Tree Related Subsidence Damage
Arboricultural Risk and Solutions, An Overview
Low rise buildings on shrinkable clay soils are vulnerable to movement and structural damage due to moisture abstraction by trees and other vegetation groups. Over the last 10 years domestic subsidence claims number between 28,000 to 55,000 costing the insurance industry £140M - £400M annually. A significant proportion of these claims relate to the influence of trees and other vegetation groups.
Tree loss due to subsidence damage claims to low rise buildings is a contentious issue in the UK. Undoubtedly tree removals can have a high negative impact on amenity and the environment at the local level. At the wider level, the impact is generally less than commonly thought. The issue is frequently covered in the national press with hysterical claims of mass tree removals instigated by insurers and exaggerated evaluations of risk from trees based on ignorance, limited data and lack of evidence.
Urban Trees in Britain
Trees in Town II - (Department for Communities and Local Government) - A new survey of urban trees in England and their condition and management completed in 2005, provides useful data on the urban tree stock in Britain. The objective of the survey was to provide up-to-date information on the national urban tree stock and urban tree management by local authorities.
Two thirds of all trees and shrubs were on private property (mainly in gardens) or on less accessible public land (e.g. schools, churchyards, allotments, etc.). Almost 20% were located in public parks and open space. Comparisons with 1992 data suggest a possible increase in the overall proportion of street/roadside trees, particularly in residential and industrial areas.
Data showing increased tree densities may be a positive, but what about species composition? What sort of trees do we want in our urban landscapes? Is scale important? The replacement of large trees with small ornamental species is likely to have a negative environmental impact and lower amenity value. Can large woodland species be accommodated in our crowded built environment? Large broadleaved tree species accounted for approximately 31% of all trees. Small broadleaved tree species made up 42% of all trees. Conifers comprised 27% of all trees.
What was the most frequently recorded species? Leyland cypress (12.3% of all trees and shrubs). Most trees in the survey were estimated to be between 2.5 and 9.9 m tall. Relatively few were taller than 10 m. In all regions except the SE only 10-20% of trees had a trunk/stem diameter (dbh) of 30 cm or more.
Most trees were estimated to be between 10 and 50 years old
- Young (14%)
- Semi-mature (41%)
- Early mature (27%)
- Mature (17%)
- Over-mature (0.2%)
Are tree removals due to subsidence claims a significant problem?
Using London as an example. The Greater London Authority published Chainsaw massacre - A review of London’s street trees (May 2007) which looked at London’s trees and reasons for tree loss. It found that there were approximately 7,000,000 (7 million) trees across Greater London (including woodlands and parks) with close to 500,000 street trees. “Many of London’s traditional broadleaf street trees (the London Planes for example) are under threat from development pressures, reduced expenditure by some London Boroughs, a peculiar mixture of public apathy and antipathy and an insurance industry risk averse to subsidence claims”. It asked the following question; “ Is London losing its street trees”? giving its own answer as “The short answer is no”. The survey included questions to the The London Tree Officers Association (LTOA) and recorded the following outcomes.
- There has been a net loss of street trees in a third of London Boroughs.
- Harrow - lost 5,000 street trees over the last five years.
- Croydon has lost 2,600 of its trees in the past five years.
- There was a net gain of over 8,000 trees planted across the Capital in the past five years = + 1.66%
Two thirds of London boroughs planted more trees than they have removed over the previous five years. So why are trees removed in London?
- Of the 39,924 trees removed over the past five years 75% were felled because of health and safety reasons.
- 5% were removed because of subsidence claims – a total of 1996 (howvere some boroughs report 10-40% - Hackney, Brent , Camden)
- No reason given to account for the other 20%.
The Mayor’s London Tree and Woodland Framework document referred to states that the perceived threat of subsidence is much greater than the actual threat and it is estimated that less than 1% of the total tree population has actually caused damage to properties.
