DFOS

Tunnel Structural Health Monitoring

Distributed sensing for continuous assessment of tunnel lining condition, deformation, cracking, convergence, ground interaction and long-term structural performance.

Distributed Monitoring
Structural Safety
Continuous Sensing
DFOS
Distributed Fiber
Tunnel Classification

Monitoring Across Tunnel Types

Different tunnel construction methods introduce different structural components, failure mechanisms and sensing requirements.

Road

Road Tunnel

A tunnel designed primarily for vehicular traffic, incorporating structural lining, ventilation, drainage, safety and operational systems.

Primary Components
Tunnel liningPrimary supportSecondary liningInvertPortal
Critical Zones
Tunnel crownSide wallsInvertPortal zonesConstruction joints
Monitoring
Longitudinal strainCircumferential strainConvergenceLining deformationCrack development
DFOS Strategy
Longitudinal fiber along tunnel lining
Circumferential fiber around tunnel rings
Fiber across crown and side-wall regions
Distributed strain • Lining deformation • Crack localization • Temperature • Ground-structure interaction
Railway

Railway Tunnel

A tunnel designed for railway traffic where structural deformation, track geometry and dynamic train loading must be monitored.

Primary Components
Tunnel liningPrimary supportSecondary liningInvertTrack slab
Critical Zones
CrownSpringlineInvertTrack slabSegment joints
Monitoring
Lining strainTrack slab strainTunnel convergenceSettlementVibration
DFOS Strategy
Longitudinal fiber along tunnel lining
Circumferential fiber around tunnel rings
Fiber along track slab
Distributed strain • Dynamic strain • Lining deformation • Track slab response • Crack localization
Metro

Metro Tunnel

An underground rail transportation tunnel constructed primarily for urban metro systems, often requiring strict deformation and settlement control.

Primary Components
Segmental liningTunnel ringsSegment jointsGasketsTrack slab
Critical Zones
Segment jointsRing jointsStation-tunnel interfaceCross passagesCrown
Monitoring
Segment strainRing deformationJoint openingConvergenceSettlement
DFOS Strategy
Circumferential fiber around tunnel rings
Longitudinal fiber along tunnel axis
Fiber across segment joints
Ring deformation • Distributed strain • Joint deformation • Crack localization • Temperature
TBM

Bored / TBM Tunnel

A tunnel excavated using a tunnel boring machine and commonly supported by precast segmental lining rings.

Primary Components
Segmental liningTunnel ringsSegment jointsLongitudinal jointsCircumferential joints
Critical Zones
Segment jointsRing jointsKey segmentsTBM tail-zone regionFault crossings
Monitoring
Ring deformationSegment strainJoint openingConvergenceGround movement
DFOS Strategy
Circumferential fiber around segment rings
Longitudinal fiber through multiple rings
Fiber across segment joints
Ring deformation • Joint strain • Distributed lining strain • Crack localization • Ground-structure interaction
NATM

NATM Tunnel

Tunnel constructed using the New Austrian Tunnelling Method, where ground behavior is actively observed and the support system is adapted to measured deformation.

Primary Components
Shotcrete liningRock boltsSteel ribsInvertSecondary lining
Critical Zones
Tunnel crownSide wallsFace influence zoneFault zonesRock class transitions
Monitoring
ConvergenceCrown settlementShotcrete strainRock bolt strainGround deformation
DFOS Strategy
Longitudinal fiber along tunnel axis
Circumferential fiber around excavation
Fiber embedded in shotcrete
Distributed shotcrete strain • Convergence-related deformation • Crack localization • Ground-support interaction • Rock bolt response
Cut & Cover

Cut-and-Cover Tunnel

An underground structure constructed by excavating from the surface, constructing the tunnel structure and subsequently backfilling above it.

Primary Components
Roof slabBase slabSide wallsInternal columnsDiaphragm walls
Critical Zones
Roof slabBase slabWall-slab connectionsExpansion jointsDiaphragm wall connections
Monitoring
Wall strainSlab strainSettlementJoint displacementTemperature
DFOS Strategy
Longitudinal fibers along roof slab
Fiber along side walls
Transverse fiber across slabs
Distributed strain • Settlement-related deformation • Crack localization • Joint movement • Temperature
Immersed

Immersed Tunnel

A tunnel formed from prefabricated structural elements that are immersed into a prepared underwater trench and connected to adjacent tunnel sections.

