How roots navigate around obstacles in the soil

Ekka

Source: How Roots Find A Route Around Obstacles In The Soil

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wulkowicz

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)

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Ekka

You know I like reality shows.


It's debatable whether or not either structure was defective ... what is apparent is the lifting capacity of the roots. In both cases the roots have grown thicker in time and it must have been less resistance to lift the structure than depress into the ground.

So, what are the PSI capabilities of roots in such scenarios I wonder? How much weight can they lift?

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newguy18

Good read Eric.

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reggie

I had a japanese maple and it was about 15yrs old,i decided to take it down as it was out of sink with the garden though now there are shoots coming up in the pavement in the street.I cant identify any major roots in the garden what should i do?

Ekka

Cut the shoots and poison with round-up ... it could kill any others in the area so only do it if all are expendable.

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reggie

seems to be the only way,was think of digging up the space where the maple was to try and find the roots if find them i was gonna try and use round up on them,do you think it would be effective?

Ekka

Are you sure they're not seeds?

Trouble is that end might be the dead end and compartmentalized off ... gotta go to the "action end"

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JohN Dee

We just completed this land clearing contract and this was a driveway in one of the yards where a camphor laurel had lifted the fence aswell as the driveway about 15-20 inches.



We were going to grind out the other side of the fence stump half, but found aspestos when raking it back so we let our client deal with the stumps being dug out and having the aspestos professionally removed. We did poison the stumps though, for the meantime.


This picture really doesn't do the justice of the destruction going on here.


This is a crappy edit by me trying to show the level ground, where it should be...

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Sean Freeman

Bob, I really enjoy your writings, always have since I first read some in the early 2000's......i find you provoke positive questioning in my mind and hopefully in the minds of others.

Now I'm not absolutely confident about the relative pressures produced by roots under rigid slabs, so I will defer to the values you suggest, however clearly in the pics posted and in almost all the anecdotal experiences i have had with roots its dynamic leverage vs inflexible, flat concrete sheets lying over poorly prepared foundation layers.

It would be very hard to calculate just what pressures are being produced by the mass of roots that you would find under such slabs/sheets. But...what I would be confident in stating is that given a crow bar I could get movement in those slabs...the slads would probably crack at the point of leverage...the mass of roots have an advatage of spreading the loading...very clear in Erics 2nd pic...if you tried to lever up the slad at the red arrow it would crack but the proposed spread of lifting root mass (yellow lines) combined with the heaving of the soil beneath has little trouble moving the slab/sheet, and this does not surprise me at all.



The claims that are made against trees in relation to building foundations, are a different thing entirely, and I have written at some length about this on the forum previously....differential settlement of the subbase is very often the real cause for subsidence under buildings....correct foundation preparationand construction eliminates the direct physical role of roots.....moisture changes in the soil surrounding and below the foundation is effected by all vegetation but again correct foundations are not as vulnerable as the commonly held view suggests.

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reggie

ahh,good point,cheers mate.:

Ekka

You're right Sean.

The tree roots only have to lift part the weight of the slab over the roots surface area, because they dont lift it off but tilt it. The slabs are just resting on the ground not anchored, be a whole different story if there were screw piers attached. Now if those slabs crack it makes it easier as it means less weight to lift.

In a recent exercise a guy was installing a 20,000L water tank near trees and worried about the weight on the roots. Here's my calculations.

So all of a sudden the weight of the water on the tree roots becomes insignificant.

Knowing what roots can exert in PSI before the cells die is the data we require. Like ole Bob says, the tree doesn't want to kill itself so there's going to be a force where cells under pressure no longer function.

Palm roots in retaining walls put side force on the wall and break it, seen that over and over.

Palms between a fence and pool, doubt the roots will push a concrete 40,000L pool so they do damage the other side if there's something there. In situations where there's say a block rendered wall with foundations and a pool and palms planted between in the 1m gap (like a huge pot) what happens? Over time the palms get root bound and start looking like crap, the under plantings look like crap as they get strangled with palm roots and robbed of resources. In effect the whole area becomes like a root bound pot.