However, the LTOA has challenged the figures and stance of the ABI regarding subsidence issues and street trees. The following statement by a London tree officer is included: “Normally we deal with insurance companies day in and day out and 100% of the time the tree is asked to be removed in quite farcical circumstances. In a lot of cases the trees can be 20 or 30 metres away from the property. A root has not been found. The soil is not desiccated, it is not even clay, and they will ask for a tree to be removed.” This is not our experience.
The data provided suggests that the number of trees removed due to subsidence equates to 0.5% of London’s total street tree population.
Trees related Damage
Trees can damage buildings directly or indirectly. Direct damage is usually obvious and spectacular. Structural failure of the tree, or a significant part of it, can result in the tree striking buildings, power lines, vehicles and in some instances people. The consequences of direct damage often make media headlines. In addition, trees can cause direct damage to buildings and light structures as a result of the pressures exerted on the structures through the incremental growth of parts of the tree Roots and main stem).
Tree related subsidence damage to property is probably the single largest tree related insurance issue in the United Kingdom and it has been statistically shown that trees and other large vegetation on clay subsoil are responsible for over 65% of all instances of subsidence damage to domestic property – that percentage is even higher during drought years.
The problem was first highlighted in the 1940’s when, Ward, (1947) wrote an article on the 'Effects of fast growing trees and shrubs on shallow foundations”, but really came to its present level of importance following the drought of 1975/1976.
The presentation concentrates on circumstances where trees cause damage to low rise buildings. This damage is a result of trees' ability to abstract water from soils through transpiration leading to changes in soil volume, of clay soils in particular, as water content within the clay fraction is reduced (Cutler & Richardson, 1998).
What is subsidence?
The Oxford English Dictionary definition of subsidence is ‘to sink to a lower level, to cave in, collapse’. For insurance purposes subsidence is defined as the downward movement of a building and its foundation caused by loss of support of the site beneath the foundations. This is usually associated with volumetric changes in the ground supporting the foundations, e.g.:
- compression of peat, other than that due directly to the self-weight of the building
- further compaction or consolidation of fill
- shrinkage or softening of clay soils
- washing away of fines in granular material
It should be emphasised that all these volumetric changes to the ground can only occur as a direct result of an external factor,
- change in ground water level
- leaking drains
- influence of trees or other vegetation
Settlement differs from subsidence in that it is the result of downwards pressure of the load of a structure as it attains equilibrium with its substrate. Settlement occurs ‘once’ in the years immediately following construction and is a unidirectional event. Vegetation related subsidence is cyclic in nature. Trees predominantly abstract moisture (in excess of rainfall) from the soil throughout the growing season (typically April till October), their effect on the soil moisture deficit (and any volumetric change) reaches a maximum when deciduous trees shed their leaves. At this point the observed damage is correspondingly at its most severe. Over the winter rainfall exceeds transpirational loss, the soil moisture deficit is decreased and the soil rehydrates. When rainfall and infiltration is sufficient the soil will return to its ‘normal’ volume and there may be ‘closure’ of the superstructure cracking. In situations where desiccation is severe and extends deep into the soil profile then seasonal recovery may not be complete and the soil moisture deficit would progressively increase but would still be cyclic in nature with the degree of damage and the soil moisture deficit varying seasonally.
The Insurance Issue
Insurance cover for subsidence, heave or landslip damage was introduced as an additional service feature in 1971.
It originated following pressure from Mortgage Lenders to protect their loan security (the property) and their customers against major catastrophe. In most instances there was no charge for this cover.
The additional cover was not considered by insurers to result in a significant increase in claims since traditionally, policyholders accepted small cracks as being part of home ownership. Given that the majority of subsidence cases cause only cosmetic damage, these cracks were not viewed as significant by the home owner, any prospective purchaser or Mortgage Lender. However, as home ownership became more common and the housing market ‘took off’, property became a major asset with equity, and claims for subsidence increased. In the majority of cases damage was relatively minor, but insurance company pressure on engineers to provide a guarantee that claims would not recur, resulted in the specifying of expensive underpinning solutions.
