Mechanics of Unsaturated Soils, Weimar, 2007
A Comparative Study of Soil Suction
Measurement Using Two Different
High-Range Psychrometers
Rafaela Cardoso1, Enrique Romero2,
Analice Lima2 and Alessio Ferrari3
1Instituto Superior Técnico, Lisbon, Portugal
2Universitat Politècnica de Catalunya. Barcelona, Spain
3Università degli Studi di Palermo, Palermo, Italy
OUTLINE OF THE PRESENTATION
• Introduction
• Equipment description
• Calibration protocols
• Water Retention Curves
• Discussion
• Conclusions
INTRODUCTION
This experimental study compares the measurement
capabilities of two suction measurement equipment:
• SMI transistor psychrometers
• Chilled-mirror dew-point psychrometer (WP4)
The water retention curves, WRC, along drying paths of
different compacted and natural clayey soils are
presented.
A wide total suction range (0.1 to 70MPa) is covered.
EQUIPMENT DESCRIPTION
Some factors that can affect the reliability of the readings:
• temperature (transistor psychrometer)
• hysteresis effect on calibration
• change of calibration with time (aging of sensing device)
• equalisation period
• drop size (transistor psychrometer)
• …..
Suggestions regarding the improvement of the equipment
performance and the calibration protocols are usually
based on controlling these factors.
EQUIPMENT DESCRIPTION
SMI transistor psychrometers
EQUIPMENT DESCRIPTION
Chilled-mirror dew-point psychrometer WP4
(Leong et al, 2003)
EQUIPMENT DESCRIPTION
Comparison between the equipment:
Equipment
SMI psychrometer
Chilled-mirror dew-point WP4
1 to 70MPa (?)
(if disconnected from
logger)
<±0.05 pF
±0.01 pF (repeatability)
±0.1MPa from 1 to 10MPa and
±1% from 10 to 60 MPa
Measuring time
Usually 1 hour
3 to10 minutes
Calibration
Multiple point calibration
Single point calibration
Sample
geometry
Ø=15mm, h=12mm
Sample cup: Ø=37mm, h=7mm
Suction range
Accuracy
1 to 60MPa (max. 300MPa)
CALIBRATION PROTOCOLS
4.0
SMI Psychrometer #10:
2
1 - ∆V = 0.0581 ∆ψ ; R = 0.9928
2
2 - ∆V = 0.0323 ∆ψ ; R = 0.9999
SMI Psychrometer #4:
2
3 - ∆V = 0.0439 ∆ψ ; R = 0.9998
2
4 - ∆V = 0.0308 ∆ψ ; R = 0.9972
Volts (V)
3.0
2
2.0
4
1
1.0
3
0.0
0
10
20
30
40
50
60
70
80
Total suction (MPa)
Calibration of the SMI for one-hour reading time, standard drop size and
along a drying path
Two linear relationships are necessary to fit the results
CALIBRATION PROTOCOLS
Influence of time in the reading:
ψ=73 MPa
ψ=39 MPa
ψ=20 MPa
ψ=2.2 MPa
distilled water
Volts
3
3000
Output (mV)
4
2
1
2000
1000
Ψ = 84 MPa
0
Ψ = 57 MPa
Ψ = 33 MPa
-1000
0
0
10
20
30
40
Time (minutes)
50
60
0
10
20 30 40
Time (minutes)
50
60
(Mata et al, 2002)
Time effects on reading are more important
at high suctions.
CALIBRATION PROTOCOLS
Possible explanations for the changes in
the readings with time:
• the measuring chamber is not completely closed and
the measuring environment is affected by the relative
humidity of the laboratory.
• at elevated total suctions
the water drop evaporates continuously, demanded
firstly by the total suction of the soil and later by the
influence of the relative humidity of the laboratory.
If the water drop dries completely, the voltage reading
should return to zero, corresponding to null total suction.
