state of the art ontensiometers and axis- translation ... · marinho et al (2003) the tensiometer:...

State of the Art onTensiometers and Axis-Translation technique

International SymposiumADVANCED EXPERIMENTAL UNSATURATED SOIL MECHANICS

Trento, 27-29 June 2005

Fernando A. M. MarinhoW. Andy Take

Alessandro Tarantino

Basic Concepts:(wettability, capillarity, metastablity, air-entry)

Presentation Outline

The Tensiometer:(design, saturation, challenges)

Axis Translation Technique:(design, use, challenges)

Conclusions

Basic Concepts: Wettability

Basic Concepts: Capillarity

Water

(-)

(+)

uw

0(-) (+)

What determines

this maximum value?

Idealisedcapillarypore

uw

ua

Τ2r

α

Wettability!

Soil Soil

VoidsVoids

Capillary phenomenaCapillary phenomena

Terzaghi (1925)


1 10 100 1000 10000 100000Suction (kPa)

0

10

20

30

40

Wat

er C

onte

nt (%

)

(a)

(b)

(c)

(d)

(a) - hypothetical single pored material(b) - single pored material(c) - double pored material(d) - multi pored material


Marinho (2005)


Perera et al. (2005)

Basic Concepts: Cavitation

Water

(-)

(+)

uw

0(-) (+)

What determines

this maximum value?

Galileo-GalileiThe first observation that water cannot rise indefinitely has been attributed to Galileo Galilei.

He found that 10m was the limit to which the water could rise in a suction pump.

This limit is determined by the pheomenia of cavitation

Stratosphere Giant112.34 m

http

://w

ww

.pla

ntph

ys.n

et/p

rinte

r.php

?ch=

4&id

=100

http://canopy.evergreen.edu/workshop02/bvp.asp


Air bubbles arising in a glass tube. (Harvey et al., 1944)


t = 0 -

ua=0

uw>0

t = 0 +

uw<0

ua<0

t > 0

ua=0

uw<0

Menisci sustain the positive water pressure and hinder air cavity dissolution

Water pressure attains negative values, the cavity expands and air pressure drops (gas state equation)

Air diffuses towards the cavity to restore atmospheric air pressure (Henry’s law) and cavity futher expands. Eventually, the air cavity detaches from the surfcae and trigger cavitation

xa xa xa

Basic Concepts: CavitationA Cavitation Mechanism (Harvey et al, 1944)A Cavitation Mechanism (Harvey et al, 1944)

Does this mean Does this mean that tensiometers that tensiometers are limited toare limited to-- 1atm?1atm?

Solid boundary

Atmosphericpressure

Pre-pressurisation

Water reservoir

Basic Concepts: CavitationA Cavitation Mechanism (Harvey et al, 1944)A Cavitation Mechanism (Harvey et al, 1944)

Fernando: A possible alternative slide to the previous one

Air diffusionDoes this mean Does this mean

that tensiometers that tensiometers are limited toare limited to-- 1atm?1atm?

Basic Concepts: Metastable State of WaterAfter Balibar (2002)

Pure water can carry tension!

Finite density at zero pressure

The Tensiometer: Design1900 20001950

Mey

er (1

911)

Mey

er (1

911)

Apf

el (1

970)

Apf

el (1

970)

Cha

pman

(197

5)C

hapm

an (1

975)

Hen

ders

on &

Spe

edy

(198

0)H

ende

rson

& S

peed

y (1

980)

Hig

ht (1

982)

H

ight

(198

2)

Ohd

e et

al.

(199

2)O

hde

et a

l. (1

992)

Porous ceramic

Pressure tranducerWater reservoir

Porous ceramic

Pressure gauge

Water reservoir

Maintenance system

The Tensiometer: Conventional Design

Pressure tranducer

Needle

System been conected


Bubble formation in ordinary tensiometer


The Tensiometer: Design1900 20001950

Mey

er (1

911)

Mey

er (1

911)

Apf

el (1

970)

Apf

el (1

970)

Cha

pman

(197

5)C

hapm

an (1

975)

Hen

ders

on &

Spe

edy

(198

0)H

ende

rson

& S

peed

y (1

980)

Hig

ht (1

982)

H

ight

(198

2)

Ohd

e et

al.

