The Concept of Effective Stress in Unsaturated Soils - Multiscale Geomechanics: From Soil to Engineering Projects - page 150

Chapter 6
The Concept of Effective Stress in
Unsaturated Soils
Very small to large strain properties of a silty sand were measured at several
water contents ranging from a few percentage points to saturation level by the use of
three triaxial devices with different types of transducers. In the analysis of the data,
attention was paid to the influence of the water content on the maximum (Young's)
modulus and the effect of the confining stress, etc. Isotropic and triaxial tests, with
the measurement of capillary pressure (suction), were also performed on the same
material. The effective stress concept was used to interpret the results. The
predictions of a micromechanical model were compared with the experimental data
in the elastic domain and on the failure criterion. The method highlights a simple
way to relate the strength and stiffness of the soil to its capillary pressure.
6.1. Introduction
The “effective stress” concept is perhaps one of the most debated notions in
unsaturated soil mechanics. It was originally defined as “…a function of the total
stress and pore pressure that controls the mechanical effects resulting from a change
in stresses…” [BIS 63] and it applies exclusively to the solid skeleton of soil
[TER 43]. Effective stress has been used to formulate both elastic and elasto-plastic
constitutive laws of the solid skeleton, where it controls stress-strain relationships,
the volume changes and strength of the solid skeleton, independently of the pore
C hapter written by Said T AIBI , Jean-Marie F LEUREAU , Sigit H ADIWARDOYO , Hanène S OULI and
António G OMES C ORREIA .