The average annual cost of building subsidence to insurers is over £400 million, which represents approximately 40,000 claims per year. The experience of the major loss adjusters is that approximately 50% to 70% of these claims are repudiated, i.e. they are rejected as invalid, mostly because damage is identified as being due to some non-subsidence related cause, not covered by the policy. Of the valid claims, around 70% are tree-related, with perhaps 10% to 15% being related to drains and the remainder due to other causes. It is a fact tree-related claims cost more and take longer to resolve than other causes of subsidence.
The peaks in numbers of claims and cost to insurers correspond with periods of drought, i.e. between 1989 and 1992, 1995 to 1997 and 2003. Over a 12-year period between 1989 and 2003 the total cost to the industry was close to £4.8 billion, which represents an annual average cost of £598 million. Because the impact of drought periods skews the data (since the peak years are not evenly distributed) the adjusted average is closer to £400 million per annum. Between 60% and 70% of the claims are in areas with shrinkable clay soils, i.e. London and the southeast and along a line between the Humber and Severn estuaries. The hidden cost of subsidence has been that until comparatively recently, the average duration of a claim was eighteen months, and thousands of households suffered lengthy disruption while insurers dealt with the repair.
The pre requisites for Subsidence
In order for vegetation related subsidence to occur the following factors must be present at the same time
- There must be a building
- The foundations must be on a shrinkable clay soil
- The soils must be capable of supporting vegetation which could act as a bio-mechanical pump
- The soil drying ability of this vegetation must (normally) extend to depths below foundations
- Rainfall and infiltration of water must be less than evapotranspiration for significant periods
The United Kingdom is one of the most densely populated areas of the world. It has a population of over 65 million with over 25 million residential homes on a land area of 244,750 km2, (DTRL, 1991).
The number of residential homes is increasing rapidly with Government approval and encouragement. The ratio of the urban to the rural population is one of the highest in the world and reflects the nature of industrial development within the UK from the industrial revolution to the present day. The urban conurbations contain trees, which are maintained as part of a sustainable green environment. The benefits of urban trees to the health and well-being of the population is recognised and the UK has the most sophisticated system of legal protection for trees in the world.
Clay soils are distributed across the UK but are concentrated in the south and east.
Clay is a complex chemical medium and its primary interest in biological terms relates to plant water use. Clay intercepts precipitation and binds water to its structure against gravitational energy. Vegetation including deciduous trees can access a percentage of the bound water. The shrinking of the clay volume as water is removed can lead to movements in any structure founded on clay. It is important to appreciate that it is the volume of water in the soil that changes and not the clay itself.
The term ‘Clay’ has three meanings in relation to its use in terms of subsidence:
- Clay minerals : specific chemical compounds
- Clays size: particle size descriptor – particles smaller than 2μm
- Clay soil: a soil with enough clay sized material or clay minerals to exhibit cohesive properties
Two distinct types of clay soils exist: normally consolidated and over consolidated. Normally consolidated clay soils are usually soft and tend to be associated with ‘settlement’ rather than subsidence problems. Over consolidated clays are usually ‘firm’ near the surface and are commonly associated with subsidence e.g. London, Gault and Boulder Clays.
The climate of the UK is influenced by the weather systems of the Atlantic Ocean and the continental land mass of Europe and both factors act on the geography of the country in a dynamic and complex manner. The west and north of the country is upland onto which the Atlantic weather systems fall. The south and east is generally at or very near sea level and its physical proximity to mainland Europe leads to a more continental climate than in the north and west with longer summers, higher average temperatures and lower average rainfall.
Despite the influence of the Atlantic, the U.K. is relatively poor in water resources. River systems are short and fed from small upland watersheds. The large centres of population, particularly in the Thames Valley, and the low average rainfall of the south and east combine to produce a water poor environment. Counties such as Essex are classified as semi-arid; an indication of the relative scarcity of available water in eastern counties. Additionally, the impact of the dry summers from the 1970’s onward, together with low winter rainfall has had an effect upon the volume of moisture in clay soils.