(199

2)O

hde

et a

l. (1

992)

Rid

ley

(199

3)R

idle

y (1

993)

Gua

n (1

994)

Gua

n (1

994)

Mar

inho

(199

5)M

arin

ho (1

995)

Tara

ntin

o &

Mon

giov

ì (20

02)

Tara

ntin

o &

Mon

giov

ì (20

02)

Take

(200

2)Ta

ke (2

002)

Ridley & Burland (1993)

The Tensiometer: High Capacity Design

High capacity tensiometer (HCT)

Pressure transducer without the porous ceramic

Porous ceramic

The Tensiometer: High Capacity Design

15 bar air entryporous ceramic

diaphragmstrain gauge

water reservoir

water pulled by the soil

The Tensiometer: Working Principle

air

water

ATMOPSPHERIC

NEGATIVE

The high air entry ceramic sustains the pressure differential between the negative water pressure in the reservoir and the atmospheric air pressure outside the ceramic

The Tensiometer: Saturation

Initial Saturation:•Evacuation of tensiometer reservoir / ceramic•Importance of initially dry ceramic (e.g. Take & Bolton, 2003)•Rotational technique (low air entry value ceramics)•Two chamber technique (higher air entry value ceramics)

Pre-pressurisation (Conditioning):•Application of a large positive water pressure•Cycles of cavitation and pressurisation (Tarantino & Mongiovì, 2001)

Difficulty of Saturation•Increases with air-entry value (AEV)•Required conditioning pressure increase with AEV

Chemical Treatment•May help in the reduction of number of cavitation nuclei?

Solid boundary

Atmosphericpressure

Pre-pressurisation

Water reservoir

The Tensiometer: Saturation

Gas

Water reservoir

Solid boundary

Atmosphericpressure

Water tensionmagnitude

NO

RM

AL

CO

ND

ITIO

NED

Installation effects

The Tensiometer: ChallengesInterpretation

Long term measurements

How do we know if we have properly saturated a device?

Premature tension breakdown

Tensiometer Challenges: InterpretationInterpretation:

Perhaps the biggest challenge associated with the direct measurement of suction is to understand when your tensiometer is giving a misleading observation of matric suction.

An example...

Tensiometer Challenges: Interpretation

“my zero offset has changed!”… NO!

Why we have gotten away with poor saturation in positive measurements.

Appears like an equilibrium reading

Tensiometer Challenges: Interpretation

The pressure in the reservoir may differ from the pressure at the soil-ceramic interface if the ceramic is not adequately saturated

Another instructive example

0 10 20 30 40 50time (hours)

-2000

-1600

-1200

-800

-400

0

400

800pr

essu

re (k

Pa)

soil paste

cavitation

TRENTO tensiometer

recovering

free water

wipedat 4MPa for 1 min

wiped

Tensiometer Challenges: InterpretationStrategy: Use multiple tensiometers at the same location

Comparing measurements of two IC tensiometers

“Blue” probe saturated only for 100 min after triggering cavitation

0 20 40 60time (h)

-1200

-1000

-800

-600

-400

-200

0

pres

sure

(kP

a)

pr2

pr4

0 10 20 30 40 50time (h)

-1200

-1000

-800

-600

-400

-200

0

pres

sure

(kP

a)

pr2

pr4

Tarantino & Mongiovì (2001)

Tensiometer Challenges: Assessing Sat.?How do we know if we have properly saturated a device?

