Suction Excavator Brand Self-Made
Four men in orange work clothes lie sprawled on their stomachs. They peer into a narrow gap under a huge concrete slab, poke into the cavity with long iron rods, rattle two plastic hoses that are also pushed under the concrete slab. A handful of other workers follow the action with some tension. The suction excavator is stuck somewhere under the floor slab of Stuttgart’s new Main Station. One of the workers follows the special device on a screen, a slim black sledge of their own design, with which the ZÜBLIN-STRABAG team will suck 40 t of grit out of a cavity hardly higher than a cigarette packet over several weeks.
The floor slab of the new Stuttgart station building lies above the suburban railway tunnel, which has already been in place for 40 years. The ceiling vault is only 30 cm thick – in order to support the enormous concrete slab, it will therefore be cantilevered, load-free and constructed in individual sections in its final state. ‘To do this, we first temporarily poured a layer of grit – then concreted the concrete slab on top and tensioned the anchors to reduce the deflection,’ explains senior site manager Alexander Behrend. Then, the grit is removed again, the slab floats and the cavity later fills with groundwater. This way, there is no load on the ceiling of the subway tunnel. The challenge was the removal of the chippings: The cavity under the slab is only 10 cm high – no commercially available device could be used in this narrow gap to extract the material.
Salt or sugar as a filler?
How do you solve the task? ‘We had a few ideas,’ Alexander Behrend recalls. ‘One was to use salt or sugar as a filler. Both would then simply dissolve in the water.’ For environmental reasons, however, this solution was not feasible. Next idea: corrugated cardboard cylinders in plastic bags that then fill with water – that was also rejected. ‘Then we considered trying it with sand, gravel or grit – and then simply sucking the material out again at the end,’ says Behrend.
In 2017, the first promising trials were conducted on this idea. The team finally decided on shell limestone. Sand would have the disadvantage that it can get wet and is then difficult to move. Gravel, on the other hand, would be difficult to vacuum because of its round grains. Grit is ideal: The grain sizes of 2-5 mm fit perfectly, and they are easy to shake even when wet. On the construction site, the suction excavator is now up and running again. Like a submarine, it circles under the concrete slab in the dark, closely followed by the workers on the screen. Dieter Rannert, also senior site manager, leads along the long plastic hoses. These lead to a truck with a huge attachment. ‘Inside the vehicle are two turbines,’ Rannert explains and adds:
‘They create an airflow that sucks or blows 44,000 m³ of air through the hose within an hour. When you hold this hose in your room at home, it shifts all the furniture.’