Geoscience Reference
In-Depth Information
Theoretically, it can be demonstrated that the placement variability increases as
the flow resistance becomes more influential. Current observations suggest the flow
resistance only exerts an influence when it is comparable to the gravity forces that
drive the fall of the geotextile container. As long as the container accelerates down-
wards, the flow resistance forces are still so small that they exert very little influence.
When the container arrives at the bed the flow forces increase and create horizontal
deviations;
h
1 corresponds to a water depth of around 6 m (under the split barge),
see formula G.3 in Appendix G. For rubble the terminal fall velocity is reached at a
much lower water depth (depending on the size of the rubble).
In conclusion, the different studies referenced in this appendix show that place-
ment variability can be a realistic phenomenon when placing geotextile containers at
greater depths (more than 15 m). It is therefore advisable to perform field dropping
tests at such locations where greater water depths are encountered and determine the
placement accuracy by means of sonar measurements before and after the drops.
As stated already, the accuracy of placement of geotextile containers is very
dependent on the water depth, the flow velocity and the wave height. From a practical
perspective, the impact of these conditions on placement accuracy, is demonstrated
by the following:
′
=
Water depth to 10 m; current velocity to 0.5 m/s; significant wave height max.
1.2 m; geotextile container circa 300 m
3
. Slope of 1:1.5 possible with stacking of
four containers;
Water depth 15 to 20 m; no current; no waves. Evenly filled container horizontal
deviation up to 3 m. Unevenly filled container horizontal deviation up to 5 m.
Slope of 1:2 possible;
Water depth circa 15 m; flow velocity to 0.5 m/s; significant waveheight
max. 1.2 m; bed flat. In this situation horizontal displacements of max. 25 m
measured.
A well-planned drop strategy based on the experience of the contractor can
contribute considerably to the end result.
In summary, it can be anticipated that the placement accuracy increases when:
the lower the water depth;
the lower the wave height and/or flow velocity;
the larger the filled cross sectional area of the geotextile container and split
barge;
the greater the density of the geotextile container [1];
the more evenly the geotextile container is filled (with homogeneous sand, in
terms of grain-size distribution and water content, applied layer by layer);
the faster the split barge is opened;
the rougher the bed surface.
