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Tunnel Instrumentation for Settlement ControlZhangwei Ning, Ph.D.Sixense Inc.

Field instrumentation is vital to the practice of geotechnics, in contrast

to the practice of most other branches of engineering in which people

have greater control over the materials with which they deal. For this

reason Geotechnical engineers, unlike their colleagues in other fields,

must have more than casual knowledge of instrumentation: to them it

is a working tool, not merely one of the components of research.

--- Ralph B. Peck

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Tunnel Instrumentation and Monitoring (I&M)

Ground MovementSurface Settlement

▪ Long duration (usually >3 years)

▪ Big Zone Of Influence (ZOI)

▪ Complex and sensitive environments

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Tunnel Instrumentation and Monitoring (I&M)

Complicated and challenging I&M Plan

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Tunnel Project I&M

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Tunnel Project I&M

▪ Spatial and Temporal Distribution

▪ Accuracy and Precision

▪ Monitoring Frequency

▪ Baseline Monitoring

▪ System Redundancy

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Important Monitoring Design Considerations

▪ Accuracy: to describe how close a measurement is to the true value.

‘The correctness with which a measured value represents the true

value.’ - ASCE (2000), Guidelines for instruments and measurements for

monitoring dam performance

▪ Precision: To describe how close a series of measurements are to

each other.

‘The ability of a sensor to repeatedly give the same reading for

the same measurand, is termed precision.’ - Barrie Sellers President

Emeritus of Geokon

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Accuracy and Precision

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Accuracy VS Precision

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The only truth is we don’t know about the truth

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The only truth is we don’t know about the truth

The monitoring frequency depends on a number of factors,

including:

▪ The rate at which change is expected to develop

▪ The stage of construction

▪ The distance to active construction zone

▪ Requirements of any contingency or emergency plan

▪ Practical response time to control a construction process

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Monitoring Frequency

▪ Miss the details in the true signal

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When Monitoring Frequency Is Low

▪ Late/Mis-detection of changes

It is tempting to over-specify monitoring frequency

requirement simply because of the available technology, but

with no real benefits and even adverse effects:

▪ Overload data transmission and management system

▪ Increase system cost

▪ Shorten system life

▪ More data ≠ more useful information

▪ Create digital-waste

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The Curse of Over-sampling (high frequency)

▪ Baseline monitoring are needed to establish the stability of any

monitoring system prior to construction. It is used as a reference

against which subsequent change is measured.

▪ The baseline measurements should also be used to determined any

existing movements which could be otherwise be attributed to the

works.

▪ A long period of baseline monitoring will increase confidence, but this

may need to be balanced against other factors such as cost or the risk

of delay to the works.

- British Tunneling Society, Monitoring Underground Construction, A best practice guide15

Baseline Monitoring

Stressmeter

Levelling

Theodolite Digital imagecorrelation

Extensometer VibrationMonitor

Total Station GSM Data

Transmission

Laser Scanner Drones

Piezometer Submarine Monitor

Data-logger Time DomainReflectometry

Fiber Optics Multi-parametric borehole system

Web-based Data

Management

Load cell Inclinometer Laser distance-meter

In-place

inclinometer

GNSS Interferometric

radar

Wireless

Monitoring

1950s 1960s 1970s 1980s 1990s 2000s 2010s

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Evolution of Geotechnical I&M

Paolo Mazzanti, 2018

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Sub-surface and Surface Monitoring for Tunneling

The primary way for sub-surface parameters monitoring are still

through those conventional sensors, such as:

▪ Piezometer

▪ Borehole extensometer

▪ Inclinometer (SAA is a new counterpart)

Improvements:

▪ Miniaturized, Lighter

▪ Built-in wireless communication

▪ Digitized

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Subsurface Instruments

▪ Multiple Point Borehole Extensometers (MPBX) and vibrating wire Piezometers

were placed together in boreholes along the entire tunnel alignment, with an

averaged 50 ft spacing;

▪ MPBX: 5 ft, 10ft and 20 ft above tunnel crown; (145)

▪ Piezometer: near the bottom anchor of MPBX; (250)

▪ Manual inclinometer: adjacent or in the path of the TBM. (110)

▪ Automatic In-place inclinometer (IPI): around the rescue shaft

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Subsurface Instruments used in SR-99 Tunnel in Seattle

▪ Piezometer installed near rescue shaft

It captured both the construction related

dewatering effect and the non-

construction related daily tidal effect.

