UKTC Archive

RE: Subsidence where foundations less than 300mm deep

Subject: RE: Subsidence where foundations less than 300mm deep
From: Alastair Durkin
Date: Sep 08 2020 07:42:12
Hi Wayne

Essentially in certain parts of the UK (particularly London) there are large 
amounts of clay present at, and below, foundation depth (clay shrinks 
significantly when water is sucked out of it, unlike sand - for example). 
Buildings that were built pre Building Regulations with shallow foundations 
can suffer differential movement due to the drying of soils underneath and 
around these foundations, particularly when trees are growing nearby. This 
can also happen to modern buildings where trees are planted within 
influencing distance, and is a particular problem when extensions are built 
with an insufficiently deep foundations, as they move differently to the 
parent building (assuming it has deeper foundations). There are lots of other 
elements to take into account, but I think that is basically it. 

Piled foundations can be the answer to all this, but they are expensive. 

Alastair


-----Original Message-----
From: uktc-request@xxxxxx.tree-care.info <uktc-request@xxxxxx.tree-care.info> 
On Behalf Of Wayne Tyson
Sent: 07 September 2020 21:57
To: UK Tree Care <uktc@xxxxxx.tree-care.info>
Subject: Re: Subsidence where foundations less than 300mm deep

Pardon the density of my gray matter, but I continue to fail to see why trees 
are blamed for subsidence. Perhaps I haven't been paying attention, but is 
this cited anywhere in the US? Are there scientific studies demonstrating the 
fact? I understand the "theory," but question the conclusion.

The engineers I know would use pilings instead of foundations. Then there's 
the famous Frank-Lloyd Wright-designed hotel in Japan.

Would anyone care to lead me out of the darkness?

Wayne

On Mon, Sep 7, 2020 at 5:15 AM Jon Heuch <jh@xxxxxxxx.co.uk> wrote:

Trevor



There is nothing magic/noteworthy about 300 mm deep foundations.



What I expect they are trying to say is that seasonal movement in a 
property should be less the deeper the foundations. With shallow 
foundations some seasonal movement can be expected & identifying the 
cause of that movement with shallow foundations can sometimes be 
problematic.



a.      Would the same or similar movement have occurred without the trees?
b.      Will removal of the trees solve the problem i.e. reduce the
seasonal
movement to within acceptable limits?



"no other cause produces a similar pattern"....well I wouldn't state 
this definitely related to a 300 mm foundation. It depends what the 
structure is so a light weight structure such as a garage might move 
seasonally on a 500mm foundation without any trees..whether that is a 
problem or not depends upon the structure and whether it can put up 
with the differential movement.




Jon






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The UKTC forum is supported by Bosky Trees arboricultural consultancy and
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