Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 251

25

20

15

10

5

0

10 6 1,000 10,000 100,000

Soil suction, kPa

Clay (drying) Clay (wetting)

Clayey silt (wetting)

Clayey silt (drying)

Figure 5.95 Drying and wetting SWCCs for clay and clayey silt
placed in vacuum desiccators (Ebrahimi-Birang et al., 2007).

Figure 5.94 Vacuum desiccator with soil specimens coming to
equilibrium in controlled relative humidity environment.

below the final equilibrium water content will define the
wetting curve branch of the SWCC. However, the differ-
ence between the drying and wetting portions of the SWCC
at high total suctions is generally quite small.
The total-suction value corresponding to any tempera-
ture for a specified salt can be computed from the Gibbs
free-energy equation. Young (1967) published equilibrium
relative humidity data for a series of saturated salt solutions.

Table 5.7 Approximate Equilibrium Relative
Humidity for Selected Saturated Salt Solutions

Salt Solution Average Relative Humidity, %

Cesium fluoride 4

Lithium bromide 7

5.10.3 Typical Laboratory Results
Small soil specimens consisting of a few grams of soil can be
suspended above a salt solution in the desiccator. It is pos-
sible to periodically weigh the soil specimens to determine
whether moisture equilibrium has been attained. Equilibrium
conditions are achieved more rapidly under low-relative-
humidity conditions than under high-relative-humidity con-
ditions. Low-relative-humidity equilibrium conditions can
be attained in about one or two weeks. However, when the
equilibrium relative humidity is high (e.g., 97% RH), several
months may be required for equilibrium to be established.
The time for equalization is also dependent upon the size of
the soil specimen and constant temperature control becomes
more important at high-relative-humidity values.
Figure 5.95 shows the wetting and drying curves for a
clay soil and a clayey silt soil. Soil specimens were initially
prepared in both wet and dry conditions. Laboratory results
showed that there was some hysteresis in the SWCC rela-
tionship. The classification properties of the soils tested are
shown in Table 5.8.

Lithium chloride 12

Potassium acetate 23

Magnesium chloride 33

Potassium carbonate 43

Sodium bromide 59

Potassium iodide 70

Sodium chloride 75

Potassium chloride 85

Potassium sulfate 98

relative humidity environments for more than one year.
Some other salt solutions can only be used for a few months.
The principal factors affecting the time required to estab-
lish vapor pressure equilibrium are (1) the ratio of free
surface area of the solution to the chamber volume, (2) the
amount of air circulation, (3) the absorbing properties of the
sample, and (4) the agitation of the salt solution.
Soil specimen sizes can be small or quite large. The larger
the specimen size, the longer the time that is required to attain
equilibrium conditions. Specimen sizes ranging from 1 to 5 g
are sufficient for purposes of measuring the water content
corresponding to high-total-suction values on the SWCC.
Specimens with initial water contents above the final equi-
librium water content will define the drying curve branch of
the SWCC. Specimens with an initial water content that is

5.11 USE OF CHILLED-MIRROR
OR DEW-POINT METHOD

Gee et al., (1992) introduced a method to measure the total
suction in the medium- to high-suction range through mea-
surement of water activity. The device was first introduced