Re: Tree’s and Building Subsidence UK

The most current topic regarding tree roots and buildings involves the removal of a proportion
of the crown/leaf area (transpirational bio-mass) in order to reduce the tree’s moisture up-take
on low permeable clay sub-soils and therefore mitigate structural damage to low rise buildings.
Limited data and very few case studies are available.

Your thoughts on the specific species biological processes, sub-soil analysis and climatic
variations are appreciated.

We are limited in our knowledge of the biological species profile (BSP), the tree’s, true rooting
profile and capacity (again limited case studies for limited species in limited areas), the specific
soil water conducting tissue types (WCTT), the seasonal variants to the tree and it’s soil water
requirements, building orientation, along with the soil moisture deficit (SMD) analysis from
various Meteorological Establishments displaying data based on non-site specific coverage
without tree cover and more-so for crop or bare soils. Limited data suggests that pruning trees
will not mitigate the problem. Some people argue that following crown pruning, re-growth of
juvenile foliage along with increased leaf area stomatal densities ? will actually increase the
tree’s water up-take ? (wind speed, pressure and temperature have to be taken into account) and
very limited data also suggests that pruning will increase water up-take by trees following a
single case study now being monitored in the south UK ? (UK Building Research
Establishment). Percentage removals of transpirational bio-mass are involved and we need to
understand that the greater the percentage area removed, the greater the likelihood of reduced
water up-take, however, what of the aesthetics and amenity value of the tree? Meetings and
publications are due in the UK this month (May) on these issues, though I do not expect
anything radical.

Trees are dynamic living organisms and innumerable variables are involved. Definitive causal
links are often made following poor analysis or understanding of the biological process, the
soils and the climate.

Trees are implicated in building damage, we often know why, but can we definitively point to
one specific species on one particular site and state YES that is the culprit ? What about the
remaining vegetation and its own role or contribution to the persistent SMD scenario?

What use is root identification, given that many roots may be in close proximity to the building
damage?
How can we successfully distinguish which tree is to be removed and which is to be retained,
or are we relying on “Tree to Damage Distances” alone ?

What use is comparing the actual relative soil moisture content for preliminary investigation
when we know full well that many soils will fully rehydrate during the winter months when
many site specific precipitation levels are high as opposed to the later summer months when the
opposite occurs in relation to localised vegetation (low winter precipitation on low permeable
clays excluded of course!!).

Over the last few years, we have had in the UK, a number of case studies following insurance
claims data (tree to damage distances), storms of 1987 data and tree’s and their water demand
data along with damage limitation exercises on cyclical pruning regimes and water up-take. It
must be realised that all the data is extremely limited, extremely site specific and open to
mis-interpretation and has and is still being used by a number of consulting Arborists as being
acceptable.

The truth is, trees are being unnecessarily removed.

To all you clever people out there. I am simply looking for answers!!

Paul H.

Reply to post by Paul H, on May 06, 1998 at 15:22:46:

Hi Paul,

and All,

Let me pose a couple of questions. Maybe they can lead to answers if they are collectively
contemplated.

[1] Do buildings sink in UK when no trees are nearby?

[2] Are clay type soils primarily involved?

[3] How deep is the water table, and does it correlate at all with subsidence?

[4] What differences exist in UK versus tundra settings in Alaska and Yukon Territories?

[5] Are the subsidence problems in those areas related to entirely different factors?

[6] Are there other areas of the world that experience subsidence more often than normal, and
are there geological explanations?

[7] Has inward or outward stress to foundations been observed and recorded?

[8] Where does the displaced soil go, or is the displacement primarily the liquid portion of the
soil?

I, like Paul hate to see trees removed or severely pruned unnecessarily. It seems to me that we
are all missing a piece of the picture of what all is happening when a building sinks. Frankly,
the subject is bothersome enough, that I for one would really like to see some definitive answers
surface.

KWK

Reply to post by Paul H, on May 06, 1998 at 15:22:46:

The most recent info I have is by a study done by Nelda Matheny & James Clark although I don't know if it has been published yet, It is very interesting in regards to clay soils, roots, and foundations. Her email is hortsci1@ix.netcom.com

Reply to post by Paul H, on May 06, 1998 at 15:22:46:


It seems to me that case studies on specific sites might be more due to the fancier lawyer than to good science. No? When water is pulled out of clay soils, the soil tends to harden. Subsidence would be more likely caused by a saturated clay soil with downward pressure exerted upon it. Why? Because water enables the flat micro-plates in clay minerals to align, and shear horizontally. This will cause a bulge on the perimeter of the pressure, and a hollow in the mmiddle, like the infamous do-nut, or government bureaucracy. P.S. I am always happy to be corrected.