•Perform a “descructive” test of saturation allow water to evaporate from the device. The maximum observed suction should correspond with the nominal air entry value (e.g. Ridley & Burland, 1999)

•Measure the response time of the device in the range -100 to -90 kPa. (Take & Bolton, 2003)

•Indicators of adequate saturation of the ceramic (Tarantino, 2004)

•Use two tensiometers at the same location (Tarantino, 2005)

Tensiometer Challenges: Long-term•Cavitation is an instability driven process. Therefore, the timeof measurement is highly variable

•Mechanism of tension breakdown is probably linked to diffusion

•Measurement duration in high-capacity tensiometers is typicallyweeks for s<800 kPa, days for 800<s<1500 kPa, hours s>1500 kPa

•Measurement duration can be significantly improved by cycles of cavitation followed by pressurisation (Tarantino, 2004)

•Could different procedures for initial saturation and pre-pressurisation, different types of ceramics, or chemical treatments improve long-term response?

700 750 800Time (min)

1480

1520

1560

1600

Suc

tion

(kP

a)

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500Time (min)

0

400

800

1200

1600

Suc

tion

(kP

a)

50 kPa 100 kPa 200 kPa 300 kPaσv=14 kPa

Boso et al (2004)

The Tensiometer: InstallationSoil paste to make contact

•The tensiometer was locked in place and soil paste was initially too wet•Paste shrinkage caused partial detachment of the paste and measurement was controlled by water vapour equilibrium •Temperature-induced fluctuations are not observed in ‘good’ contact•Measurement precision is typically less than 10 kPa in ‘good’ contact

Air room temperature:20±0.5°C

Boundary Conditions

Finte element mesh Materials

SOIL SAMPLE

SOIL PASTE

POROUS CERAMIC

The Tensiometer: Installation

Tensiometer Challenges: InstallationInstallation Effects:

•The way the tensiometer is installed will affect the time until an equilibrium measurement is obtained.

•Dry soil vs wet soil (Ridley et al. 2003)•Amount of paste (Fernando – put your ref here)

0

50

100

150

200

250

300

350

400

0.00 5.00 10.00 15.00 20.00 25.00 30.00

Time (min.)

Suc

tion

(kP

a)

Experimental datapaste r=0.3 mm, paste thickness=0.1mmpaste r=0.6 mm, paste thickness=0.1mmpasta r=0.3 mm, paste thickness=0.2mm

Tensiometer Challenges: Premature Cav.Premature Cavitation:

•Definition: Caviation prior to reaching the air-entry value of the device

•The design of tensiometers must eliminate the space for large bubbles to “hide”.

•Example from Take (2003) in which an elastomer trapped bubbles

0 200 400 600 800 1000 1200 1400 1600 1800Confining Pressure (kPa)

-400

-300

-200

-100

0

100

200

300

400

500

600Po

re W

ater

Pre

ssur

e (k

Pa)

SD-4 Loading

SD-7 Loading

SD-4 Unloading

SD-7 Unloading

Marinho et al (2003)

The Tensiometer: Applications

0 100 200 300 400 500

Suction (kPa)

0

100

200

300

400

500

(σ1−

σ 3)/

2 ( k

Pa)

Conf. 0 kPaConf. 50 kPaConf. 100 kPaConf. 200 kPaConf. 300 kPa

Compacted Residual SoilOptimum W/C

Oliveira (2004)


0

100

200

300

400

500

Suct

ion

(kPa

)

Optimum moisture content

0

100

200

300

400

500

Suct

ion

(kPa

)

Dry of optimum

0 5 10 15 20 25 30 35 40 45 50

Time (min.)

0

100

200

300

400

500

Suct

ion

(kPa

)

Wet of optimum

Oliveira & Marinho (2005)



Suction-monitored static compaction

Post-compaction suction increases as degree of saturation increases !!

0 200 400 600 800 1000 1200Matric suction, -uw : kPa

0.2

0.4

0.6

0.8

1

Deg

ree

of s

atur

atio

n, S

w=0.311

w=0.299w=0.275

w=0.259w=0.254 w=0.236

w=0.215

De Col & Tarantino (2004)

Loading-unloading cycles to 300, 600, and 1200 kPa


Suction-monitored shearbox

Caruso & Tarantino (2004)

silicon grease

latex membrane

specimen

silicon grease

specimen

tensiometer

brass retaining plate

brass loading pad

O-ringupper half

lower half

clamp

silicon grease

latex membrane

specimen

silicon grease

specimen

tensiometer

brass retaining plate

brass loading pad

O-ringupper half

lower half

clamp

Investigation of the transition from unsaturated to saturated states

0 1 2 3 4 5 6 7 8Horizontal displacement: mm

-80

-40

0

Por

e w

ater

pre

ssur

e: k

Pa

0

100

200

Mea

n sh

ear s

tress

: kP

a

Motivation:

Tensiometers are subject to cavitation (no long-term measurement in mechanical and hydraulic testing)

Tensiometers are not (yet?) commercially available

What can be done to measure the pressure differential between the pore-water pressure and the pore-air pressure?

