UKTC Archive

Re: Subsidence where foundations less than 300mm deep

Subject: Re: Subsidence where foundations less than 300mm deep
From: Wayne Tyson
Date: Sep 09 2020 18:49:42
 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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