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CONSTRUCTION MATERIALS
Geotextiles are a family of synthetic materials such as polyester, polyethylene and polypropylene that are becoming increasingly used for coastal applications. The materials (usually polyester or polyproylene for coastal applications) are used to form flexible and durable sheet fabrics that resistant to tension and tear.
These sheet fabrics may then be sewn to form bags or containers designed to contain fine grained materials, these units can then be used as coastal construction elements. Probably the major difference between different geotextile materials is their method of manufacture, woven or non-woven. Woven fabrics are manufactured by weaving high tensile yarns in an orderly pattern on large industrial looms. Woven geotextiles have a high tensile strength, but a low elongation capacity which can limit their use for some applications. Non-woven geotextiles are manufactured as sheets of directionally or randomly orientated fibre or filaments, thermally bonded or mechanically needle punched. Non-woven fabrics have excellent filter characteristics, a high elongation capacity but a lower tensile strength than woven's. Both woven and non-woven fabrics are becoming increasingly popular as alternatives to 'hard' coastal construction.
The use of geotextiles has several advantages over conventional construction materials like rock and concrete. Probably the major benefit is that they allow the use of locally available fine grained materials such as sand, which would usually be useless for coastal construction. This can help to reduce costs and minimises the impact of the construction works on the local area as there is less need to bring in materials by road.
Either woven or non-woven material could be used successfully on the Lyall Bay reef, the final choice of material will eventually come down to who wins the tender to supply the materials. The tenderer would have to prove that their material is suitable for the application. While it is true that geotextiles are not as durable as rock or concrete, with correct care they can be very effective and long lasting materials especially when used to construct submerged structures. Modern geotextiles are durable materials, with a guaranteed service life of around 20 years and a postulated life of up to 100 years when submerged, even in a challenging marine environment. Tests by Naue Fasertechnik and others have shown that the materials are resistant to chemical and biological influences and can be effectively protected from ultra violet degradation by applying UV stabilised products. It is unlikely to be necessary for UV stabilised products to be used on the Lyall Bay reef as the structure will be submerged and so should be at no risk from UV. However it is generally the case that all geotextiles designed for engineering use have some degree of UV stabilisation. I would not think it necessary to specify any extra resistance to UV.
