Engineering of an underground laboratory in clay

Objective

When investigating the possibility to dispose of nuclear waste in an underground repository, the feasibility to construct such a repository evidently has to be demonstrated. The construction of the HADES underground facility was of key importance in this perspective. Not much knowlegde and experience on excavating in a deep plastic clay formation was available in the first construction phase (1980-1987). The work during this phase can therefore be considered to be really pioneering. Throughout time the excavation techniques and geomechanical understanding have evolved, which is reflected in the subsequent sections of HADES. The later construction of a second shaft and new galleries by industrial techniques (1997-2007) have demonstrated the feasibility to build an underground repository in the Boom clay.

Achievements

First construction phase: 1980-1987

The first shaft has an internal diameter of 2.65 m and consists of a lining of poured concrete. The concrete was poured in two layers of 0.4 m thick. In the water bearing sand layers a PVC-membrane was placed in between the concrete layers to ensure the watertightness of the shaft. The design of the shaft (1980-1982) was based on the geomechanical knowledge at that time. Boom clay was expected to respond very plastically at depth (like toothpaste). It was therefore chosen to freeze the clay formation before excavating.

Also the first gallery (URL, 1982-1983) was excavated in frozen clay. The URL has an internal diameter of 3.5m and is lined with 0.2 m thick cast iron segments. A small experimental shaft (1983) and a gallery (1984) were then constructed from the URL in unfrozen clay. This proved excavating without freezing the clay was feasible. Both the shaft and the gallery have an internal diameter of 1.4m and are lined with 0.30 m thick non-reinforced concrete segments.

The excavation technique was demonstrated on a larger scale by the construction of the Test Drift (1987) which has an internal diameter of 3.5 m for a lining thickness of 0.6 m. Steel sliding ribs (TH) were used in one part of the Test Drift.

Up to this point, the excavation of galleries in deep unfrozen clay was demonstrated, but it was only performed semi-manually by means of hand pneumatic hammers. The demonstration of such an excavation by industrial techniques remained open.

Second construction phase: 1997-2007

The second construction phase started by the construction of a second shaft (1997-1999). Ground freezing was only applied for the water bearing sands above the Boom Clay. The wall was secured by a 0.2 m thick concrete primary lining. At the top of the Boom Clay a reinforced concrete foundation was built to support the secondary lining consisting of prefabricated concrete rings with an outer steel casing. In the Boom Clay itself, the primary and secondary lining consisted respectively of steel sliding ribs and poured concrete. At the bottom of the shaft, two starting chambers were constructed.

The northern starting chamber was enlarged to create a mounting chamber (2001) for the assembly of a tunnelling machine. From there a connecting gallery (2002) with an internal diameter of 4.0 m was excavated towards the existing part of HADES. A mean excavation rate of 3m/24h was reached. The clay front was excavated using a roadheader and the advancing shield was equipped with a cutting head to ensure a smooth excavation profile. The lining consisted of 0.4 m thick wedge blocks which were placed by a bird-wing erector.

In 2007 the Praclay gallery was constructed: a 45 m long drift perpendicular to the connecting gallery with an internal diameter of 1.9 m. A similar construction technique as for the connecting gallery was used. To ensure the structural stability of the crossing of the two galleries, a reinforcement structure had to be build before the excavation. The lining thickness amounts 0.3 m. The PRACLAY gallery will host the large scale and long-term (10 years) PRACLAY heater experiment.

The construction of the second shaft and the connecting and Praclay galleries demonstrated the feasibility to construct a HLW repository in Boom Clay at a depth of 225 m by using industrial techniques. Achieving a mean excavation rate of 10m/24h is assumed to be realistic.

More information:

More information on the underground laboratory HADES can be found in the section Our Research - Research facilities - HADES.