

με scale

1500 με = 100kN (or 10 tons)

Four Instrumented Bolts at the Mid Pillar of a Room and Pillar

mine

Long Base-length – Rebar Arrays

Long Base-length – Rebar Arrays

1 2 3 4 5

Nod

e Lo

catio

n

1

2

3

4

5

6

7

Heading #6

Heading #7 Intersection

100555077

100555074

100555071

100555075

100555078

Nod

e po

sitio

n

1

2

3

4

5

6

7

Heading #6

Heading #7 Intersection

100555078

100555074

100555075

100555071

100555077

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

με scale

1500 με = 100kN (or 10 tons)

Steel rebar

Steel rebar

Readings taken: 09/07/2010 11:00

Readings taken: 09/07/2010 17:00

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?

Objective

• Validate the use of fiber-optic technology for rock bolt instrumentation

• Develop a superior marketable product for monitoring and safety services

Testing• Developing a prototype

Diametrically opposed grooves along the length of a Rebar Bolt

Run fiber- optic instrumentation along the grooves in Rebar

Testing• Point Load Bending (Symmetric and Cantilever)

• Axial Pull-Test (Short Embedded Length)

• Double Shear Configuration

Symmetric Point Load

Symmetric Point Load

Experiment Theory

Cantilever Load

0.2m Embedment length in concrete block (held in place)

Direction of applied load

Cantilever Load

Experiment Theory

Pull-Test

Pull-Test

Full Length Embedment Length

Pull-Test

Double Shear Configuration

Direction of applied force

Double Shear Configuration

Shear Couplet

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

AIMS 2012 Rock Bolting and Rock Mechanics in Mining

Comparison of Methods

Foil Strain GaugeLong 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 YesYes: but only two

instruments/readout

Data-Logging Frequency 1/s 0.1/sec 10/sec

Intrinsically Safe Yes Requires InvestmentYes: Analyzer 120ft

away in clean airType Monitoring Monitoring Inspection

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

Thank YouQuestions?