M D Bolton

W A Take

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Take, W. A. & Bolton, M. D. (2004). Géotechnique 54, No. 3, 229–232
229
DISCUSSION
Tensiometer saturation and the reliable measurement of soil suction
W. A. TAKE and M. D. BOLTON (2003) . Géotechnique 53 , No. 2 , 159 – 172
A. Tarantino, Università degli Studi di Trento, Italy
The authors address two important issues relating to tensi-
ometer measurements: the initial saturation of the high air-
entry ceramic and the unreliable response of the tensiometer
due to the inadequate saturation.
Ridley & Burland (1999) suggested a procedure for initial
saturation where the first stage consisted of removal of air
from the ceramic. This was achieved by applying a vacuum
in a chamber partly filled with de-aired water. The authors
assume that the ceramic may adsorb water molecules during
porous ceramic installation and evacuation, and this is likely
to reduce the wettability of the ceramic during the subse-
quent inundation. To cope with this problem, the authors
propose a new initial saturation process where air removal is
completed in two steps: oven-drying and evacuation in the
absence of water.
A very similar procedure has already been suggested by
Tarantino & Mongiovı̀ (2002) using the vacuum chamber
shown in Fig. 15. The only difference lies in the drying of
the ceramic, as the tensiometer was placed in a desiccator
containing silica gel and not in an oven. The use of this
double-compartment vacuum chamber was to overcome an-
other supposed limitation of the procedure proposed by
Ridley & Burland (1999). As free water is present in the
vacuum chamber, water vapour continuously develops during
evacuation. This prevents the attainment of very low abso-
lute pressures, and hence reduces the effectiveness of air
removal from the ceramic.
The authors have shown that the initial saturation was
sufficient to saturate the tensiometer fitted with the 100 kPa
air-entry porous ceramic. However, the initial saturation
alone was not sufficient to saturate the ceramic with a higher
air-entry value (300 kPa), and a pressurisation was required
to measure water tension. Similarly, the discusser found that
initial saturation alone was not sufficient to saturate the
Trento tensiometers, which use a ceramic with a very high
air-entry value (1500 kPa). After saturation using the satura-
tion chamber shown in Fig. 15 and a vacuum pump capable
of a measured minimum absolute pressure of 0.03 kPa, the
tensiometers could withstand a minimum gauge pressure of
about 300 kPa for only 1 or 2 s.
It is then possible that the initial saturation adopted by the
authors and the discusser does not significantly enhance the
procedure proposed by Ridley & Burland (1999). The dis-
cusser has saturated two Imperial College tensiometers fol-
lowing the procedure suggested by Ridley & Burland
(1999), and using a large triaxial cell as a saturation
chamber and a vacuum pump capable of a minimum abso-
lute pressure of about 3 kPa. In spite of the poor vacuum
applied, these two tensiometers later exhibited a satisfactory
performance in terms of measurement precision (tensi-
ometers pr3 and pr4 in Tarantino & Mongiovı̀, 2000, 2001)
that is comparable to that of the best-saturated Trento
tensiometers (precision of about 3 kPa relative to the
average measured value). The experience of the discusser is
that saturation of the ceramic is achieved mainly through
cycles of cavitation and subsequent pressurisation (Tarantino
& Mongiovı̀, 2001; Tarantino, 2003). An inadequate initial
saturation simply increases the number of cycles necessary
to obtain a satisfactory performance.
The second issue is the reliable measurement of pore
water tension. Tarantino & Mongiovı̀ (2001) and Tarantino
(2003) showed evidence that inadequate saturation might
result in readings that appear reliable and stable although
actually incorrect. The authors have demonstrated this in a
very rigorous fashion, and have addressed perhaps the main
problem in tensiometer measurement, which is how to
discriminate a good measurement from a bad one.
The authors state that ‘tensiometer response times under
low absolute pressures are an excellent indicator of tensi-
ometer saturation’, implicitly referring to the tests shown in
Figs 12 and 14. However, in these tests, water in the
tensiometer was subject only to positive absolute pressures
and for a limited period of time (35 min). The tensiometer
response may not be as good as supposed when water is
subject to tension (gauge pressures less than 100 kPa) and/
or when measurement lasts for longer periods (from hours to
days). Under these conditions, air cavities may form within
the tensiometer without triggering cavitation, but causing a
faulty response of the instrument. Cavity formation in the
tensiometer is illustrated by the test shown in Fig. 16, where
the pressure recorded by an Imperial College tensiometer is
plotted against time. The ceramic was wiped after about
11 min to let water pressure drop to about 1900 kPa, and
then the tensiometer was immediately replaced in free water.
The time required for the tensiometer to return to atmo-
spheric pressure was about 0.5 min (Fig. 16(b)).
A kaolin paste was then applied to the porous ceramic,