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reggie

the garden centre recommended SKB as a root killer,though it dosent say how often to use it,after 24hrs the shoots still apear alive and well,should i use it daily for reinforcement or would it loose its effect?,thanks in advance

Ekka

Vitax Limited

Most take around a week to show effects.

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wulkowicz

Can someone help me get a clean copy of this as a pdf ?

bobw@enteract.com

thank you, I'll use it as the reference for this discussion


Agris Repository Search Results

http://www.fao.org/agris/search/disp...7/v2303/GB9634...
1 of 1 5/31/08 9:36 PM

AGRIS record

Record number GB9634677

Forces exerted by tree roots

Personal Authors MacLeod, R.D.; Cram, W.J. (Tree Root Investigations Ltd, 3 Langley Drive, Kinnoull Hill,
Perth PH2 7XA (United Kingdom))
Publisher Arboricultural Advisory and Information Service, Forestry Commission Research Station
Publisher Place Farnham (United Kingdom)
Publication Date 1996

AGRIS Subj. Cat. Forestry production
AGROVOC Terms TREES, ARBORICULTURE, RECREATIONAL AREAS, ROOTS, DAMAGE, ARBRE,
ARBORICULTURE ORNEMENTALE, AIRE DE LOISIR, RACINE, DEGAT, ARBOLES,
ARBORICULTURA ORNAMENTAL, ZONAS RECREATIVAS, RAICES, DANOS
Other subjects HORTICULTURE, PLANT ANATOMY, PLANT VEGETATIVE ORGANS, PLANTS, WOODY
PLANTS, ANATOMIE VEGETALE, HORTICULTURE, ORGANE VEGETATIF DES PLANTES,
PLANTE, PLANTE LIGNEUSE, ANATOMIA DE LA PLANTA, HORTICULTURA, ORGANOS
VEGETATIVOS DE LAS PLANTAS, PLANTAS, PLANTAS LENOSAS, AMENITY TREES

Language En
Notes 15 ref.
Collation 8 p.
Availability DEFRA (United Kingdom of Great Britain and Northern Ireland)
Availability number 9634677

Serial Title Arboriculture Research and Information Note - Department of the Environment (United Kingdom)

Serial number no. 134/96/ext

This document is available from the following AGRIS Centre:
DEFRA (United Kingdom of Great Britain and Northern Ireland)
London SW1P 3JR, United Kingdom of Great Britain and Northern Ireland
Contact:Kevin JACKSON
Tel:+44 (0)20 7238 3324
Email:Kevin.Jackson@defra.gsi.gov.uk
URL:Defra, UK - Department for Environment, Food and Rural Affairs
© AGRIS 2006 - FAO of the United Nations

wulkowicz

I did get in touch with Kevin Jackson who will help me. So we shouldn't disturb him following the request I made above.

thank you all

tubs

sustainability

Jason Grabowsky has conducted this study with Rutgers. Interestingly enough, it is your conditions not the trees that create these circumstances.
Everyone wanting to cut the tree or the root for the mistake the tree has made... noone looking at the mistake the human made. There are a few reasons why roots won't delve deep and one is oxygen. The forces exerted by a root diminish the deeper it grows. If given an environment where a root can't grow it requires oxygen and so will become surface or as much so as it can. This then is complained about.... seriously! You make a mistake and the tree suffers! Learn about trees you might find that working with them helps you.

Raven

Thanks for the informative article.
Education is the key . No?

chadd

thats a very useful bit of info.
a fe years ago i removed a cedar hedge along a drive way, they replaced the back half as it was severally damaged and lifted from the roots. a couple of months ago i was calle dback to prune some other trees on the property and the owner showed me how the front half of the drivaway (which was still level after the removals) had sunken and was as bad as the back had been from lifting. so the oppoite can happen from removing them where they had ample room to grow.

wulkowicz

Two librarians in the UK, responding to my plea for the study, Forces exerted by tree roots, were professional enough enough to send me clean crisp copies of the report, and I do plan on publishing it here in the spirit of sharing, and to save anyone else the ordeal of another search.

I also need to double check on copyright and I would like to blend that report with my comments that have developed over the years to let readers better understand my own conclusions and future mutterings.

