30 August 2016
Next up on our featured innovators list is Andrew Mather, a professional civil engineer from Durban, South Africa. Mather deals with the city’s coastal environment on a strategic level, making decisions in terms of intervention, protection, and tourism. One of his more interesting strategic decisions was to use geotextile bags for sea defense.
Geotextile bags have been used around the world for several decades. A geotextiles is permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain. Durban has been using them as an alternative to sea walls for about a decade. They cost roughly as much as traditional construction; however, they offer far more flexibility and construction time and environmental impacts are much less.
We sat down with Mather to learn more about geotextile bags and why the city decided to use them.
How did you get started using geotextile bags?
We had a very large storm in Durban in 2007. Up until that time, most of the engineers, their approach to sea defense was basically to build some form of concrete wall – very rigid, very intrusive. After this big storm, one of the consequences of the damage we had was all these manmade structures were basically broken up by the sea and they contaminated the beaches. We had rubble all over the beaches. The recovery and the cleanup was extensive.
So we started looking at what sort of systems we could put in place where even if we do have a [infrastructure] failure (and I do need to say, engineers, unfortunately, are not given the budgets to design something that will never fail). Generally, we try to design for a one in fifty year storm event. We know that there is about a 2% chance we’ll have a [infrastructure] failure.
We wanted to find something we could use that when it does fail, there is far less of an impact on the environment in terms of clean up. We also looked at moving away from a very rigid system to one that’s more flexible where we can go in and do repairs and monitor.
After the 2007 storm, we put in a lot of those bags all up the coastline in places it was damaged. We were using the parent material from the beach so we were able to quickly fill up the bags. We didn’t have to bring in tuck loads of cement or rubble or other materials because they were basically already present on the beach. With minimal resources, we were able to put in place a defense system which would meet our design requirements. To date we’ve got about 20,000 bags.
How do they work?
At low tide, we send a machine in to take sand from the beach below the high water mark so we’re not affecting the visible beach. We dig down below that, use the sand to pack the bags and then place them during high tide. You can fill up a bag (4 tons of sand) in half an hour. There’s a special machine that sews up the bags before they’re placed.
The defense system is normally inland from the high water mark – we’re trying to preserve a beach in front and then trying to protect infrastructure which is normally located behind the beach.
The smaller bags measure 1.5 x 1 meter and weigh 4 tons when fully loaded. They have two layers fused together, each about ¾ in thick. There is a UV protection layer on the outside. It also protects against abrasions. Then on the inside, there is a white layer and that’s the strength component. It’s 10 solid tons per running meter. Manufacturers estimate the durability of these for 25-50 years, depending how much UV light they get.
The beauty of the bags is that they pack down. You can get all your construction materials to the site in just a few trucks. It makes working in inaccessible areas much easier. Provided your sand is already there, you basically just bring these things to the beach and start filling them.
What are some of the challenges in using these?
Like any construction there’s always a couple of glitches. We had one failure where the bags were stacked on top of each other going up the side of an embankment and the structure was too flat. There wasn’t enough friction generated between the layers so when the waves hit the structure, the down force would suck out these 4 ton bags. So we modified the slope to create more overlap between the layers. A one hundred percent overlap makes the structure stronger.
Another problem is that they get vandalized, even though they are almost vandal proof. The UV layer on the outside absorbs sand so they start to look like the beach. If you take a knife and stab it in, you get quite a lot of resistance from the coarse grains of sand. Driftwood fires on the beach also pose a problem – they can melt the synthetic fabric.
You can control the vandalism side, obviously, by having a responsible population who are aware of what it’s there for.
Do you conduct any public awareness campaigns around the bags?
We put up boards where we have these projects telling the public what we’re doing and why. Generally, it’s pretty well received. The other problem we have in South Africa is that we have quite a lot of illiterate people so that message isn’t always received. We have 13 native languages which makes it even more complicated. The municipality predominantly uses Zulu. Communication wherever you are in the world is always a problem.
You recommended a couple suppliers to us – why them?
When we first started looking at bags, there actually wasn’t a supplier in South Africa. There were a few geotextile manufacturers but they weren’t making this specific product. So they came to us and opened up discussions and from there started manufacturing South African devices. There was originally one company but recently two more have joined it. The demand is there. We’re using extensively in Durban but they’re being used up and down the coast now. They seem to be more environmentally friendly than rock and concrete. It’s sort of the new kid on the block.
Any final thoughts?
We can’t design for every eventuality and climate change is going to cost all of us money. So, to be quite frank, even if you don’t believe in climate change, it would be completely irresponsible not to take some of the elementary changes that you can do virtually at no cost so that there’s less damage in the future.
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