and the paste was allowed to dry out slowly. After reaching
a pressure of about 2600 kPa (595 min) the tensiometer
was again replaced in free water (Fig. 16(c)). The equalisa-
tion time was much larger in this case, being about 7 min.
The reduced permeability is probably due to the expansion
of the air cavities in the porous ceramic. Here, it was
necessary to reach a very high tension to expand the
cavities, but lower tensions might be sufficient to enlarge
Vacuum Vacuum
De-aired water
Screw valve (closed position)
O-rings
Tensiometer
Fig. 15. Schematic layout of the Trento double-compartment
vacuum chamber
cavities if the initial degree of saturation of the ceramic is
low. These cavities may then cause false tensiometer read-
ings.
Air cavities may expand not only under tension but also
under high positive pressures. Tarantino & Mongiovı̀ (2001)
reported the case of a tensiometer stored for 1 year in a
saturation chamber under a pressure of 4 MPa. After re-
moval from the chamber, the tensiometer could sustain a
tension of 650 kPa for some seconds only. However, the
same tensiometer could sustain very high tensions (greater
than 2000 kPa) after just one cycle of cavitation and subse-
quent pressurisation at 4 MPa. Despite the high positive
pressures, air cavities had expanded in the tensiometer when
stored for 1 year because of air diffusion.
These two examples suggest that the degree of saturation
of the ceramic must be regarded as a ‘dynamic’ variable, in
the sense that it might change with time. As a result, the
types of test shown in Figs 12 and 14 may or may not be
indicators of adequate saturation for the case where longer
measurements are carried out and/or a tension is applied to
water.
To verify the reliability of suction measurement, Tarantino
& Mongiovı̀ (2001) and Tarantino (2003) have suggested
indicators of adequate saturation to be checked before and
after measurement. These indicators were defined on the
basis of long-term measurement using more tensiometers
placed on the same sample. Nonetheless, we consider that
the straightforward way to verify measurement reliability is
to measure suction using at least two tensiometers simulta-
neously and checking that they give the same readings.
Authors’ reply
The authors would like to thank Dr Tarantino for his
interest in the paper. The discusser has provided useful
additional information regarding the independent develop-
ment of a similar technique for the initial saturation of
tensiometers. Although motivated by different intentions—a
very dry filter element in the authors’ case and the mini-
misation of the absolute pressure at saturation by the dis-
cusser—these two techniques share the key features of a
dried filter element and the evacuation of the tensiometer in
the absence of water. Despite providing a higher degree of
initial saturation, the discusser has debated the merits of
these techniques as ‘inadequate initial saturation simply
appears to increase the number of cycles necessary to obtain
a satisfactory performance of the instrument’.
This difference in opinion perhaps arises from the differ-
ences in the magnitude of the positive pressures used in the
second stage of saturation. One objective of the paper was
to investigate the process of saturation to permit the meas-
urement of soil suction with more sensitive, low-pressure-
range devices that cannot be subjected to large saturation
pressures. For example, one version of the tensiometer
designed specifically for the accurate measurement of low
suctions has a full-scale range of 100 kPa. Even if a portion
of the safe over-pressure range is used to apply a saturation
pressure of 200 kPa, a consideration of the physics of
saturation (equation (2)) indicates that the initial degree of
saturation, Si, must be greater than 0.96 to saturate the
ceramic filter sufficiently . In contrast, saturation using posi-
tive pressures of the order of 4 MPa (e.g. Tarantino &
Mongiovı̀, 2001) is likely to be capable of saturating all but
the most poorly saturated of devices, given time. Thus it
follows from equation (2) that the significance of the high
degree of initial saturation will be diminished if very large
positive pressures are applied to saturate the device.
The typical pressure range of the base device upon which
the new tensiometer is built is 700 kPa. At the higher
saturation pressure of 1 MPa, the initial degree of saturation
will be less important, but it is the experience of the authors
that the higher degree of saturation ensures the saturation of
a device fitted with a 3 bar air-entry ceramic on the first
short pressurisation. When saturating up to a dozen of these
devices for inclusion into model tests, the authors are of the
opinion that this benefit alone is worth the extra effort of
ensuring a high initial saturation.
Second, the discusser raises the important issue of stable
bubble growth in the ceramic filter element under high
absolute tensions, and questions whether tensiometer re-
sponse times under low absolute pressures are therefore
useful indicators of tensiometer saturation. This scenario has
been investigated in a further set of experiments using a
tensiometer fitted with a nominal 3 bar air-entry ceramic
