Hydro-Mechanical Properties of an Unsaturated Frictional Material

2.7. VOLUMETRIC BEHAVIOR OF PARTIALLY SATURATED SOILS 63
investigated the stress-strain behavior of a silt using dimensional compression and rebound
tests and their results confirm that increasing suction has a beneficial effect on its mechanical
behavior. That means with increasing suction the compressibility decreased and the stiffness
increased.
Considering granular materials most experimental studies were performed on saturated
(S = 0, S = 1) specimen as done by Oda et al. (1978) who performed plane strain and triaxial
compression tests on samples of naturally deposited sands. Desrues et al. (1996) performed
triaxial tests on sand. Gennaro et al. (2004) investigated the influence of different loading
path on the undrained behavior of saturated sand (Hostun sand) carried out in several triaxial
tests. Schanz & Vermeer (1996) derived elasticity moduli of dry sand specimen (Hostun sand)
from one-dimensional compression and triaxial tests. The influence of void ratio was studied.
Many researchers studied the influence of void ratio and loading path on the stress strain
behavior of sand. The influence of suction or water content was not from interest even it is
well known that the suction (in this case the matric suction) is the main force influencing the
hydro-mechancial behavior of granular materials.
2.7.2 Collapse Behavior
An important feature of the volume change behavior of unsaturated soils is the collapse
phenomenon. The oedometer apparatus usually is used to predict the collapse potential of a
soil. The collapse phenomenon is defined as the decrease of total volume of a soil resulting
from a induced wetting and the breakdown of the structure of the soil at unchanging vertical
stress. The collapse potential CP conducted in a oedometer apparatus is the change in
specimen height ∆h resulting from wetting, divided by the initial height h0 of the tested
specimen, expressed in percent:
� �
∆h
CP = · 100 (2.26)
h0
The collapse can be slow or fast and its magnitude can vary between less than 1% and up
to 10% (Lawton, Fragaszy & Hardcastle 1991). The literature review by Lawton, Fragaszy
& Hetherington (1991) showed, that nearly all types of compacted soils are subjected to
collapse under certain conditions. But also naturally deposited soils, clean sands and pure
clays and also soils containing gravel fractions can undergo collapse (Lawton et al. 1992).
Whereas compacted soils generally collapse at high stress levels, naturally deposited soils can
collapse at low stress levels. Among others the amount of collapse increases with the applied
vertical stress, decreases with initial water content as well as decreases with dry density (Cox
1978, Lawton et al. 1989). Some conditions that are required for a soil to collapse are an
open partially unstable, unsaturated fabric in the soil and vertical stress. A suction that is
high enough to stabilize the unsaturated specimen or cementing agent between the grains are
required for a soil to collapse.