Clay Desiccation Demystified - CILA/The Chartered Institute of Loss ...

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Clay Desiccation Demystified
When a homeowner discovers cracks in his property it is a cause for great concern.
Public knowledge of subsidence is now widespread and confirmation that what is
probably your most valuable asset is suffering subsidence damage results in considerable
anxiety.
Subsidence did not always provoke such emotion. Prior to 1971 UK household insurance
policies did not include this peril. This is not to say that buildings did not suffer from
ground movement and consequential fracturing but these seldom resulted in structural
instability and the risk of collapse. Fractures would generally open and close seasonally
and filling them in preparation for redecoration was regarded as a normal maintenance
task.
Insurers added subsidence cover to their domestic policies largely at the behest of
mortgage lenders, believing it exposed them to little additional risk. It was not until the
1975/6 drought that widespread damage resulted in the submission of a large number of
claims. Understandably insurers then became wary of offering cover to new
policyholders if their properties exhibited any indications of movement whatsoever, i.e. if
they perceived there could be a greater than normal risk.
Valuation surveyors, mindful of their exposure to professional indemnity claims, reported
all minor fractures regardless of whether they considered them to be serious. The
unintended consequence of this was that a house with any cracks was effectively
uninsurable and therefore unmarketable. To enable such properties to be sold without
suffering a diminution in market value (to reflect the cost of possible underpinning)
insurers came under pressure to ensure either the cause was removed (this generally
applied to trees whose roots were encroaching beneath the affected foundations) or the
foundations were improved by underpinning. Policyholders would often remove their
own trees but to get a third party’s tree removed to avoid underpinning, often requiring
the consent of the Local Planning Authority for protected trees or cooperation of the
Highways Department, proved very problematic.
If the third party’s tree remained and insurers agreed to meet the cost of underpinning,
adjusters were invariably required to seek recoveries from the owners whose trees they
considered responsible for the damage. Liability was rarely conceded and recoveries
were often based upon superficial technical considerations.
This was the situation that adjusters faced in the late 1970s and early 1980s when Richard
Driscoll published a paper titled “The influence of vegetation on the swelling and
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shrinking of clay soils in Britain”. This provided an objective measure for “significant
desiccation” and was utilised by adjusters to demonstrate the involvement of trees.
How Does Subsidence Differ From Heave And Soil Recovery?
Subsidence refers to downward movement. Heave and recovery are the opposite.
As a tree grows it will abstract moisture from the underlying subsoil and if that medium
is a highly shrinkable, and therefore low permeability, soil it can cause a ‘persistent
moisture deficiency’ to develop . That is, the soil does not fully re-hydrate during the
autumn and winter before it undergoes a further bout of clay shrinkage subsidence during
the next growing season.
The accumulative subsidence is locked in place until the tree is felled or dies. The
subsoil will then recover from its desiccated state and if the swelling forces generate
sufficient pressure they will lift a building founded upon that soil. Typically the swelling
pressures generated in London clay as that medium reverts to its equilibrium natural
moisture content will be sufficient to lift a two storey domestic building. Other clays do
not always possess the same ability to ‘bounce back’.
Generally this will result in soil recovery, with the damaged building being lifted back
towards its previous position. This reduces the distortion within the structure, often
sticking doors and windows again move freely and cracks close. With homeowners
becoming more conscious of subsidence problems it is unlikely that persistent moisture
deficits will develop to the extent that recovery will continue for an unacceptable period.
It is important to differentiate between heave and recovery. Heave lifts the affected
foundations to a higher level than where they were constructed. It can take many years
for rehydration to occur, the period principally being dependent upon the degree of
desiccation and the permeability of the clay. However, heave is only a risk when a
building has been erected above soil with a severe moisture deficit and the cause of that
deficit has been removed.
Clay Desiccation
In June 1983 Richard Driscoll’s paper, “The influence of vegetation on the swelling and
shrinking of clay soils in Britain”, was published in Geotechnique, a research journal for
geotechnical specialists. It offered “a tentative means of determining the likelihood of
significant desiccation by relating in-situ moisture contents to liquid limits”.
It is important to note that the objective of this paper was to help geotechnical engineers
decide if desiccation existed and thus ascertain whether costly foundations were required
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for new buildings erected on sites from which trees were to be removed or if
underpinning was required to stabilise buildings that had already suffered some heave
damage. Driscoll addressed whether significant desiccation existed at the time of
sampling and thus “it could be taken to have occurred (developed) when the soil suction
was sufficiently large that on its release a low-rise building could be lifted”. In other
words, the suction by tree roots overcomes the inherent internal negative pore pressure
within the clay and increases this suction beyond equilibrium levels. When the tree is
removed these increased suctions draw in any available water and the clay rehydrates and
swells to its original volume.