The Met Office has advised (within the last two weeks) that the UK must become more resilient to both drought and flooding.
The Met Office has suggested that the UK could experience a severe short-term drought, similar to the drought experienced in 1976, once a decade.
The Arboricultural Mortgage Report
A vegetation related risk of a clay shrinkage subsidence event requires:
- Needs to be a building (housing distribution across UK)
- Needs to on a shrinkable clay soil (soils distribution) – why do clay soils shrink?
- Needs to have trees / veg
Two of these are variables… the weather and the trees…. The others are constants although we may need to make assumptions as we don’t know what the figures are!!
We have to assume (however unlikely it seems at the moment) that the weather is at some point in the future going to be hot and dry?
In terms of assessing – how hot and dry do we assume?
What about the influence of global warming?
In reality we tend to work on - it is going to be hot enough and dry enough.
The risk assessment needs to be tempered with reality – and the reality that matters most is could I/we be considered negligent for not anticipating the scenario or would the event only be likely to occur in extreme (unforeseeable) circumstances.
Shrinkable clay soil
Determining whether the subject property is founded on a shrinkable clay soil is done on various levels. In deciding whether a report is required it is normal to determine the soil type by either local knowledge or by reference to British Geological Survey Maps (Accuracy is reasonable but it must be remembered that in some areas there is significant localised soil variability and also that the exact boundaries of each soil type may not be mapped perfectly).
In determining the risk of subsidence we need to assess ‘how shrinkable the clay is’. i.e if you were to dry the clay exactly how much would it shrink.
The measurement which best reflects this property is the ‘Plasticity Index’.
As you can see from the British Geological Survey map the areas of high plasticity clay soils are predominantly found south of a line drawn between Bristol in the south west and Hull on the east coast (although there are other areas!).
What is important to note is that just because a property is on a low plasticity soil does not mean that it won’t have subsidence.
The plasticity of the soil is only ONE factor.
There are three levels of soils assessment available in terms of arboricultural mortgage reporting.
- Reference to the BGS data
- Site soil sampling and analysis (basic testing)
- Full soils analysis (Atterberg limit testing)
- There are numerous factors relating to the actual building which can affect the possibility of subsidence.
- The age of the building
- The foundation design
- The foundation depth
- The build quality
- Any modifications e.g. extensions / underpinning
The age of the building is relatively easy to estimate, determining the foundation design/ depth less so. The foundation depth is important as we are trying to determine the likelihood of tree roots drying soil underside of foundations… if we don’t know the depth of foundations then we have to make an educated guess.
We will assume that the house has been constructed broadly in line with accepted building practice at the time of construction and that foundation depth was increased in the knowledge that the building is located on a shrinkable clay soil.In addition we will assume that the property was constructed in accordance with NHBC guidelines.Typically the older the building the shallower the foundations.
A significant number of subsidence claims relate to damage at the interface between extensions (particularly conservatories) and the original building. This is due to differential movement.
The BRE established that foundations in excess of 900mm deep can typically withstand the effects of normal climactic drying and the influence of minor vegetation.From this you can say that properties with foundations below 900mm may be affected by climactic drying and the influence of vegetation.
If you have a house which is founded on 600mm deep foundations and a conservatory which is founded on 300m foundations then… more of the soil underside of foundations to the conservatory will be affected than the soils underside of foundations to the house…
Accordingly you could expect the conservatory to ‘subside’ more.
As its movement is greater than that of the house to which it is attached it must either…
Hang in mid-air supported by its attachment to the main house (unlikely)
Fracture at the interface with the main house.
Some 70% of all subsidence problems have been found to be attributed to trees and large shrubs extracting moisture from a shrinkable clay substratum. As a consequence, a property constructed on clay in the proximity of trees can suffer structural damage of a subsidence nature. In a drought year this can be up to 80% of all claims.
