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 info@stemat.com

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 royal@boskalis.nl

• 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