Measurement and Estimation of State Variables - Unsaturated Soil Mechanics in Engineering Practice - page 109

CHAPTER 4

Measurement and Estimation of State Variables

4.1 INTRODUCTION are still significant challenges to overcome. The geotechnical

engineer should understand the physics associated with soil
suction and the range of suction values that can be measured
with various suction devices.

The state variable of greatest relevance to unsaturated soil
mechanics is soil suction. Soil suction is the general term
which may be used when referring to matric suction, osmotic
suction, or total suction. There are also other state variables
associated with unsaturated soils that may need to be mea-
sured or estimated, namely, the amount of water in the soil
or its natural water content.
The variables of soil suction and water content measure-
ments combine to form the SWCC, a function that is used
for the assessment of unsaturated soil properties. Unsaturated
soil mechanics engineering designs can be verified in the field
through in situ measurement of soil suction and water content.
In situ measurements can verify that the actual response of an
unsaturated soil in the field is consistent with the SWCC used
during design. There are some situations where it is sufficient
to simply measure soil suction in situ or water content in situ.
These situations are discussed later.
There are other state variables that might need to be mea-
sured in the field for verification purposes. There are vari-
ables that may be related to ground movements in the hor-
izontal and/or vertical directions. These measurements are
related to the deformation state variables. The instrumen-
tation required for the measurement of field movements is
quite similar for both saturated and unsaturated soils. Lim-
ited attention will be given to the measurement of ground
movements in this chapter.

4.2.1 Theory Related to Measurement of Soil
Suction Components
The theoretical concept of soil suction was developed in soil
physics in the early 1900s (Buckingham, 1907; Gardner and
Widtsoe, 1921; Richards, 1928; Schofield, 1935; Edlefsen
and Anderson, 1943; Childs and Collis-George, 1948; Bolt
and Miller, 1958; Corey and Kemper, 1961; Corey et al.,
1967). The soil suction theory was mainly developed in rela-
tion to soil-water-plant systems. The importance of soil suc-
tion in explaining the mechanical behavior of unsaturated
soils encountered in civil engineering was introduced at the
Road Research Laboratory in England (Croney and Coleman,
1948; Croney et al., 1950). In 1965, the review panel for the
soil mechanics symposium “Moisture Equilibria and Mois-
tureChanges in Soils” (Aitchison, 1965) provided quantitative
definitions of soil suction and its components from a thermo-
dynamic standpoint. These definitions have become accepted
concepts in geotechnical engineering (Krahn and Fredlund,
1972; Wray, 1984; Fredlund and Rahardjo, 1988).
The components of soil suction have a common context
when considering the free-energy state of soil-water (Edlef-
sen and Anderson, 1943). The free energy of the soil-water
can be measured in terms of the partial vapor pressure adja-
cent to the soil-water (Richards, 1965). The thermodynamic
relationship between soil suction (or the free energy of the
soil-water) and the partial pressure of the pore-water vapor
can be written as follows:

4.2 MEASUREMENT OF SOIL SUCTION

It is paramount to be able to measure soil suction in the lab-
oratory and in the field. The application of unsaturated soil
mechanics in engineering practice is closely related to the
ability to measure soil suction. Soil suction (i.e., matric suc-
tion and total suction) has proven to be a difficult variable
to measure. The cavitation of water in soil suction measur-
ing systems has made it difficult to measure soil suctions
that exceed 1 atm of tension. While much progress has been
made with regard to the measurement of soil suction, there

ln u v
u v 0

RT K
v w 0 ω v

ψ =−

(4.1)

where:

ψ = soil suction or total suction, kPa,

R = universal (molar) gas constant [i.e., 8.31432

J/(mol K)],