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difficult due to unavoidable sample disturbance and this leads to a rather
conservative attitude: too many dikes may get the qualification unsafe. Not only in
the Netherlands is this problem prevalent. In China, for more than 50% of the flood
disasters along the Yangtze River, piping is the culprit. The many sand boils along
the Mississippi also show the relevance of piping. During recent storms, large
problems by piping occurred in California and New Orleans.
The demand for fundamental insight in the piping process and in the liquefaction
phenomenon is urgent. A suitable prediction model for piping and/or liquefaction
in loose sands is clearly needed and sound investigation, including time effects,
may prove that temporary higher water levels are not that dangerous. A major issue
is the localisation of weak (geological) spots in the dike rings and a sound method
for the determination of the local density. A promising innovation seems using
biogrout (bacteria that cement the sand) in order to fix loose sand preventing
liquefaction and piping or to avoid sample disturbance during in-situ testing the
density. The ultimate goal is a thorough knowledge of the phenomena and practical
and adequate prediction tools, by which the safety of flood defence systems can be
guaranteed without unnecessary over-sizing.
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Figure 17.11 Four inside-dike enforcement techniques
Inside
Four innovative dike redesign methods have been evaluated by different
consortia: (1) short sheet dowel, (2) mixed in place, (3) nailing, and (4) pin
doweling. The challenge is to strengthen existing dikes without occupying more
space, because in many situations the existing buildings and infrastructure do not
allow expanding the dike geometry. Each of the consortia consisted of participants
from contractors, consulting firms, institutes, universities and governmental
authorities. The different techniques have been investigated by physical tests,
numerical predictions and field-testing. Environmental aspects, sustainability and
economics have been an important part of the studies as well. For these methods a
pilot test has been performed focusing on construction, quality and monitoring. The
methods are judged to be sufficiently mature to be applied in practice.
Flood control of urban areas
As shown in Fig 17.1 most of the world's large cities are positioned in delta
areas, and their size and number is expected to increase in the near future. The
vulnerability of these areas to flooding and the complexity of flood prevention and
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