Environmental Engineering Reference
In-Depth Information
Table 4.3 Properties of High-Air-Entry Disks
Manufactured by Soilmoisture Equipment Corporation
(Manufacturer's Results)
is limited to approximately negative 90 kPa due to the
possibility of cavitation of the water in the tensiometer.
The measured negative pore-water pressure is numerically
equal to the matric suction when the pore-air pressure is
atmospheric (i.e.,
u
a
=
zero gauge pressure). When the
pore-air pressure is greater than atmospheric pressure (i.e.,
during axis translation), the tensiometer reading can be
added to the ambient pore-air pressure reading to give the
matric suction of the soil. The measured matric suction must
not exceed the air-entry value of the ceramic cup on the
tensiometer. The osmotic component of soil suction is not
measured with tensiometers since soluble salts are free to
move through the porous cup.
There are several types of tensiometers commercially
available from Soilmoisture Equipment Corporation and
other companies. The tensiometer tube in Fig. 4.7 has a
diameter of approximately 20mm and various lengths up to
1.5 m. The tensiometer cups can be installed in the field to
a depth of 1.5m below the ground surface. However, the
negative water pressure recorded at the ground surface must
be corrected for the elevation head difference between the
tensiometer tip in the soil and the gauge at ground surface.
The elevation correction results in a more negative water
pressure being measured at ground surface than that being
experienced by the soil. A length of 1.5m corresponds to a
pressure correction of 15.2 kPa.
The tensiometer must be properly serviced prior to usage in
order to obtain reliable results. Details regarding the prepara-
tion, installation, and usage of a tensiometer are presented by
Cassel and Klute (1986). The ceramic cup should be checked
for signs of cracks and air bubbles should be removed from
the tensiometer prior to installation. The response time of
the tensiometer should be checked by placing the empty ten-
siometer upright in a pail of water and allowing the cup to
soak in water. Later, the ceramic tip of the tensiometer can
Coefficient of
Approximate Permeability
Air-Entry
Pore
with Respect
Value,
Diameter
to Water,
(u
a
−
u
w
)
d
,
10
−
3
mm)
Type of Disks
(
×
k
d
, m/s
kPa
10
−
7
1
/
2
Bar,
high flow
6.0
3
.
11
×
48-62
10
−
9
1 Bar
2.1
3
.
46
×
138-207
10
−
8
1Bar,
high flow
2.5
8
.
60
×
131-193
10
−
9
2Bars
1.2
1
.
73
×
241-310
10
−
9
1
.
73
×
3Bars
0.8
317-483
10
−
9
5Bars
0.5
1
.
21
×
>
550
10
−
11
15 Bars
0.16
2
.
59
×
>
1520
Note
: Soilmoisture Equipment Corporation, Santa Barbara,
CA.
plastic due to its low heat conduction and noncorrosive
nature. The tube and the cup are filled with deaired water.
The cup can be inserted into a predrilled hole. It is important
that there be good contact between the high-air-entry ceramic
and the soil. It is also important that the soil particles not
be too large; otherwise there may be inadequate contact
between the ceramic and the water in the soil.
The water in the tensiometer will have the same negative
pressure as the pore-water in the soil once equilibrium is
achieved between the soil and the measuring system. The
pore-water pressure that can be measured in a tensiometer
Table 4.4 Permeability and Air-Entry Value Measurements on High-Air-Entry Disks from Soilmoisture Equipment
Corporation
Air-Entry
Coefficient of
Diameter of
Thickness of
Value of Disk,
Permeability of
Type of Disks
Disk, mm
Disk, mm
(u
a
−
u
w
)
d
,m/s
Disk,
k
d
,m/s
10
−
8
1 Bar, high flow
19.0
6.4
115
5
.
12
×
10
−
8
19.0
6.4
130
3
.
92
×
10
−
8
19.0
6.4
110
3
.
98
×
10
−
8
19.0
6.4
130
5
.
09
×
10
−
8
19.0
6.4
150
5
.
60
×
10
−
8
>
200
4
.
20
×
101.6
10.0
10
−
9
5 Bars
56.8
6.2
1
.
30
×
10
−
9
15 Bars
56.8
3.1
8
.
41
×
10
−
10
57.0
3.1
6
.
82
×
Source
: From Fredlund, 1973a; Rahardjo, 1990.
Search WWH ::
Custom Search