Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 165

Figure 4.88 Osmotic suction versus as-compacted water content
for Regina clay (from Krahn and Fredlund, 1972).

appears that osmotic suction is relatively constant over a
range of water contents. It would appear to be reasonable to
use osmotic suction as a relatively fixed value that can be
subtracted from total suction in order to provide an indica-
tion of matric suction.
Water is being removed from a soil as it dries. However,
the salts are left behind in the soil. Consequently, there can
be a significant increase in the salt content of the pore-water
near the evaporation front. As a result, the osmotic compo-
nent of suction increases. The increase in salts on or near the
evaporation surface of a soil is known to “shut off” or greatly
decrease the evaporation rate of water from the soil surface.
The decrease in evaporation rate is closely related to a signif-
icant increase in the osmotic component of soil suction. The
increase in osmotic suction increases total suction of the soil.

Figure 4.86 Osmotic suction versus electrical conductivity for
pore-water containing mixtures of dissolved salts (from USDA
Agricultural Handbook No. 60, 1950).

Figures 4.87 and 4.88 present the results of osmotic suction
measurements on glacial till and Regina clay, respectively.
The measurements were conducted using the squeezing tech-
nique. The measured osmotic suctions are shown to agree
closely with the difference between independently measured
total suction and matric suction values. The discrepancies
shown at low water contents for the glacial till (Fig. 4.87)
appear to be primarily related to inaccurate measurements of
matric suction (Krahn and Fredlund, 1972).
Figures 4.87 and 4.88 indicate that the squeezing tech-
nique can be used for the measurement of osmotic suc-
tion. The results also support the validity of the relationship
between matric and osmotic components of soil suction. It

4.5 MEASUREMENT OF IN SITU WATER
CONTENT

The void ratio and the water content of a saturated soil
bear a fixed relationship by the specific gravity of the soil.
However, water content and void ratio become independent
variables for unsaturated soils. Consequently, it is often nec-
essary to measure or monitor water content when dealing
with unsaturated soils.
Both soil suction and the water content may need to be
measured in the field when monitoring performance of soil
cover systems. However, some projects may only require
that the water content be monitored while on other projects
it might only be necessary to measure soil suction. For
example, it might be preferable and sufficient to measure soil
suction when dealing with clay soil and it might be prefer-
able and sufficient to measure water content when dealing
with sand soils. On other occasions it might be preferable
to measure the changes in both soil suction and water con-
tent. Each situation needs to be studied and an engineered
instrumentation plan designed for each situation.

4.5.1 Measurement of Water Content
Gravimetric water content (i.e., ratio of mass of water to
mass of solids) is the easiest variable to measure in the

Figure 4.87 Osmotic suction versus as-compacted water content
for glacial till (from Krahn and Fredlund, 1972).