An investigation of the root barriers used with the new Laman Street figs

I was showing some friends from Sydney around Newcastle the other day when one of them posed an interesting question to me. This post is my attempt at answering it.

We were walking down Laman street and I was pointing out the newly planted fig trees and the wonderful Brett Whitely egg sculpture (“one of only two large form Whitely sculptures in existence, and we’ve got it here, in Newcastle!” I gushed).

I got to the bit about the new figs being planted within some sort of underground cage aimed at containing their root system (my understanding of the situation). My friend piped up and said “how does that work? I mean, when the roots grow big enough to fill the cage, what happens then? Do they burst through or do they just keep growing and keep filling the space until there’s no dirt left, just deformed roots?”

It was an interesting question. Because it was one I’d never thought to ask. How do those root cages work? And so I decided to do some internet research and see what I could come up with in the way of an explanation.

A note of warning – I ended up with more questions than answers. If you keep reading this, you might too. On the other hand, maybe you’ll know something I don’t, in which case please help fill in the (vast) gaps in my own understanding by sending me an email or commenting on this post.

So basically what I’m trying to find out is, what are the root boxes exactly, what are they meant to achieve and how do they work?

My first port of call, for a general overview, was Wikipedia. It turns out the site had a page dedicated to what I was after, albeit with a warning at the top that the article needed more sources.

According to the root barrier Wikipedia article:

Root barrier is a physical underground wall, placed so that structures and plants may cohabit happily together.

Okay so far so good. That’s pretty much what I expected.

Development of root barriers, their construction and applications has progressed since 1992 when technology progressed from a concrete barrier which cracked and failed, to specific plastics with the capacity to handle major stresses and loads created by matric suction, soil and moisture movement.

Well, this is more interesting. Assuming the information in the article is correct it provides a good line of enquiry regarding the Laman Street fig root barriers. Are they made out of outdated concrete? Or are they up to modern standards and made out of some sort of plastic?

A search through back issues of the Newcastle Herald seemed to point in the worrying direction of outdated materials being used in the construction of the Laman Street barriers:

Despite being planted in concrete vaults to prevent their roots growing into the surrounding utilities, they will eventually grow to about 25 metres tall. Much of the work involved in the street’s reconstruction has taken place below ground. (‘Laman Street ready for grand opening’ – Newcastle Herald Oct 25, 2013)

I tried to confirm this information on the Newcastle City Council website but to no avail. There’s a large amount of information on the site seemingly aimed at supporting Council’s decision to remove the figs, very little information regarding dissenting views and almost no detail as to the actual make up and function of the tree “vaults”.

The only mention of anything related to the tree root barrier situation that I could find was in this paragraph:

The legacy of this management approach is that many of the trees have become increasingly unstable due to decaying and poorly developed root systems despite the fact that most trees appear green and healthy.

And this paragraph:

It is imperative to provide adequate space below ground for new trees to establish a stable root system at maturity without compromising infrastructure and the stability of the existing trees. These requirements could not be met under the current constraints in Laman Street.

And in a subpage on risk management:

Road works and the subsequent installation and maintenance of underground public utilities including electricity, stormwater and water supply have created structurally compromised root plates resulting in greatly reduced ability to withstand wind loading.

So the Council website, as far as I can tell, doesn’t provide any information on the root barriers themselves. It does however provide a fairly clear statement of why the Council decided to remove the figs – for public safety. The safety issue being that there was something wrong with the root systems of the trees and therefore the branches were more likely to randomly fall off the trees and injure people. It would follow logically then that the purpose of the root barriers was to encourage a healthier root system in the new figs, to achieve greater public safety.

General Manager Ken Gouldthorp confirms and extends on this reasoning in an NBN News Report:

So the idea of this is to create space for a healthy root system and reduce the interaction and impact on the infrastructure.

I go back to the Wikipedia article to see if Council’s use of the root barriers are inline with common practice. I find this:

 Initial development of this product was based on stopping trees from affecting buildings, but as the research evolved it was discovered that what was actually being done was stabilizing moisture in reactive clay. A flexible waterproof cut off wall placed around a building will allow the building foundations to float on a block of stable clay and moisture, reducing the need for piers and other structure and proving to be very economical.

So apparently the root barriers aren’t meant to be made out of concrete, and they’re meant to surround nearby structures not the trees themselves:

 Normal placement of the barrier is to locate it around the structure, out from and parallel to the footings of the structure. Try not to surround the tree. The preferred method is placing the root barrier along beside the building, path, road etc. so that the tree roots cannot gain access to the structure. Root barrier works as a waterproof seal protecting the soil under the structure from moisture loss laterally. The structure prevents loss of moisture vertically and so the moisture content of the soil can be stabilized and will stay constant. After installation the soil under the building can be rehydrated if necessary to return it to the moisture content that it was when the building was built.

So according to Wikipedia the root barriers shouldn’t surround the trees at all. This seems to be very much at odds with the term “vaults” which has been used almost exclusively in the media. The idea being put out by every piece of media I’ve read on the issue is that the fig trees were placed INSIDE custom built concrete vaults. So as far as I can tell, in this configuration, the material is wrong and likely to break apart and the placement of the barriers is also wrong.

Wrapping the foundations of the art gallery, library and other surrounding infrastructure (although probably exorbitantly expensive) makes more sense when considered alongside Council’s purpose of giving the tree more space to grow a healthy root system. Surely boxing the roots of a huge fig tree in a small concrete vault would have the opposite effect to the one intended by Council?

I know that my source on this matter is possibly unreliable, and I accept that I’m no expert. But I don’t think the burden of proof should lie with a layman resident like myself on this issue. The removal of the original figs was heartbreaking to myself and many others. It was also very expensive for Council. The public should have access to a clear explanation of the process being undertaken so they can feel safe in the knowledge that history is not going to repeat itself thirty years down the track.

If the $5million spent on removing the beautiful fig cathedral was because of something wrong with the roots and the new figs are going to avoid that same fate because of a specific mechanism – shouldn’t that mechanism be clearly explained to the public?


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