Coastal & Waterways

Coastal & Waterways

Many coastal and waterway applications commonly use TERRAM Geosynthetics including

Foreshores, dams and flood bunds, river and canal banks, lagoon, lake and reservoir shores, culverts and outfalls, ports, breakwaters, artificial islands and causeways. 

TERRAM Defencell geocells are used for temporary flood protection where a fast response in required and local fill materials are available.

Coastal, Waterway & Marine Geotextile Selection

TERRAM Robust Geotextiles provide an innovative alternative to traditional materials, but the careful selection and specification of the correct geotextile is vital, in the same way as the stone size and grading were important for the more traditional methods

The correctly specified geotextile must:

i) be sufficiently robust to withstand installation and service-life loads.

ii) have a suitable pore size and be capable of providing sustained permeability and filtration

iii) be more permeable than the underlying soil.

iv) possess the necessary extensibility to conform intimately to the profile of the rocks above it and to adapt to point loads, in order to avoid puncture and tearing. 

TERRAM Robust Geotextiles have been engineered to provide this combination of properties for this application.

Filter properties

The performance of a geotextile filter is dependent on a number of principal factors:

• the size, number and uniformity of the geotextiles pores in relation to the soil particle sizes

• the magnitude of the disturbing forces (the higher the forces - the greater potential for soil migration)

• the structure and inter-particle bonding of the in-situ soil particles - the greater the soil density and the greater the interparticle forces, the less potential there is for the soil to migrate

• the particle size distribution, structure and pH of the soil

• the permeability of the geotextile in relation to the soil

The universal approach to filtration is that each layer must be more permeable than the layer beneath. Rules adopted using a geotextile within a wave environment suggest that the coefficient of permeability of the geotextile used should be 10-100 times greater than that of the underlying soil. It is also important that the geotextile maintains or exceeds its required index permeability whilst under load, i.e. any reorientation of the fibres should not decrease permeability.

The characteristic pore size of the geotextile should be less than the average grain size of the soil to be filtered to ensure that any loss of material through the geotextile is mitigated. Established design rules for reverse flow applications, and for a typical geotextile, state that the opening size of the geotextile (O90) should be less than the sieve size (D50) of the soil to be filtered.

kg > 10-100ks

where: kg = coefficient of permeability
            for the geotextile

            ks = coefficient of permeability for the soil


O90 < D85

where: O90 = apparent opening size for the geotextile
            D85 = particle size fraction for the soil

TERRAM Robust Geotextiles are designed to provide sufficiently low pore size to prevent the migration of fines whilst ensuring the necessary permeability required for a wave environment.

Resistance to Damage

The geotextile incorporated within the design must be able to withstand the stresses placed upon it both during the installation and in service. Loads placed upon the geotextiles by the overlying armour stone will be uneven, with the highest stress concentrations at localised points.

TERRAM Robust Geotextiles are engineered to have an elongation at break which allows deformation around the armour stone without rupture or loss in hydraulic performance.

The mass, angularity and drop height of the armour stone being placed directly upon the geotextile, together with the method of installation, play a contributing factor in the potential for the geotextile to be punctured. Additional factors for consideration include movement of armour stone during wave action and differential settlement. 









Coastal Defence Breakwater - Terram Robust Geotextiles
TERRAM geotextile used in a revetment breakwater
Coastal robust geotextile / waterway geotextile

TERRAM Robust Geotextiles are engineered to provide isotropic strength and elongation. This is important to ensure that point loads do not have any detrimental effect of filter performance.

It is recommended that, where possible, the designer performs a trial installation with the proposed rock. A designer can specify a site damage test such as a rock drop test where a pad of geotextile is laid on a prepared beach material and held taught around its perimeter. A rock similar to those proposed should then be dropped onto the surface from its
maximum construction height. There should be no damage to the underlying geotextile. For further advice please contact the TERRAM technical team for assistance with the selection of an appropriate TERRAM Robust Geotextile.


In the selection of a geotextile, a major consideration should be its ability to perform within its environment for the full duration of its design life. 

TERRAM Robust Geotextiles are manufactured from UV Stabilised virgin polypropylene fibres that have been designed to perform in coastal applications with a predicted durability of 100 years in standard specified acid and alkaline conditions.