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Comment by Animats | Hacker News Reader

Of course, you need "forever plastics" to make this work long term. It's like everything else where you have a material with decent compressive strength but weak tensile strength. You need something with some tensile strength to hold it together. That's what rebar does. It's why rebar rusting out destroys concrete structures. The same problem applies to geotextile containment of dirt. If the textile degrades, it's just a pile of dirt, in a place where a pile of dirt isn't enough.

The more traditional approach is to surround dirt or rubble with something that has more tensile strength. Some Egyptian pyramids were built that way. One of them fell down because the outward pressure from the load above was not sufficiently contained. Traditional stone construction of large structures is all about containing that outward push. That's what flying buttresses and bridge abutments do.

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> It's why rebar rusting out destroys concrete structures

That isn't really why. The loss of tensile strength from rebar decomposition isn't what kills it.

Steel expands as it rusts. It breaks apart the concrete via expansion just like freezing water would. If you had a mix of steel and stainless steel rebar, the rusting of the normal steel would destroy the concrete all the same.

Good point. The steel rusts, expands, more water gets in, more rust, concrete cracking, exposed rebar, rebar rusts away, no tensile strength, collapse.

(I was going to link to a picture, but at least half the images of corroded rebar found by search are AI-generated. Bogus technical pictures are getting out of control. Just saw some of the construction of the Panama Canal. No, steam shovels there did not have cowcatchers.)

I'm sort of OK with this use of plastic. Unlike all the single-use plastics we make (bottles, forks, food-wrap - the list is endless) this is going to be in use and providing value for decades. And can likely be made from recycled HDPE.

There appears to be a bamboo-derived plastic out there with high tensile strength, but I don't think it'd work for this application, because it degrades within 50 days of being buried in soil. I wonder if being in gravel would delay that, due to the different microbes present?

It seems like there are usually biobased solutions out there, but they aren't as cost-effective or easy to procure as the forever plastics, so organizations don't bother with them. Perhaps some kind of incentive would get us moving in the right direction.

You can have material that biodegrades, or material that doesn't biodegrade. But you can't have material that doesn't biodegrade that biodegrades. Plastic is a wonder material because it's a forever material.

That reminds me of a fun research presentation I saw a decade ago. They engineered a bioplastic which would break down on sustained contact with water. While trying to come up with applications, they proposed reducing plastic consumption by using it for ... water bottles.

There's a bit of nuance here. A material which doesn't biodegrade but is inert, a material which biodegrades but only when exposed to light or moisture or heat, a material which biodegrades into something else which is in turn inert, etc.

Not really relevant to geocells but quite relevant for e.g. timber, which might last a hundred years in a roof but start to rot in six months of composting.

Plastic is inert, but microplastics harm us anyway, and don't go away, because it's inert.

Plastics tend to be more chemically active than a lot of rocks, for example.

Although that's not going to protect you from crystalline dust from silica and asbestos.

The problem with plastic is that it often doesn't biodegrade but loses capability with time. So you end up with it turning to rubble after few decades but still polluting the environment

Plastic that disappeared after 50 or 100 years would still be very useful for making things. Plastic for packaging only needs to last months or years.

You need to avoid certain kinds of biodegradation but you don't need "forever".

And in particular if you could make very small pieces biodegrade quickly, and only those, you'd instantly solve microplastics.

There are species of wood that do very well buried. Black locust is known to last 100yr buried. I wonder if you could scale up the effect using a larger grid of black locust 2x8s [edit] or even smaller dimension mill scrap.

>If the textile degrades, it's just a pile of dirt, in a place where a pile of dirt isn't enough.

It's now a pile of dirt with loads of bits of plastic and slowly leeching plasticisers.

Yep, my first thought when I saw this was "and everywhere this is used, microplastics out to wazoo."

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