I re-read this thread because it was brought to mind by an article in Tree Care Industry IX(11) November, 1998: 'Trees and Subsidence Damage' by Dr. Giles Biddle.

It goes into a great deal of technical detail which might be interesting re: the earlier thread.

It's a new one to me. Neither roots nor soils are my area of expertise. Real estate is so I'm intrigued.

A few questions come to mind:

1. p.9 suggests that low permeability clay soils rehydrate slowly causing roots to go progressively deeper looking for water. We keep hearing that roots don't like hard soils low in organic matter and go elsewhere. What's the real story here. Are roots going into the low permeability clays?

2. What is the geographic relationship of building codes to damage? Are some foundation types less susceptible (or more susceptible) to damage. I recognize this is only of use in understanding where problems may occur, not in remediation strategies.

On reading p.18 I see that deep basements minimize risk of subsidence. Here in Northern U.S. we traditionally have basements as a result of needing footings below the frost line. Slab on grade or shallow foundations are less common. I'm told that's not the case, for example, in much of CA. Any comments?

I seem to recall something about expansive clays being a problem with regard to pushing in foundation walls (the opposite of subsidence) in the midwest U.S. Any comments?

3. Dr. Biddle suggests (p.19) A) that the UK Arboricultural Association's SRA methodolgy (which he helped develop) is an unreliable predictor of damage and B) that it should be expected that trees 'occassionally' cause damage. Any comments, particularly on what 'occassional' is?

Reply to post by Scott Cullen, on November 24, 1998 at 16:47:02:

I don't know much about codes, but here in SE Wisc. we have a big problem with basement walls buckling and our hard clay soils.

The problem comes from improper backfill during construction, if they would put in a snady clay or gravel backfill then tere would be no problem, but usualy they use the trash pulled out of the hole.

As for will roots penetrate the clay, they are hydrotropic if there is water deep(oxy too, i think the bottom limit is 8% for most) they will grow into it, this is why the will invade sewers, a plumbe of nutrient rich water (+O) coming from a cracked/separated pipe 10-15 ft below grade.

jps

Reply to post by Scott Cullen, on November 24, 1998 at 16:47:02:

Caveat domumemptor: Homebuyer Beware

Given the wretched summer that most of us have endured it will come as no surprise that the Environment Agency has officially announced the end of the worst drought to affect the UK for over 200 years. Although this declaration means that the immediate threat to short term water supplies has been lifted, it should not detract from the reality that the climate of the south and east of Britain has been getting warmer and drier; a trend which is forecast to continue.

The Meteorological Office has recorded a drop in average rainfall of 20% in the last 20 years and temperatures in the south are 1.5 – 2(C higher than normal and rising. Meanwhile, in 1991 the then Department of the Environment (DoE) estimated that a drought comparable with the summer 1976 will be a 1 in 10 year event by the year 2020. Whereas it was considered a 1 in 1,000 year event at the time it occurred. These statistics are all indicative of an insidious long-term drought that goes beyond having enough water to wash the car or water the lawn. This drought affects people’s homes and health. It has already influenced buying and selling property, and resulted in the wide scale persecution and premature death of many trees.

That Sinking Feeling

Surveyors and Valuers will be only too aware of the problems that the climate has had on properties. In particular the increased vulnerability of low rise buildings to subsidence damage as a consequence of trees desiccating shrinkable clay soils. The inexorable growth of subsidence claims is not only a function of climatic trends but represents an increased awareness and an intolerance of minor damage by homeowners. An attitude which is hardly surprising given that a previous insurance claim against subsidence can reduce the value of the affected property by up to 20%; reports the Subsidence Claims Advisory Bureau (SCAB). Moreover, many properties are effectively unsaleable whilst a claim is being investigated.

