115 metres underground:

the St. Anna shaft was "sunk".

Difficult geological conditions

The actual shaft is circular and leads into the depths in the centre below the headframe. Its size and position are marked by the ‘object frame’.

The geological conditions here are problematic, because down to a depth of approx. 80 metres, the soil structure consists of non-load-bearing and unstable layers of earth and sand. A load-bearing layer of light-coloured, chalk-like limestone does not begin until a depth of approx. 80 metres from the planned bottom of the shaft.

Innovative construction of the shaft wall

The upper part of the shaft had to be built on a stable foundation on the natural rock at a depth of 80 metres. To achieve this, 42 concrete pillars had to be driven close together in a circular pattern and then fully lined with sprayed concrete from the inside for the purpose of sealing.

Further sinking between 80 and 120 metres deep was then to follow, as usual, by blasting and drilling into the rock.

Major problems were encountered during sinking

The sinking work was carried out in 1956 and 1957. First, the boreholes for the concrete posts were drilled and filled with concrete.

The temporary wooden winding tower was then erected, and the winding machine installed. Excavation of the soft earth and sand within the shaft wall then began.

This revealed that due to the uneven subsoil and the considerable drilling depth, the lower ends of some of the concrete pillars were positioned either inwards or outwards. In addition, gaps of different widths remained between some of them.

Over pressurisation process against water infiltration

Water-bearing layers were reached at greater depths during sinking. The water flowed in through the gaps between the concrete in such large quantities that the sinking work could not be continued.

The problem was solved by continuing the work in the shaft under greatly increased air pressure. This meant that the water pushing in from the side could be held back. For this purpose, a pressure lock was installed at the top of the shaft, through which heavy rock, material and personnel had to be transported. To protect the health of the miners and construction workers, they had to adapt to the changing pressure long enough to enter and exit a pressurised chamber, similar to divers.

Once the earth and sand had been removed down to the solid rock layer and the shaft wall lined with concrete, no more water could penetrate. From a depth of 80 metres, the shaft was then sunk further to its final depth by conventional blasting and removal of the loose rock.

Erection of winding tower and shaft hall

In 1957, the current pithead and shaft hall were erected above the shaft in place of the wooden winding tower.