Exposing the Piston
The piston is exposed using circular milling so that there is approximately a 2-3 meter distance between the outer surface of the piston and the shaft, in which the piston will later move. Therefore there is enough space so that this region is accessible and can be supplied with seals (geomembranes, metal cladding).
For mechanical reasons, the easiest way to expose the trench between the piston and the shaft is from above. After the upper loose rock has been removed by digging, a stone mill operates continuously in a downward spiral. In parallel to this, the top surface is stabilized, if necessary, and coated with metal or geomembranes. Depending on the depth of the construction, the bottom surface of the piston is reached after a few months. The last few meters can be broken through from below so that the structural safety is maintained for the personnel and machines.