Primary Components
Tunnel elementsImmersed jointsWaterproofingFoundation bedApproach structures
Critical Zones
Immersed jointsElement connectionsFoundation interfaceApproach transitionsTunnel corners
Monitoring
Joint displacementTunnel strainDifferential settlementWater pressureTemperature
DFOS Strategy
Longitudinal fiber along tunnel elements
Circumferential fiber around sections
Fiber across immersed joints
Joint deformation • Distributed strain • Settlement-related deformation • Crack localization • Temperature
Drill & Blast

Drill-and-Blast Tunnel

Tunnel excavated through controlled drilling and blasting followed by installation of ground support and lining.

Primary Components
ShotcreteRock boltsSteel ribsPrimary liningSecondary lining
Critical Zones
CrownSpringlineInvertFault zonesRock class transitions
Monitoring
Ground deformationConvergenceShotcrete strainRock bolt strainCrack development
DFOS Strategy
Fiber embedded in shotcrete
Fiber along rock bolts
Circumferential fiber around excavation
Support strain • Rock bolt response • Distributed lining strain • Crack localization • Ground-support interaction
Structural Parameters

What Should Be Monitored?

Tunnel health is determined through multiple structural, geotechnical and environmental parameters.

Lining Strain

Unit: με
critical

Indicates

Load redistribution
Lining stress concentration
Flexural response
Ground-structure interaction

Conventional Sensors

Electrical resistance strain gaugeVibrating wire strain gaugeFBG strain sensor
DFOSSuitable
BrillouinRayleighOFDR

Provides continuous strain information along the tunnel lining instead of isolated point measurements.

Tunnel Convergence

Unit: mm
critical

Indicates

Tunnel deformation
Ground pressure changes
Support performance
Long-term deformation

Conventional Sensors

Tape extensometerTotal stationConvergence meterLaser scanner
DFOSindirect

Distributed strain measurements can be used for deformation reconstruction when the fiber geometry and structural boundary conditions support reliable displacement estimation.

Crack Development

Unit: mm
critical

Indicates

Concrete deterioration
Lining distress
Ground movement
Joint deterioration

Conventional Sensors

Crack gaugeLVDTDigital image correlationVisual inspection
DFOSSuitable

Localized strain concentrations and abrupt changes in distributed strain can be used to identify and track regions associated with crack development.

Settlement

Unit: mm
critical

Indicates

Ground movement
Foundation movement
Tunnel-induced settlement
Differential deformation

Conventional Sensors

Precise levelingGNSSTotal stationSettlement markersHydrostatic leveling
DFOSindirect

DFOS can monitor structural strain resulting from settlement, while absolute ground or structural elevation should generally be verified with geodetic or displacement instrumentation.

Joint Displacement

Unit: mm
critical

Indicates

Segment movement
Joint opening
Tunnel deformation
Waterproofing risk

Conventional Sensors

LVDTCrack meterJoint displacement sensor
DFOSSuitable

Fibers routed across joints can provide localized strain/deformation information associated with joint opening or movement.

Temperature

Unit: °C
critical

Indicates

Thermal expansion
Thermal gradients
Temperature compensation
Environmental loading

Conventional Sensors

PT100ThermocoupleThermistor
DFOSSuitable
RamanBrillouin

Distributed temperature measurement can provide continuous thermal information over long tunnel sections.

Vibration / Dynamic Response

Unit: Hz / m/s²
high

Indicates

Train-induced vibration
Traffic loading
Construction activity
Blasting response

Conventional Sensors

MEMS accelerometerPiezoelectric accelerometerGeophone
DFOStechnology-dependent
Phase-sensitive OTDRRayleigh-based dynamic sensing

Dynamic DFOS performance depends on interrogator architecture, sampling rate, spatial resolution, sensing range and required frequency bandwidth.

Water Ingress / Leakage

Unit: engineering-specific
critical

Indicates

Lining waterproofing failure
Joint leakage
Groundwater migration
Drainage system problems

Conventional Sensors

Leak detection cablePressure sensorFlow sensorVisual inspection
DFOStechnology-dependent

Distributed temperature or strain responses may assist in identifying water-related anomalies, but dedicated distributed leak-detection systems may be preferable for direct leakage detection.

Rock Bolt Strain

Unit: με
critical

Indicates

Ground-support interaction
Load transfer
Rock mass movement
Support utilization

Conventional Sensors

Instrumented rock boltsLoad cellsStrain gauges
DFOSSuitable

DFOS-compatible instrumented fiber arrangements can provide distributed strain information along selected support elements.

