UKTC Archive

Re: Subsidence

Subject: Re: Subsidence
From: Wayne Tyson
Date: Dec 06 2018 19:37:42
Yes. Thank you David.

Here in my part (earthquake country) of the western US, I know of no modern
cases where foundations are permitted to be built upon clays or, for that
matter, *any* incompetent* materials--with, to my knowledge, the sole
exception of structural (compacted to 90% of maximum dry density) fills.
Can you fill me in on current practice/regulations with respect to said
fills in Britain? If others care to chime in with respect to their part of
the world or knowledge of other places, I would be interested in learning

*At least this was true in the 1970's when I was a park construction
inspector. There may have been changes and miraculous innovations since
then somewhere in the world. I was co-author with two prominent engineering
geologists of a paper I presented at the Association of American Geologists
(AEG) meetings around the turn of the millennium (1999?) wherein I
ad-libbed something to the effect that all structural fills were failures
waiting to happen. None of the august experts present challenged me on the
statement, but my co-authors never spoke to me again and the paper wasn't
printed in the Proceedings. The genesis of the paper (*The Relationship of
Vegetation to Slope Stability and Instability)* was my showing my
co-authors an ecosystem restoration project that I had completed in the
early 1980's on a disposal fill that had survived the 1994 Northridge
earthquake. The project was about 6.87 miles NNE of the epicenter. The
*only* lesson to be learned by the fact that no failure occurred was the
unusual step taken by the Los Angeles Department of Engineering in
approving three feet of lesser-compacted fill material on the surfaces of
the project. Structural fills are intentionally resistant to the entry of
water; in fact that is the justification for my contention that compacted,
unconsolidated (therefore incompetent) materials are failures *waiting* to
happen. The function of the vegetation here is the *removal* of water by
evapotranspiration, thus providing a degree of protection from the very,
very slow percolation of water into the unconsolidated, if highly-compacted
mass. This is valid only in areas of relatively low precipitation combined
with a permanent (capable of reproducing itself in perpetuity) indigenous
ecosystem, the net result of which will be extracting all of the water from
the soil (which forces drought-dormancy on the vegetation, indicating that
there can be no percolation below the root zone). I doubt that any such
conditions exist in Britain. I simply do not know if "cut and fill" grading
is practiced in Britain, but if it is, I am horrified. What I would call
"subsidence" does, however, exist here, where deep clay deposits are rare
and, to my knowledge, never built upon.

On Wed, Dec 5, 2018 at 2:33 PM David Lloyd-Jones <>

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
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
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
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
highly shrinkable modified Plasticity Index. That building and trees aint

Hope this helps


On Wed, Dec 5, 2018 at 7:41 AM David Lloyd-Jones <>

Hi Jon

All valid points that I take on board.

The homeowner and neighbour both would like to keep the tree so I am
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
keep and enjoy the tree.


-----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
to tree removal where just one small brick built porch is slightly
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
has been tried on several occasions by building professionals..watering

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

i)                    What's caused the dryness in the first place? A
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
this respect...but:

iv)                 Once the top layer of the soil is wet, it will swell
not let the water go any deeper. If the roots causing the problem are
metres down all you will get is run-off getting nowhere near the dry

v)                   If you have mechanisms for getting the water deeper
the soil profile you still have to work out what depth and the volumes do you do that?

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


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The UKTC is supported by Bosky Trees arboricultural consultancy