Hydro-Mechanical Properties of an Unsaturated Frictional Material

66 CHAPTER 2. STATE OF THE ART
- Janbu’s method (1969)
Janbu (1969) proposed that the preconsolidation pressure can be determined from the
plot of tangent modulus M versus effective vertical net stress (M − σ ′ ):
M = ∆ε
∆σ ′
(2.27)
where: ε is the strain and σ ′ is the effective vertical net stress (see Fig. 2.32). He found
a drop in stiffness modulus near the preconsolidation pressure signifying a breakdown
of the structural resistance. He defined the preconsolidation pressure as the point just
before the stiffness modulus levels out.
A detailed review of evaluation of of the preconsolidation pressure and a investigation of these
various methods is given by Grozic et al. (2003). Investigations on silt (Rahardjo & Fredlund
2003) showed, that soil has not only a memory with respect to effective stress, but also with
respect to suction. In the present study the methods given by Casagrande (1936) and Janbu
(1969) are used to estimate, if there is an influence of the applied matric suction on the stress
history behavior of granular material. The maximum stress to which the soil has been exposed
(i.e. due to suction) is determined equivalent to the preconsolidation pressure and called here
yield stress ˜σ0.
2.8 Summary
This chapter summarizes important literature referring to unsaturated soils. In the beginning
unsaturated soils are introduced and differences in hydro-mechanical behavior of several types
of soils are given. Further stress state in saturated as well as unsaturated soils is explained.
Several authors suggested single valued stress equations and other authors suggested the
approach of independent stress state variable for describing unsaturated soils. In the present
study the approach of independent stress state variables is preferred and used to perform
experimental investigation on unsaturated sand. That is the application of suction and total
net stress with reference to the air-pressure or water pressure.
Phases in unsaturated soils as well as soil suction and its components are discussed. In
case of granular materials as sand the matric suction is the main component influencing its
characteristics.
The definition and meaning of both hydraulic functions, namely the soil-water character-
istic curve and unsaturated hydraulic conductivity function is given. Mainly the chapter is
focused on the methods for determination of hydraulic functions. Most common methods are
the one-step, multistep method and the continuous flow experiment that are used tradition-
ally in combination with inverse methods to determine hydraulic functions. However, in the
present work parallel measurements of water content and matric suction are carried out for
the direct identification of hydraulic functions for drainage and imbibition cycles.