Driscoll’s tentative assessment of desiccation compared the actual moisture contents of
high plasticity clays with their liquid and plastic limits. He suggested that if the moisture
content exceeded 50% of its liquid limit there was no desiccation. At 50% the onset of
desiccation occurred and below 40% the soil sample had attained significant desiccation.
Significant desiccation could also be identified if the actual moisture content was less
than the plastic limit plus 2%, although he considered this was a less reliable indicator,
with increasing unreliability for soils with higher plastic limits.
Driscoll’s formula, as it is sometimes referred to, therefore provides a method (but not the
only method) of assessing desiccation. In situ shear vanes were available prior to 1983
and various methods of assessing soil suctions have been developed since Driscoll’s
paper was published in Geotechnique
Application of Driscoll’s “Formula”
At a time when subsidence was a new peril with limited understanding of the
mechanisms underlying it, pursuing recovery actions against tree owners was extremely
difficult. The tree owners vigorously resisted claims against them and only claims
supported by very good evidence were submitted.
Driscoll’s ‘formula’ was eagerly seized upon as providing the best available supporting
evidence of desiccation and hence clay shrinkage subsidence via root action. It appears
that the conclusion that ‘significant’ desiccation according to Driscoll enhanced a
recovery gave rise to the corollary for some that in the absence of such desiccation a
recovery could not be pursued, because the nearby tree(s) were not implicated. Although
Driscoll’s formula actually indicates a very high level of desiccation, with the passing of
time some practitioners now mistakenly believe it represents the minimum level of proof
necessary.
Regrettably it is still misquoted and misreported in some ‘expert’ reports when
considering whether or not a tree has caused subsidence damage. The fact that the
formula was only offered for high plasticity clays is frequently overlooked and it is often
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erroneously applied to low and intermediate plasticity clay samples. In the BRE Trust
guide Subsidence Damage to Domestic Buildings: A guide to good technical practice
published by IHS BRE Press in 2007, Richard Driscoll and Hilary Skinner warn about the
indiscriminate use of 0.4w L. and the publishers have kindly authorised us to include a
relevant extract at the end of this paper.
What is Desiccation?
Clay soil comprises clay mineral particles that are molecularly bonded with water to form
a cohesive material. London clay, by way of example, typically contains 50% water by
volume.
A clay subsoil can be considered to behave similarly to jelly. The overall volume of the
subsoil (or jelly) is largely dependant upon the amount of water that is bonded at any
given time to the clay minerals (or gelatine). The soil (or jelly) will shrink if water is
removed and then subsequently expand if water is reabsorbed. This expansion ceases
when the clay achieves its former or a new state of equilibrium, i.e. when there is no
longer a negative pore pressure to draw additional moisture into the soil. It is important
to appreciate that the solid particles do not shrink. Volume change is totally dependant
upon variation in moisture content.
For a clay subsoil the natural (or equilibrium) moisture content is dependant upon its
geological history. If any water is withdrawn the clay by definition becomes desiccated –
that is drier than in its natural state. It follows that a reduction in moisture content will be
accompanied by clay shrinkage and the soil does not have to attain a state of ‘significant
desiccation’ before clay shrinkage commences. Fig. 2 in BRE Digest 240, 1993,
illustrates this.
Although in every day usage desiccation generally refers to the complete removal of
moisture, e.g. desiccated coconut, when used to describe a clay soil it means the subsoil
has attained a lower moisture content than the soil’s natural/equilibrium moisture content
(Institution of Structural Engineers, Subsidence of low rise Buildings 2000 and BRE
Digest 412, 1996). Damage will usually be very noticeable in a domestic property that
has suffered differential foundation movement where the clay has attained ‘significant
desiccation’ but the onset of property damage will be dependant upon the type of
construction and thus varies from property to property. Sometimes a 1% moisture
reduction (or less) in a clay subsoil will induce sufficient differential foundation
movement to cause damage. It is not always possible for a laboratory to identify such
small reductions and in some instances the soil may have recovered from its earlier
deficiency prior to sampling.
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The apparent lack of desiccation by reference to Driscoll’s formula does not dismiss the
possibility of clay shrinkage subsidence damage. BRE Digest 412 states: ‘Clearly, w <
0.4w L should be used only as a rough guide and it is unwise to base an assessment of
desiccation solely on this criterion, particularly if desiccation is slight.’ The evidence
should be considered in the round and Driscoll’s formula should not be regarded as ‘the
Holy Grail!’
Conclusions
This article contends that desiccation and the associated clay shrinkage process occurs
due to any reduction in the clay’s natural moisture content and is not restricted nor
limited to the predefined limits of Driscoll’s ‘formula’ . Shear vane results or soil
suctions provide indications of desiccation but failure to satisfy such criteria at the time
of testing does not preclude the possibility (in many cases the probability) of clay
shrinkage subsidence.
