Inspection and Monitoring of Gallery Linings
Our Operations & Safety team and the RD&D team collaborate in monitoring the stability of the underground galleries. These structures have remained remarkably stable for decades, with the newest over 20 years old and the oldest exceeding 40 years. We need to make sure that it stays that way, by meticulously monitoring the stability and possible movements of the galleries. Not only is this necessary for ensuring the safety of our workers in HADES, it also generates unique data about the different types of lining under large pressures at 225 meters depth. Unique, because HADES is the oldest underground research lab in clay, which allows us to investigate the impact of the pressures and clay interaction on the gallery lining as a function of time over more than 40 years. These data are investigated by our researchers, as this is valuable input for the final safety case of a deep disposal facility.
Why do gallery linings need to be inspected?
HADES is excavated in Boom Clay, a poorly indurated clay formation. Because this is a relatively soft geological medium, the underground galleries require structural support to ensure their long-term stability. When HADES was first constructed, cast-iron lining segments were installed to support the galleries. As the facility expanded, concrete lining systems were introduced because they are more cost-effective and easier to install while providing the required structural performance. Today, the linings in the Test Drift, access shafts, connecting gallery and PRACLAY Gallery are regularly inspected and monitored. These inspections help assess their structural integrity, verify safe operation of the underground laboratory, and provide valuable information on the long-term interaction between the clay formation and engineered support structures.
Why inspections matter
Monitoring the condition of gallery linings allows the HADES team to:
- Ensure the safety of personnel working underground.
- Verify the long-term stability of the galleries.
- Detect changes such as cracking, deformation or movement at an early stage.
- Improve understanding of clay-lining interactions over decades.
- Generate knowledge relevant for the design and safety assessment of future geological disposal facilities
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Cast-iron linings
The earliest galleries in HADES were supported using cast-iron lining segments, designed to withstand the pressures exerted by the surrounding clay formation. These structures have now been performing underground for several decades and provide valuable information on the long-term behaviour of engineered barriers in clay.
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Concrete linings
More recent gallery sections were constructed using concrete linings. These systems are widely used because they are economical, durable and easier to install. Their performance is continuously assessed through visual inspections and dedicated monitoring programmes.
How do we monitor the galleries in HADES?
Two types of measurements are performed on the galleries by an external surveyor company. Our team accompanies this external firm to assist where necessary. They perform three-monthly topography measurements on the convergence of the galleries, using prisms that are incorporated into the lining segments at certain intervals to measure the exact position of the lining segments. This has been done in each of the galleries since the beginning of the construction, meaning that we have a very good overview of the movement of the galleries. In total, the concrete-lined gallery test drift has converged (became more concave due to the pressure of the overlying earth layers) by about 4 cm. Most of this ovalisation occurred in the first years after construction.
The external surveyor company has also started performing 3D scans in recent years. This is done once per year and fully maps the galleries in a way that can be implemented for 3D visualization and modelling.
Moreover, the team performs several measurements independently. Managed by our RD&D team, the following tasks are covered by the EURIDICE technicians:
- Complete Mapping and Inventory
Detailed mapping of the gallery linings, including identification of any cracks or fractures and assessment of their impact.
- Installation and Monitoring of Crackmeters
Crackmeters are placed at strategic locations to measure cracks in concrete segments. Data are read out periodically, to be used for stability assessment and modelling purposes.
- Periodic Measurement of Deviations and Movements
We measure shifts between concrete segments (in the test drift) or metal sliding ribs (in the ANDRA gallery). Frequency of the measurements depends on findings: sliding ribs are checked monthly, while overall displacement measurements occur annually.
- Photographic Documentation
Full photographic surveys of gallery linings provide snapshots for periodic comparison and long-term monitoring.
- Concrete Hardness Testing with Schmidt Hammer
A non-destructive method to check the relative hardness of concrete quickly and efficiently.
- Integrated Strain Gauges
Strain gauges installed in strategic segments measure stress levels on the structure. These sensors convert force, pressure, and tension into measurable resistance changes. Data is typically read every three months.
Based on the combination of all these measurements, HADES is mapped in detail. The periodic measurements provide the input necessary to possibly adapt or update this mapping in function of time, due to the pressures at a depth of 225 meter. Here you see the 2D map of the Test drift, on which the boreholes and instrumented rings are indictated.
Our collaborations
We collaborate with other organisations in various projects to analyse the stability of the galleries in HADES, in developing methods that can be used to monitor the stability of a future repository.
- ANDRA: collaboration to develop monitoring tools
- Laborelec: characterisation of cracks