Recent Subsidence Claims



Year Subsidence Number of
Gross Claims (£M) Claims
1993 34,000 134 1994 27,600 125 1995 44,700 326
1996 47,700 333 1997 45,900 393
Association of British Insurers (ABI)

The Root of all Evil

According to the Institute of Structural Engineers (ISE), at least 80% of all subsidence claims on shrinkable soils are associated with trees and shrubs being too close to the building. However, subsidence can only be caused by trees if the wrong tree is in the wrong place and those that are correctly implicated as the cause of damage represent only a tiny fraction of the overall population. Nonetheless, trees are habitually incriminated by default; analysis by the London Tree Officers Association (LTOA) concluded that 70% of claims made against trees are unfounded. Therefore, it's essential to distinguish between those trees that have the potential to cause problems and those that don't. Unfortunately, poor advice by many professions working outside their field of expertise, including far too many tree surgeons and arboricultural consultants, and a public fear of subsidence, has panicked people into felling or mutilating many innocent trees.

The public perception of tree induced subsidence as a threat has been fuelled by very alarmist 'safe distances' that have been widely publicised. 'Safe distances' that are based on a fundamental misinterpretation of the root spread data from the Kew Tree Root Survey. So widespread has the misuse of this data been, that one of its principal researchers, David Cutler, expressly stated that “…use of the maximum root spread data in this way is foolish from both a statistical and biological point of view and shows a lack of professionalism.” Furthermore, a recent paper in the Arboricultural Journal, qualified this data as being useful for suggested planting distances only. Moreover, the distances cited should not be taken as an indication that trees are a risk to buildings, unless all other factors are considered.

The deeply sad and worrying consequences of this arboricultural witch-hunt are that many fine trees are being needlessly felled or badly pruned. These are the very same trees that are becoming increasingly important to our health and wellbeing. Trees, and especially large trees, are very much on the front line in the battle with poor air quality because they play such a vital role in checking the spiralling levels of pollution that are now commonplace in our towns and cities. Recent research into the affects of traffic emissions on health has established that ambient levels of pollution remain significantly above background levels within 50m of a main road. Furthermore, trees can reduce the impact of these traffic emissions by 30% at times of peak loading when traffic congestion is at its greatest. Their capacity to do this is directly correlated to the foliage area; in other words the size of the tree.

To add insult to injury the poor homeowner frequently suffers the double whammy of being out of pocket because they've had to pay for unnecessary tree-work, whilst their property value may diminish because trees have been removed or mauled. And it could get much worse. Many homebuyers are blissfully unaware that although their own insurance policy may cover them for subsidence damage, the minimum excess is usually £1,000, and may be even more in high risk areas. What is more, many Mortgage Lenders and Insurers accept reports from Arboriculturists irrespective of whether they are sufficiently trained or qualified to comment on such matters. Finally, a word of caution, attempting to resolve the problem by merely felling the tree may result in heave as the soil rehydrates, which is frequently even more damaging than subsidence.

What’s the Crack?: AMIUG v SRA

Consulting on trees and the potential for subsidence is a specialist subject and until recently arboriculturists were neither screened nor trained in this discipline, irrespective of their qualifications. However, for reasons that are beyond the scope of this article there now exists two organisations that offer training for pre-purchase mortgage reports.

The Arboricultural Mortgage and Insurers Users Group (AMIUG) was founded in 1995. It is a professional affiliation of the International Society of Arboriculture (ISA) and works with the support and co-operation of the Building Research Establishment (BRE). The AMIUG method uses a template report format and evaluates the possibilities of subsidence as well as the health and safety of significant trees and shrubs. It represents a preliminary assessment based on field evidence with options to increase the depth and scope of an investigation as circumstances dictate.
The Arboricultural Association (AA) launched the Subsidence Risk Assessment (SRA) in 1997 after meetings with the Association of British Insurers (ABI) and the Council of Mortgage Lenders (CML). The SRA utilises a mathematical methodology, where site data and laboratory analysis are entered into a spreadsheet and a Subsidence Risk Factor (SRF) is calculated; a SRF below 100 represents an insignificant risk.
The two approaches are very different and may superficially seem to confuse the issue still further. However, in what can only be described as a remarkably bizarre turn of events, following the launch of the SRA, its author tested the efficacy of the methodology against case studies where subsidence damage was attributable to trees. “…in half the cases where damage has occurred, the SRF would indicate an insignificant risk…the objective of a method of this sort is to identify and predict those situations where damage is likely to occur; it clearly fails to achieve this…” In other words, it quite simply doesn’t work.

