Subsidence and trees – A soils perspective
By Adrian Howorth BEng CEng MICE MCIWEM and David O’Sullivan BSc
MSc (Eng) MCIOB.
Synopsis
This paper considers the relationships between Policyholders, Insurers and Local
Authorities as tree owners and reviews the positions taken by each when settling
Insurance claims arising from subsidence damage. The paper seeks to promote faster
and more amicable resolution of claims between the parties based upon better
engineering assessment of soils.
The new Local Authority Position
A Risk Limitation Strategy for Tree Root Claims 3rd Edition May 2007 published by
the London Tree Officers Association (LTOA) is the current recommended guidance
for use by all Local Authorities and by extension, all Local Authorities in the London
clay basin, when considering subsidence claims involving their trees. The document
signals a shift in the consideration over the role of tree roots in subsidence claims. The
document is partly a response to the approach by Insurers and their Solicitors who
have vigorously pursued Local Authorities with claims for potential recovery by
building cases based upon the Rylands v Fletcher ruling and the decision in Paterson
v Humberside County Council (1995). These cases tend to offer subjective
arboriculture evidence in particular to substantiate cause. Under the terms of the
Strategy, Local Authorities as tree owners will now resist tree nuisance claims
without good quality evidence to support such a claim. This change in emphasis from
an argument based on legal precedent to an engineering evaluation on a case by case
basis should prompt engineers and loss adjusters to take a fresh look at the underlying
causes of subsidence damage as well as encouraging all sides to be pro-active in
reaching a satisfactory conclusion.
The ‘value’ of trees
The LTOA have responded to the many subrogated liability claims instigated through
insurers and their solicitors by considering the ‘value’ of the individual tree on a
sliding scale. This takes into account its position and amenity value and raises the
level of evidence required in support of removal or reduction measures according to
its value. Articles published in The Times and other newspapers indicated that Tree
Officers had valued some London trees as high as £750,000. The Strategy
recommends that each Local Authority provide dedicated resources to defend claims
affecting council controlled trees including privately owned trees within a
Conservation Area. Considered and objective tree retention is encouraged wherever
possible and the Strategy recommends that unwarranted or unsubstantiated claims are
robustly defended. The Strategy also calls for a cyclical regime of maintenance with
pruning and selective removal of Council controlled tree stock where appropriate with
provisions for the removal of lower value and diseased trees. Loss Adjusters should
obviously continue to take such opportunities to negotiate with and the Tree Officer
concerned if tree removal is considered a desirable risk improvement measure.
1.
Effectiveness of Tree management
When confronted by a claim from Insurers, Local Authorities may offer to reduce the
tree or make reference to ongoing maintenance in mitigation.
Current research shows however that vegetation control is only effective if local water
demand by trees upon the soil is permanently reduced or eliminated. A study by the
Cambridge LINK consortium completed in 2006 has shown that crown thinning or
removal of selected tree branches has no measurable effect on soil moisture demand
as the rate of transpiration through the remaining branch and leaf area will simply
increase to compensate. A substantial crown reduction of all leaf producing branches
on the tree is required to have a measurable effect on water demand by its roots. The
study concludes that a minimum of a 40% crown reduction is required; more if the
tree can stand it. This is obviously only an effective measure until leaf growth is reestablished and the timeframe for this could be as little as one growing season.
From the Insurers point of view the only satisfactory permanent solution therefore is
removal of the tree and this is very often the recommendation of Arboriculturists.
Insurer’s dilemma
Insurers are currently faced with a dilemma when it comes to trees. On the one hand
there is little or no tolerance of even minor cracking by homeowners and property
surveyors alike so Insurers are bound to act in support of their policyholders and the
premises on risk. Also, Insurer’s rights of subrogation under the terms of the
insurance policy are seen to be compromised in law if immediate notice of potential
tree damage is not given at the outset of the claim. The Paterson v Humberside
County Council (1996) decision requires only that the neighbours tree is shown to be
the ‘effective and substantial cause’ and other possible contributing causes including
soils, shallow foundations or secondary vegetation are effectively ignored in the legal
ruling. Enthusiastic use of this principle has often led Insurers into conflict with Local
Authorities to the detriment of claims settlement and at times, has lead to unnecessary
tree removal
The LTOA requirements now require substantially more high quality and targeted
investigation to support the removal of high value trees so in such cases the claim
settlement period could be excessive from the Policyholders viewpoint leading to
distress and complaint. So Insurers dilemma is how to maintain Policyholders
expectations whilst considering the time and cost to settle the claim and the prospect
of a recovery of their outlays, and whether the claim justifies the removal of a tree.
The Soils Perspective
A geotechnical soils assessment in the above context could be used to greater effect in
our view. The roots of a mature tree are very often present within the soil for a long
period of time before any cracks are present but will not directly damage a building
unless other conditions prevail. The mineralogy of the clay fraction within the soil
matrix remains a constant but ambient heat and precipitation levels can be considered
variables.
2.
Subsidence in the shrinkable clay regions is invariably associated with hot dry
weather which triggers the chain of events leading to damage. Irrespective of whether
a tree is reduced or removed or not, full re-hydration of the soil will cause the cracks
to recover or close. This sequence of events suggests that the proximate cause of the
damage is climatic with clay mineralogy determining the magnitude of any damage.