CALIBRATION PROTOCOLS
Observations consistent with drop evaporation:
-the mass of the high-suction soil samples
slightly increases along the reading period
-the water drop evaporates and progressively
vanishes
Two approaches to minimize drop evaporation
effects:
• increase the size of the water drop
(Woodburn and Lucas,1995)
• change the measurement time
(Mata et al., 2002)
CALIBRATION PROTOCOLS
Measurement time reduced (high suction range)
5000
4
Measured values
Output (mV)
3
Volts
4000
∆V = 0.0403∆ψ; R2 = 0.9994
20min
2
30min
1
60min
0
0
10
20
30
40
50
60
Total suction (MPa)
70
80
1 hour:
42.1Ψ r2 = 0.998
0≤ψ<33MPa
20 Min
3000
30 Min
2000
1000
33MPa 57MPa
Measured values
0
60 Min
30 minutes:
33 ≤ψ< 57MPa
20 minutes:
33MPa 57MPa
ψ≥57MPa
0 10 20 30 40 50 60 70 80 90
Total Suction (MPa)
(Mata et al, 2002)
The reduction in the measurement time depends on the
prediction of the soil total suction before doing the
measurement, which is not always easy to assume.
In this paper, the bi-linear calibration with standard drop
size (1 hour reading time) was the one selected.
CALIBRATION PROTOCOLS
Calibration of the WP4 psychrometer:
Single-point calibration performed in the low total
suction range (2.2 MPa at 20ºC).
The calibration slope is fixed during factory calibration
and the user only fits the zero offset.
Independent offset calibrations were performed at three
different total suctions (2.2, 19.4 and 38.2 MPa (22ºC)):
Applied total suction
(22ºC)
Measured values for different offset point
chosen, ψcal (22ºC)
2.2 MPa
19.4 MPa
38.2 MPa
6.0 MPa (NaCl 1.3 mol/kg)
5.5
4.4
4.8
10.5 MPa (NaCl 2.2 mol/kg)
9.8
8.7
9.1
19.4 MPa (NaCl 3.7 mol/kg)
20.4
19.5
19.7
38.2 MPa (NaCl 6.1 mol/kg)
37.4
37.5
38.1
CALIBRATION PROTOCOLS
The slope of the readings (sensitivity) in each
offset point is different:
Which offset
point is the most
adequate?
calib 2.2MPa; sensitivity b=0.9960; R2=0.9966
calib 19.4MPa; sensitivity b=1.0414; R2=0.9984
calib 38.2MPa; sensitivity b=1.0335; R2=0.9978
38.2 MPa
35
40
35
30
25
19.4 MPa
b=1
20
15
measured
suction
30
30
35
40
applied
suction
1.05
b=0.0163Ln(ψcal) + 0.9834
10
R2 = 0.9957
1.04
10
38.2MPa
5
19.4MPa
5
0
0
5
10 15 20 25 30 35 40
applied suction (MPa)
sensitivity:
0
0
⎛ 1⎞
ψ = ψ WP 4 − (ψ WP 4 − ψ cal ) ⎜ 1 − ⎟
⎝ b⎠
5
10
sensitivity,
b
Sensitivity, b
measured suction (MPa)
40
1.03
1.02
1.01
1.00
2.2MPa
b=1 approx.
in the offset point
suggested by the
manufacturer
0.99
0
5
10
15
20
25
30
Calibration point, ψcal (MPa)
Calibration
point
35
40
RESULTS
The drying branches of the WRC were determined with the
equipment for different soils.
Two different compacted clayed soils were studied:
Sample
wini (%)
γd (kN/m3)
wL (%)
PI (%)
γs (kN/m3)
A- Compacted
destructured argillite
12.4
17.2
37
16
27.4
B- Compacted scaly clay
15.3
17.4
58
30
27.7
RESULTS
Soil A - Compacted destructured argillite
100
SMI -Drying
WP4-Drying
Curve SMI (drying)
Total suction (MPa)
Curve WP4 (drying)
10
0.5-7 MPa:
good
agreement
between
results
1
0
0
5
10
15
water content (%)
20
25
RESULTS
Soil B - Compacted scaly clay
100
SMI -Drying
WP4-Drying
Curve SMI (drying)
Some
shiftening
observed in
the higher
total
suctions
Total suction (MPa)
Curve WP4 (drying)
10
1
0
0
5
10
15
water content (%)
20
25
RESULTS
Similar results were obtained for two natural soils studied
more recently.
