Coastal protection

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 stabilisation.

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.