Tree roots will merely optimise the potential for shrinkage already present within the
soil. There are exceptions of course such as the case of a young tree planted close to
an older building with increasing water demand upon a clay soil due to growth of its
crown.
A simple model illustrates the point. Figure 1 below shows three foundations bearing
at different depths in the soil. Research by Richards et al (1983) and more recently by
The Clay Research Group and others suggests a seasonal desiccation zone (SDZ) in
clay soils to about 1.0m depth where hot dry weather alone will lead to shrinkage of
certain clay soils irrespective of whether tree roots are present. The root zone of a tree
will vary with species and soil type but research has shown that roots are rarely
encountered below 3 to 3.5m depth because they cannot proliferate under anaerobic
(no oxygen) conditions or where the soil is waterlogged. The zone from 1.0m to 3.0m
could then be said to be the tree root desiccation zone (TRDZ). For foundation F1, it
is clear that removing a tree will not stop the effects of clay shrinkage subsidence
following a hot dry summer. In the case of foundation F2, the extra depth will provide
little resistance to shrinkage within in the root zone. Only foundation F3 is effectively
resistant to both root damage and seasonal damage.
Effective desiccation zones
F1
F2
F3
SDZ
TRDZ
GL
1.0m
3.5m
F1, F2 and F3 = Foundation depths
SDZ = Seasonal Desiccation Zone
TRDZ = Tree Root Desiccation Zone
FIGURE 1- Foundation depths and desiccation zones
3.
Mineralogy and Activity
Clay mineralogy varies with location and not all clay soils exhibit shrink/swell
potential. Montmorillonite for example is a clay mineral which produces large volume
changes in response to moisture whereas kaolinite produces negligible volume
change.
Skempton’s Activity value introduced in 1953 gives an indirect method of clay
mineral identification. The Activity value PI / C is the ratio of PI (plasticity index)
divided by C (percentage of 0.002mm clay particle size). The Activity value is specific
to the clay fraction and therefore a distillation of the Atterburg limit. A value of less
than 0.75 is classed as inactive indicating a clay mineral with little shrink/swell
potential. The contribution by tree roots to foundation damage in an inactive clay soil
is likely to be negligible and there might therefore be no case for removing the tree
irrespective of age. Other causes of damage would need to be reviewed and
eliminated if possible. Activity value is not related to moisture content of the soil
sample and is therefore less subjective or time dependant than an assessment of
desiccation. Where evidence of desiccation by the popular array of laboratory and
interpretative tests is inconclusive, then an assessment of the clay mineralogy of the
sample should prove more decisive.
U-line
PI
60
A-line
50
Montmorillonites
40
Illites
30
Kaolinites
20
Halioysites
10
0
LL
Chlorites
0
10
20
30
40
50
60
70
80
90
100
FIGURE 2 – Casagrande plasticity chart showing Atterburg limits for principal clay minerals (from
Holtz and Kovacs 1981) Illustrative only.
4.
Figure 2 shows the standard plasticity (Casagrande) chart in use today with the
principal clay groups superimposed. It can be seen that montmorillonite clay minerals
are present if the Atterburg limit of the clay sample is close to the U-line. The A-line
separates the illite from the kaolinite groups and other inactive clay minerals are
below. Most soils contain more than one mineral however hence the Atterburg limit
may not coincide with a specific shaded area on the chart. The chart demonstrates that
soils plotting below the A line are likely to contain stable clay minerals with no
significant shrinkage potential. Soils plotting between the A-line and the U-line
should be possible to further differentiate by reference to their Activity value. As is the
case with all site investigations however, these results should be interpreted in the
context of other site investigation information obtained.
There are obvious practical examples such as the old building / young tree scenario
where the presence of active clay minerals within the tree root desiccation zone would
confirm the geotechnical evidence in support of tree removal. There are many cases
however where evidence of inactive soils would indicate that the presence of a tree is
less of a dominant cause and that other causes of cracking need to be considered.
There is also the concept of tolerable minor or cosmetic cracking which is
commensurate with age or design of the building on shallow foundations and the
presence of inactive clays should enable the engineer to quantify any potential for
significant subsidence when considering cause.
Conclusions
Insurers should expect to receive the best specialist advice and recommendations from
their appointed Engineers, Arboriculturists, Loss Adjusters and Solicitors. They must
also balance their legal position in respect of their rights of recovery against their
commitment to their Policyholders and claims philosophy.
Increased use of interpretative data such as Activity value of a clay soil would assist
Engineers/Loss Adjusters when giving guidance on costs and duration of claims. In
terms of the recent LTOA Strategy and the quality of evidence this implies, this
approach will either help to justify such further costs and delays or if these costs may
be better directed to claim resolution. Early negotiation with Tree Officers to confirm
the Local Authority position on a case by base basis is also strongly recommended to
assist in reducing settlement times.
The Authors
Adrian Howorth is a chartered Civil and Structural Engineer with over 25 years
experience dealing with soils and subsidence claims in the London region. David
O’Sullivan is a qualified Surveyor and Geotechnical Engineer and has dealt with
subsidence claims in the London region for over 15 years. David O’Sullivan works at
the West London office of Ellis May Loss Adjusters, Adrian Howorth has recently
left the Company.
5.