[TECHNOTE] PET geotextiles offer greater alternatives for silt curtains

PET geotextiles offer greater alternatives for silt curtains
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PET GEOTEXTILES OFFER GREATER ALTERNATIVES FOR SILT CURTAINS

PET geotextiles offer greater alternatives for silt curtains, as their flexibility and improved tensile strength make them better options for marine environments.

Ecocoast was the first company to manufacture the full range of silt curtains in the EMEA region, under its Ecobarrier brand, creating customised products that suit specific conditions and project requirements.

These vertical barriers can be positioned in the water to contain fine particles of silt that are discharged into the water from dredging, construction or reclamation activities. The aim of these silt curtains is that they can act as a settlement pond and allow the silt time to settle within a contained area to reduce their spread. In doing so, the chances of the silt being re-suspended is reduced dramatically.

PET geotextile versus PVC silt curtains
PET geotextiles offer greater alternatives for silt curtains. PHOTO: Ecobarrier Silt Curtains, Doha Corniche, Qatar.

PET geotextile versus PVC silt curtains

However, while there is no doubt about the efficacy of silt curtains, questions have been raised about the suitability of the materials used to manufacture them. At Ecocoast, we manufacture our curtains with woven polyester geotextiles that have an opening size of 150µm, as it allows water to pass through the fabric, without the silt particles also getting through. We find this is a better option than PVC curtains, as they restrict the free flow of water due to being nonporous.

This is important for particular applications – such as in areas with strong currents and larger waves, while also aiding in the longevity of the silt curtain. Because water is allowed to pass through a silt curtain, it reduces the pressure exerted on the skirt, which helps both with the ability to anchor the silt curtain in place, while also extending the usable lifespan of the barrier.

At Ecocoast, we are always mindful about the impact our products can have on the marine environments they’re deployed in. One of the reasons we decided against using PVC coating on polyester for our silt curtains is because they are made by adding plasticisers in order to make the PVC softer and more flexible. These plasticisers have been proven to have an adverse effect on the environment, and although the probability of these chemicals leeching into the water is relatively low, over an extended period of time, in unpredictable water conditions, it is possible.

Silt curtain permeability

There is a common misconception that for a silt curtain to be completely effective, it must be completely impermeable, or that at the minimum, the opening size of the fabric should be smaller than the size of the particles suspended in the water. However, we believe that before attempting to reduce the permeability of the materials, it is important to consider the strength of the materials, so as to ensure that they are sufficient for environmental conditions.

Studies have shown that PVC-coated silt curtains are typically deployed where water currents are light and wave conditions are minimal. Other factors to consider are the depth of the water and the contours of the bottom where it is being deployed, as well as how the curtain will be anchored. Given the majority of our silt curtain projects are offshore in oceanic conditions, we believe that our PET geotextiles are far more adaptive and effective in these environments.

PET geotextiles offer greater alternatives for silt curtains
PET geotextiles offer greater alternatives for silt curtains. PHOTO: Ecobarrier Silt Curtains, Berri Pipeline Replacement, Saudi Arabia.

Furthermore, one of the most commonly used test methods for determining the Apparent Opening Size (AOS) of a geotextile is ASTM D4751, which uses glass beads of a known diameter and determines the 095 size by standard dry sieving. This sieving is done by using beads of successively larger diameters until the weight of beads passing through the test specimen is 5%. This defines the 095 size of the geotextile openings in mm.

We find that this is a poor test as it is conducted in a dry environment, whereas filtration and drainage always involve liquids. Secondly, the glass beads often become trapped in the geotextile and do not pass through at all, while electrostatic charges can result in finer glass beads clinging to the sieve.  Also, yarns in geotextiles, particularly woven ones, that shift and allow beads to pass through enlarged voids are not representative of the sample.

Specifically for the treatment of silt curtains, this test is flawed and provides a very basic, one dimensional view that does not take into account that silt curtains installed in marine environments will quickly become effectively impermeable, regardless of material, due to the expansion of geotextile fibres in the water, marine growth, formation of filter cake on the material, and particle clogging of pores, which can result in a reduction in coefficient water permeability by up to 84% after 180 minutes of artificial clogging.

By basing silt curtain material selection only on permeability assumptions defined by ASTM D4751, incorrect conclusions can be drawn that these particles will pass through the fabrics and as a result lead to incorrect selection of fabrics to weaker fabrics.

It is our belief that the above factors void requirements for skirt fabrics to have a minimum opening size or flow rate, and the sole deciding basis for skirt fabric should be tensile strength, designed to suit the environment of the installation. This will ensure the curtain a) performs in the environmental conditions for which it is selected, and b) does not fail, which can cause a much larger environmental incident than a few particles transferring through the fabric.

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