Soil C – Natural Boom Clay
Soil properties
Density, ρ
Dry density, ρd
Gravimetric water content, w (ASTM D2216)
1.99 to 2.05 Mg/m3
1.71 to 1.65 Mg/m3
21% to 25%
Density of soil solids, ρs (ASTM D854)
Void ratio, e
Porosity, n
Degree of saturation, Sr
Liquid limit (SBCW), wL (ASTM D4318)
Plasticity index, PI
2.67 Mg/m3
0.56 to 0.618
0.358 to 0.382
91 to
99100%
%
(55.7 ± 0.9) %
(26.9 ± 1.0) %
Soil D – Marl from Abadia
Sample
wini (%)
γd (kN/m3)
wL (%)
PI (%)
γs (kN/m3)
D- Marl from Abadia
14.5
21.4
50
25
27.5
RESULTS
Soil C – Natural Boom Clay
200
100
80
60
Total suction (MPa)
40
20
SMI psychrometers
Drying (curve fitting)
Wetting (curve fitting)
10
8
6
4
WP4 psychrometer
Drying (sample 2)
Wetting (sample 2)
2
1
0.8
0.6
0.4
0
5
10
15
Water content, w (%)
20
25
(Lima and Romero, 2006)
RESULTS
Soil D – Marl from Abadia
SMI- drying
SMI- drying curve
WP4- drying
WP4 - drying curve
vapour equilibrium - drying
Total suction (MPa)
1000.00
100.00
10.00
1.00
0.10
0
2
4
6
8 10 12 14 16 18 20 22
water content (%)
(Cardoso, 2008)
RESULTS
Soil C – Natural Boom Clay
200
100
80
60
SMI psychrometers (ψ < 15 MPa)
Drying (curve fitting)
Wetting (curve fitting)
Total suction (MPa)
40
20
10
WP4 psychrometer
Drying (sample 2)
Wetting (sample 2)
8
6
4
Vapour equilibrium technique (22 oC)
Drying
Wetting
2
1
0.8
0.6
0.4
0
5
10
15
Water content, w (%)
20
25
(Lima and Romero, 2006)
DISCUSSION
Since the equipment were carefully calibrated, the
differences observed cannot be only due to errors in the
measurements.
The differences may be explained in terms of the
hydraulic paths undergone by the soils during the
measurement period.
DISCUSSION
Inside the equipment chamber:
SMI
HRb
HR0=40%
HRSOIL
HRb> HR0 (fast process)
HRSOIL> HR0
HR1
HReq SMI
HRSOIL
HR1> HRSOIL
HR1> HR0
HR1> HRSeq SMI > HRSOIL
WP4
HR0=40%
HReq WP4
HRSOIL
HRSOIL> HR0
HRSOIL > HRSeq WP4 > HR0
DISCUSSION
Hydraulic paths followed by the soil:
Total suction, ψ (MPa)
HR0= 40%
ψ ≈ 124MPa (22ºC)
ψWP4
ψSOIL
WP4
3
2
WP4 – Drying
ψSMI
SMI
1
3
SMI - Wetting
1
wWP4 wSOILwSMI
1 – Previous reading
2 – Sample prepared for new reading
3 – End of new reading
Main drying curve
(unstressed)
Water content, w (%)
The total suctions measured and the
final water contents are different.
CONCLUSIONS
• Summary
SMI psychrometer
Calibration protocols suggested:
• To use a bi-linear calibration relationship if the
same measurement time with the standard drop
size is adopted.
• Calibration along a drying path.
Measuring protocols suggested:
• To determine the water content of the sample
after the measuring period. To assign this water
content to the total suction reading.
CONCLUSIONS
WP4 psychrometer
Calibration protocols suggested:
• Set the zero offset at a value near the expected
total suction. If not, an expression to correct the
readings was suggested based on the verification
study performed.
Measuring protocols suggested:
• To determine the water content of the sample after
the measuring period.
• To assign this water content to the total suction
reading.
CONCLUSIONS
• Results of the study (along drying paths of the WRC)
- Good agreement between the equipment readings in
the low total suction range.
- Differences between the readings of both
psychrometers were observed in the high-suction
range (usually over 7 MPa), which increased with the
total suction of the soil.
A possible explanation for these differences was based
on the different hydraulic paths undergone by the soils
along the measurement period:
• some drying in the WP4 chamber;
• some wetting in the SMI chamber.
ACKNOWLEDGEMENTS
Dr Francesc Ferrer,
for lending to the Geotechnical Laboratory (UPC) the
WP4 equipment and also for his useful comments
regarding its calibration and use.
Portuguese Foundation for Science and Technology, FCT
(Ref. POCTI/ECM/59320/2004).
ESV EURIDICE GIE Belgium contract (Ref. EUR-04-248).
The end
Thank you