▪ MPBX and Piezometer installed along

TBM alignment

MPBX measured the above crown

settlement and its partial rebound while

piezometer responded to and tracked

shield pressure

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Subsurface Instruments used in SR-99 Tunnel in Seattle

(Cording et al., 2017)

Shape Accelerometer Array (SAA)

Image courtesy of Measurand

Horizontal SAA▪ A low overburden railway tunnel constructed in Toronto, with 11 Horizontal

SAA installed 1.5 m below surface to monitor Highway (21 lane) settlement

Image courtesy of TTP

Horizontal SAA

Image courtesy of GKM

Horizontal Shape Array (SAA)

Image courtesy of GKM

▪ Drilling

▪ Uncertainty in subsurface

▪ Installation quality control

▪ Difficulties in verifying results

▪ Non-repairable

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Subsurface Instruments - limitations

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Data Acquisition and Wireless Transmission

Lower Power Wide Area Network (LPWAN)

Image courtesy of Loadsensing

Automatic Motorized Total Station (AMTS), or

Robotic Total Station (RTS)

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Surface Monitoring - AMTS

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Evolution of AMTS

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Theodolite Total Station[Thodolite + EDM]

1920s 1980s 1990s 2010s

• Angle measurement • Angle measurement• Distance measurement

Automatic Motorized Total Station[Total Station + ATR + Motorization]

• Angle measurement• Distance measurement• Motorized• Automatic target detection

• Reflectorless EDM • Imaging • Piezo-motorization• Integrated scanner

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HOW AMTS Settlement Monitoring Works

Xi, Yi, Zi

Xi, Yi, Zi

Xi, Yi, Zi

Xi, Yi, Zi

Xi, Yi, Zi

Settlement Zone?,?,?X, Y, Z ?i, ?i, ?i

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?i, ?i, ?i

?i, ?i, ?i

Xi, Yi, Zi

Xi, Yi, Zi

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Xi, Yi, Zi

Xi, Yi, Zi

Xi, Yi, Zi

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AMTS Remote Control and Data Communication

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Cloud Server

Computation and data/alert management

Control boxAMTS control and communication module

Engineer

Monitoring data reporting

AMTS

Internet connection

Instrumentation Specialist

Remote Control and Programing

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AMTS Grouping

510

510

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AMTS + IoT

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AMTS + IoT

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SR-99 (Alaskan Way Viaduct Replacement)- Seattle

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NorthLink - Seattle

▪ NO ATR (Automatic Target Recogntion)

▪ Fixed Horizontal and Vertical Angle

▪ Dz at (X0, Y0) is approximately = |Z1-Z0|

▪ The flatter the surface and the bigger the shooting angle, the closer the

approximation

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AMTS in Reflectorless Mode

X0, Y0, Z0

X1, Y1, Z1

▪ A 350ft long 42-inch OD casing pipe by micro-tunneling under the Interstate 405

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Reflectorless AMTS Monitoring I-405 Los Angeles

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Reflectorless AMTS Monitoring I-405 Los Angeles

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Reflectorless AMTS Monitoring I-405 Los Angeles

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Reflectorless AMTS Monitoring

▪ Started early in the 1990’s

▪ Take a long way to becomes popular today [In many instrumentation

and monitoring specification]

▪ High precision in 3D: 1mm within 100 m distance

▪ Fast Measurement: 80 - 100 targets per hour/one AMTS

▪ 24/7 operations with remote control

▪ Near-real time data processing and control

▪ Harsh weather resistance

▪ No traffic closure for prism installation in reflectorless mode

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AMTS Features

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Monitoring from the Sky - Satellite Radar Interferometry▪ Phase comparison to reveal surface deformation at mm-scale.

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Satellite Radar InterferometryHistorical studies/Baseline Monitoring Monitoring

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InSAR for Tunneling Monitoring

Satellite monitoring

Construction Active Ground-based instrumentation + wide coverage low frequency InSAR Monitoring

Post-Construction Long term Residual settlement monitoring by InSAR

Pre-constructionBaseline monitoring through InSAR

Active Ground-based monitoring Manual Survey

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InSAR in SR-99 Seattle

Cosmo-SkyMed (X Band) 130 images from Jul-2012 to Dec-

2017

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InSAR in SR-99 Seattle

▪ Over 85,000 monitoring points

▪ Nearly 5-year continuous monitoring

with mm-scale accuracy

▪ Measures actual extent and

magnitude of the effect of the

project, even beyond the theoretical

ZOI

▪ Identifies settlements areas adjacent

to project but not directly related to

project

▪ Complements and validates other

ground-based instruments

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InSAR Monitoring in CrossRail - London

Area of interest 36 km2

902,394 measurement points

24,800 PS/km2

Courtesy of Mike Black, Crossrail

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Satellite Radar Interferometry

▪ City-scale coverage extending outside ZOI.

▪ Settlement accuracy < 3mm

▪ Up to 10,000 points/km2

▪ Historical ground analysis prior to construction

▪ No ground installation required

▪ Absolute and Large Scale. Identify

▪ Validation of ground instrumentation

▪ Litigation mitigation

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More Eyes in Future Sky

More planned SAR satellite constellation will:

▪ Reduce the cost

▪ reduce revisit from days to hours

Photo credit: Capella Space

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Tunnel Monitoring Visualization

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Integrating TBM Data (Bertha)

• TBM Parameters display

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Future of Visualization – Mixed Reality

Photo Credit: BGC Engineering Inc.

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Future of Visualization – Mixed Reality

Photo Credit: BGC Engineering Inc.

▪ Fusion of monitoring and numerical modeling for back-analysis

and prediction

▪ From construction to operation (Tunnel Structure Health

Monitoring)

▪ Artificial Intelligence techniques in tunnel construction

(intelligent early warning, TBM auto-piloting etc.)

▪ Education

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Future Developments