The likelihood of damage arising from moisture extraction because of tree roots growing close to, or beneath a property, is very complex and depends upon:
- the species, crown size, and condition of the tree or trees
- their proximity to the property and depth of the foundations of the property
- the climatic conditions
- the nature of the substratum
Each of these factors can be very variable, and their interaction becomes even more variable and unpredictable. As a result, very occasionally roots can cause building movement leading to damage at very considerable distances from a tree.
While assessing potential risk thought to be imposed by trees, on buildings on shrinkable clays is complex and involves a number of variables, the data published by Cutler & Richardson (1981 & 1989) is referenced by practitioners in this field.
If trees are at a greater distance than the maximum recorded by Cutler & Richardson, they are unlikely to present a significant risk and can probably be ignored. f they are closer, their potential involvement in damage should be considered. Small ornamental trees and shrubs may also cause damage if they are very close to a building with shallow foundations.
Even with fairly detailed investigations, it is not possible to make an accurate prediction as to which trees from amongst the total population will actually initiate damage.
Why do trees need water?
A Simple Model Tree
It is important at this juncture to understand how trees function and grow in order to appreciate how they can cause subsidence damage and to understand the management options for repairs where trees are present. To do this we can consider this ‘simple model’ for how a tree functions in regard to water use.
Although not defined in law, trees are generally considered to be large dynamic woody perennials that are self-optimising. In effect they are dynamic pumping mechanisms and have essentially three component parts:
- Roots for anchorage, storage and absorption;
- Scaffolds containing branches, sub-branches and twigs that carry leaves;
- Trunk (bole or stem) that connects the other two parts.
Roots absorb water and the three major nutrients Nitrogen, (N) Phosphate (P) and Potassium (K) together with between 13 and 17 micronutrients. Water and nutrients are transported from the roots to the shoots through the wood (xylem). Leaves contain chlorophyll where the process of photosynthesis combines water from the soil with carbon dioxide from the atmosphere and energy from sunlight to produce sugars (photosynthate) and releasing oxygen back into the atmosphere. The sugars are simple carbohydrates such as glucose, fructose and sucrose that are transported from the shoots to the roots and other areas in the inner bark or phloem tissue.
Photosynthesis 6CO2 + 6H2O + Energy (Sunshine) = C6 H12 O6 + 6O2
Trees store energy in the form of starch (a long chain molecule comprised of multiples of glucose molecules). The presence of starch in tree tissues can provide an indication of its state of health, its presence in roots recovered from underside of foundations is generally taken as an indication of live material.
Leaves also provide the mechanism by which water leaves the tree. Water is absorbed through the roots and moved up the wood or xylem to the leaves where some is used in photosynthesis and the rest evaporated to the air through structures called stomata. This process is called transpiration. Thus the leaves regulate the water use of trees:
It is difficult to believe but tree growth in the United Kingdom is limited by the lack of water available to the plant! In fact most plant suspend photosynthesis during the middle of the day when the available light is most plentiful – in order to conserve water.
Arboricultural input in the claim process
Arboriculturists are not structural engineers (generally) and should not be diagnosing the cause of damage. This is for the engineer to evaluate. Our starting point when instructed to advise on a subsidence damage claim assumes clay shrinkage subsidence is the cause of damage. It is our remit to advise if vegetation is a factor in the damage and what if any vegetation management is appropriate or necessary to restore stability. This does not mean the wholesale removal of all vegetation.
Many low rise buildings on plastic clay soils will undergo a degree of movement – most of which is not significant to the structural integrity of the building and does not result in cracking. Movement becomes more significant when there is an external force acting upon soil moisture and thus soil volumes i.e. vegetation. Whilst ambient evaporative pressures will cause drying and shrinkage at near surface levels, it is moisture abstraction by vegetation which causes shrinkage at depths beyond 500 – 1000mm below ground level. It is differential movement that generally results in damage.
Influencing vegetation may comprise shrubs, trees or climbers, or a combination of all three and can be relatively straight forward or complex in nature.
