Prosper-Haniel colliery: tight in the shaft

The Prosper-Haniel colliery was the last active mine in the Ruhr area. Three years after the closure, the filling of the last shaft has started. How does this work? How long does it take and how safe is it?

The concrete flows 1000 meters deep, at a speed of 17 meters per day. It will take until the end of the year for the concrete to fill the entire Haniel 2 shaft in Bottrop. The same thing happens with the Haniel 1 shaft, which is about half as deep.

The fact that shafts that are no longer used are filled with concrete is part of the normal dismantling of a mine. But this backfill is a special one. “The last coal was handed over to the Federal President at the Haniel shaft in 2018. It is therefore also of historical significance. It is the last shafts of the last mine in the pot that are now being closed,” says Christof Beike from RAG Aktiengesellschaft, which runs the mine operates.

Permanent backup

But before the concrete is poured into the shaft, the mine must first be cleared. Ropes, conveyor baskets, machines – everything that can move has to go. However, what is permanently installed can remain, such as the guide rails – a kind of rails for the baskets – explains Thomas Ahlbrecht, an expert in manhole filling. Only then can the concrete come.

In the past, rock from the mines or partially recycled material was also used for backfilling. “But that has been completely abandoned for several years because you want to be sure that this manhole filling is permanent,” says Ahlbrecht. In his view, permanent means to fill the shaft safely for 150 to 200 years.

Everything has to be tight

“As things stand at present, this manhole filling is made permanent, but it cannot be ruled out that such backfilling will have damage at some point, so that it has to be refilled or one has to consider how to deal with this damage,” explains Ahlbrecht. That is why shafts that have not been filled according to today’s standards are regularly closed with concrete. Because it has to be tight. The closed pits must later no longer pose any danger – for example, from a burglary in the ground or from evasive gases that can ignite.

A specially produced concrete mix is ​​used. It should completely fill all cavities. “With full backfilling, the shaft is completely backfilled from the deepest point to the surface of the earth. Where there are transitions from the shaft to the mine, walls may be put in place so that the material cannot drain off,” says Ahlbrecht. Most of the time, however, the material is touched in such a way that it does not need such walls.

Costs in the millions

Even if the principle of backfilling is simple, it is not cheap. Christof Beike from RAG estimates that it will cost around 14 to 15 million euros to close the last two Prosper-Haniel shafts. Prosper Haniel should be completely full by the end of the year.

What happens then is not yet clear. There are ideas and discussions, says Beike. There could then be new commercial space or leisure facilities on the site, but “time is not running out”. Backfilling the shafts is an important step in making something new possible. But it also ends irrevocably with the history of mining, which shaped the Ruhr area for centuries.