Early warning of geotechnical limit states: Slope ALARMS ... · Slope ALARMS: Benefits •AE monitoring provides an early warning of slope failure •Information on slope displacement

© Loughborough University

Professor Neil DixonSchool of Civil and Building Engineering

Loughborough University

Early warning of

geotechnical limit states:

Slope ALARMS (a slope

displacement rate sensor)


Outline of presentation

• Monitoring geotechnical limit states

• Known unknowns

• What, where and when?

• Continuous, real-time and robust

• Trigger levels and who to share the data with

• Slope ALARMS: Displacement rate sensor

• Acoustic emission (AE) monitoring

• Case studies, including comparisons with traditional techniques

• Summary: The benefits


Specification for an

early warning system

• Sufficient warning to enable action to be taken

(implement emergency plan)

• No false alarms (undermines confidence)

• Provide information on rates and magnitude of

movement (assess likelihood and significance)

• Identify mode of failure (assess significance and inform

action)


Geotechnical limit states

• Ultimate limit states

Complete loss of stability (e.g. shear failure)

First-time failures – fast moving & large displacements

Reactivated – slow moving & often (but not always)

small displacements

• Serviceability limit states

Loss of function (e.g. track alignment, debris on track)

• Both are a function of deformations (i.e.

magnitude and distribution)


How difficult can it be?

• Can complex mechanisms be identified by a ‘simple’

trigger level from a measured value?

Complex relationships and processes: Site specific behaviour

such as weather, vegetation, geometry, material, history,

engineering interventions, time…

Causes: Rainfall pore water pressures & moisture content –

leading to reduced strength (possibly increased destabilising

forces), and through shearing processes these result in…..

Effects: Deformations (when, where, how fast, magnitude)?

Time frame: Rate of stability decrease from initiation to collapse?

• It is not easy and there is no magic wand


• Known unknowns

‘Every instrument on a project should be selected and

placed to assist with answering a specific question, if

there is no question then their should be no

instrumentation’ Dunnicliff (1988)

Do you know what you don’t know (i.e. what question

do you need to answer)?

What, where and when?


Deep shear surface


Shallow shear surface


Wash out/fall


Wash out


Must haves

• Reliable measurements (robust with no false alarms)

• Low/little maintenance

• High confidence levels

Tried and tested (…but how do you achieve this for

new ideas?)

• Measurements that are: Relevant, accurate, reliable,

affordable, accessible

• Continuous and real-time?

• Verification – Other instruments, cameras, site visit?


Trigger levels – so what?

• Alarms (information) – who should it be sent to?

If interpretation is needed, send to expert Geotechnical Engineer to

turn the data into useful information!

If a clear/simple response can be made (no interpretation required),

send to central control (but how to minimise/deal with false alarms?)

Simple trigger levels must be reviewed and revised over time based on

experience (engineering judgement needed)

• What does real-time monitoring mean?

Arguably, real-time requires processing on the instrument so output

(trigger) can be actioned immediately


Slope ALARMS: AE monitoring technique

• AE are relatively high-frequency stress waves which

propagate through materials surrounding the

generation source (10s of kHz)

• The AE monitoring technique is well established in

other industries

• In soil, AE is generated by inter-particle friction and

hence the detection of AE is an indication of

deformation

• Research over a 50 year period has shown that AE

can be used to detect deforming soil bodies (slopes)


Monitoring approach: Active waveguide

Assessment of

Landslides using

Acoustic

Real-time

Monitoring

Systems

ALARMS

communication

node

Active waveguide

Transducer

ALARMS sensor node

Amplitude (V)

Time

Threshold

level (V)

Ring-down counts (RDC)

If RDC > Trigger

value,

send warning

WSN

WSNGSM

A slope displacement

rate sensor

Slope ALARMS

• Collaboration between

Loughborough University and

British Geological Survey

• Unitary battery device

• Piezoelectric transducer

detects AE

• Sensor measures AE ring-down counts (RDC) and logs

number for a set time period (e.g. 15, 30, 60 minutes)

• Automatic SMS messages sent when thresholds are

exceeded

• Trials are underway at sites in UK , Austria, Canada and

Italy


BIONICS, NewcastleSpa, Scarborough

Flat Cliffs, Filey

Players Crescent

Hollin Hill

Ruthlin/Dyffryn

UK trial sites


Hollin Hill trial


Source: Smith A, Dixon N, Meldrum P & Haslam E (2014) Inclinometer casings retrofitted with

acoustic real-time monitoring systems. Ground Engineering, October Issue.

Instrumentation


Cumulative AE: Cluster 2


Cumulative AE RDC


Peak velocity

After Dixon et al. (2014)

Typical AE response to reactivated slope movements (Hollin Hill)

Source: Dixon N, Spriggs MP, Smith A, Meldrum P & Haslam E (2014) Quantification of

reactivated landslide behaviour using acoustic emission monitoring. Landslides, 1-12.

DOI: 10.1007/s10346-014-0491-z


Cumulative

displacement

derived from AE

Quantification of velocity and displacement from AE (Hollin Hill)

Source: Dixon N, Spriggs MP, Smith A, Meldrum P & Haslam E (2014) Quantification of

reactivated landslide behaviour using acoustic emission monitoring. Landslides, 1-12.