Reply to post by peter t, on May 06, 1998 at 15:22:46:


Soil Textural Classifications: Are a means in which the characteristics of the soil can be recognised. Textural type relates to the soil particle sizes, sand particles are relatively large, meaning soils with a high percentage of sand have a coarse texture. Silt soils have a finer texture because particles are smaller, while the clay particle is the finest. Soil shrinkage and heave is usually associated with clay, although some peaty and fen soils can exhibit shrinkable characteristics. Clay soils have differing lattice structures that can relate to their shrinkability. Montmorillonite lattice structures exhibit large moisture volume changes when compared to Kaolinite or illite lattice structures.

Soil Plasticity: Soils which are categorised as shrinkable clay have to display more than 34% fine particles (silt or clay) with a plasticity of more than 10% The clay component of the soil sample is tested using the methods as described in British Standard 1377 Part 2 ’Methods of Tests for Soils’ 1990. This method analyses the soils upper and lower plastic limits (i.e. too wet to paste or too dry to retain a moulded form). The difference between these two limits is the Plasticity Index.

Soil Permeability: Permeability is the factor which controls the rate of water movement in the soil. Soil particle size’s are significant in modulating permeability. Clay soils that have a low permeability are usually linked to high shrinkability. Consequently a low permeable soil means the time taken for rehydration is considerably longer.


Clay soils move, amplitude of movement is variable. Read some stuff, get some CPD and you will see why!! I can let you have the data if you wish?

Paul H.

Reply to post by Paul H, on May 06, 1998 at 15:22:46:

Maybe it's not the trees. Maybe its shoddy building practices i.e. poor excavating for the foundation. Where I live in Sidney, B.C. Canada and we have very clayey soils. If a building shifts or sinks we look to the building contractor not the trees! Dorothy

Reply to post by Dorothy Hartshorne, on May 06, 1998 at 15:22:46:


Dorothy

With respect, we may be dealing with the building contractor who is long dead!! or at best someone who is younger but difficult to trace.

Consider the facts:

Scenario:

Vigorous trees abstracting moisture from a low permeable clay sub-soil/high shrinkability, within proximity and underlying the foundations of a property. Rehydration of the clay component maybe slow and during drought periods and soil subsidence and building movement may occur. OK, building design is bad, and is maybe 100 years old, we are now talking about resolving the situation in 1998. Please forget the mistakes of the past, we are on to that problem, give us the answer to predicting the potential for mature trees to cause damage to properties in the future? If damage is occuring, define which trees are most likely to be exacerbting the situation, given climatic variations and the species biological profile eg. rooting cababilities, water conducting tissue types? moisture abstraction rates linked primarily to leaf area transpirational loss.

As Arborists, we need to look at the tree and its biological process, along with site specific soils and localised climatic variations along with the differentials from species to species. One cannot simply point the blame at the dead builder or many cases the tree!!

Paul H.

Reply to post by Dorothy Hartshorne, on May 06, 1998 at 15:22:46:


Dorothy

As an after thought and having read the reports of some of the so-called best Arboricultural Consultants in the UK, it would seem to me that a conclusion for tree related building subsidence is based on extremely limited and spurious data. I feel that the insurance companies should allow further monitoring periods in order to let the Arboriculturist and Structural Engineer, determine a definitive causal link. Otherwise we have the usual scenario of "unnecessary tree removal."

Your thoughts are welcomed.

Paul H.

Reply to post by Kerry Knorr, on May 06, 1998 at 15:22:46:

Yes, I am afraid it is true that trees (as well as other vegetation) can contribute to structural damage by extracting moisture from the soil. The required conditions are 1) soil type that shrinks as it dries, i.e. expansive clay; 2) tree of sufficient size to use relatively large amounts of water; and 3) structure that relies on the underlying soil for stability. This problem certainly occurs in the U.S., but is not very well recognized, which, based on the U.K. experience, is probably better for our tree! Holtz (1983) estimated that 20% of all damage that occurs on expansive osils can be attributed to desiccation effects by vegetation. We have consulted on several cases involving this issue. In some cases the trees have been largely responsible for the damge; in others, not at all. It is a very complex issue that involves biology, soils, engineering and climate. Fascinating to investigate and study; difficult to simplify and diagnose accurately.

Reply to post by Nelda Matheny, on May 06, 1998 at 15:25:54:

Nelda

Some recent UK info on subsidence damage

Figures for 1997 which have recently been released by the Association of British Insurers (ABI) and they follow a trend for escalating claims. According to the Building Research Establishment (BRE), around 70% of all subsidence claims on shrinkable clay soils are associated with trees and shrubs being too close to the building. Recent Subsidence Claims Data:

Year Gross Claims (£M) Number of Claims

1993 134 34,000
1994 125 27,600
1995 326 44,700
1996 333 47,700
1997 393 45,900

Reply to post by Paul H, on May 06, 1998 at 15:22:46:

See new post 11/24/98.