Soil-Water Characteristic Curves for Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 250

u w = - ρ w gH

Cover on the top

H

Hydrostatic line

(a) (b)

Silty sand, SM
(drying)

0.4

Silty sand, SM
(wetting)

0.3

0.2

Fine sand, SP
(wetting)

Gravelly
sand, SP
(drying)

Fine sand, SP
(drying)

0.1

Gravelly sand
SP (wetting)

0.0

0.01 0.1 1 10 100 1,000

Matric suction ( u a - u w ), kPa
(c)

Figure 5.93 Column test performed on coarse-grained materials to measure the soil-water char-
acteristic curve. (a) Equilibrium hydrostatic matric suctions in the column test, (b) Column test
showing capillary rise for the wetting test. (c) SWCCs for coarse sands measured using simple
soil column for adsorption curve and pressure plate apparatus for desorption curve (after Yang
et al., 2004a).

Saturated salt solutions are able to liberate and adsorb con-
siderable quantities of water with negligible changes to the
equilibrium relative humidity. Consequently, the use of sat-
urated salt solutions is preferable to the use of unsaturated
salt solutions.
Salt solutions that are commonly used to create a range
of relative humidity environments can be found in ASTM
(2003) Designation E 104-02. Table 5.7 provides a list of
suggested saturated salt solutions along with the general
range of relative humidity created.
The generated relative humidity values created are also
dependent on the temperature of the salt solutions.

5.10.2 Laboratory Test Procedure
A convenient sized, leak-proof container for creating a
constant relative environment is the common laboratory
desiccator. Salt solutions can be placed in the bottom of
a desiccator of the type shown in Fig. 5.94. The salts are
mixed with water until there are some free salts that appear
in the bottom of the desiccator in an undissolved state.
The saturated salt solution should be prepared at room
temperature (or at a temperature slightly higher than room
temperature) in order to ensure that the salt solution is
saturated. The salt solutions suggested in the ASTM (2003)
Standard E 104-02 can be kept and used to create fixed