UKTC Archive

Subsidence & NHBC 4.2

Subject: Subsidence & NHBC 4.2
From: Paul Hawksford
Date: Aug 02 1999 18:26:44
UK Tree Care -

The questions you must ask yourself here are simple:

What is your experience of and how many reports have you been
commissioned to produce in relation to localised vegetation implication
of soils subsidence induced building movement/damage (it ain?t just
trees you know)?

Following on from the original structural engineers report, and given
his seasonal building movement analysis, soils PI and root analysis,
were you confident that a definitive causal link was accurate and that
the right tree was removed?

Did you at any time specify pruning to abate the problem, if so, why
and what substantial data on this practice do you refer to? Prove your
case to your client, its easier than trying to prove it in court
against some shit-hot Arb consultant.

Did you consider the following criterion: Species type and their
rooting morphology (major problems of accuracy here), localised
climatic variations, building orientation, foundation depth, soils
analysis in detail, other sources of soils influence/depletion of
localised moisture,  tree/vegetation physiology i.e. their ability to
abstract moisture at normal rates (whatever they may be). Distance to
buildings is not relevant, influencing distance is extremely site
specific and impossible to calculate as are a number of the variables

If the tree/s vegetation are on a neighbouring property and covered by
a TPO, your investigation will have to be extremely accurate in order
to convince the LA that the tree was the culprit otherwise some
hot-shot switched on Tree Officer will simply blow you away either in
or out of court.

You have five semi-mature trees in proximity to the damage in the
building: Which tree is the culprit, can you definitively link one, one
or two, a few more or all as culprits. If those trees were on
neighbouring land and I owned those trees, I would be waiting with
baited breath, as would my lawyer.

You find roots in a trial hole taken by the engineer and identified by
a credible source as Rhododendron species. No roots of a nearby
early-mature Poplar are found. Which do you implicate, given the solid
evidence provided by analysis. After all, you can only take so many
trial holes, without physically undermining the building? I don?t know.

NHBC 4.2 - The Facts

This is a guide for NEW Building in Relation to Trees (not trees in
relation to existing buildings).

To produce a report based on some perverted reversal of NHBC is
obscene. If people produce it, then they are extremely stupid. If
people continue to produce it, they obviously they have no real angle
on the variables and need a tap on the head. Looking at SRA and other
specific models attributed to the tree as a living dynamic organism,
I?m lost for words, how come myself and others see the obvious and the
rest remain ignorant?

Consider the NHBC data provided:

Water Demand of Species - INACCURATE
Mature Height of Species - INACCURATE
Plasticity Index Analysis - Soils are not generally homogeneous and
therefore, INACCURATE
Climatic Zones - (compare overall summer precipitation rates in various
parts of North Yorkshire in relation to its zoned cousin  in  parts of
western peripheral north Wales) - INACCURATE
Distances to Proposed Foundations - Where is the solid data to support
"influence" - INACCURATE

NHBC assumes:

If you don?t know the particular species assume maximum height
If you don?t know the particular water demand assume "High Water
If you don?t know the soils plasticity index assume "high shrinkablity"

Assumptions do not provide definitive conclusions to an investigation.
Get it wrong now, and the lawyers will chase your arse into retirement.

I know colleagues who have carried out thousands of these particular
reports, but very few even get near the mark. Some have produced but a
few, but seem to know it all!! So be wary. I for one tend to avoid them
if I can, some cases are easier than others, though COMMON SENSE must
always prevail. I have a case on Monday morning and I?m as wary as
ever. If in doubt, well maybe your cleverer than I am???? Can you
enlighten me with new data or provide your insight that I can use to
make the analysis work with without unnecessarily removing the vital
sustainable urban vegetation resource????

There are no experts in this field and there are no specialists either.
Its too damn variable for that. What you write today is OK, though what
comes back to bite you in 5+ years is YOUR nightmare (especially if
you?ve ever carried out a subsidence investigation at 17 Elm Street,
made worse by the presence of a wicked Goat, who?s eating habits of
localised vegetation closed the cracks in the building for a short
while, so some people would think!!  ahhhhahhh).

Paul H.

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