performance of the durham university – wykeham farrance tensiometer

Performance of the Durham University
– Wykeham Farrance tensiometer
Sérgio Lourenço1, Domenico Gallipoli1,
David Toll1, Fred Evans2
1Durham
University, UK
2Wykeham Farrance, UK
The Durham University – Wykeham Farrance Tensiometer
1
2
3
1.
2.
3.
4.
5.
6.
(Stannard, 1992)
4
5
6
High Air entry stone 15bar 10mm diameter 10mm thick
Glue line
Stainless Steel Housing 14mm diameter x 35mm long
Shim to allow flexing of the diaphragm
Ceramic transducer 15bar full scale
Four core screened cable
Saturation
Procedure:
Vacuum stage
Saturation stage (with the tensiometer under vacuum)
Pressurization stage
Suction measurement range (1)
0
1
2
3
0
pressure (kPa)
-200
-400
-600
-800
-1000
-994.9 kPa
-1200
time (m)
4
5
6
7
Testing program
Pressurization stage
Tensiometer
identification
Test nr
Sequence
Maximum
pressure
(kPa)
Cavitation triggering
stage
Duration
(hr)
Material
Duration
(min)
Maximum
suction
(kPa)
T11
1st
800
72
Kaolin
45
-981.9
T12
2nd
800
24
Kaolin
60
-917.3
T13
3rd
800
5
Kaolin
60
-990.3
T34
4th
1000
-
Air drying
2
-1043.0
T15
1st
800
60
Kaolin
120
-858.6
T16
2nd
800
24
Kaolin
60
-792.1
T25
3rd
Continuous use*
Kaolin
150+8
-1230.9
T18
1st
800
Kaolin
30
-903.7
T27
2nd
Continuous use*
Kaolin
150
-818.2
T29
3rd
Continuous use*
Air drying
12
-1047.7
T20
1st
-
-
-
-
-870
T28
2nd
800-1000
4-14
Air drying
1
-1032.1
T30
3rd
Air drying
3
-994.9
II-1
II-2
II-3
II-4
48
Continuous use*
Suction measurement range (2)
II - 1
II - 2
II - 3
II - 4
Suction at cavitation (kPa)
-700
T16
-800
-900
-1000
-1100
-1200
-1300
T12
T27
T15
T20
T18
T11
T13
T29
T34
T25
tensiometer identification
T30
T28
Post-cavitation response (1)
1200
1000
pressurization
pressure (kPa)
800
600
400
200
tensiometer inserted in free water
0
-200 0
1500
3000
4500
-400
-600
-800
-685.6 kPa
time (m)
6000
7500
Post-cavitation response (2)
0
2 1st cavitation
4
6
trigger
8
10
12
14
2nd cavitation trigger
-100
tensiometer inserted in free water
suction (kPa)
-300
-500
-700
-900
-1100
-1032.1 kPa
time (m)
Calibration: with Axis translation technique (1)
600
500
400 kPa
400
299.4 kPa
suction (kPa)
300
200
200 kPa
99.6 kPa
100
0
-100 0
-200
1500
3000
4500
95.7 kPa
191.4 kPa
-300
6000
284.9 kPa
-400
381.6 kPa
-500
time (m)
ua
uw
7500
Calibration: with Axis translation technique (2)
0
-50
0
50
100
150
200
250
300
-100
uw (kPa)
-150
-200
-250
-300
-350
-400
-450
ua (kPa)
expected
measured
350
400
450
Calibration: by Isotropic unloading
400
333.9 kPa
300
pressure (kPa)
200
100
0
0
500
1000
1500
-100
-200
-300
-308.2 kPa
-400
time (m)
confining pressure
tensiometer
2000
Applications: Air circulation technique (1)
Applications: Air circulation technique (2)
0
2000
4000
6000
-50
pressure (kPa)
-150
pump on
-250
-350
pump off
pump on
-450
-550
-650
pump off
-750
time (m)
sample air pressure
uw (tensiometer)
Conclusion
• A new miniature tensiometer has been
developed by Durham University and Wykeham
Farrance.
• It is able to measure suctions up to 1200 kPa,
without requiring complex saturation procedures.
• The tensiometer is being calibrated in the
negative range, by the axis translation technique
and isotropic unloading. An error below 5 % was
measured. It is not clear yet if such error is
related to the tensiometer itself or the techniques
used.
• The tensiometer will be used to measure suction
directly in triaxial tests. Suction will be controlled
by circulating air at different relative humidity.