DIKE ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 314

discharge to be tested. At irregular time intervals, the valve is opened, releasing a
water volume over the inner slope in such a way that a realistic storm is simulated.
The drop height of the water volume is calibrated to generate the correct flow
velocity at the dike crest for the large overtopping volumes.
(a) wave-overtopping simulator (van der Meer) (b) infiltration tests
Figure 17.9
Twenty-five wave-overtopping tests were performed on actual dikes at 8
locations in the Netherlands and Belgium, in winter when the grass is relatively
weak and the probability of storm conditions is high. The tests show a strong
erosion resistance of the grass cover. Most covers did not fail until an average
overtopping discharge beyond 50 l/sm. Hence, small overtopping volumes (as in
standard practice today) will cause no significant damage. If there are no mole and
rat holes, only the largest overtopping volumes produce damage.
Parallel to the research in the erosion resistance, sliding of the cover layer was
investigated. The mechanism is governed by the shear strength of the cover layer,
the fixation to the dike core, and the accumulation of induced pore pressures in and
underneath the cover layer. Field infiltration tests (Fig 17.9b) were performed on
the same locations where the overtopping tests were conducted. Based on the test
results, a simple model was created for the assessment of the pore pressure build-
up taking into account the infiltration capacity of cover layers and the overtopping
characteristics.
With the tendency of increasing hydraulic loads to be considered in dike design
due to (anticipation of) expected climate change and the desire for higher safety
levels without more expenses, the acceptance of more overtopping becomes
opportune. In some cases, the acceptance of a lower crest and more overtopping
can be a real option, particularly if space to build or extend the dike is limited.
Peat dikes
In the summer of 2003 a peat embankment suddenly slid away with a loud snap,
after months of severe drought. The water of the canal inundated a village. The
responsible water board took immediate action, dammed the canal and evacuated
part of the population. No one drowned, but the damage was later put at 60 million
euro. During that period some 15 other similar embankments were also found to be
in danger, showing local collapse, cracks and leakage. Even after a century of soil
mechanics in the Netherlands, particularly on dike technology, this was unforeseen.