Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 113

Table 4.2 Devices for Measuring Soil Suction and Its Components

Name of Device Suction Component Measured Range, kPa Comments

100 a

Psychrometers (Peltier type) Total to ∼ 8000 Constant-temperature

environment required

Filter paper Total Entire range May measure matric suction

when in good contact with
moist soil

Tensiometers Negative pore-water pressures

or matric suction when
pore-air pressure is
atmospheric

0-90 Difficulties with cavitation

and air diffusion through
ceramic cup

Null-type pressure plate (axis
translation)

Matric 0-1500 Range of measurement is a

function of the air-entry
value of ceramic disk

Thermal conductivity sensors Matric 10 to ∼ 1500 Indirect measurement using

variable-pore-size ceramic
sensor

Pore fluid squeezer Osmotic Entire range Used in conjunction with

psychrometer or electrical
conductivity measurement

0.001 ◦ C.

a Controlled temperature environment to

±

curve. The electrical output of the indirect sensor must first be
calibrated against a series of applied or known suction values.
The sensors are then ready for usage in measuring matric
suction in the soil. The indirect methods of measuring matric
suction based on the thermal properties of a standard porous
ceramic are described later in this chapter.

4.2.4.1 High-Air-Entry Ceramic Disks
High-air-entry ceramic disks have small pores of relatively
uniform size. The disk acts as a membrane or separator
between air and water (Fig. 4.5). The disk is generally made of
a ceramic * material such as sintered kaolin. Once the high-air-
entry disk is saturated with water, air cannot pass through the
disk since the contractile skin resists the flow of free air.
The ability of the ceramic disk to withstand the flow of
free air is related to the surface tension T s of the contractile
skin. The contractile skin acts as a thin membrane joining the
small pores of radius R s on the surface of the ceramic disk.
The difference between the air pressure above the contractile
skin and the water pressure below the contractile skin is
equal to matric suction. The maximum matric suction that
can be maintained across the surface of the disk is called its

Figure 4.5 Operating principle of high-air-entry disk described
in terms of capillary model.

where:

air-entry value u a − u w d . The air-entry value of the disk
can be illustrated using Kelvin's capillary equation:

(u a − u w ) d = air-entry value of the high-air-entry disk,

kPa,

T s = surface tension of the contractile skin or the

air-water interface (e.g., T s = u a − u w d = 73 . 75 mN/m

2 T s
R s

(4.4)

at 20 ◦ C), and

R s = radius of curvature of the contractile skin

or the radius of the maximum pore size, m.

* Manufactured by Soilmoisture Equipment Corporation Santa Barbara, CA.