DOI: 10.1007/s10346-014-0491-z


Hollin Hill landslide, UK: Comparison of

continuous AE and SAA measurements

Source: Smith A, Dixon N, Meldrum P, Haslam E & Chambers J (2014) Acoustic emission

monitoring of a soil slope: Comparisons with continuous deformation measurements.

Géotechnique Letters 4(4), 255-261. OPEN ACCESS DOI: 10.1680/geolett.14.00053

http://www.icevirtuallibrary.com/content/article/10.1680/geolett.14.00053


Retrofitted inclinometer casing


Source: Smith A, Dixon N, Meldrum P & Haslam E (2014) Inclinometer casings retrofitted with

acoustic real-time monitoring systems. Ground Engineering, October Issue.

Retrofitted inclinometer waveguide vs SAA


Monmouthshire: Retrofitted standpipes


Players Crescent rail cutting


Source: Dixon N, Smith A, Spriggs MP, Ridley A, Meldrum P & Haslam E (2014) Stability

monitoring of a rail slope using acoustic emission (Accepted for publication).

AE and inclinometers: Installed Feb. 2011


Displacements and sensors


Slope deformations (April/May 2012)

SAA - Geotechnical Observations Ltd


Cumulative AE vs deformations

0

5000

10000

15000

20000

25000

30000

35000

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

17/03/2012 11:02 27/03/2012 11:02 06/04/2012 11:02 16/04/2012 11:02 26/04/2012 11:02 06/05/2012 11:02

Cu

mu

lati

ve R

DC

Dis

pla

cem

en

t (m

m),

Ho

url

y

rain

fall/1

0 (

mm

)

Time (days)

Players Crescent - Deformation event - 19/04/2012 to 05/05/2012, 1.2mm over 2 weeks (~ 0.09mm/day), SAA and AEWG1

Cumulative slopedisplacement(mm)

Hurn hourlyrainfall (mm)

Cumulative RDC

0

100

200

300

400

500

600

700

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

17/03/2012 11:02 27/03/2012 11:02 06/04/2012 11:02 16/04/2012 11:02 26/04/2012 11:02 06/05/2012 11:02

RD

C r

ate

(p

er

ho

ur)

Dis

pla

cem

en

t (m

m),

Ho

url

y

rain

fall/1

0 (

mm

)

Time (days)

Players Crescent - Deformation event - 19/04/2012 to 05/05/2012, 1.2mm over 2 weeks (~ 0.09mm/day), SAA and AEWG1

Cumulativeslopedisplacement(mm)

RDC rate (perhour)


Events sequence for movements at

bottom and top of slip


Triggering rainfall event


International Trial Sites

Peace River, Canada

Passo della Morte,

Italy

Ripley, Canada

Grossreifling, Austria

Passo della Morte, Italian Alps

Instruments

installed

from tunnel

Boreholes



AEWG2 event, November 2011


AEWG2 event, November 2011

RDC response to rainfall

AE response to changing water level


Trial for the Austrian Railways

• SART – Sentinel for Alpine Rail Traffic

• In partnership with German Company INGLAS

• Combined system of monitored debris catch

fence for immediate warning of threat to line and

Slope ALARMS for early warning for inspection/

remedial action

• System linked to OeBB Operation Control Centre

• Ongoing extended trial in progress


Weak conglomerate Instrumented fence

Vertical waveguide Horizontal waveguides

Event

example

Ax Ay Az Tx Ty Tz31.05.2014 16:07:23 SW1 1 13 3,62 11 0 -74 46 62 31 31 0 1.031

31.05.2014 16:08:19 SW3 1 13 3,58 12 0 -79 31 93 78 62 0 1

31.05.2014 16:13:47 SW2 1 13 3,53 10 0 -83 15 15 31 46 -15 1.015

31.05.2014 16:14:18 SW3 1 13 3,51 10 0 -94 15 15 31 46 0 1

Pull Out

StatusRSSI [dBm]

max. dyn. accelertion [1/1000 g] static acceleration [1/1000 g]Timestamp Type Domain Device Voltage [V] Temp. [°C]


Slope ALARMS: Benefits

• AE monitoring provides an early warning of slope failure

• Information on slope displacement rates is instantaneous, continuous and in real-time

• AE rates are proportional to slope displacement rates over many orders of magnitude

• AE rates can detect very slow displacement rates and continue to operate at large displacements (>>500mm)

• Inclinometer casings can be converted into continuous and real-time displacement rate sensors


Acknowledgements

Funders: EPSRC, FFG and stakeholders

Partners: Sensor electronics and operation developed with

British Geological Survey

Collaborators: Geotechnical Observations Ltd; INGLAS,

Germany; CNR-IRPI, Italy; Queen’s University, Canada;

University of Alberta, Canada, Newcastle University;

CH2M; Thurber Engineering; Parsons Brinckerhoff

Stakeholders: Network Rail, OeBB (Austrian Railway),,

Alberta Transportation, Scarborough Borough Council,

Monmouthshire County Council, Canadian Pacific Railway,

Canadian National Railway

[email protected]© Loughborough University

www.slopealarms.com

Documents

Early warning of geotechnical limit states: Slope ALARMS ... · Slope ALARMS: Benefits •AE monitoring provides an early warning of slope failure •Information on slope displacement