Geotextile mattresses - Geosystems: Design Rules and Applications - page 43

Anchoring
Since the geotextile mattress cannot be supported by a toe provision, the mattress
must be anchored adequately on the upper side. For the strength and stability of these
anchoring structures, general calculation rules have been derived. For the maximum
tensile load on the anchoring in still water, the following applies (see also Figure 4.4):

(4.3)

T δ L bw ⋅ ρ gL ( ) gD )

L ow − − ⋅ ⋅ g

w w k

gD

(sin α cos α tan )(

ow bw

where:

T = tensile load per unit width in the geotextile mattress [N/m];

bw

= length of the geotextile mattress above water [m];

ow

= length of the geotextile mattress under water [m];

α = slope angle [deg];

δ = surface friction angle between the unanchored geotextile mattress and the

subsoil [deg].

If the bottom geotextile layer is fairly rough, the friction angle between the under-
side of the geotextile and the bank is of the same order as the bank material itself. For
bank materials consisting of sand and other granular materials, . If the

frictional behaviour of the bottom geotextile layer cannot be established (e.g. as in the
case of geotextile bags), it is advisable to adopt a conservative value for the friction
angle, (

δ = 30 ° -35 °

for a slope of sand).
During wave attack, when the waves recede, the mattress may make reduced con-
tact with the subsoil due to the water flowing out and this can result in a reduction in
stability. The location where the mattress makes reduced contact can only be calcu-
lated when there is knowledge of the hydraulic conductivity of both the mattress and
the subsoil. This manual has opted for a conservative approach to the assessment of

δ = 20 °

T

H s

W cos α tan δ

W sin α

W cos α

α

W

D k

Figure 4.4 Tensile loads in the geotextile mattress as a result of anchoring. L ow and L bw are the lengths
along the mattress.