UKTC Archive

Re: Subsidence

Subject: Re: Subsidence
From: David Lloyd-Jones
Date: Dec 05 2018 22:33:12
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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The UKTC is supported by Bosky Trees arboricultural consultancy
http://www.boskytrees.co.uk/