The report in its time, was very ambitious and did struggle with a lack of information and foundational research *

Thanks for being patient, and thanks again to my new friends in the UK who were kind enough to help Injun Joe scratching at the door to the cave.


Bob Wulkowicz


*(No pun intended, it just popped out of the voice recognition program, which I sometimes think is conspiring with the left part of my brain in an effort to make me a better writer. So far, nobody has written me to suggest that they were successful.)

Ekka

Extract from Arboricultural Research Note
134/96/EXT. Forces Exerted by Tree Roots by R D MacLeod
and W J Cram.

The forces generated by tree roots: some examples

Roots can generate forces capable of lifting paving slabs, lifting and cracking
boundary walls and driveways (Biddle, 1981). Kopinga (1994) observed that
"thumb" thick poplar (Populus spp.) roots had caused extensive damage to cycle
tracks within a few years. Such damage showed itself as raised areas and cracks
above roots which had undergone secondary growth. The asphalt was 8-12 cm
thick. It had been laid on top of a mechanically compacted layer of sand, 30 cm
deep, in which the penetrometer resistance was 2.5 MPa at 5 cm and 4.5-5 MPa at
10 cm - too compacted for roots to grow through (appendix 2). The poplar roots
had grown beneath the track by utilising any space that was present, or developed,
between the base of the asphalt and the surface of the sand. Similar paths
constructed of concrete slabs (30 cm x 30 cm x 5 or 7 cm) had also been disrupted
by the underlying roots which had grown after the path had been laid.

On the other hand, roots cannot generate sufficient force to affect heavier structures.
In each of the cases that follow the force necessary to cause deformation or breakage of the built structure
has been calculated, and compared with the force that one or more roots
could exert, calculated from a representative root pressure x area of root(s)
in contact with the structure.

A 1cm diameter root growing beneath a concrete paving slab of dimensions 30 cm x 30 cm x 7 cm

• The slab weighs about 13 kg (29 Lbs), and so exerts a downwards force of about 13 x 9.8(4)=
130 N.

• The root will become deformed so that it is flat against the slab above. It will exert a
pressure of 800 kPa (or 120 lb per square inch). This will be applied over the area of root in
contact with the slab - 1 cm wide x the width of the slab (30 cm) = 30 cm² or 0.003 m². The
upwards force that the root could therefore exert is 800 kPa (or 5x105 N/m x 0.003 m²
One root could therefore generate an upwards force over 10 times that needed to lift a
free-lying concrete slab of this size. In practice a greater force would be needed
because of the friction between closely bedded slabs.

A root mass growing beneath a non-reinforced concrete floor slab 4 m x 3 m x 14 cm

• The modulus of rupture (the stress needed to cause a non-reinforced concrete slab to crack)
for a 1:2:4 concrete mix is less than 2760 kPa (400 lb/square inch). For the purposes of
simplifying the calculation, consider a strip of concrete 4 m long and 29 cm wide. The
bending moment necessary to introduce a stress level equal to the modulus of rupture is
calculated to be 273 kN/cm (24,145 lbs/inch).

• For the case of a root mass present 50 cm on either side of the centre of the slab, generating
an upwards pressure of 800 kPa over this area, the bending moment is calculated to be 17120
kN/cm (1,522,000 lbs/inch). (In calculating the bending moment 2.57 x 10³ kg/m³ (160 Lbs
per cubic ft) have been allowed for the concrete floor and timber acting in the opposite
direction).

An important consideration is that the root mass must be present in sufficient quantity to physically raise the floor slab. This in turn implies that there must be
sufficient moisture and nutrients beneath the floor to allow root proliferation. While
this appears unlikely, it is not impossible, and at least one suspected case has been
reported. The conclusion from this calculation is that the bending moment generated
by root pressure in such a root mass could be substantially greater than that
needed to cause the slab to crack. If the roots were in contact with only 1/10 of the
area, they could still crack the slab. Conversely, if the slab was of reinforced
concrete, with a much greater modulus of rupture, then it could withstand such a
force from roots.

