lab experiment reality - tik-old.ee.ethz.ch

I ceI ceU d t di f t th i i itor .. Understanding frost weathering in-situ

Samuel Weber1, Lucas Girard2, Stephan Gruber2, Jan Beutel11ETHZ TIK 2UZH Geography 3G1ETHZ, TIK -- 2UZH, Geography, 3G

Lab experiment Reality

2006

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What stresses near-surface bedrock ?What stresses near-surface bedrock ?What stresses near surface bedrock ?What stresses near surface bedrock ?

A. Hasler


Constant mechanical loads:Constant mechanical loads:• Gravity• (Overburden pressure)

A. Hasler


Constant mechanical loads:Constant mechanical loads:• Gravity• (Overburden pressure)

Fluctuating loads:Fluctuating loads:• Water pressure, Ice formation in pores/cracks• Thermo-mechanical forcing

E h kEarthquakes

A. Hasler

State of the artState of the artState of the artState of the art

Theoretical & lab. insights:Theoretical & lab. insights:

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Theoretical & lab. insights: Transfer to field conditions, where rock Theoretical & lab. insights:already contains flaws/cracks ?

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Theoretical & lab. insights: Transfer to field conditions, where rock Theoretical & lab. insights:already contains flaws/cracks ?

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) Range of temperature ? Sensitivity to liquid water content ?

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Sensitivity to liquid water content ? Nucleation of new cracks or

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propagation of existing ones ?

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Measurement strategyMeasurement strategyMeasurement strategyMeasurement strategy

Acoustic emission (AE) monitoring:Acoustic emission (AE) monitoring:detect small “damage increments”

F 20 100 kHFrequency range: 20-100 kHz• Chose for expected source size Detection range:p• Length scale 1-10cm• Displacement scale 0 1-1μm

g• controlled by freq. range• expect most signals from <0 5 to 1mDisplacement scale 0.1 1μm

• Eq. magnitude -6 to -5expect most signals from <0.5 to 1m

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AE monitoring system: instrumentation and field set-upAE monitoring system: instrumentation and field set-upAE monitoring system: instrumentation and field set upAE monitoring system: instrumentation and field set up

12 V

AE node

AE Sensors(at 10 & 50 cm depth)(at 10 & 50 cm depth)

Temperature probe

Capacitance probe

10

Capacitance probe

WSN d 10cmWSN nodes

50cm

100cm

Depth

The measurement positionsMeasurement site at Jungfraujoch: 2 complete installationsMeasurement site at Jungfraujoch: 2 complete installationsMeasurement site at Jungfraujoch: 2 complete installationsMeasurement site at Jungfraujoch: 2 complete installations

September MarchM1September MarchM1M1M1Spur

M2

M2Densely fractured crystaline rock: GullyDensely fractured crystaline rock:• 5 - 20 clefts / meter• Crack opening .1-1mm

AE signal characteristicsAE signal characteristicsAE signal characteristicsAE signal characteristics

Signal waveform Spectrogram

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emissions(real ones!)

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1year timeseries: rock temperature AE activity1year timeseries: rock temperature AE activity1year timeseries: rock temperature, AE activity1year timeseries: rock temperature, AE activity

Rock temperaturep

10cm5 cm

50cm

Temp. gradientTemp. gradient

10-20 cm5-10 cm

10 20 cm20-50 cm

Spur site (M1)10cm

Spur site (M1)

50cm

Gully site (M2)Gully site (M2)


Simple facts:p


Simple facts:p

Energy rates under frozen conditions are roughly 100 times larger than g y gunder thawed conditions

d f hSustained freezing vs. Freeze-Thaw:• February= sustained freezing +1 cycle• March = 17 cycles• E(February) > 15*E(March)( y) ( )


Simple facts:p

Energy rates under frozen conditions are roughly 100 times larger than g y gunder thawed conditions

d f hSustained freezing vs. Freeze-Thaw:• February= sustained freezing +1 cycle• March = 17 cycles• E(February) > 15*E(March)( y) ( )

Energy rates in the gully (M2) are 10 times larger than on the spur (M1) under frozen conditionsu de o e co d t o s

Rock freezing characteristic curveRock freezing characteristic curveRock freezing characteristic curveRock freezing characteristic curve

Decrease in liquid water content during freezingDecrease in liquid water content during freezing

Gully siteGully site(M2)

S itSpur site(M1)( )

Measurements from capacitive probes (Envirosmart)Data: Oct 2011 Oct 2012Data: Oct 2011 – Oct 2012

Influence of freezing on AE activityInfluence of freezing on AE activityInfluence of freezing on AE activityInfluence of freezing on AE activity

Only events at 50cm considered here

Spur site (M1)5050cm

Gully site (M2)50cm50cm

Are AE scaling properties affected by the loading mode ?Are AE scaling properties affected by the loading mode ?Are AE scaling properties affected by the loading mode ?Are AE scaling properties affected by the loading mode ?