The potential impact on a building of any influencing vegetation is further modified by soil characteristics, foundations design and depth (including differing foundation depths).
The most significant factor in influencing claim numbers is weather. Claims rise rapidly in hot dry years. The timing of drought periods is also significant with August and September being critical months. 2012 was the wettest year on record which resulted in very low claim numbers.
All of the above factors are taken in to account when considering remedial vegetation management and future management of retained vegetation.
Options for vegetation management.
At its simplest there are two options; manage or remove.
Management will generally involve reduction of the relevant elements, the objective being to reduce the moisture uptake and therefore soil drying effects of the vegetation concerned. However, this is
species dependent – not all trees (especially some large mature trees) will tolerate heavy crown reductions and excessive pruning may result in the steady decline of the tree and ultimately its death. In these circumstances management may not be an appropriate option. Research has shown that for pruning to be an effective control of water use in trees, reductions in the order of 90% are required.
A further consideration and limiting factor is the need to comply with guidelines set out in BS3998 Tree Work Recommendations. Heavy crown reductions will usually contravene recommendations given in the standard.
Some species (e.g. willow, poplar, cherries) will respond to pruning by producing vigorous new growth which quickly restores the trees soil moisture uptake to pre-pruning levels within a couple of seasons.
A detailed knowledge of tree biology is required to make the necessary judgements on tree management.
Removal is obviously straight forward however consideration needs to be given to the potential for heave.
Clay soils shrink when moisture is removed. They swell when moisture is added. Volume increases in soil moisture up to field capacity beyond which any added water is ‘free water’ – i.e. not chemically bonded with the clay mineral particles. Most soils recover to field capacity on an annual basis. Where they do not, a permanent soil moisture deficit may develop where the soil exists in a persistent shrunken state from year to year. This usually occurs in the presence of very large individual trees, large numbers of trees or in some cases where access of precipitation to the soil is restricted (e.g. hard standing). The umbrella effect.
Heave in this case is defined as soil volume recovery over and above that on which the foundation was laid. In the authors experience this is a relatively rare cause of damage in low rise buildings but when it does occur can be uncontrollable and result in severe damage. Where a heave risk has been identified, protective measures can be adopted for new foundations.
Trees (and larger shrubs) are protected under the Town and Country Planning Act either by Tree Preservation Order (TPO) or where the trees are located within a Conservation Area.
Where trees are subject to a TPO any tree works requires an application to the relevant planning authority detailing the proposed works and the reasons for that work. The council will require evidence substantiating the role of the relevant tree(s) as a causal factor in the damage. That evidence is obtained from site specific investigations and laboratory testing of samples (soils and roots). Where trees of the same species are implicated, a DNA test may be required.
In addition, monitoring data confirming a cyclical pattern of movement consistent with vegetation induced soil volume changes will normally be required, and in some cases 12 months of data is requested. Preferred method of monitoring is precise levels as this gives a direct measure of building movement as opposed to crack monitoring which is an indirect measure.
Even where a full suite of evidence is provided, consent to carry out the works may be refused. Where the evidence is strong the option to appeal the decision and/or claim costs from the council associated with additional damage is available.
Root barriers are a potential alternative option to tree removals and underpinning in subsidence damage claims. They usually offer considerable cost advantages relative to underpinning and offer significant amenity and environmental benefits where trees are retained as a result of their installation. However, Root Barriers are not always viable in urban and suburban situations.
Consideration needs to be given to the future health and stability of retained trees requiring expert Arboricultural advice. Below ground services need to be accommodated in the design.
The major advantage over partial underpinning is that the whole building is generally protected from the influence of the offending vegetation and differential movement associated with partial underpinning is avoided.
Many root barriers have failed usually because of poor design and choice of materials, but if done correctly this can be an extremely effective solution in clay shrinkage subsidence claims.
David Mahon B Sc. Hons MICFor M. Arbor A
MWA Arboriculture Ltd
Bloxham Mill Business Centre
T: 0844 243 7899