Shaft structures – 40 metres downwards
They must be circular!
The selection of suitable shaft locations also influenced the routing of the Emscher wastewater sewer. During the construction phase the shaft structures serve as start and destination locations for the pipe jacking works. They are also needed for connecting the lateral infeeds during the operation of the Emscher wastewater sewer. For geotechnical and static reasons, it was decided to use circular shaft excavation pits and circular shaft structures. During the design and planning phase until 2006 the distance between the shafts was defined to be 600 metres max. A later technical analysis showed that distances of up to 1,200 metres are also possible. This was made possible by the advanced development of the sewer inspection system which determines the distances. In total, 113 shaft structures are to be built for the Emscher Wastewater Sewer project. They have a diameter of 6 to 23 metres and a depth position of 10 to 40 metres.
Designed to prevent buoyancy
On completion of the jacking work the construction pit walls are lined with an inner shell made of structural concrete. When installing the interior walls, a ridgepole will create a load bearing connection between the shoring (bored pile wall or diaphragm wall) and the structure proper either at the shaft bottom or at the shaft head.
The reason for this design is: By activating the own weight of the shoring, the shafts are additionally secured against buoyancy in the event of high groundwater levels. Therefore it is not necessary either during the construction phase or thereafter to operate a drainage system outside the shaft, i.e. equipment causing a lowering of the groundwater level in the environment of the structure.
During the operating phase the shaft structures are also utilised as an entry point for inspection and maintenance work. As a rule, maintenance personnel will access the structures via mobile transport systems.
Preparing the construction pit – leak-tight toward the outside, dry on the inside
Bored pile walls and diaphragm walls: Seals for the construction pit
The construction pits for the structures must be planned depending on the local geology and the groundwater situation. During the construction of the Emscher wastewater sewer, shorings with overlapping bored pile walls or, in the case of large depths, with diaphragm walls are erected, which are executed as watertight constructions to keep the construction pit dry.
Where there are extreme depths such as at the pumping station construction pits in Gelsenkirchen, Bottrop and Oberhausen, the diaphragm walls can also be replaced by staggered bored pile walls. The diameters of the bored piles are between 80 and 180 centimetres, the wall thicknesses of the diaphragm walls are 80 to 120 centimetres, and on the pumping station Oberhausen up to 200 centimetres.
When the construction pit shorings have been manufactured, the construction pit is excavated. The earth masses are dug up by hydraulic excavators and conveyed by means of bulk skips and crane, rope excavator or telescopic excavator.
Relief wells dissipate water pressure
The shoring and bottom constructions sealed against pressure water avoid a work-intensive and costly lowering of the groundwater level influencing the environment. There only needs to be a residual water retention facility within the shaft construction pits. During the excavation work relief wells within the shoring provide for keeping everything dry and that the excavation bottom does not break up. When the excavation bottom has been reached, a gravel layer will be applied by means of which the water pressure is directed to two to three relief wells. Then the bottom of the structure is concreted and the relief wells are moved to the hydrostatically required height above the bottom.
The shoring of the excavation pit is installed so deeply into the largely leak-tight stone layer of the marl – three metres as a rule –, that the lateral inflow of groundwater into the space below the bottom of the construction pit is almost completely prevented.