Stabilisation measures

Date

06/10/2009

According to the present state of knowledge, the Asse mine will remain stable until approximately 2020, however, only on condition that the inflow of saline solutions will not increase any further. Stabilisation measures are to contribute to gain sufficient time for a methodical decommissioning of Asse II. This includes the so-called backfilling of roof clefts and short term measures such as the security of the waste and the fortification of the sealing to emplacement chambers. All works will be carried out in such a way that none of the three decommissioning options will be excluded.

Backfilling of Roof Clefts

Already in earlier times, open cavities were backfilled with weak stowing material. Meanwhile this stowing material has set, so that new, narrow cavities have formed at the roofs of the respective chambers. These roof clefts will now be backfilled with Sorel concrete to stabilise the mine. Thus, the deformation of the mine and the risk of an increased inflow of solution will be reduced. The first roof clefts to be backfilled are located in three chambers on the 532-m level.

To backfill the roof clefts, galleries located above the respective chambers need to be broadened to be able to place a mobile device producing building material. From there, pipe lines will be drilled to the roofs of the chambers.

Backfilling of the Area below the 850-m Level

Part of the cavities below 850 m was already backfilled with salt grit and magnesium chloride in the past. Sorel concrete is now to be pumped into the cavities that are still open. It is intended to use influent saline solutions which cannot be delivered to other mines for mixing this concrete. First backfilling works have already started. On account of the pressure in time under which the stabilisation must be carried out, new methods cannot be tested by preceding, time-consuming test series. There is also a technical challenge involved insofar as the optimum mixing ratios of concrete and salt water needs to be determined.

Covering of Drums with Salt Grit

Until 2008, emplacement chamber 7 on the 725-m level was still accessed by visitors and was only sealed in May 2009. Cracks indicate that pieces of rock can break from the roof at any time and damage the drums. As a first stabilisation measure these drums were protected with a layer of salt grit.

Strengthening the Chamber Sealings

The sealings of emplacement chambers 4 and 5 on the 750-m level were strengthened with concrete. Microseismic measurements had shown that larger amounts of salt could fall from the roof in these chambers. The purpose of the additional concrete sealing is to avoid that radioactivity can enter into the mine through the blast wave resulting from such a fall.