Stemat Boskalis
Founded on underwater precision
BY STEMAT/BOSKALIS V.O.F.
Stemat bvQuarantaineweg 10P.O. Box 545113008 KA RotterdamThe NetherlandsT +31 (0)10 299 655 5F + 31 (0)10 299 654 3E [email protected]
Stemat/Boskalis v.o.f. Boskalis International bvRosmolenweg 20P.O. Box 433350 AA PapendrechtThe NetherlandsT +31 (0)78 69 69 011F +31 (0)78 69 69 555E [email protected]
• Placement of a variety of granular bulk materials, such as stones up to a Dmax ≈ 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;
• 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 applicationThe 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.
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;
• 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.
Capabilities of the Stemat Scradeway® multi-purpose pontoon: The Scrader concept®
MultiPurposePontoon
Spud
ScradeFoot Tunnel ElementGravel Berms
Laser Receiver
Fall Pipe
InclinationMeasurement
HydraulicCylinders
Laser Transmitter
Measuring Tower
1.65 m
42 m
56 m
1.00 mPattern of Gravel
Slope ofthe Trench
www.stemat.com
Founded on underwater precision
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.
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 under-
water foundations for projects such as caisson walls and
immersed tunnels, and can be used with a wide variety
of granular bulk materials. It has been successfully
employed in major projects.
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.
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.
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.
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.
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.
Granular bulk material is fed into the fall pipe by a conveyor belt.
The Scrader concept:®
Reference list executed projects:
Gravel bed for Øresund tunnelCopenhagen, Denmark
Gravel bed for second Benelux tunnelRotterdam, The Netherlands
Bachfilling powercable trench with slacksWesterschelde, The Netherlands
Renewal of roof cover of Elbe tunnelHamburg, Germany
Gravel bed for caisson quay wallCartagena, Spain
Construction of gravel bed foundation for breakwater in
Cartagena, Spain