Distributed Fibre Optic Strain Sensing for Monitoring Civil Infrastructure – A practical guide

Contents Acknowledgements ix
About the authors xi
Notation xv
Introduction 1
References 6
Part A. General concepts 9
1 Physical principles of fibre optic strain sensing 11
1.1 Basics of light propagation in optical fibres 11
1.2 Distributed Brillouin sensing principles 22
References 29
2 Fibre optic strain analysers and specifications 31
2.1 System specifications 31
2.2 Commercial Brillouin analysers 38
2.3 Basic Brillouin spectrum analysers operation 47
References 52
Part B. Practical considerations 53
3 DFQS hardware and testing equipment 55
3.1 Strain and temperature compensation cable
characteristics 55
3.2 Splices 63
3.3 Connectors 69
3.4 Testing and integrity checking equipment 71
References 82
4 Installation and operation of DFOS systems 83
4.1 Preparation of fibre optic cables for a DFOS
system 83
4.2 Installation of a DFOS system 90
4.3 Taking measurements from a DFOS system 99
4.4 Health and safety considerations 101
References 104
Part C. Data processing, analysis and
interpretation 105
5 Calculating strain from Brillouin frequency data 107
5.1 Influence of strain and temperature on
Brillouin frequency shift 107
5.2 Thermal expansion of structure 108
5.3 Temperature compensated strain 109
5.4 Associated data processing routines and
implementation into software 112
References 114
6 Engineering interpretation 115
6.1 Axially loaded piles 115
6.2 Retaining walls 118
6.3 Tunnels 120
6.4 Slopes 123
6.5 Example of data interpretation: axially loaded
pile 125
References 129
Part D. Case studies in geotechnical
applications 131
7 Tunnel monitoring 133
7.1 Thameslink tunnel at King’s Cross:
Deformation monitoring during proximity
tunnelling 133
7.2 Royal Mail tunnel at Liverpool Street:
Monitoring of a cast iron tunnel during
proximity tunnelling 141
7.3 Singapore Circle Line 3: Monitoring twin
tunnel interaction 146
7.4 National Grid tunnel: Segment lining
instrumentation 153
7.5 Crossrail Liverpool Street Station: Sprayed
concrete tunnel lining monitoring during
cross-passages excavation 159
7.6 CERN TT10 tunnel section: Long-term
structural health monitoring 163
References 168
8 Pile monitoring 169
8.1 259 City Road: Preliminary load test 169
8.2 Francis Crick Institute: Osterberg cell pile test 178
8.3 Isle of Dogs: Osterberg cell pile test 186
8.4 Continuous flight auger (CFA) pile: Preliminary
pile testing 191
8.5 Lambeth College: Pile loading and thermal
response test 197
8.6 6 Bevis Marks: Monitoring and re-use of piles 203
References 207
9 Diaphragm walls 209
9.1 Abbey Mills pumping station: Shaft
monitoring during excavation 209
9.2 Paddington Station: Diaphragm wall
deformation monitoring during deep
excavation 216
9.3 Stepney Green Plate Junction: Shaft
monitoring during excavation 221
Reference 227
10 Slopes and embankments 229
10.1 A2/A282 Widening project: Steep highway
cut slope stabilised with soil nails 229
10.2 Slope monitoring 235
Reference 240
Index 241