The Scrader concept - Dredgepoint

Capabilities of the Stemat Scradeway ® multi-purpose pontoon:
®
The Scrader concept
• Placement of a variety of granular bulk materials, such as
stones up to a D max ≈ 300 mm, for installation of rubble
foundations and scour protection works;
• Backfilling of trenches with sand after the placement of units
such as tunnel elements. In these operations, a special sand
chute is mounted at the bottom of the fall pipe;
As foundation base for structures, such a construction offers
several benefits in comparison with a closed integral gravel plane:
• Berm construction with
intermediate grooves avoids
the risk of high spots at
overlapping tracks of gravel;
Pattern
of Gravel
1.65 m 1.00 m
Founded BY STEMAT/BOSKALIS V.O.F.
on underwater precision
• Placement of armour stone on top of elements for protection;
• Cleaning of undesired silt from trenches by applying a
mountable suction unit, in combination with an underwater
pump, fitted to the lower end of the fall pipe;
• Survey of constructed works using a high accuracy survey
system mounted at the bottom of the fallpipe.
Special application
The capability to construct a rubble stone structure of an
intermittent pattern of identical berms, alternated by grooves in
stead of a closed plane, opens up a special application of the
Scrader concept. The interval, height and angle of the berms can
be varied depending on the specific design applications.
• Possible high spots of gravel
can be moved sideways into
the grooves;
• At the joint of the elements or
caissons, the distance between
the berm can be increased,
minimising the risk of aggregate
particles being trapped
between the elements;
• Water and siltation are more
easily dissipated through the
grooves when lowering the
element onto the foundation.
Slope of
the Trench
42 m
56 m
Multi
Purpose
Pontoon
Laser Receiver
Laser Transmitter
Measuring Tower
Spud
Fall Pipe
Inclination
Measurement
Scrade
Foot
Hydraulic
Cylinders
Gravel Berms
Tunnel Element
Stemat/Boskalis v.o.f.
www.stemat.com
Stemat bv
Quarantaineweg 10
P.O. Box 54511
3008 KA Rotterdam
The Netherlands
T +31 (0)10 299 655 5
F + 31 (0)10 299 654 3
E info@stemat.com
Boskalis International bv
Rosmolenweg 20
P.O. Box 43
3350 AA Papendrecht
The Netherlands
T +31 (0)78 69 69 011
F +31 (0)78 69 69 555
E royal@boskalis.nl
Stemat
Boskalis

Granular bulk material is fed into the fall pipe by a conveyor belt.
The Scrader concept proved
extremely valuable in the
construction of the second
Benelux tunnel across the
port entrance of Rotterdam,
for which the foundation
accuracy requirements were
very stringent.
immersed tunnels, and can be used with a wide variety
of granular bulk materials. It has been successfully
employed in major projects.
With its telescopic fall pipe, the Stemat Scradeway can
operate in water depths of up to 28 metres, laying gravel
foundations with a tolerance of only 25 mm.
Scrading allows for incredible accuracy. In a water depth
of up to 28 metres, the tolerance in laying a gravel
foundation can be less than 25 mm (!). The screadhead is
fitted with five multi-transducer echosounders for
continuous survey of the newly 'scraded' layer.
Combining their know-how and equipment in a joint venture, Stemat and Boskalis have set a new
world standard in seabed treatment for underwater construction: the Scrader ® concept.
Using the specially developed multi-purpose pontoon Stemat Scradeway, the company is capable of
constructing gravel beds with extremely high accuracy in waters up to 28 metres deep.
®
The Scrader concept:
Founded on underwater precision
The precision performance of the Scrader method allows
for ballasting and placing tunnel elements right onto a final
foundation of gravel, saving time and minimising
construction risks. Besides tunnel projects, the technique
can also be used for other underwater construction works,
such as foundations for bridges, caissons, windmills and
offshore outfalls.
A common technique for building underwater foundations
is known as 'grading', a two step operation of depositing
and subsequent levelling of a gravel bed. Scrading
combines these two operations: the Stemat Scradeway is
equipped with a telescopic fall pipe to deposit and level
the material at the same time. The fall pipe is positioned at
the side of the multi-purpose pontoon and can be moved
47 metres, horizontally across the trench. A conveyor belt
on the pontoon feeds the fall pipe with scrade material.
As the fall pipe moves across the trench, it is kept at the
required level by means of a laserguided hydraulic
'screadhead' and remains in constant contact with the
gravel berm. With the vertical flow of the material confined
within the fall pipe, segregation is virtually eliminated, while
substantially less gravel is used.
The Scrader method is ideal for construction of underwater
foundations for projects such as caisson walls and
Construction of gravel bed foundation for breakwater in
Cartagena, Spain
Reference list executed projects:
Gravel bed for Øresund tunnel
Copenhagen, Denmark
Gravel bed for second Benelux tunnel
Rotterdam, The Netherlands
The Scrading process is fully
automated. The system
operator observes the
process on a computer
monitor, displaying the
underwater actions as well
as the positioning
information.
Bachfilling powercable trench with slacks
Westerschelde, The Netherlands
Renewal of roof cover of Elbe tunnel
Hamburg, Germany
Gravel bed for caisson quay wall
Cartagena, Spain