UKTC Archive

Re: [EXTERNAL] Tree failure potential assessment study WAS Civil cases for nuisance or damage to property

Subject: Re: [EXTERNAL] Tree failure potential assessment study WAS Civil cases for nuisance or damage to property
From: Wayne Tyson
Date: Jan 16 2020 00:26:00
I am unfamiliar with "subsidence" in the UK. I have gotten the impression
that expansive clays are believed to contract because of roots removing
water from them. Or is it that they have such low bearing strength? In my
experience here in my part of the US, we are not permitted to build upon
them; we must excavate and replace with non-expansive, compacted materials
to 90 percent of maximum dry density or go down to competent rock, on piles
if necessary. Perhaps its clay (turtles?) all the way down in the UK?

As I understand it, expansive clays gain and lose water and expand and
contract based on their water content. Water can be gained or lost in a
number of ways, and that means other than by tree roots. Roots take up
water at their tips, not through their suberized or woody tissue, so if
roots are to have an effect upon the shrink/swell behavior of expansive
clays, it should occur at the extremities of the root system, not closer to
the trunk. Other plant roots might be even more responsible for water loss
than trees in a given context.

Clays need to be pretty low in strength for root penetration, and, as water
is lost, reach a point where the *available *water is zero, a lot of water
remains held under tension by the clay particles. Beyond that, further
water loss can occur only by *evaporation* (not transpiration). This can
occur at any exposed surface, including contraction cracks or other
openings in the soil. Most tree roots (except those having the ability to
translocate O2) cannot grow in anaerobic environments, so the envelope
within which most tree roots can withdraw water from clays is rather
narrow.

What actual science supports the idea that tree roots are at fault when
"subsidence" (other than loss of strength due to free water in the
clay--if, in fact, that is the case) due to water loss occurs?

Is it not folly to build on expansive materials in the first instance?

I shall look forward to being further educated. I'll leave what I have
sacrificed to brevity to be filled in by my betters.

Wayne



On Wed, Jan 15, 2020 at 12:28 PM Simon Pryce Arboriculture <
info@xxxxxxxxxxx.co.uk> wrote:

If you look at the subsidence data from Cutler & Richardson and Mercer
et.al. roots can clearly spread well beyond RPA distances e.g. 30m for
species oak and poplar.  Those are distances to which roots have spread
in sufficent quantities to damage buildings, so the farthest edge of the
root systems will no doubt be well beyond that.  Basing RPAs on trunk
diameters is attractive because the diameter is the about the easiest
thing to measure, but that assumes that the root spread / trunk diameter
relationship is linear.  One of my forestry lecturers used to point out
dbh is pretty inaccurate as a measure of trunk volume, but can at least
be measured easily while the tree is still standing.

I played with some of my own subs data a while back, which suggests that
it isn't linear, at least with the sort of established trees that get
implicated in subsidence cases.  Distance / diameter is greatest in
relatively small early mature trees and less in older, larger ones,
which is consistent with our understanding of tree biology.

The NHBC assessment of the range at which trees might damage buildings
is based on their height, while BS5837's assessment of building work
damaging trees is based on their trunk diameters.  Go figure.
Fortunately both systems have sufficient safety margins that either can
be used with reasonable success without engaging the brain too hard and
the trees frequently forgive us, which isn't bad compared with many
other human endeavours.

By the way please stop putting "nature" and "design" too close together
in the same post.  The way the internet works we'll be getting
creationists, flat earthers and maybe even Richard Dawkins joining in.

Simon





--
The UK Tree Care mailing list
To unsubscribe send mailto:uktc-unsubscribe@xxxxxx.tree-care.info

The UKTC is supported by Bosky Trees arboricultural consultancy
http://www.boskytrees.co.uk/




-- 
The UK Tree Care mailing list
To unsubscribe send mailto:uktc-unsubscribe@xxxxxx.tree-care.info

The UKTC is supported by Bosky Trees arboricultural consultancy
http://www.boskytrees.co.uk/