early warning of geotechnical limit states: slope alarms ... · slope alarms: benefits •ae...
TRANSCRIPT
© 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)
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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
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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)
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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)
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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
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• 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?
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Deep shear surface
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Shallow shear surface
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Wash out/fall
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Wash out
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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?
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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
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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)
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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
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BIONICS, NewcastleSpa, Scarborough
Flat Cliffs, Filey
Players Crescent
Hollin Hill
Ruthlin/Dyffryn
UK trial sites
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Hollin Hill trial
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Source: Smith A, Dixon N, Meldrum P & Haslam E (2014) Inclinometer casings retrofitted with
acoustic real-time monitoring systems. Ground Engineering, October Issue.
Instrumentation
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Cumulative AE: Cluster 2
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Cumulative AE RDC
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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
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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
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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
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Retrofitted inclinometer casing
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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
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Monmouthshire: Retrofitted standpipes
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Players Crescent rail cutting
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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
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Displacements and sensors
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Slope deformations (April/May 2012)
SAA - Geotechnical Observations Ltd
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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)
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Events sequence for movements at
bottom and top of slip
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Triggering rainfall event
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International Trial Sites
Peace River, Canada
Passo della Morte,
Italy
Ripley, Canada
Grossreifling, Austria
Passo della Morte, Italian Alps
Instruments
installed
from tunnel
Boreholes
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AEWG2 event, November 2011
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AEWG2 event, November 2011
RDC response to rainfall
AE response to changing water level
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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
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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]
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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
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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