Delta Life Nr. 3 February 2015

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18
‘WE ARE GETTING
BETTER AT
PREVENTING
DAMAGE FROM
VIBRATIONS AND
SUBSIDENCE’
Mandy Korff has been working for more than
ten years now on the risks associated with
large projects that involve moving large
amounts of earth, such as the construction of
a newmetro line beneath the centre of Am-
sterdam, and a range of underground car parks. Mandy also
looked at the impact on foundations, houses and dikes of
drilling for gas in the Dutch province of Groningen. She has
built up a sizeable practical database that she can draw on
to advise which techniques are suitable, and which aren’t.
How similar are the cases of damage caused
by vibration and subsidence that you have
studied?
‘All human activities have an effect on the subsurface. As
a result of vibration and subsidence, there are knock-on
effects. In areas with soft and weak soils, such as clay or
peat, building or tunnelling can cause vibrations and sub-
sidence. That wouldn’t be a particular problem if we weren’t
working closer and closer to existing buildings.’
Won’t we just have to accept these conse-
quences?
‘No, because a sound risk analysis in advance allows us to
make quite a good assessment of the possible impact. For
example, if necessary, we can use alternative construction
methods that generate fewer vibrations, and less deforma-
tion and subsidence. Advanced measurement systems can
help us to control and adapt the construction work.’
Why isn’t that the case already?
‘There is usually a risk assessment, but that often doesn’t
‘We build and tunnel closer and
closer to existing buildings, and
that can sometimes cause damage
because of vibrations or subsidence.
We have more and more ways of
tackling this problem,’ reports senior
specialist Mandy Korff. ‘We can’t
prevent all damage, but we can do a
great deal.’BY KARIN STONE / PHOTO SUZANNE DORRESTEIN
DELTARES, FEBRUARY 2015 19
cover the entire construction process. For example, people
forget to extend the risk assessment to activities that are
conducted early in the process. If the damage occurs in
the early stages, a project can get a bad name even before
it has really got started. Limitations in risk management
are often linked to cost considerations. Alternatively,
project developers may be concerned that they may have
to switch to a more expensive building method. But my
experience is that investments of this kind always turn
out to be cost-beneficial later on. If it ain’t broke, you don’t
need to fix it.’
And what about the impact of earthquakes
caused by gas extraction?
‘Predicting the consequences is a complex business, but
we are getting increasingly better at identifying the areas
at risk. Limiting or even preventing damage still represents
a major challenge. In Groningen in the Netherlands, for
example, people are thinking about injecting water into
the subsurface. Elsewhere in the world, there are studies
looking at ways of counteracting vibrations. We still don’t
know whether those methods are effective, or in what
conditions.’
There is a lot of talk about damage to homes,
but shouldn’t we be just as concerned about
damage to important infrastructure like
bridges and dikes?
‘When there are earthquakes, people are mainly worried
about their homes, which is of course entirely under-
standable. But there is also an impact on flood defences
and people still don’t realise that enough. The agencies
responsible for the dikes are, fortunately, well aware of the
risks. That is why Deltares has teamed up with the Dutch
water authorities to work on this problem, for example by
upgrading dikes that are susceptible to earthquakes. That
may involve widening them, installing a sheet pile wall or
strengthening the soil.’
Does your expertise help to reassure local
residents?
‘I am often called in by agencies who want to reassure
people, or address their concerns. These are interesting
processes. The most fascinating part for me is attending
information evenings for local residents. I often hear quite
weird claims. Local residents are, due to the Internet,
often very well informed, almost to an expert level at
times. Even so, they sometimes have theories that simply
don’t match the technical and scientific realities. As an
expert, I can answer their questions and put them into the
right context so that they focus on the right things. I can’t
always reassure people, but I can make sure that their
concerns are realistic. Honesty like that is always very
much appreciated.’
More information?
WHAT
WENT
WRONG?
The collapse of the historical
weavers’ houses on the Vijzel-
gracht in Amsterdam was a
low point, and also a turning
point, in the construction of
the North-South Metro Line.
A lot has happened since then:
extra precautions have been
taken to mitigate the risks,
and the communications
culture has been transformed.
Mandy Korff discovered that
old buildings with wooden
piles are particularly suscep-
tible to disturbances in the
subsurface. These buildings
were severely affected by
the moving of pipes and
the removal of obstacles
but - fortunately - less by the
digging work for the stations.
The newer the foundations,
the less the piles settled.
The construction work on the
North-South Line has now
been completed. The lessons
learnt will be used to manage
the risks better when building
new parking garages and
tunnels.