Environmental Engineering Reference
In-Depth Information
The pore-water in a soil generally contains dissolved salts.
The water vapor pressure over a flat surface of solvent (i.e.,
salt solution),
u
v
1
, is less than the water vapor pressure over
a flat surface of pure water,
u
v
0
. In other words, the relative
humidity decreases with increasing dissolved salts in the
pore-water of the soil. The decrease in relative humidity
due to the presence of dissolved salts in the pore-water is
referred to as the osmotic suction
π
.
4.2.3 Typical Suction Values and Measuring Devices
Table 4.1 shows typical matric, osmotic, and total suction
values for two soils often used as subgrade material for roads
built in the province of Saskatchewan, Canada (Krahn and
Fredlund, 1972). Regina clay is highly plastic, inorganic clay
with a liquid limit (LL) of 78%and a plastic limit (PL) of 31%.
Glacial till has a liquid limit of 34% and a plastic limit of 17%.
Suction values are given in Table 4.1 for soils compacted to
standardAASHTO conditions at water contents near optimum
water content and 2% dry of optimum water content.
Figure 4.4 shows experimental data illustrating that matric
suction plus osmotic suction are essentially equal to the total
suction in a soil. The glacial till data presented in Fig. 4.4 are
for specimens compacted to modified AASHTO conditions
at various initial water contents. The components of soil
suction (i.e., matric suction and osmotic suction) and total
suction were independently measured.
Several devices commonly used for measuring total,
matric, and osmotic suctions are listed in Table 4.2 along
with the range over which measurements can be made.
Figure 4.4
Total, matric, and osmotic suctions for glacial till
(from Krahn and Fredlund, 1972).
are made of negative pore-water pressure. Properties of high-
air-entry ceramic disks are presented to assist in understanding
the operation of direct measurement devices. Conventional
tensiometers measure matric suctions up to almost 1 atm.
More recently, direct high-suction tensiometers have been
developed based on a special treatment procedure where the
water in the measuring device is subjected to high pressures
prior to usage in the measuring device.
Several types of porous material sensors have been used
for making
indirect
measurements of matric suction. A fea-
ture common to all indirect methods of measuring matric
suction is the need to calibrate the sensors. Calibration is
required because a property other than the pore-water pres-
sure is being measured. The thermal or electrical properties
of a specially designed porous material are a function of its
water content. The water content of the ceramic, in turn, is a
function of matric suction or the tension in the water phase.
Ameasurement of the electrical or thermal properties of the
porous sensor provides an indication of the matric suction in
the sensor and the surrounding soil through use of a calibration
4.2.4 Measurement of Matric Suction
Matric suction is the difference between the pore-air and
pore-water pressures (
u
a
−
u
w
) and can be measured either
in a direct or indirect manner. Direct methods measure the
negative pore-water pressure. The pore-air pressure compo-
nent of matric suction is generally atmospheric in the field.
Indirect methods measure a variable other than the negative
pore-water pressure (e.g., thermal conductivity) through use
of a specially designed ceramic material. A calibration of
the measuring device is required to determine the matric
suction in the soil when using an indirect method.
High-air-entry ceramic disks are used as the primary separa-
tor between air andwater pressureswhen
direct
measurements
Table 4.1 Typical Suction Values for Compacted Soils
Water Content
Matric Suction
Osmotic Suction,
Total Suction,
Soil Type
ω
,%
(
u
a
−
u
w
), kPa
π
, kPa
ψ
, kPa
13
.
81 kN/m
3
Regina clay,
γ
max
=
30.6 (optimum)
273
187
460
28.6
354
202
556
19
.
24 kN/m
3
Glacial till,
γ
max
=
15.6 (optimum)
310
290
600
13.6
556
293
849
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