Remote boreholes may provide some assistance in identifying the pre-damage moisture
content but in practice these are often of limited value due to variations in plasticity
indices across the site and the moisture deficiency being too small to identify by
comparison. In addition, it is often difficult to site a sufficiently close remote borehole
that is completely unaffected by vegetation.
Key factors in identifying clay shrinkage subsidence damage are distortion, the associated
pattern of damage in the building and the season in which this occured. Such factors
should be considered in conjunction with the subsoil (shrinkable or not), roots (present or
not) and any information provided by monitoring. Significant closing of fractures and
increases in level throughout the winter are uniquely peculiar to clay re-hydration
following earlier desiccation but the absence of such movement ‘after the event’ does not
dismiss the possibility of clay shrinkage subsidence.
Desiccation is a process rather than a state with only empirically defined parameters as to
when it becomes significant. It is over simplistic to suggest a clay with a moisture
content of 39% of its liquid limit has the ability to cause damage whereas moisture losses
between 50% and 40% have no effect. If a reduction in moisture content results in a
degree of clay shrinkage, as confirmed by BRE Digest 240, then any moisture loss has
the potential to cause subsidence damage.
At its simplest, subsidence damage occurs when a building is “bent” by differential
foundation movement and the stresses generated during that process exceed the
building’s ability to withstand them. Masonry is very good in compression but cracks
under tension. Provided the crack damage indicates subsidence damage, the fundamental
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question is ‘What, on the balance of probabilities, is the cause of the differential
foundation movement beneath that part of the building at this time’?
A tick box mentality is no substitute for expert appraisal.
Graham Rex, BSc(Hons), MRICS, FCILA, ACII, MCIArb, Cunningham Lindsey
(e-mail:- graham.rex@cl-uk.com Tel 01206 754229)
Richard Thomas, BSc(Hons), C.Eng, MICE, ACILA, RTG Expert Services
(e-mail:- rthomas@rtgexpertservices.co.uk Tel 01245 402196)
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Copyright BRE, reproduced by permission.
Acknowledgement
Following the initial publication of this paper we have incorporated some helpful
comments from Richard Driscoll and he now endorses remarks in the paper concerning
the use of 0.4 x wL. We thank him for his assistance and IHS BRE Press for permission
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to append the following extract from Subsidence Damage to Domestic Buildings: A guide
to good technical practice published in June 2007, reference FB13, and available from
www.ihsbrepress.com.
Using Moisture Content and Soil Plasticity Measurements
Moisture content may be reliably measured in disturbed soil samples taken from trial pits
(BRE Digest 381) or boreholes; samples must be carefully sealed prior to transporting
and testing in the laboratory or the results can lead to pessimistic assessments of
desiccation. This potential error and the difficulty of determining in a single profile what
is the normal moisture content are reduced by comparing moisture content values in
samples taken at the suspect location and at a control location remote from it and the
suspect tree; however, such control measurements are often difficult to make owing to
the restrictions of many urban sites. There is often reluctance to pay for this more reliable
comparison.
An objection to the use of moisture content profiles to assess desiccation is that, in highly
plastic clays such as London Clay, large changes in soil moisture suction occur with only
relatively small changes in soil moisture content, making desiccation detection unreliable
when moisture content deficits are small, even when the deficit is detectable by reference
to a control profile. Some people claim to be able to detect desiccation from a difference
between the measured moisture content and a 'natural' value for saturated clay. These
claims should be treated with caution as the 'natural' moisture content, even for a well
known soil such as London Clay, can vary quite appreciably from place-to-place,
depending on a variety of factors including plasticity and density.
The difficulties and costs of comparative measurement have led to attempts to identify
desiccation by comparing sample moisture content with some arbitrary value that is
deemed to indicate desiccation. Such an approach, based on shrinkage curves for a
number of UK overconsolidated clays (ref to Driscoll, 1983), suggested a relationship
between moisture content, w, and Liquid Limit, wL, at different suction values. The
relationship suggested that the onset of desiccation could be defined as a suction of 10
kPa at which pressure many of the clay samples tested exhibited a moisture content of w
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= 0.5 wL. The relationship further suggested that significant desiccation (defined as a
suction of 100 kPa) occurred where w = 0.4 wL.
Largely because of its simplicity, this method is often given unwarranted authority when
assessing suction and establishing tree causation; its indiscriminate use can lead to the
mistaken belief that desiccation is present ( ‘mistaken desiccation’). Particularly, the
method takes no account of the stress regime of the soil before it was sampled, and can
also give erroneous results in other than high plasticity soil and where the plasticity
varies locally. Consequently, it is now generally considered that this approach should not
be primarily relied upon to indicate desiccation. However, it can provide additional
confirmation of desiccation when comparing moisture content profiles at the suspect site
and at the control site.
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