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Originally Posted by Ekka  |
Well thank you very much Ekka. it is a much pleased and prideful Wulkowicz who responds to your post on roots meeting obstacles. I've argued for years about the poor science and misapplied thinking that went into claims that roots poked through concrete and brick walls, and generally vandalized our structures.
Here, from the glory days of 2001, is a snippet of the debate I had in Russ Carlson's Tree-Tech which includes the bolded statement that roots didn't push their way through much of anything. They don't have the strength.
The tip of a root is about the size of a period at the end of this sentence.The numbers bandied about as a pressure possible for a root tip were given in pounds per square inch, which is absolutely reasonable as a unit of discussion. The problem was no one really noticed that it took an awful lot of period-sized tips (500 microns) to make up that square inch.
In my clumsy grammar school arithmetic way, I calculated about 25,000 of the wee beasties all had to push at something at the same time with everything they had to generate the touted 145 psi. If herding cats is a problem, who's going to step forward and wrangle 25,000 root tips?
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<Bob Wulkowicz>
Posted Tuesday August 28, 2001 12:34 PM
Reply to post by Russ Carlson, on August 24, 2001 at 22:24:48:
Bob, you pointed out that roots may be "on their way somewhere else" in some cases.
This is how a wall can be damaged The root tip will find its way into a tiny crack or crevice. It may be turned aside, yet still survive. The root tip that survives expands into a root, which grows laterally.
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Russ,
Roots are notoriously on their way somewhere else. That is what accounts for root-bound plants in pots and girdling roots on trees. Roots search, explore, and exploit; indeed, it is a critical function of what they must do. Once past the buttress area where root sections serve a structural purpose, roots sick to the business of being the water and mineral transport mechanisms.
Encountering an obstacle is an everyday occurrence for roots and so wrestling for survival at an obstruction is rarely a consequence. When the path for an root apical meristem is blocked, the meristematic tissue further back simply creates a new apical point (obviously at right angle initially) and begins to grow in that direction. It is a wonderfully simple adaptive mechanism that works quite well. The root doesn't waste its time fighting, it goes somewhere else as a new lateral root.
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This root then exerts a lateral pressure on the objects adjacent In most cases of damaged walls, curbs, etc. that I've inspected, the structure was defective in some way. It had cracks or crevices for the roots to enter.
This also implies a weakness at that point. So regardless of the compress or tension ratings of the concrete or other material, the structure's ability to withstand pressure at the defective point is less Roots can and do exert pressure.
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I agree that the radial growth of roots creates a pressure, but again it is not in a range that can threaten concrete--even concrete weakened by other conditions. I also can't agree with your statement that compression or tension ratings adjacent to a crack or hole are somehow less at that defective point. The general integrity of the wall may change to some degree, but the strength of the surrounding concrete is essentially unaltered.
Mattheck estimated a possible pressure of over one mega-pascal (one million newtons per square meter). Now obviously, the root is pressing over a smaller area, but it's still a lot of force. Combined with other forces such as freeze/thaw cycles, slight shifting of the roots or soil due to tree movement, shrink/swell of the soil, etc., a wall with a weakness can be damaged.
You are aware that I am very troubled by many of Mattheck's pronouncements and the interpretations attributed to him. If I look in my conversion tables, the megapascal pressure you quote translates into 145 psi (1MPa). The water pressure from my pump at the cabin and at most city faucets is about 45 psi. This is not a big time pressure.
If I tripled the water pressure on my pump and held a garden hose nozzle against my foundation wall, does anyone think I would push a hole through the wall? I might blow a hole in the hose, but not the pump, the fittings, the nozzle or the concrete. That pressure attributed to the growing tips of roots is nowhere near the pressure necessary to affect concrete.
What is needed? Here's a typical rule of thumb today for cutting concrete: we find "a ratio of 3.5 psi. water pressure for every 1 psi. compressive strength of concrete is used to calculate optimal operation pressure. If a concrete core test indicated nominal compressive strength of the concrete to be 5,000 psi., a minimum water pressure of 17,500 psi. is indicated. "
This is what I mean about throwing the theoretical away in a second when presented with a practical consideration. A strength of 5000 psi is pretty close to residential foundation strengths and the 145 psi pressure from the tip of a root ain't beans to those walls.
The photo at the top of this piece is of a tree root showing the cap, the meristem, and the elongation zone. If you can see and count the cells, the structure is obviously microscopic. The force claimed for it are available only at its microscopic tip. This size is what enables it to slide through pore spaces and if it encounters a concrete wall, this is the area of pressure exerted against that wall.
Even if 100,000 root tips arrived at the wall at the same time, would they penetrate the wall? How many angels can dance on the head of a pin?.
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Now, let's consider a lateral pressure as you suggest from roots growing horizontally and parallel in relation to a wall:
In a study sent to me, the authors attempted a compilation of papers on tree root pressures. I don't have it in front of me, but I remember a quoted primary growth forward pressure of 128 psi. The authors couldn't find a lateral growth pressure (secondary growth) anywhere, so they assumed a lateral pressure of at least the same 128 psi. The need to have both pri. and sec. answers for their paper prompts them to assume a number just to fill in the blank(?)! I don't find this great science, and I certainly don't find it a benchmark for pressures from diameter increases in roots adjacent to walls.
But let's use the Mattheck 145 psi. Even if it were true, this is even less likely a meaningful factor in distressing foundation walls. Our roots will probably exist mostly in the approximate 18" root zone that they occupy everywhere else. And even if they were laying right next to the wall, they would first compress the adjacent soil until it was incompressible before they would exert any subsequent pressure on the wall. The contact point of the root circle to the contact point of the next root circle against the wall will be determined by the root's diameters as an "automatic" spacing depending on root age that reduces the potential pressures..
I have only seen vertical and diagonal cracks in foundation walls, which would be consistent with vertical displacement and settlement. I have never seen horizontal fractures or cracks that would imply lateral pressure from lengths of pressure increasing structures like roots.
And if these roots went under a foundation and tried to lift it based on a 145 psi capacity, we're expecting a living expanding cylinder using its ability to divide cells to raise a foundation wall, the floor loads, the building contents, the framing, the roof timbers and the shingles? What pressure would that take? But more importantly, would the cells stay alive at that pressure to continue their division, expansion and subsequent growth.
I can crush the cells of a root or branch with a pair of pliers. I can impede radial growth in a woody cylinder with remarkably small counter pressures. I have hundreds of photographs illustrating the reluctance of meristematic cells to kill themselves in any kind of expansion that hardly approaches the pressures needed to lift a foundation and its vertical loads.
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It may not be the tree that caused it, but it certainly can contribute to the problem. In Mike's case, the foundation wall is probably new enough and well enough constructed that there will not be a problem. Old foundations with crumbling mortar would be an entirely different story, however.
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Trees really don't cause the problems--and in my judgment, don't really contribute either. A mega anything sounds big. The explanations seem plausible until they're examined more closely. Our prejudices and ignorance do not change the laws of physics or biology. Our deadly inclination to repeat myths and clichés will condemn more trees.
I keep sputtering. Please tell me where I'm wrong.
Bob Wulkowicz
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"In all things it is a good idea to hang a question mark now and then on the things we have taken for granted." Bertrand Russell
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The root taking a sideways step. http://www.sciencedaily.com/images/2...0228143543.jpg
A root growing vertically, with a root hair cell emerging horizontally. (Credit: John Innes Centre)