filter element. This tensiometer was initially saturated in the
two-chamber saturation apparatus before being subjected to
a saturation pressure of 1 MPa.
Following the experimental methodology described in the
paper, the response time of the instrument was then assessed
by subjecting the device to 10 kPa pressure increments of
100 s duration between 0 and 100 kPa, and between 100
and 0 kPa. The device was then removed from the chamber
and allowed to generate a pressure of 250 kPa under the
action of evaporation (Fig. 17(a)). At this point, the device
was submerged in free water and allowed to equilibrate to
atmospheric pressure before the response time was again
assessed.
The lower half of this stepwise calibration exercise is
shown in Fig. 17(b). The response of the device was ob-
served to be indistinguishable from that observed before the
generation of the absolute tension, and—as shown in the
1000
0
1000
2000
3000
W
at
er
 p
re
ss
ur
e:
 k
P
a
0 200 400 600 800
Time: min
(a)
0
500
1000
1500
2000
2500
W
at
er
 p
re
ss
ur
e:
 k
P
a
12 16 20 596 600 604
Time: min
(b) (c)
Fig. 16. (a) Response of the tensiometer associated with slow
drying of the paste applied to the porous ceramic; (b)
equalisation in free water before measurement; (c) equalisation
in free water after measurement
230 DISCUSSION
figure—indicates little lag-time observed between the applied
and measured pressures. The device was then subjected to a
further pressure of 500 kPa by evaporation (Fig. 17(c)),
before being plunged into free water. Upon pore pressure
equalisation, the device was again assessed for its response
time under low absolute pressures. These results, presented
in Fig. 17(d), indicate that even the short-term generation of
this higher tension has reduced the permeability of the filter
element, despite not having initiated nucleation.
Rather than the absolute magnitude of the applied tension,
the authors would suggest that it is the fact that the air entry
value has been exceeded that gives rise to this response, and
that reported by the discusser (Fig. 16). Although the air-entry
value of a ceramic is often quoted as a single value, the pore
size distribution will ensure that air entry is not an instanta-
neous process. Rather, as the tension is increased past the
nominal air-entry value, the menisci at the exposed surface of
the tensiometer will start retreating into the ceramic, beginning
with the largest of exposed pores. The thicker the filter
element, the longer this process can continue before nucleation
occurs, the more unsaturated the surface of the ceramic can
become, and the larger the tension that can be recorded in the
reservoir of the device. The difference in response time ob-
served by the discusser is a result of a longer period of
desaturation. This observation, in combination with the experi-
ment described herein, indicates that largest measurable ten-
sion in the reservoir should not be the defined as the largest
measurable suction, as applied suctions greater than the
air-entry value of the ceramic will initiate the process of
desaturation. As the process of desaturation will always be
accompanied by a reduction in permeability, any technique
evaluating this property either quantitatively or qualitatively
(as in the technique proposed by the authors) will provide a
good indication of tensiometer saturation.
0
100
200
300
400
500
600
u w
: k
P
a
Submerged in
free water
Nominal air-entry value
0 20 40 60 80 100 120 140
Elapsed time: s
(a)
0
100
200
300
400
500
600
u w
: k
P
a
0 20 40 60 80 100 120 140
Elapsed time: s
(c)
50
55
70
75
80
95
100 95 90 80 75 65 55 50
Actual pressure: kPa
(b)
85 70 60
90
85
65
60
O
bs
er
ve
d 
pr
es
su
re
: k
P
a
50
55
65
75
90
95
O
bs
er
ve
d 
pr
es
su
re
: k
P
a
100 95 90 85 70 65 60 50
Actual pressure: kPa
(d)
80 75 55
85
80
70
60
Submerged in
free water
Nominal air-entry value
Fig. 17. Reduction in tensiometer response times with stable air entry
DISCUSSION 231
REFERENCES
Ridley, A. M. & Burland, J. B. (1999). Discussion: Use of tensile
strength of water for the direct measurement of high soil
suction. Canadian Geotechnical Journal, 36, 178–180.
Tarantino, A. (2003). Panel report: Direct measurement of soil
water tension. Proc. 3rd Int. Conf. on Unsaturated Soils, Recife
3 (in press).
Tarantino, A. & Mongiovı̀, L. (2000). Experimental investigations
on the stress variables governing unsaturated soil behaviour at
medium to high degrees of saturation. In Experimental evidence
and theoretical approaches in unsaturated soils (eds A. Taranti-
no and C. Mancuso), pp. 3–19. Rotterdam: Balkema.
Tarantino, A. & Mongiovı̀, L. (2001). Experimental procedures and
cavitation mechanisms in tensiometer measurements. Geotechni-
cal and Geological Engineering 19, Nos 3–4, 189–210.
Tarantino, A. & Mongiovı̀, L. (2002). Design and construction of a
tensiometer for direct measurement of matric suction. Proc. 3rd
Int. Conf. on Unsaturated Soils, Recife 1, 319–324.
232 DISCUSSION


Tensiometer saturation and the reliable measurement of soil suction

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Tensiometer saturation and the reliable measurement of soil suction

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