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)
Basic Concepts: Capillarity
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
Basic Concepts: Capillarity
Marinho (2005)
Basic Concepts: Capillarity
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
Basic Concepts: Cavitation
Basic Concepts: Cavitation
Air bubbles arising in a glass tube. (Harvey et al., 1944)
Basic Concepts: Cavitation
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
The Tensiometer: Conventional Design
Bubble formation in ordinary tensiometer
The Tensiometer: Conventional Design
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)
The Tensiometer: Applications
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)
The Tensiometer: Applications
The Tensiometer: Applications
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
The Tensiometer: Applications
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
The Axis-Translation Technique
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!
The Axis-Translation Technique
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)
The Axis-Translation Technique
Scholander et al (1965)
SOIL
Air pressure
Water(Atmospheric pressure or known pressure)Pressure transducer
porous ceramic
The Axis-Translation Technique
porous ceramic
Air pressure
Air pressure higher than the air entry pressure
The Axis-Translation Technique
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.
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