Delta Life Nr. 3 February 2015

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12 DELTARES, FEBRUARY 2015 13
DOSSIER
LAND SUBSIDENCE
the consequences are so far-reaching, and
definitive, if nothing is done.
There is a problem in that the solutions are
expensive and you only see an effect after at
least ten years. By that time, someone else
is often in charge. Effective action requires
vision, conviction and the courage to look
ahead. So you want to be sure that the
problems are actually being caused by land
subsidence. That requires sound, thorough,
scientific research.’
Let us suppose we are convinced,
and that we want to get stuck in
straight away. What should we
think about doing?
‘There is no straightforward solution; a tailored
approach is always needed based on how the
area is used, the area itself and the impact. We
calculate the costs of land subsidence if policies
remain unchanged. They are often quite a bit
higher than people expect. Then we offset them
against the costs of structural interventions,
giving users of an area a basis for making
decisions. Broadly speaking, there are two
types of measure: reducing land subsidence or
adapting to the situation. The first approach
may involve thinking about using light filler
materials, such as pumice stone, or diverting
heavy traffic away from roads without
foundations. Another important way of
counteracting land subsidence is raising the
water table. But that is a far-reaching process
in areas with agriculture and housing. Sound
teamwork and smart water-level management
will be needed.
Adaptive strategies may involve raising houses
and roads, having cables and pipelines on the
surface, or strengthening the soil, possibly with
geotextiles. Alternatively – and this is a very
drastic approach indeed – you can decide to
stop building in a particular area.’
That sounds like quite an action
list. It’s a lot to ask.
‘Yes, it is. But we can’t just ignore it. For some
time, we have acted as though we can keep
on using peat areas without any problems
if we just pump hard enough. But the harder
we pump, the faster we fall. We really have to
break out of that vicious circle.’
More information?
Soft soil is mainly a prob-
lem because it is precisely
deltas that are home to a
lot of economic activity. In
the Netherlands, at least
6 of the 17 million inhabi-
tants live on soft soil.
Laboratory tests deter-
mine how weak soil is. But
you will feel it immediately
when you walk on soft soil.
Peat is always soft, and so
are certain types of clay.
6 17
Po valley
Kalimantan
Sumatra
East Anglia UK
Sacramento Delta Mississippi Delta
Everglades Florida
EXAMPLES OF LAND SUBSIDENCE IN PEAT AREAS
In Kockengen, old, thick layers of asphalt are being removed from below the road and replaced by light materials such as
pumice stone. Differences in height have been created near houses that apparently rest on sound foundations. That is clear to
see in the photo on page 13. The light row of bricks shows how the ground has subsided. Many homeowners build a few steps.
North-German coastal plain
The west and north
of the Netherlands
kg
PHOTO’S: GUUS SCHOONEWILLE
P HOTO: GUUS SCHOONEWILLE