UKTC Archive

RE: Subsidence where foundations less than 300mm deep

Subject: RE: Subsidence where foundations less than 300mm deep
From: Jim Quaife
Date: Sep 10 2020 06:04:04
My machine does not allow your link to open Wayne.
Provided that any changes to a tree's growing environment happen a, slowly 
enough and b, do not cause catastrophic harm, the responsive adaptive growth 
will accommodate the changes.
This is evident from observation.
The way in which, or even if, one builds on "incompetent" soils is down to 
engineering = cost.
The textbook description of unimpeded root morphology is entirely correct, 
but on its own it does not explain why trees grow in extraordinary places 
(not least busy city centres).  The explanation for that is adaptive growth.
I would hope that all thinking arbs understand the principles of tree growth, 
but applying those principles to individual circumstances is the "art". 
Jim

P.S. In the UK we use the term "shrinkable" soils not "expansive". 

-----Original Message-----
From: uktc-request@xxxxxx.tree-care.info 
[mailto:uktc-request@xxxxxx.tree-care.info] On Behalf Of Wayne Tyson
Sent: 09 September 2020 19:49
To: UK Tree Care
Subject: Re: Subsidence where foundations less than 300mm deep

 The fundamental point is whether or not it is wise to build foundations on
incompetent soils. Expansive soils are not "competent." This is a
long-established fact; it matters not how "old" the statement is, but
whether it is true or false, based on competent *science*. It's not a
matter of one bias against another. Evidence, evidence, evidence!

Any root (tree or no tree) able to penetrate any soil is going to remove
water from it--that's what they do--in addition to maintaining the
structural integrity of the tree. Planting trees near foundations is not
optimal for a tree, in any soil--roots that radiate 360 degrees from the
trunk (as they normally do) tend to ensure greater resistance to the
*various* forces that destabilize the tree's structure. Digging foundations
through tree roots, even at a considerable distance from the trunk does not
make the tree more stable--yea, it weakens the tree, shortening its life
and creating the potential for uprooting. The resistance ("strength") of
the root system comes from the root/soil matrix of many small roots, not so
much the bigger ones, but big roots are attached to that matrix, which is
killed when severed. Barriers like foundations and impenetrable cloth
essentially duplicate said effect upon tree stability.

Starting at about 25 seconds into this video, note the very straight line
of the edge of the root-plate at top-right and below (barely visible) where
the normal root radial development had been stopped by the adjacent
building's foundation and even the pavement at bottom.
https://www.cbs8.com/article/news/point-loma-heights-tree-falls-on-house-killing-couple-from-north-dakota/509-5d85162b-ee6e-4401-9060-b006cfa16453

I have better photos if anyone needs more clarity.

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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To unsubscribe send mailto:uktc-unsubscribe@xxxxxx.tree-care.info

The UKTC forum is supported by Bosky Trees arboricultural consultancy and
Stockholm Tree Pits
https://www.stockholmtreepits.co.uk