Geoscience Reference
In-Depth Information
The slope factor K s is a function of the influence of the angle of shearing resistance
between the geotextile mattress and the subsoil:
2
=−
sin
sin α
(4.8)
K s
1
δ
where:
α
=
slope of the structure [deg];
δ
=
surface friction angle between the geotextile mattress and the subsoil [deg].
In most cases the mattresses are anchored at the top of the slope. If this anchoring
is sufficient to prevent sliding K s
1. Using the formulae above, it is apparent that
in normal conditions, geotextile mattresses will not be subject to critical loads under
normal current conditions.
=
4.5.5 Geotechnical stability
Geotextile mattresses are mostly used for bank protection. In this situation they also
have to be evaluated for geotechnical stability. In chapter 3 the potential mechanisms of
shearing of the subsoil and liquefaction within geotextile bags are described (see 3.5.6).
These potential mechanisms may also occur when geotextile mattresses are used. For
this the design graphs provide in Appendix C, can be used which have been derived for
both geotextile mattresses and geotextile bags. In these graphs the geotechnical calcula-
tion assumes that the geotextile mattress is not anchored and that it has to remain stable
over its full design life. For geotextile mattresses this is a conservative assumption, while
for geotextile bags, without interconnections, this is more realistic. A slope protected
with an anchored geotextile mattress will not necessarily suffer a loss of strength if the
design graphs shown in Appendix C indicate that the structure is unstable. In this state,
strain will occur in the geotextile mattress and cause deformation. Since this may be an
undesirable condition and affect serviceability, reference should be made to Appendix C.
If the deformations are acceptable, then the structure may be also stable under higher
wave heights, although this depends on the extent of the deformations.
4.6 CONSTRUCTION ASPECTS
For geotextile mattresses, the following aspects require attention during construction:
￿
With the in-situ filling method, mattresses can be sewn together which enables,
larger sections to be laid, minimizing the material losses due to overlaps. It is
advisable, however, to apply an overlap of approximately 1.5 metres after each
4th or 5th mattress so as to limit the loads on the seams due to possible subse-
quent settlements;
￿
During in-situ filling the width of the mattress will decrease in the direction
perpendicular to the tube. This may lead to extra tension loads in the mattress
depending on the applied filling procedure;
 
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