Hi, Glyn Onione the Senior Technical Consultant for Aquatic Engineering in the UK here again to answer another great question we are often asked. Which is – What Are Silt Curtains Made Of?
I’m often asked here at Aquatic Engineering as the Senior Technical Consultant: what are silt curtains made of and how are they made? Well that’s an interesting question and I hope I can answer it to your satisfaction. But it is largely down to application, for instance if we compared protective silt curtains with say shoes, which are also protective, then we can better see the huge variance; sandals wouldn’t be suitable for mountaineering; just as wellington boots would be odd if worn indoors whilst watching TV.
At Aquatic Engineering we have produced curtains up to 30 metres deep for protecting sea grasses in marine habitats for periods of up to 5 years in duration; but then again we produce fish curtains for biomanipulation projects or to prevent fish being sucked into pumps or machinery. These are not the same products we developed for keeping E. coli out of the Triathlon course at the Glasgow 2014 Common Wealth Games. These significantly vary from the products we use to contain suspended silt curtains in the water column on civil engineering sites. However, with that said they all have a lot of things in common.
Silt Curtains or Silt Barriers, or just about any other curtain system designed for aqueous application are made up of usually 4 basic components: A floatation system; a ballast chain along the bottom; various ironmongery – nuts, bolts, washers, reinforcing and attachment plates, straining wires, attachment shackles (according to the design specification) and finally the most important item the curtain membrane itself. Let’s look at each of these in turn.
- Floatation: This can be by means of an air inflated bladder running along the top of the curtain; plastic or fibreglass pods or floats and most commonly in the UK a solid foam float inserted inside of a pocket running the length of the top of the curtain. Many companies in the UK and abroad go for the cheapest option, which is HDP, High Density Polystyrene. Polystyrene is an excellent buoyancy aid, however from an environmental standpoint it is a disaster! When polystyrene is exposed to rough conditions it is prone to breaking up into a granular form. If then for some reason the curtain float pocket is compromised the polystyrene escapes and continues to break down through abrasion and such like creating those small balls we can have stuck through static to our clothes when it is used for packaging. Unfortunately it becomes yet another man made pollutant of our rivers, seas and oceans and is ingested by a wide range of marine and bird life. At Aquatic Engineering we made a decision over 8 years ago not to use HDP but rather polyethylene, which is much more stable in water and is probably the most environmentally acceptable manmade foam for this application. However, we currently also have several R&D projects running looking at other forms of floatation. Floatation can be as small as circa 50mm diameter through to 500mm for large silt barrier installations.
- Ballast chain: The ballast chain helps the silt curtain sit vertically in the water, it weighs the curtain down to help hold it in place. Sometimes the ballast chain is enough of a weight if the curtain is installed in a calm water environment, but usually this is not the case. The ballast chain can then become the attachment point for anchors (anchorage will be dealt with another time). The chain can be quite small for low energy curtains, say a 6mm chain, but right up to 32mm, what we would call ship chains, for marine applications.
- Ironmongery: This varies a lot from silt curtain to silt curtain, many components were designed by AE and are made by specialist engineers on the Isle of Wight where our main factory is located. Suffice to say if you are building floating silt barriers 30m deep or in one piece 1.2km long then you need to be able to rely on all of your fixtures and fittings – integrity of product is key.
- Curtain material: We probably offer more choice than any other silt curtain fabricator, because we have developed so many applications for our silt containment products we rely on probably 30-40 varying materials according to the application. However, we can sum up these into 3 or 4 categories: PVC, Woven Polypropylene, Neoprene and another couple of unique materials, we do not generally advertise, for the specialist applications we are renowned for.
Because we use such a wide range of materials, so that we can produce the toughest silt curtain required at the cheapest price for our clients and partners, we also have to have a wide selection of silt curtain fabrication systems available to us. We find each have their merits and advocates, however by selecting the right curtain material then marrying it up with best fabrication practice we can avoid getting set into a narrow way of thinking and production. I always say to the team – adaptation is often the key to innovation – so we adapt to the needs of our clients and the environments they work in which results in innovative new ways of production. We are fortunate that we work out of a modern 6000ft2 factory, so we can quickly alter our production system to suit each days silt curtain challenge. A large open space is vital when dealing with thousands of square meters of heavy fabric for a specialist silt curtain application.
Silt retention curtains and the wide range of other applications such as fish curtains or algae deflector curtains and baffles etc. can be: Stitched; High Frequency (HF) welded; hot air welded; wedge welded; riveted, bolted or vulcanised, depending on the application and environmental restraints and conditions.
I hope you’ve found this interesting and informative – I still keep being asked other questions about our Silt Curtains, such as: What other applications do you use your silt curtains and floating silt fences for? I will definitely be answering that one in an upcoming blog post – you’ll be amazed!
If you have any questions or would like to talk through your civil engineering projects please get in touch.
Glyn Onione, Senior Technical Consultant at Aquatic Engineering
Call 01983 616668 Keep up to date with our silt curtain installations here