WASSERBAU-KUESTENSCHUTZ

Coastal protection measures are designed to protect inland flooding
and minimise the erosion of the coastline caused by the constant motion
of the sea. Geosynthetics are used in various coastal protection
applications such as filters in dykes and dams, for foundations under
groynes and breakwaters, as well as by using geotextile containers as
structural elements in groynes, seawalls, breakers or for bed and
embankment stabi lisation.

Stabilisation of dykes

Terrafix® needle-punched geotextiles are used to protect the
coastline when used in the toe area of sea walls and dykes. They improve
the construction efficiency if the sea currents cause surface erosion
or unacceptable soil displacement. The three-dimensional, labyrinth-like
pores and channels of Terrafix® nonwovens are not only similar to the
soil structure itself, but, if correctly designed, also increase the
stability of the revetment against impact stress caused by the motion of
the sea. For coastal hydrodynamic forces and the typical fine to medium
sands in these areas, the following simplified engineering approach can
be used to determine the effective opening size 090,W of a filter geotextile when one is used below an open revetment:

090,W ≈ d50 of the subsoil

The minimum thickness of the filter geotextile should be 4.5 mm
(please refer to chapter “Filtration”) in order to ensure the stability
and permanency of the filter. Due to their extraordinary robustness,
Terrafix® geotextiles with a mass of about 600 g/m² are used below
blocks or riprap.

A geotextile with a minimum mass per unit area of 600 g/m² is necessary wherever type II (LMB5/40) or III (LMB10/60)
armor rock with individual weights ≤ 60 kg are placed directly on
Terrafix® geotextiles, or where concrete revetments for high-stress
applications have been installed. Where individual stone weights exceed
60 kg, Terrafix® geotextiles with yet a higher mass per unit area are
recommended. In the case of low-stressed dykes, Terrafix® filter
geotextiles with a minimum mass of 500 g/m² and minimum thickness of 4.5
mm serve to encapsulate and stabilise the sand core from erosion. When
flooding occurs, they prevent washout of the sand and ensure the
stability of the dyke. Top soil as well as concrete blocks can act as an
effective cover layer over the geotextile.

Groynes, breakwaters and seawalls

In some cases, dykes alone are insufficient for coastal protection.
Structures such as groynes, breakwaters and sea walls are used to
prevent the erosion of coastal areas and directly influence the
localised sea currents. Terrafix® sand mats provide stability as well as
scour protection for these structures. Even in deeper waters, the
underwater installation of Terrafix® B 813 is easy due to the integral
sand ballast layer. Depending on the subsoil conditions, Terrafix®
filter layers that are at least 4.5 mm thick (for soil types 1 to 3) or
6.0 mm thick (for soil type 4), ensure the filter stability of the fine
grained subgrade. Secugrid® geogrids are used as additional reinforcing
elements where foundation failure is possible. The reinforcement
measures required for transverse structures depend on the prevailing sea
current stresses to which they are subjected. For low-current stresses,
a Terrafix® needle-punched nonwoven geotextile or a Terrafix® sand mat
can be placed on the sandy dam core and covered with riprap. For
mediumcurrent stresses, it is recommended that the dam core is
reinforced with two or three layers of Terrafix Soft Rock® sand
containers prior to installing the revetment. Groynes can be built
entirely with Terrafix Soft Rock® sand containers, and if required, be
covered with the appropriate revetment.

Terrafix Soft Rock® sand containers consist of needle-punched
nonwoven geotextiles with a maximum tensile strength of ≥ 30 kN/m. Over
80 % of the nonwoven geotextile strength is required for all seams. One
of the remarkable features of Terrafix® is its robustness and elongation
capacity. Terrafix Soft Rock® sand containers can be placed in their
exact position and over granular terrain without a high risk of becoming
damaged by using specific excavator grabs (“grippers”). When properly
designed, even the dynamic impact of dumping does not reduce the
performance of Terrafix Soft Rock® due to the flexibility of the
Terrafix® nonwoven geotextile. Mussels, algae and other underwater life
attach to the crimped fibres that are partially in a vertical position,
so that the Terrafix Soft Rock® sand containers integrate quickly and
easily into the sensitive sea ecosystem.

Erosion control of the coast

Storm tides can set the entire sea in motion, with its energy reaching the coastal breaker zone inducing erosion and

morphological displacement. In such cases, Terrafix Soft Rock® sand
containers used as artificial reefs serve as underwater breakers to
reduce wave energy. Depending on the requirements, Terrafix Soft Rock®
sand containers can have a volume of 1 m³ to 250 m³. Due to the high
elongation capability needle-punched Terrafix® nonwoven geotextiles are
used to handle the critical stress that can occur during the
installation process. Terrafix® sand containers easily accommodate
irregular surfaces.

Terrafix® nonwoven filter geotextiles are also used for the
construction of artificial dunes. Using a unique wrap-around method,
Terrafix® nonwoven geotextiles encapsulate a sand layer and are then
planted with locally available bushes and grasses. Due to the high
abrasion resistance and elongation capacity of Terrafix® nonwoven
geotextiles, this construction technique provides excellent resistance
to wave impact year after year – preventing erosion of the coastline.

Conclusion

Terrafix® system solutions allow the implementation of new and
innovative coastal protection measures. Terrafix® needle-punched
nonwoven geotextiles, sand mats and sand containers can be effectively
employed in dykes, transverse structures, as well as breakers for
erosion control. Placement of the robust and resistant Terrafix®
products can be carried out under dry conditions, as well as from ships
or underwater. Terrafix® products allow new approaches to coastal
protection that are close to nature.