Hydro-Mechanical Properties of an Unsaturated Frictional Material

2.7. VOLUMETRIC BEHAVIOR OF PARTIALLY SATURATED SOILS 65
stress and the suction. The addition of water to the unsaturated specimen reduces the suction
in the soil and weakens or destroys the bonding or cementing agent and thus causes shear
failure at the interaggregate or intergranular contacts (Casagrande 1930, Barden et al. 1973,
Mitchell 1993).
2.7.3 Influence of Stress History on Mechanical Behavior
The preconsolidation stress σ0, is the maximum effective stress to which the soil has been
exposed. Estimation of stress history characteristic are most often based on one-dimensional
compression tests. The interpretation of the stress-strain curve during initial loading gives
information, if the soil was already compressed by a greater overburden pressure. The pre-
consolidation pressure can be determined using several methods. Common used methods are
that proposed by Casagrande (1936) and Janbu (1969). Both methods are briefly described
below:
Void ratio
- Casagrande’s method (1936)
As presented in Fig. 2.32 Casagrande (1936) proposed a graphical method using the plot
of void ratio versus effective vertical net stress (e − log σ ′ ). First the point of maximum
curvature (point A) on the recompression part is determined. Then the straight-line
part BC is produced. A Tangent is drawn to the curve at point A and the horizontal is
drawn through A. The angle between the horizontal and tangent is bisected. The point
of intersection of the bisector and BC gives the approximate value of the preconsolidation
pressure σ0.
A
σ
Effective 0
Effective vertical net stress
B
C
Tangent modulus
σ 0
vertical net stress
Figure 2.32: Casagrande’s (1936) and Janbu’s method (1969) for prediction of preconsolidation
pressure