UKTC Archive

Re: Subsidence

Subject: Re: Subsidence
From: Jerry Ross
Date: Dec 06 2018 21:13:42
As Tim says, a big subject and lots of cans of worms!
But the (over-)simple answer to your question is yes - it's a matter of the differential shrinkage (and sometime expansion) of certain type of clays in response to changes in soil moisture content, which can be affected by tree root action.

The UK has a wonderfully complex geology (https://images-na.ssl-images-amazon.com/images/I/91gdvcQOZsL.jpg) and there's quite a lot of clay. You say that in (your part of) the States there's little or no building permitted on clay soils. That would mean that large swathes of London and the SE of England would be devoid of houses (which some might consider no bad thing!)

Interestingly, tree-related building subsidence was no big deal in this country until insurance companies rashly started including it in the polices they offered some time around the 1970. About the same time that large numbers of shoddy buildings with hard, brittle mortar and inadequate foundations were being flung up around the country.
Then we had some hot dry summers...

The attached old (1979) article by Giles Biddle gives a brief outline.


On 06/12/2018 19:45, Wayne Tyson wrote:
Jerry and Group:

I'm still struggling with understanding just how trees cause subsidence. Am
I to understand that the extraction of water from clay soils is the primary
issue?

Wayne

On Thu, Dec 6, 2018 at 4:25 AM Jerry Ross <trees@xxxxxxxxxx.co.uk> wrote:

Writing as someone who lives and works in an area without heavy clays,
I'venonetheless been asked to look at numerous tree-related subsidence
claims. They are usually for trivial and certainly for non-structural
damage and not uncommonly the evidence presented is, to say the least,
dubious.
But there's always a claims management company going after the tree (or
the tree owner) like a dog after a bone...
Poor building practice certainly doesn't help, but I'd suggest that a
lot of our subsidence 'problems' have been created by the industry
that's grown up to make money out of them.



On 06/12/2018 12:05, Bill Anderson wrote:
NHBC are the national house building council Wayne. A sort of insurance
company for the development industry. They publish Building Standards,
which their clients are supposed to follow.
Debatably, our subsidence problems follow from poor building practices in
the recent past more than anything else..

Bill.

On Wed, 5 Dec 2018 at 22:33, David Lloyd-Jones <
dlj@xxxxxxxxxxxxxxxxxx.com>
wrote:

Hi Wayne

My replies are in context...

-----Original Message-----
From: Wayne Tyson
Sent: Wednesday, December 5, 2018 8:22 PM
To: UK Tree Care
Subject: Re: Subsidence

I am interested in this subject, but know nothing of the issue in
Brittan.
Would anyone be interested in explaining the phenomenon in simple terms?
The following questions probably reflect the depth of my ignorance,
being
mere guesses. Am I to understand that this issue is common in Britain?

1. Are foundations built upon materials (e.g. clay) subject to excessive
expansion and contraction?

Yes. Clays vary in volume when moisture is abstracted and if a tree
exacerbates the relative degree of dessication close to it, differential
shrinkage occurs and subsidence can result.

2. Are growing tree roots demonstrated to, or suspected of, causing
damage
to foundations?

It's trees abstraction of moisture that is the problem, another variable
is
the relative depth of foundations, variations in the depths of
foundations
and yet another set of variables the tree's rooting strategy, age and
relative vitality.

3. Are dead tree roots demonstrated to, or suspected of, causing damage
to
foundations by leaving voids which result in subsidence?

Only in terms of their ability to transfer wind loadings if they are
close.
In practice, unlikely

4. Are there any laws requiring that foundations be built upon firm
strata?
No, but there are NHBC Guidelines that an engineer can use to calculate
the
relative depth to which foundations should extend so that they are
founded
in what is likely to be moisture stable clays

5. Are there regulations requiring specially-designed foundations for
construction on  "soft" materials? If so, what are they?

See last answer.

6. What am I missing?

Historical variability of building design. I recently surveyed a damaged
building whose foundations extended just 37CM under ground on a clay
with
a
highly shrinkable modified Plasticity Index. That building and trees
aint
friends!

Hope this helps

David


On Wed, Dec 5, 2018 at 7:41 AM David Lloyd-Jones <
dlj@xxxxxxxxxxxxxxxxxx.com>
wrote:

Hi Jon

All valid points that I take on board.

The homeowner and neighbour both would like to keep the tree so I am
merely
trying to enable that to be the outcome along with some checks and
contingency plans so that until it happens again (if it ever does) they
can
keep and enjoy the tree.

Thanks

-----Original Message-----
From: Jon Heuch
Sent: Wednesday, December 5, 2018 12:05 PM
To: UK Tree Care
Subject: Re: Subsidence

"I am just about to specify an automated watering system as an
alternative
to tree removal where just one small brick built porch is slightly
affected.
It will be the

first such system that I have designed so this one might be
interesting."




Sounds like you are trying to re-invent the wheel...this is an approach
that
has been tried on several occasions by building professionals..watering
the
foundations.



It's not really a solution in any general sense:



i)                    What's caused the dryness in the first place? A
tree?
If you leave it there the water you put on will disappear next summer.

ii)                   How much water do you put on? Too much and you
will
make matters a lot worse. Remember wet clay has no bearing capacity.

iii)                 Where do you put the water? Precisely. Do you just
leave a hose running? A damaged porch with no other damage may be
simple
in
this respect...but:

iv)                 Once the top layer of the soil is wet, it will
swell
and
not let the water go any deeper. If the roots causing the problem are
1.5-2
metres down all you will get is run-off getting nowhere near the dry
soil.
v)                   If you have mechanisms for getting the water
deeper
in
the soil profile you still have to work out what depth and the volumes
required..how do you do that?



And if you want to talk to someone who has tried it contact Chris
Kawecki
http://www.subsidencemanagementuk.co.uk/  but please don't use this as
my
endorsement of the method. I don't know anyone of any organisation
attempting this routinely.

http://www.subsidencemanagementuk.co.uk/company-information/reversal/



Jon






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http://www.boskytrees.co.uk/







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