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The countdown to the National Garden Show 2011 in Koblenz has begun. All along the Rhine and Moselle riverbanks and at Ehrenbreitstein fortress the last construction work is in full swing. The first National Garden Show in Rhineland Palatinate is preparing to show around two million visitors the new face of one of the oldest towns in Germany. One of the many highlights: the terraced garden behind the Kurfürstliche Schloss that has never been open to the public before. Basic idea: the models of the Prussian horticultural master, Peter Josef Lenné, are to be brought back to life with a modern twist.

Walter & Radke GbR Straßenbau from Steimel was awarded the contract to construct the waterbound surfacing on the paths going through this terraced garden. This kind of surfacing consists of multiple aggregate mixtures; it is highly functional, permeable and environmentally friendly, since it does not need artificial binding agents such as cement or bitumen.

Firstly, a base course of local aggregate or hardcore was laid as the foundation; this type of base course consists of crushed rock size 2/32 or 0/32 with a specified fines ratio and a layer thickness of between 15 to 30 cm, depending on the load.

Next, both of the top layers were laid: a dynamic layer of Sabadyn (0/16) with a thickness of 6 cm after compaction and the top surface layer of Sabalith (0/6-0/8) with a thickness of 4 cm after compaction. Walter & Radke GbR Straßenbau decided to use the BOMAG BF 300 road paver with chain drive. For good reason, because the geometry of the total 3000 m2 job area was extremely varied and ranged from wide stretches of up to 10 metres to very narrow niches and strips of 1.90 metres.

What was required was manoeuvrability, a flexible screed width, a large capacity hopper, an excellent view of the site and an even paved surface. The BF 300 road paver screed is an innovation in this class. It offers laying widths of 1.70 to 3.40 m and can be extended from 30 cm to 4 m with two side plates. An special feature is the hydraulic extension of up to 100 percent: a considerable advantage for easy operation and rapid completion of work.

The hopper on the BF 300 finisher can hold 4.8m3 of material; a real benefit in this weight class. The scraper belts are individually controllable and reversible, just like the bin flaps and auger drives. The auger sensors can be remotely controlled from the screed operator station. This ensures great ease of operation, a regular flow of material and an even pavement surface.

The operator's platform provides ease of operation and safety to the highest standard. The intuitive operating console can be rotated and the operator's platform has the patented SideView System, i.e. the entire operator's platform can be shifted to both sides to provide the best view of the job in hand.

 

It took only two days to complete construction using the BF 300. In the case of new surfacing such as this, the waterbound paths have to set before they can take heavy loads. This was achieved with purely static compaction; i.e. vibration-free rolling with the soil in a damp condition, but not wet. This provided basic cohesion in the surface layer before rain-related settlement processes stabilises the surface.

The interaction of moisture and drying enables the aggregate mix to restructure, in that fine particles from the top few millimetres are taken deeper. This ensures that the surfacing mix is extremely stable. During this settlement period, the waterbound surface is not yet stable and cannot be fully loaded. It needs handling with care. It must not be driven on if possible so that the loose top layer is not damaged and reworking or repairs are avoided.

 

In the terraced garden at the Kurfürstliche Schloss in Koblenz, the path surfaces need around another 7 months to stabilise before visitors come from all around Europe to not only appreciate the gardens, but also enjoy readings and music events.

During these months, the stability of the path surfaces and their load-bearing capacity will increase until the coarse grit near the surface starts to appear as a rolling upper layer. The top layer of a path has reached sufficient density (DPr ~ 95%) and shearing strength for use, if the heel of a shoe pushed in at an angle does not sink into the top layer by more than 1.5 cm. If the layer is softer, it is still not suitable for full use. If it is stable, its use will only be restricted during frost/thawing conditions and extreme weather, e.g. heavy downpours – and everyone hopes that will not happen during the National Garden Show 2011. Nevertheless, the condition of the surface layer will be checked regularly.