A large root growing beneath a wall

• For a wall 2 m high and 11 cm thick, the downwards force exerted by a 3 m section due to
its weight alone is about 13,200 N. The force needed to crack the wall would be very
much larger. It would involve rupturing the wall or overcoming the forces of attachment
to the rest of the supporting structure on either side. An estimate of the force necessary to
crack the wall could be calculated from the vertical bending moment and modulus of rupture
as in the previous example.

• The force upwards generated by a 10 cm diameter root, flattened against the base of the
wall, would be 800 kPa x [10x11] cm²
Consequently it is highly unlikely that a single root, even if large, could cause
damage to such a wall simply by lifting it.
Tipping the wall, however, would demand much less force. This is because the wall
would not have to be lifted and the force needed would be that to overcome only the
bending resistance of the wall perpendicular to its height and length. A wall
adjacent to a tree trunk from which a large buttress root is growing might easily be
tipped outwards due to uneven expansion of the root upwards.

Consequently it is highly unlikely that a single root, even if large, could cause
damage to such a wall simply by lifting it.
Tipping the wall, however, would demand much less force. This is because the wall
would not have to be lifted and the force needed would be that to overcome only the
bending resistance of the wall perpendicular to its height and length. A wall
adjacent to a tree trunk from which a large buttress root is growing might easily be
tipped outwards due to uneven expansion of the root upwards.

A 1 cm diameter root growing between two bricks in a wall
or 8800 N. This is half the force needed to support
the weight of the free-standing wall, so that the root could not even begin to overcome the
bending moment in the vertical plane.
• The force generated by the root would be 880 N in each direction (800 kPa x [1 x11] cm2
• Imagine a large crowbar inserted between two bricks in a waft A lateral force of 150 N applied
with a 5-fold leverage would generate 750 N. Experience suggests that this would not crack
the wall vertically.

Thus roots could not generate anything like the force necessary to crack a wall apart.
In fact, they would simply be constrained by the size of the pre-existing hole they
grew through in the first place, expanding as they grew on both the near and the far
side of the wall.

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glennak

This force is rediculous 800 kPa (or 120 lb per square inch). The internal pressure in a mountain ash Euc regnanns has been measured at 15,000 psi. This is the max pressure that the wood can handle that is why the heart is usless, it has been crushed by the out side of the tree. So why can roots only exert less than a 100th of this force?
I have seen trees growing in holes in concrete slabs split the slab (in fact I may go and take a picture of one near by) and this takes more than120psi a lot more, more like 15,000psi and why should roots be any weaker?
Root hairs may not exert great pressure but if a root gets into a gap, hole or crack and expands its going to take a lot to hold it, up to 100,000 KPa.
After all they used to use wooden pegs to split rocks just by adding water, and Euc regnans has shown that it can crush wood so it has the power to split rocks.

Ekka

Hey, I'm all ears but I need facts and data.

Cells will self destruct above what? 200psi?

Root growth is extremely limited past what, 200psi on the ole penetrometer.

Roots are living, heartwood is dead.

Force exerted, as per text above, is per square inch, so amplification occurs when more than 1 square inch is in contact. Think like this ... I have a crow bar, the tip of the crow bar is 1 square inch and I get it under a slab and heave like crazy, I coincidentally happen to exert 120psi. Then you come along and give me a hand with your own identical crow bar, you do the same right next to me, now whilst we both are exerting 120psi next to each other there's 240 pounds pushing up on the slab. But the slab still hasn't lifted so JayD dives in and does the same as us, another crowbar and bingo the slab lifts. whilst we were all exerting 120psi in total we lifted 360 pounds.

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glennak

Lifting slabs is not hard it would have to be very thick concrete to be heavier than 200 pounds /square". But cracking a slab with a side ways push is a different thing all together, it is dependant on the strength of the concrete and trees can do it. So they push in the MPa range. The cells grow then the walls thicken and crack they make more room for themselves.

Ekka

I know, that's mentioned above also. It's just number crunching, but also is there existing hairline fractures/imperfections like when bridges/freeways get cracks and fall?

The root or tree becomes flat as the cells grow along not into the concrete.

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