The Future

The projected changes in climate for the UK are for it to become warmer and drier in the south and east. When considered alongside the increase in greenhouse gases, and especially CO2, then the following consequences are likely:
Subsidence claims will increase.
Trees will grow faster, the growing season will become extended, and consequently more moisture will be lost to transpiration.
Tree cover will become increasingly important for public health and the economic benefits provided by shade and amelioration of micro-climates.
Trees are undoubtedly an asset, bringing many tangible benefits that enhance the quality of life and even increase the value of property; adding as much as 12%. Conversely, they could be a liability that causes much grief and undermines a homeowner’s investment. Determining whether a tree is an asset or a liability requires an objective and comprehensive understanding of numerous site specific variables. It necessitates specialist training and is all too frequently the subject of misinformation and bad practice. Consequently, if Surveyors and Valuers wish to mitigate the possibilities of future legal action for contributory professional negligence than it is essential that they are aware of what is best practice.

David Evans at the Arbor Centre Bath UK

Reply to post by Paul Hawksford, on November 24, 1998 at 16:47:02:

As a follow up to the SRA and its author's comments:

SRA - Meetings Between the Arboricultural Association (AA) and ABI - Letter Received 22/11/98

"...As I mentioned, no final agreement was reached on the insurer requirements from an Arborist and thus the AA are incorrect to say that their handbook has been developed after consultation with the ABI. Secondly, the ABI guidelines to which they refer only exist in draft form, they have never been formally adopted by the ABI membership and are not, therefore, being used by insurers or presumably by anyone else.

We have now written to the AA expressing concern that they have given the impression that the ABI are still working with them and, indeed, have formally endorsed their output. We have asked that all reference to the ABI and ABI draft guidelines are removed from the SRA document and are not reinstated or reissued unless or until formal agreement between the organisations has been reached.

It is indeed somewhat strange that Giles Biddle, who acted as principle spokesman in discussions with the ABI, has now effectively dismissed the SRA method that the AA developed as a body while they continue to promote and train their members in its application..."

The Association of British Insurers (ABI)


Paul Hawksford

Reply to post by Paul Hawksford, on November 24, 1998 at 16:47:02:

Are the structures slab built or on a basement? seems to me that is it only happens with certain clay soils, excivation and backfill with a sandy clay aor gravel would eliminate the problem.

Reply to post by John S, on November 25, 1998 at 18:49:00:

What!! Please read the thread. You have obviously missed the plot!! Le'ts have some good replies.

Paul H.

Reply to post by Paul Hawksford, on November 24, 1998 at 16:47:02:


Paul,

perhaps I miss the point in all this but ...

It seems a tragedy that people own houses that were designed and built a long time
ago. These buildings were poorly built forthe conditions.

The trees are not using more significantly more water. In fact during
dry periods they may be using less.

The number of trees is not changing significantly.

Damage is greatest during periods of low rainfall.

It seems the only thing that seems to have changed is the rainfall and yet you Britts seem
obsessed with the tree continuing its natural consuption of water. What inane madnes. Why dont we
add in moisture loss through storm water, global warming due to cars and factories.

There can be no doubt that the problem is inappropriate construction on reactive clays. The trees
have not caused the shortage of water. This is purely a climactic issue. Even without trees these buildings
would suffer damage, perhaps even more damage.

\Lets be insensed at this continual blaming of trees and start becoming their defendants.

Why not beat up on meteorologists or the church (acts of God) instead it's just as crazy.

Thats my 2cents worth

Mark