Lining Temperature

Unit: °C
medium

Conventional Sensors

PT100Thermocouple
DFOSSuitable

Distributed temperature sensing can provide continuous temperature profiles along the lining.

Ground / Rock Pressure

Unit: kPa / MPa
critical

Indicates

Ground loading
Support loading
Stress redistribution
Excavation effects

Conventional Sensors

Pressure cellEarth pressure cellLoad cell
DFOSindirect

DFOS measures structural strain rather than ground pressure directly. Pressure should generally be measured using dedicated pressure instrumentation.

Tunnel / Lining Tilt

Unit: mrad / degrees
high

Indicates

Differential settlement
Lining rotation
Ground movement
Structural deformation

Conventional Sensors

TiltmeterInclinometer
DFOSindirect

Distributed strain measurements may contribute to deformation reconstruction, while direct angular measurements are better obtained from dedicated tilt instrumentation.

Structural Hotspots

Critical Tunnel Zones

Structural discontinuities, load-transfer regions and geological transitions require higher-resolution monitoring.

Tunnel Crown

The crown is a critical load-transfer region and can experience ground pressure, bending, cracking and settlement.

Parameters
Longitudinal strainCircumferential strainCrack developmentSettlementTemperature
Recommended DFOS Layout
Longitudinal fiber along crown
Circumferential fiber
Grid around crack-prone regions

Springline / Side Wall

Side walls participate strongly in transferring ground pressure and may experience lateral deformation and cracking.

Parameters
Horizontal deformationVertical strainCrack developmentTemperatureGround pressure response
Recommended DFOS Layout
Vertical fiber
Longitudinal fiber
Circumferential fiber

Tunnel Invert

The invert closes the structural ring and may experience heave, cracking, bending and differential deformation.

Parameters
Longitudinal strainBending strainHeaveCrackingTemperature
Recommended DFOS Layout
Longitudinal fiber
Transverse fiber
Circumferential loop

Segment / Ring Joints

Segment joints are critical locations for differential movement, leakage and load transfer.

Parameters
Joint openingJoint rotationLocalized strainWater ingress
Recommended DFOS Layout
Fiber crossing the joint
Circumferential fiber around ring
Local zig-zag routing

Portal Zone

The transition between the underground structure and the surrounding ground can experience differential movement and complex loading.

Parameters
SettlementLongitudinal strainCrackingGround deformationTemperature
Recommended DFOS Layout
Longitudinal fiber
Transverse fiber
Grid around transition zone

Geological / Fault Transition Zone

Changes in rock or soil properties can produce significant differential deformation and localized structural response.

Parameters
Distributed strainConvergenceGround movementCrackingWater ingress
Recommended DFOS Layout
Dense longitudinal fiber
Circumferential loops
Grid layout

Cross Passage / Connection

Connections between tunnel structures create geometric and stiffness discontinuities where stress concentrations can occur.

Parameters
Local strainCrackingJoint displacementTemperature
Recommended DFOS Layout
Fiber around connection
Grid pattern
Zig-zag routing

Tunnel–Station Interface

Changes in geometry, stiffness and structural loading can create differential movement between the tunnel and station structure.

Parameters
Differential strainJoint displacementSettlementCrackingVibration
Recommended DFOS Layout
Longitudinal fiber
Transverse fiber
Dense grid at interface
Technology Comparison

Conventional Sensors vs DFOS

Compare point-based instrumentation with distributed fiber-optic sensing for tunnel structural monitoring.

Lining Strain

Conventional
Electrical strain gauge
Vibrating wire strain gauge
FBG
DFOS
Distributed strain sensing
Brillouin
Rayleigh
OFDR
Replacement Potential
High

DFOS provides spatially continuous strain information over long tunnel sections.

Tunnel Convergence

Conventional
Tape extensometer
Total station
Convergence meter
DFOS
Distributed strain-based deformation reconstruction
Replacement Potential
Medium

DFOS can provide distributed structural response but absolute convergence should be validated against appropriate geometric measurements.

Crack Monitoring

Conventional
Crack gauge
LVDT
Visual inspection
DFOS
Distributed strain concentration monitoring
Replacement Potential
High

DFOS can identify localized strain changes and their spatial location over continuous fiber routes.

Temperature

Conventional
PT100
Thermocouple
DFOS
Distributed temperature sensing
Replacement Potential
High

A single fiber can provide temperature information over long tunnel sections.

Settlement

Conventional
Precise leveling
GNSS
Total station
Settlement markers
DFOS
Structural strain response
Deformation reconstruction
Replacement Potential
Low / Medium

DFOS can identify structural effects of settlement but absolute elevation should generally be measured using dedicated geodetic systems.

