UKTC Archive

Re: Subsidence where foundations less than 300mm deep

Subject: Re: Subsidence where foundations less than 300mm deep
From: Wayne Tyson
Date: Sep 11 2020 00:31:09
As "they" say, "one can't prove a negative." Please prove the positive.
Appeal to authority is a well established fallacy. Please show, on the
basis of evidence, how expansive soils like clay *can* move a building.
Ironically, I respectfully submit the following authority: 'One of the
great commandments of science is, "Mistrust arguments from authority." ...
Too many such arguments have proved too painfully wrong. Authorities must
prove their contentions like everybody else.' --Carl Sagan

Actually, I'm interested in learning from the individuals on this forum,
not from countless textbooks. However, a specific quote from a primary
source (peer-reviewed research) would contribute to a polite discussion on
the subject. If anyone has a technically valid, specific point to make,
I'll be happy to become better educated.

I *suspect* that the real culprit is differential settling or compression
caused, perhaps, by variations in soil moisture content, physical
structure, and texture. I also suspect that unconfined clays will tend to
exert pressure where there is the least resistance. Confined clays, as in
lab experiments may not translate to the real world where any confinement
is variable and limited by the strength of the confining materials or
structures. I can imagine how one might make a valid observation but reach
an invalid conclusion, which can lead to persistent myths. What we may need
here, is a real research scientist in the area of soil mechanics, but even
they apparently have differences. Of course, *context* always is crucial. I
will be delighted to be enlightened.

Wayne

On Thu, Sep 10, 2020 at 2:28 PM oliverhayes <olivernghayes@xxxxxx.com> wrote:

Hi Wayne,

Vegetation certainly can conserve soil moisture, but what is your reason
for saying that clay is “too plastic” to move a building? Especially after
having read the paper linked in the previous reply.

See also this quote from an important book on trees and soil in relation to
buildings:

“If the building is constructed on soil which already has a persistent
[moisture] deficit, and if inadequate precautions are taken in foundation
design, it is inevitable that at some stage the soil drying will cease and
the soil will rehydrate and swell. The swelling pressure which is generated
will be equal to the suction, and is usually many times greater than the
typical 100 kPa load of a two story building. The swelling of the soil will
therefore lift the foundations; this upward (and sometimes lateral)
movement of the foundations is known as heave.” (Biddle, ‘Tree Root Damage
to Buildings: vol. 1’, p111)

Soil drying may cease because vegetation has died or been removed; some
other cause of drying has been removed; or because of inundation. If
rewetting is extreme the soil may dissolve or become displaced, which could
cause subsidence. Up to that point, clay is capable of moving a building *
precisely because* it is plastic.

Oliver

On Thu, 10 Sep 2020 at 20:58, Wayne Tyson <wt750mv@xxxxxx.com> wrote:

I did make an error, but that wasn't it. Clays are too plastic to move
much

of anything.



CORRECTION: It is not so much roots, but the shade and wind protection

provided by the vegetation that slows shrinkage.



Wayne



On Thu, Sep 10, 2020 at 3:52 AM Jerry Ross <trees@xxxxxxxxxx.co.uk>
wrote:



"Expanding soils do not move buildings. "

https://is.gd/8BHnSz





On 10/09/2020 10:44, Wayne Tyson wrote:

Shrinkable soils without roots result in cracks, which allow
evaporation

to

the depth of the cracks in warm weather. They serve to replace lost
water

when water enters the cracks. This cracking is largely prevented by
the

presence of plant roots. Plant roots are opportunists that develop
where

conditions are favorable (*available* water, very low soil strength
or

pores at least as large as the root tip, and sufficient oxygen for

respiration). Wet clay soils tend to be of variable strength, and the

weight of a building can cause differential settling. Expanding soils
do

not move buildings.



The soils may not be the only factors lacking competence. It's about

*engineering*, not guessing and presuming. Sometimes correct
observations

can lead to incorrect conclusions.



Wayne



On Mon, Sep 7, 2020 at 4:04 AM "theapsy@xxxxxx.com" <

uktc@xxxxxx.tree-care.info> wrote:



Dear all,

I came across this unexpected sentence in a recent subsidence report
and

wondered what they were trying to say:

"Where vegetation is involved it produces a characteristic
'seasonal'

pattern of foundation movement (subsidence through the summer,
recovery

through the winter); no other cause produces a similar pattern. If
it
is

occurring - soil drying by vegetation must be involved, unless the

foundations are less than 300mm in depth, which in this case they
are

not."

Any thoughts?

Cheers



Trevor







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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
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