The Axis-Translation Technique

Atmospheric pressure 1.4 bars

Saturn

Atmospheric pressure 92 bars

Venus

Atmospheric pressure (bars) 1.013

Earth

http://www.solarviews.com/

Atmospheric pressure 0.007 bars

Mars


There is nothing There is nothing “magical” about “magical” about 1.013bars! 1.013bars!

We can artifically We can artifically raise the “atmospheric raise the “atmospheric pressure” of a sample pressure” of a sample in the lab!in the lab!

The Axis-Translation TechniqueA

bsol

ute

Pres

sure

(kPa

)

0Metastable!

Atmospheric

Matric suction, P1 A

bsol

ute

Pres

sure

(kPa

)0

Metastable!

“New Atmospheric”

Matric suction, P1

Cavitation is now not an issue!


Conceptual justification

For incompressible liquid, an increase in air pressure does not alter the meniscus curvature and, hence, the difference uw-ua

θ=θr θ=θr

uw-ua= -2T cosθr /ruw-ua= -2T cosθr /rua=0 ua>0

uw<0 uw>0

Hilf (1956)


Scholander et al (1965)

SOIL

Air pressure

Water(Atmospheric pressure or known pressure)Pressure transducer

porous ceramic


porous ceramic

Air pressure

Air pressure higher than the air entry pressure


Tensiometer versus Axis-Translation

Similarities•A pressure differential is measured across a high air entry ceramic•Expansion of air cavities in the ceramic may also take place in axis-translation technique, but this problem is (probably?) negligible if the ceramic is adequately saturated •Water pressure in the water reservoir may differ from pore-water pressure if the ceramicisnot adqualtely saturated

Differencies•Water in axis-translation is NOT in a metastable state•In axis-translation, relatively large air cavities in the ceramic may slowly expand (because of air diffusion) but there is not rapidexpansion of cavitation nuclei.

Validity of Axis-Translation Technique

High air entry ceramic

Water reservoir

σ0 = 0

ua0 = 0

uw0 < 0

w ua > 0

At the same water content, can we state that

uw = 0

ua0 - uw0 = ua - uw

and hence chamber air pressure ua equals negative pressure –uw0?

w

Validation of the Axis-Translation

Medium degree of saturation (Sr < 0.85-0.9)•Bishop & Donald (1961)•Tarantino, Mongiovi, and Bosco (2000)

High degree of saturation (Sr > 0.85-0.9)•Fredlund & Morgenstern (1977) (data not really consistent) •Tarantino and Mongiovi (2005) (one single test)

Is axis translation reliable at high degrees of saturation? We have not clear and convincing experimental evidence!

Conceptual limitation of Axis-Translation

0.85-0.90 < Sr < 1

uw < 0 ua=0

uw<0

ua>0

uw<0

At high degree of saturation, we have to figure out a capillary tube with compressible liquid. The increase in air pressure increase the boundary meniscus curvature and, hence, the difference uw-ua

Axis translation might overestimate suction at high degrees of saturation

Conclusions: Tensiometers:

•The technology of the high capacity tensiometer (HCT) is maturing and becoming more widely adopted

•Challenges still remain. Most notably: interpretation and length of measurement.

Conclusions: The Axis-Translation Technique:

•Axis translation appears to be a reliable technique to measure/control suction.

•Doubts arise at high degree of saturation and more experimental investigation is needed.

state of the art ontensiometers and axis- translation ... · marinho et al (2003) the tensiometer:...

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