PIANC E-Magazine - PIANC AIPCN welcome

6. COMMENCEMENT OF
CONSTRUCTION
Construction commenced on March 4, 2011. The
contract period was six months. The first step of
the works was to demolish the 30 m portion of
concrete roadway (Ch 66 m to Ch 96 m). However,
upon demolition of the concrete roadway it
was discovered that although the remainder of
the existing core under the demolished concrete
roadway consisted of 0-500 kg material, this material
had an incorrect grading. The grading was
skewered to the left to such an extent that the material
can basically be classified as consisting of
fine quarry run with some isolated larger rock in
between ranging up to 500 kg in size. The grading
did not comply to filter rules. As to be expected
over time all these fines washed out resulting in
the massive cavity under the concrete roadway.
This in turn caused the deck to collapse. This
wash out of core material is referred to as Failure
Mechanism 2. This is illustrated clearly Figure VI-
2-40 in the Coastal Engineering Manual.
Designing and/or constructing breakwaters with
sand cores only and no geomembrane or filter
layers is risky. Breakwaters with a sand core are
classified as having either geometrically open filter
systems or geometrically closed filter systems.
With geometrically open systems the core is only
considered sand tight if the rock layer overlaying
the sand core is of ample thickness to prevent the
mitigation of fines through the material. Currently
limited research had been conducted on geometrically
open filter systems. In this case, however,
sand washed out over time. This breakwater
therefore consists of a geometrically open filter
system being of the non-sandtight type.
After removal of concrete deck:
Test hole at 1 m deep
After removal of concrete deck:
Test hole at 1.5 m deep
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PIANC E-Magazine n° 144, November/novembre 2011