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Sharply increasing damage and an unsuitable
prevention and compensation regime
Communication to the National Assembly public policy
evaluation and audit committee
February 2022
Executive summary
The shrinkage-swelling of clay soils, or the shrink-swell phenomenon, consists of a series
of movements caused by variations in the water content of clay soils; in France, its harmful
effects on housing have been known and documented for more than 30 years. Compensation
for the damage it causes has been included since 1989 in the natural disaster regime, known
as “Cat Nat”. However, the phenomenon has attracted recent attention from the public
authorities due, on the one hand, to the growing magnitude of the risk and the subsequent
claims and, on the other, to the increase in dissatisfaction expressed by the exposed
Referred to by the National Assembly Public Policy Evaluation and Audit Committee
concerning the prevention and compensation of damage caused by the shrink-swell
phenomenon, the Court conducted an investigation taking into account the analyses,
observations and recommendations from past studies and from recent parliamentary and
administrative work. These included, in particular, the report by Ms Bonnefoy, senator for
Charente, from 2019
, the work carried out during the drafting of the Act aimed at reforming
the natural disaster regime
(Cat Nat), and the report submitted to the Government in March
2021 by the General Council for the Environment and Sustainable Development (CGEDD), the
General Inspectorate of Finance (IGF) and the General Administration Inspectorate (IGA),
dedicated to the management of damage related to the shrinkage-swelling of clay for existing
Following this investigation, the Court is presenting three main findings.
First of all, the phenomenon and its causes have been identified for many decades. It is
the result of the combination of a geotechnical situation and a series of climatic episodes: on
certain soils, containing clay, a series of wetting periods and drying periods can cause damage
to buildings. The Court notes that the effective implementation of preventive measures was
delayed and remains insufficient today
Knowledge of the phenomenon, although long-
standing, was not accompanied by any effective prevention policy
until the entry into
force of Act No. 2018-1021 of 23 November 2018 on changes in housing, land management
and digital technology, known as the ÉLAN Act, which established construction rules for high-
risk areas.
Secondly, the Court analysed, taking into account the reasoning and recommendations
formulated by the CGEDD, the IGF and the IGA last March, the strengths and weaknesses of
the current compensation regime. Integrated into the natural disaster regime, the current
compensation rules would be sustainable, no doubt at the cost of some administrative
adaptations, if the damage caused by the shrinkage-swelling of clay soils was, in the medium
term, constant in terms of its frequency, severity and financial impact. However, the extent of
the effects of climate change, taken as a whole, and the various forecasts and projections
made, in particular by the french public reinsurance group (CCR) and the French Insurance
, show that this is not the case. Indeed,
since 1989, the shrinkage-swelling of
clay soils has accounted for 36% of the claims observed for natural disasters
and half
of the most expensive events recorded over the same period. Originally limited to certain
, it now affects all areas of metropolitan France and more than half of the stock
of single-family homes
All of the studies undertaken recognise that the costs associated
with the shrink-swell phenomenon will increase sharply over the next 30 years.
Information report no. 628 (2018-2019) by Ms Nicole Bonnefoy, on behalf of the information mission
on climate risk management, submitted on 3 July 2019.
Act No. 2021-1837 of 28 December 2021 on compensation for natural disasters.
The french Insurance Federation has taken the name of
France Assureurs
since 5 January 2022.
Thirdly, the damage compensation mechanism, based on the natural disaster regime, is
currently reaching its limits: it is now unfair and unsuitable and should lead the public
authorities to question this phenomenon's very classification as a natural disaster.
The spread of the phenomenon across metropolitan France, its intensity and its
social and financial consequences require that the responses provided by the public
authorities be adapted.
The Court is therefore making recommendations aimed at
strengthening the
of disasters, not only through the rigorous application of building requirements but
also by intensifying research and development with regard to remedial measures likely to
prevent damage to existing buildings and ultimately facilitate financially sustainable
However, it has chosen not to draw up recommendations aimed at changing the current
compensation system; rather, it has decided to propose guidelines underlying policy choices,
taking note of the recent promulgation of the Act of 28 December 2021 on compensation for
natural disasters and the adoption of the Act on differentiation, decentralisation and
deconcentration introducing various measures for the simplification of local public action, which
pave the way for a reform of the compensation regime.
It invites the State to reconsider whether the phenomenon should be classified as a
natural disaster. If it is considered that the shrinkage-swelling of clay soils is not a natural
disaster characterised by its unforeseeable and irresistible nature
, removal from the regime
may be possible. If compensation continues to fall under the natural disaster regime, some
major changes should be considered, once they have been anticipated and evaluated through
meticulous impact studies.
The shrink-swell phenomenon: some key data
We usually speak, indistinctly, of “shrink
swell risk” and “drought risk”. The shrinkage
swelling of clay soils is not caused by drought but rather by a series of episodes of drought
and re-wetting. Rainfall deficits contribute to the phenomenon, but they are not the sole
cause. Moreover, the phenomenon should be distinguished from agricultural droughts,
which are covered by different recognition and compensation procedures.
Drought risk in the context of the shrink-swell phenomenon accounted for 36% of Cat
Nat claims
over the 1989-2019
. It was the second largest item behind flooding.
Of the 20 most costly events over the 1989-2019 period, 11 were linked to episodes
of drought, including that of 2003 which cost €1.9 billion, that of 1990 which cost €1 billion
and those of 2017 and 2018 estimated at around €800 million.
The average cost of a clay shrinkage-
swelling claim is estimated at €16,300
this kind of “damage” guarantee appears as the most expensive.
The phenomenon affects all of France (all regions have been the subject of Cat Nat
drought recognition at least once), but some regions have historically been more affected
than others: Ile-de-France, Occitanie, PACA, and Nouvelle-Aquitaine.
Over the past nine years, an average of 50% of municipal requests for Cat Nat
recognition in relation to the shrink-swell phenomenon have been unsuccessful.
According to the definition in the Act of 13 July 1982, a natural disaster is characterised by the abnormal
intensity of a natural agent when the usual measures to be taken to prevent this damage have not kept
it from occurring or could not be taken.
Audit recommendations
Set up a control and sanction system for measures resulting from the Act on changes in
housing, land management and digital technology (ÉLAN), for new buildings in areas
exposed to the shrink-swell phenomenon (
Directorate of housing, urban planning and
Add the shrink-swell phenomenon to the natural and technological risk report (ERNT),
made accessible to any potential buyer or tenant of a property as provided for by Article
236 of the Act of 22 August 2021 on fighting climate change and building resilience to its
effects (
Accelerate research and development projects in order to have, with a view to experiments
in 2025, a set of remedial measures applicable to buildings erected before 2020 that are
exposed to clay shrinkage-swelling, while ensuring their best cost-effectiveness ratio
DHUP, Cerema, Geological research mining bureau [BRGM]