Frozen = T<0 in the whole rock column (5cm – 100cm)Thawed = T>0 at 5- 100cmThawed = T>0 at 5- 100cm

3 03.0

i i i iMicro-seismic events in large rock slide: 2.6Earthquakes: 2.0(Helmstetter & Garambois 2010,

)Kagan 1999)

Energy, duration, waiting times also power-law distributed

ConclusionsConclusionsConclusionsConclusions

At our site, frost cracking is the largest damage driver N f t ki i d No frost cracking window Strong control of water availability on frost cracking Strong control of water availability on frost cracking

Factor ~10 here

Rock damage dynamics appear independent of the loadingRock damage dynamics appear independent of the loading mode (i.e. freezing vs. thermo-mechanical forcing)o ( g g) Similar flaws activated by both loading modes D i d d b ti f i ti fl / k Damage induced by propagation of existing flaws/cracks Suggests the importance “Macro-gelivation” Suggests the importance Macro gelivation

Frost heave in soils: growth of ice lenses through cryo-suctionFrost heave in soils: growth of ice lenses through cryo-suctionFrost heave in soils: growth of ice lenses through cryo suction Frost heave in soils: growth of ice lenses through cryo suction

IceUnfrozen water

ColderColder

OverburdenOverburdenpressure

IceDepression= Ice

Liq water

Depression=Suction of liq. water

Liq. water

Disjoining

GrainSoil particle

Disjoiningforces

Warmer

(Taber 1930, Rempel 2007, …)

Frost heave in soils: growth of ice lenses through cryo-suctionFrost heave in soils: growth of ice lenses through cryo-suctionFrost heave in soils: growth of ice lenses through cryo suction Frost heave in soils: growth of ice lenses through cryo suction

IceUnfrozen water heave

ColderColder

Rupture Ice lens

Grain

Warmer

(Taber 1930, Rempel 2007, …)

How can freezing deform materials?How can freezing deform materials?How can freezing deform materials?How can freezing deform materials?

When freezing, Under sustained freezing:When freezing,water expands by 9%

Under sustained freezing:growth of ‘segregated ice’

- expulsion of liquid water towards unfrozen zones

- fueled by suction of water towards already frozen zonesunfrozen zones

- in rapid freezing conditions: already frozen zones

- can fracture rock and heave soilsp gincrease of pore pressure damage ? damage ?

(Murton et al. 2006)

let rock crack, let cracks rock 25

AE monitoring systemAE monitoring systemAE monitoring systemAE monitoring system

AE sensorAE sensor– Piezoelectric sensor R6α-sensor:

35-100 kHz, resonant at 55 kHzAE rodAE-rod

– Houses the piezoelctric sensorHouses the piezoelctric sensor– Rock-sensor acoustic coupling

with ~2 dB lossAdjustable sensor depth– Adjustable sensor depth

– Other rod materials reflect & attenuate acoustic wavesOther rod materials reflect & attenuate acoustic wavesAE-node

– acquisition, processing andt i i f AE d ttransmission of AE data

– 2 channels 500 kHz sampling rate2 channels, 500 kHz sampling rate– Wireless data transmission– Threshold remotely adjustable

Estimating rock liquid water contentEstimating rock liquid water contentEstimating rock liquid water contentEstimating rock liquid water content

Sentek Envirosmart:Sentek Envirosmart:Material dielectric constant as a proxy for liquid water content

Generate electrical field in radio‐freq range measure resonant frequency (and material capacitance)obtain material dielectric constant εmatεmat strongly depends on liquid water content

Freq. range 100‐300 MHzDielectric constants involved:•εrock=2‐5•εair=1•εice=3•εwater=80‐100

Fabbrriet al. 2006

AE signals: how to select the right ones ?AE signals: how to select the right ones ?AE signals: how to select the right ones ?AE signals: how to select the right ones ?> 650 000 events detected> 650 000 events detected

Analogy to lab. rock fracturing experiments:D= k1 * A + k2

fracturing experiments:1 2

(Cox & Meredith 1993)

AE signals: how to select the right ones ?AE signals: how to select the right ones ?AE signals: how to select the right ones ?AE signals: how to select the right ones ?> 650 000 events detected> 650 000 events detected

Analogy to lab. rock fracturing experiments:fracturing experiments:D= k1 * A + k2

(Cox & Meredith 1993)1 2

Event generated by an artificial rock fall

+an artificial rock fall(~50 liters)

We cannot strictly exclude the effects of rock falls,l h f d t t!or snow avalanches from our dataset!

Results: AE signalsResults: AE signals103

Results: AE signalsResults: AE signals

102

10Class 1Class 2Class 3Cl 4

+ Class 1Cl 2

101Class 4Class 2

Class 3

10−1

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PD

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Class 1Class 2Class 3Class 4

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101 102 103 104 105

10 10 10 10 10Duration (s)

Are AE scaling properties affected by the loading mode ?Are AE scaling properties affected by the loading mode ?Are AE scaling properties affected by the loading mode ?Are AE scaling properties affected by the loading mode ?1 96 2 30 1 99Frozen = T<0 in the whole rock column (5cm – 100cm)

102expo:3.07, expo:3.72, expo:3.35,

frozen

102expo:1.96, expo:2.30, expo:1.99,

frozenth d

( )Thawed = T>0 at 5‐ 100cm

101

frozenthawedall 100

thawedall

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3.0 10−2

2.010−1

PDF

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PDF 2.0

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Energy (V2)

expo:3 20 expo:3 28 expo:3 2310 10Amplitude (V)

Energy (V )100

expo:3.20, expo:3.28, expo:3.23,

frozenthawedMicro‐seismic events in rock slide: 2.6

10−2

thawedallEarthquakes: 2.0

(Helmstetter & Garambois 2010,

F 3 2

Kagan 1999)

10−4

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F 3.2

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102 10310−8

Duration (s)

Freezing vs thermo-mechanical forcingFreezing vs thermo-mechanical forcingFreezing vs. thermo mechanical forcingFreezing vs. thermo mechanical forcing

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