Geoscience Reference
In-Depth Information
σ
i
′
=
average effective stress of the sand in the container [kPa];
φ
s
=
angle of internal friction of the sand in the container [degrees];
A
=
filled cross-sectional area of the geotextile container [m
2
].
The magnitude of this component is only significant for a high value of the effec-
tive stress
and the resulting friction that occurs between the sand grains. Two cir-
cumstances can be distinguished:
σ
i
′
a situation where the sand is close to full saturation;
a situation where there is still plenty of air in the voids.
These two situations are discussed below. However, it will become clear that the
energy dissipation in the contained sand depends on other factors (as well) that can-
not always be calculated, although guidelines can be given to optimize the energy
dissipation in the sand layer.
Influence of the level of saturation
If the condition occurs where the pores are saturated with water an effective stress
may arise during impact. This arises partly due to the effect of dilatancy and partly
due to the impact itself. During dilatancy the grain skeleton increases in volume,
under a shearing deformation, with a resulting tendency to absorb water. Because of
the speed of the deformation and the low water permeability of the fill that hinders
the flow of water into the dilating voids, negative pore pressures occur in the con-
tained fill voids. A precondition for dilatancy is a sufficient high density of the sand.
This precondition is normally not fulfilled, because the sand has been deformed to a
large extent when the container leaves the split barge and therefore the density will
be close to the critical density. However, some dilatancy has been observed through
modelling and field measurements, though it is by no means always present. During
impact, there is an increase in total stress. From measurements it appears that this is
not taken by the pore pressure, but mainly by the grain skeleton increasing the effec-
tive stress directly. In field measurements at the Kandia dam [7] two instrumented
containers were dropped and the effective stresses measured. For one with saturated
sand the effective stresses remained limited, because the impact was mainly taken
by the pore pressures. The other, which was made with unsaturated sand originally,
showed an increase in effective stress during impact of 30-100 kPa. 25 kPa of this
increase in effective stress was caused by dilatancy, the rest was caused by direct
increase of the total pressure during the impact. The potential for dilatancy to occur
is greatest if the sand has few fines: the percentage of fines smaller than 63
μ
m
should be less than 20%.
Influence of enclosed air
The influence of air enclosed in the voids of the fill material (unsaturated sand)
has been studied in [18]. The conclusion is that the enclosed air increases the und-
rained shear strength. During the fall the external pressure increases, and since the
