a smooth, non-compacted granular layer
10-15cm thick. They are lifted into place
by an excavator using a pneumatic clamp
attachment that picks up a 1m square
group by clamping them at the sides, and
places them as group against kerbing or
another row of Hillblocks. Once the top
row is concreted into position, pebbles
are sprinkled over the top, wedging them
together. Still, they are free to settle; and
that’s an important feature for dyke safety,
Mughal argues. If they were concreted in
place, they would be stuck rigid, unable to
sink into the telltale dip that might mean
potentially serious subsidence or erosion.
However, the frictional force that keeps
Hillblocks together still exceeded the
destructive abilities of the maximum 5m
simulated wave height at the Deltares
Delta Flume during the last trial in 2016.
As a result, testers added a safety factor
to the design, but the makers remain
unsure of the exact strength of the system.
(However, in case of breakage, Hillblock
and its manufacturing partner have devised
a removal method that involves a low-tech
method of prising up individual blocks
using a crowbar). Flood defence materials
suppliers such as Hillblock must have their
revetment blocks tested at this facility to
win approval for national deployment on
the defence network. Hillblock’s rst trial
was in 2012, and won nationwide approval
after that.
HIGH UP
The block is generally intended for the
higher part of the dyke, the wave run-up
area. Test evidence and operational
experience has shown that the fast water
ow through the blocks tends to clear out
any otsam and jetsam. At sea level and
below, an alternative design, Ecoblock, can
be installed that has a rougher concrete
nish, facilitating attachment of seaweed
and marine life such as barnacles. Those
crevices are not cleaned out by the action
of the waves, but instead can foster
marine life.
Dutch concrete parts manufacturer
CIVIL ENGINEERING – FLOOD DEFENCES
Martens Beton exclusively makes the
Hillblock, from a mix said to be very similar
to that used for paving stones. A standard
whole 35cm-high by 25cm-diameter-wide
block weighs 32kg, although ferrous
additives can be included optionally to
increase density from 2,300kg/m to
2,900kg/m, which can help reduce the
size of Hillblock installed.
The rst prototype models were wetcast
into plastic moulds. But the delays
caused by drying time were impractical,
so production switched to a different dry
cast mix whose consistency was more like
wet sand, and which is pressed into the
moulds. That enabled blocks to be formed
in about 20 seconds, although they
still require a 28-day cure to
reach maximum hardness.
The blocks are split in two
to simplify the casting
operation, but the makers
are currently researching a
single-piece casting method
that is slower than the current
one, but still expected to improve
manufacturing ef ciencies.
Hillblocks have been designed in
heights ranging from 20cm to 70cm.
The Somerset installation was the
family-owned company’s rst export order,
and was aided by a Dutch government
grant called ‘Partners for Water’.
But even as Hillblock gears up for
foreign expansion, not only in the UK
but also in Germany and France, it has
changed its home market, according to
Mughal. “A revetment stone is a short
column of concrete. It’s actually quite
simple. For a long time, there was really
no evolution in the shape until Hillblock.
Nowadays, you can see competing
companies are also trying to evolve their
products to nd better ways of making
these revetment blocks and see how
they can increase wave reduction. In that
sense, the introduction of Hillblock was a
stimulus for other producers of revetment
blocks to nd innovations and ways of
making better ood defences.”
BRISTOL CHANNEL PROTECTION
The UK Environment Agency has been
maintaining two old ood defences on a fourmile
stretch of the Bristol Channel coastline
near Stolford, running between rock armour
at Hinckley Point nuclear power plant and the
Steart Marshes some four miles away. The rst
is a 7m-high ridge of shingle over a clay core.
Some 150-200m behind that lies a 9m-high
clay embankment built in 1977. They protect
some 20 homes, a site of special scienti c
interest and agricultural land. High waves from
storms tend to thin the shingle ridge, requiring
annual – and sometimes biennial – restoration
work involving a bucket excavator to raise
15t of material to restore the ridge’s
trapezoidal pro le. The average
maintenance bill has totalled
£40,000 in the last few
years.
As it happens, the longtime
framework contractor
for this part of Somerset
is a joint venture Team Van
Oord (consisting of Kier, Mackley
and Dutch companies Van Oord and
engineering consultancy Royal Haskoning DHV).
When the Environment Agency decided to hold
a value engineering workshop to devise a longterm
ood defence plan for the area, Hillblock
got involved.
In the end, the Agency’s £1.4m budget
wouldn’t stretch to the traditional rock
armour solution, so it decided to focus on
protecting the earth barrier. On advice from
Royal Haskoning DHV, contractors changed
its slope from 1:4 to 1:7.25, and laid 200m
of Hillblock on a key section of the barrier
where the risk of overtopping was seen to be
the most signi cant. “In terms of performance,
it’s a passive scheme; we don’t need to do
anything. That’s what we want,” says Meritxell
Fernandez-Garcia, senior user and technical
advisor responsible for South Somerset asset
management.
Meanwhile, the Environment Agency has
stopped shoring up the shingle ridge, which the
sea will breach at some point, creating a new
salt marshland on the channel.
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