Joint Displacement

Conventional
LVDT
Crack meter
Displacement transducer
DFOS
Fiber crossing joints
Distributed strain monitoring
Replacement Potential
Medium / High

Strategically installed fibers can continuously monitor deformation around multiple joints.

Rock Bolt Load

Conventional
Instrumented rock bolt
Load cell
Strain gauge
DFOS
Distributed fiber along selected rock bolts
Replacement Potential
Medium

DFOS can provide distributed strain along the support element rather than a single measurement point.

Vibration

Conventional
Accelerometer
Geophone
DFOS
Dynamic distributed fiber sensing
Replacement Potential
Medium

Suitability depends strongly on interrogator bandwidth, spatial resolution and dynamic sensing capability.

Ground Pressure

Conventional
Earth pressure cell
Pressure cell
DFOS
Structural strain response
Replacement Potential
Low

DFOS does not directly measure ground pressure and dedicated pressure cells remain appropriate.

Water Leakage

Conventional
Leak detection cable
Flow sensor
Pressure sensor
Visual inspection
DFOS
Distributed temperature sensing
Distributed anomaly detection
Replacement Potential
Medium

DFOS can assist in identifying thermal or structural anomalies associated with leakage, but specialized leak detection may still be required.

Fiber Deployment

DFOS Deployment Methods

Fiber geometry determines which structural response can be captured and how distributed measurements are interpreted.

Longitudinal Layout

Fiber follows the tunnel axis to monitor longitudinal strain and deformation.

Best For
Lining strainLongitudinal deformationPortal transitionsSettlement-related responseLong tunnel sections

Circumferential Layout

Fiber follows the tunnel circumference to monitor deformation around the tunnel cross-section.

Best For
Tunnel convergenceRing deformationCrown responseSide-wall responseInvert behavior

Cross-Sectional Loop

A continuous fiber loop follows the tunnel cross-section, providing distributed information around the structural ring.

Best For
Ring deformationCross-sectional distortionConvergence-related behaviorTunnel lining response

Zig-Zag Layout

Fiber is routed repeatedly across a structural surface to increase spatial coverage and sensitivity to localized deformation.

Best For
Crack-prone liningSegment jointsConstruction jointsLocalized damageConnection zones

Grid Layout

Longitudinal and transverse fibers form a two-dimensional sensing network.

Best For
Large lining surfacesStation interfacesPortalsCross passagesComplex structural connections

Rock Bolt Fiber Layout

Fiber is integrated with or attached along selected rock bolts to monitor distributed strain and load transfer.

Best For
NATM tunnelsDrill-and-blast tunnelsRock support monitoringGround-support interaction

Segment Joint Layout

Fiber crosses segment or ring joints to detect localized deformation and joint opening.

Best For
TBM tunnelsMetro tunnelsSegmental liningsImmersed tunnel joints

Embedded Shotcrete Layout

Fiber is embedded within or installed on the shotcrete support layer to monitor distributed strain and cracking.

Best For
NATMDrill-and-blastRock tunnelsPrimary lining

Helical Layout

Fiber follows a helical path around a tunnel-related structural or support element.

Best For
Rock boltsSupport elementsSpecialized deformation monitoring
Structural Condition

Tunnel Health States

LEVEL 0

NORMAL

Measured tunnel response remains within the established baseline behavior.

LEVEL 1

WATCH

Persistent deviation from baseline behavior has been detected and requires engineering review.

LEVEL 2

ALERT

Measured deformation, strain or other monitored parameters exceed predefined engineering thresholds.

LEVEL 3

CRITICAL

Rapid or severe structural or geotechnical response requires immediate engineering assessment and potentially operational intervention.

Intelligence Layer

From Fiber Data to Structural Intelligence

Distributed measurements can be processed into trends, anomalies, structural indicators and early-warning information.

Baseline establishment
Temperature compensation
Distributed strain analysis
Tunnel convergence analysis
Cross-sectional deformation reconstruction
Crack localization
Joint opening detection
Segment deformation analysis
Ground-structure interaction analysis
Rock-support interaction analysis
Settlement trend analysis
Time-series trend analysis
Anomaly detection
Spatial anomaly localization
Vibration analysis
Dynamic response analysis
Event correlation
Water ingress anomaly detection
Fatigue analysis
Threshold monitoring
Early warning generation
Structural health index calculation
Remaining performance assessment