Geoscience Reference
In-Depth Information
the tensile load in the mattress where it is assumed that over one wave height there
is no loss of contact under the water line between the mattress and the subsoil. The
formula is then:
T
δ
L
ρ
D
g
⋅
D
bw
(sin
α
cos
α
tan
)
bw
k
k
⎛
⎝
⎞
−
⎛
⎝
⎞
H
L
an
(4.4)
sin
1
H
s
δ
(
)
⎛
⎝
LD
⋅
+
⎛
⎝
t
−
⎛
⎜
⎛
⎝
s
s
c
o
g
⎠
⋅
⋅
⋅
⋅
−
⋅
−
k
w
w
w
α
⎝
α
ow
ow
With formula (4.4) it is possible to calculate a negative value for
T
, which means
that the geotextile mattress remains on the slope under its own weight, with no tensile
load occurring at the anchoring point.
For equilibrium along the slope:
T
=
W
⋅
sin
α
-
W
⋅
cos
α
⋅
tan
δ
(4.5)
In the event of unforeseen conditions, e.g. flow shearing along the underside of
the geotextile mattress, the loads on the anchoring point can be high (up to a maxi-
mum of the total weight of the mattress) and be coupled with substantial tensile loads
in the geotextile mattress.
In [14, in Dutch] a falling apron is also given as a possible application. In such
a structure the subsoil can erode from under the mattress as there is reduced contact
between the mattress and the subsoil. When determining the tensile loads in the mattress
in this case the friction between the mattress and the subsoil is disregarded. The anchor-
ing must therefore be able to support the weight of any part of the mattress that is in free
suspension. Only the component of that part of the mattress where the subsoil does not
erode, can contribute to the stability through friction between the subsoil and mattress.
Application phase
External damage to the upper geotextile layer is another possible cause of mechanical
damage to the geotextile mattress. This can occur by vandalism or recreational dam-
age, e.g. by impact from a boat. However, the compartmentalisation attributes of the
mattress limit the extent of any damage:
With damage caused by impact from a boat, the deeper part of the geotextile mat-
tress remains filled with sand;
The use of a combination of materials with a protection layer on the upper geo-
textile layer strongly limits mechanical damage and hinders vandalism to the
upper surface;
Where the upper mattress surface is damaged, there is still approximately 50% of
the strength in the lower mattress surface before the slope becomes exposed.
For the application phase, the actual density of the fill material is important. Wave
loads and water flows may cause the fill material inside the mattress to compress
further. If the original density is too low, further compression will reduce the mattress
thickness and this may cause gaps to occur between the mattress and the subsoil,
which may result in local deformation of the subsoil.
