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CONNECTING GALLERY |
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| Global context of the experiment - Background | |||||||||||
The research on geological disposal of radwaste in clay layers has been carried out for over 20 years in the Underground Reasearch Facility (URF) HADES at MOL. Work on URF HADES started in 1980 with the construction of a first shaft, followed by the excavation, in frozen Boom Clay ,of the URL (Underground Research Laboratory) in 1983. During the excavation, it was found that freezing the clay before excavation was not necessary and even detrimental. A small-diameter shaft and a small-diameter gallery were therefore excavated – as a test case - in non-frozen clay in 1984. This led to the excavation of the first extension of the URF HADES in non-frozen clay: the Test Drift, which was completed in 1987. Due to requirements of the mining regulatory body it became mandatory to construct a second shaft before executing any new large-scale work in the URF HADES like for instance the construction and installation of the PRACLAY- gallery. The decision was then made to extend the URF HADES with a 80 m long gallery, connecting the existing URF HADES to the newly built second shaft. The second shaft was built between 1997 and 1999 and the connecting gallery in 2001 and 2002.The construction (exploratory phase) of the earlier parts of the URF HADES was done manually reaching a relatively low construction rate (2meters a week). It is clear that a real repository shall have to be realised in a much faster and more industrial way. The feasability of industrial excavation in Boom Clay however was never before demonstrated at such large depths. Therefore it was decided that this extension (demonstration phase) of the URF HADES could be considered as a contribution to the demonstration of the feasability of deep disposal of radioactive waste. It was indeed the first time that an industrial technique would be used for the realisation of a gallery in Boom Clay at such depth making the realisation of the gallery an important milestone in the demonstration programme. Furthermore a large instrumentation programme in and around the gallery was set up in order to improve the understanding of the hydro-mechanical response of the rock during and after its construction.
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| Objective of the experiment | |||||||||||
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| Description of the experiment | |||||||||||
The gallery was excavated
starting from the bottom of the second shaft, towards the test drift.
The technique that was used for the realisation of the connecting gallery
is the technique of the excavation of the clay rock under protection of
a shield and the construction of the lining, using the "Wedge Block
technique", behind the shield. A minimal realisation rate of 2m/day,
24 hours a day and 7 days a week, was imposed. Figure 2 shows the tunnelling
machine and the lining system. |
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The tunneling machine: The tunneling shield has a rear diameter of 4.82m and is equipped with a cutting head at the front. The clay front is excavated by means of a roadheader and the shield is pushed into the clay by hydraulic jacks on a regular basis, thus enabling a smooth excavation profile thanks to the cutting head. A bird-wing erector (fixed at the end of the shield) applies the wedge-block system: 12 segments are used to build a lining ring, which has a nominal external diameter of 4m80, a wall thickness of 0.4m and a length of 1m. 83 rings are installed; in the last few meters of the connecting gallery the tunneling shield itself acts as lining. The lining system: For the lining of the gallery a lining system using the wedge block technique was considered. This technique consists of the placement of lining segments in contact with the excavated host rock. By pushing in (one or more) wedges the lining expands and presses itself against the excavated wall.Fig 2c explains the principle of the wedge block system. The advantage of using this technique in comparison with other tunnelling techniques was that it causes a minimal disturbance of the clay massif. To apply the "Wedge Block System" however, the "immediate convergence” of the clay massif had to be known quite precisely. This was one of the experimental aspects of the execution of the connecting gallery, because the "immediate convergence" was not known at that moment as digging in clay at that depth was never done in a rather fast and industrial manner. During the execution of the connecting gallery, a lot of complementary information – including on the "immediate convergence" - on the behaviour of the clay massif was gained. |
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 Fig. 2. General design of the excavation technique; a) schematic view of the tunneling equipment, b) the same equipment during a test assembly on surface, c) the wedge-block principle, d) hydraulic jacks and bird-wing erector, e) adjustable cutting head and roadheader. |
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| Results of the experiment : | |||||||||||
As it is impossible to describe al results and conclusions of the realisation of the connecting gallery on this file, only some major results and conclusions are listed below, a complete repport on the construction of the connecting gallery can be ordered on www.EURIDICE.be. Shield design The construction works have overall been successful. The accuracy of the numerical predictions of the displacements of the massif and of the pressures on the lining has allowed the design and the dimensions of the tunnelling machine and of the lining segments to be appropriately defined. The two major challenges have thus been met: |
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The wedge-block technique has been preferred to a technique using bolted segments, on the one hand because it is fast to install and is expanded against the clay massif and, hence, disturbs it less, and on the other hand for economical reasons. It could be improved through a few simple modifications and remains a privileged option for future underground construction works in the Boom Clay. Concrete-concrete contact between adjacent lining rings led to spoiling when the shield was pushed forward. The damage was probably due to small misalignments during placing. The spoiling was immediately observed and from that point onward, high density PE plates (3mm thickness) were inserted between rings and it no longer occurred. Though the technical problems encountered during the whole construction process and related to the design aspects were only minor, there has been one major, unexpected problem: the extent of the detachment of clay blocks from the front and from the unsupported sidewalls. This has been both a safety issue and a construction issue. The measurements and research programmes carried out before and during the construction of the mounting chamber and during and after the construction of the connecting gallery have led to a very comprehensive characterisation of the fracturation pattern and to an equally comprehensive picture of the instantaneous hydromechanical response of the Boom Clay to an excavation using an industrial tunnelling technique. The intensive characterisation of the fracturation resulted
in a description, in terms of orientation and shape, of the fractures
around the gallery and in a better understanding of how these fractures
are formed. All the fractures observed at the Mol site have been induced
by excavation. They originated at some 6 metres ahead of the excavation
front. Cored borings performed after the completion of the gallery indicated
that the fractures extend up to about 0.5 metre in the radial direction.
Future cores and the construction of the PRACLAY gallery, at right angle
to the connecting gallery, will provide more information. Natural, pre-existing
fractures were not observed, though it is impossible to prove their absence.
An important remaining issue is the impact fractures can have on the long-term
performance of geological repositories. This impact will probably be limited
by the healing and sealing mechanisms that have already been identified
qualitatively in various ways.
The conclusions regarding the CLIPEX programme
are discussed in other files. Finally, the petrographic study has shown that the Boom Clay only oxidises within fracture planes: the only evidence of pyrite oxidation, under the form of newly formed minerals, is indeed within fracture planes, microfractures, and discontinuities. Such oxidation effects are the visual print of the geomechanical disturbances induced by excavation. They are important in the framework of the migration studies and for performance assessments |
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| Conclusions : | |||||||||||
The feasability of realising underground infrastructures in Boom Clay at large depth has been demonstrated. In general, the tunneling technique performed adequately and the successful construction of the connecting gallery using this industrial technique is an important contribution to the demonstrating of the feasibility of disposal of nuclear watse in Boom Clay. The biggest problems were caused by fracturing of the host rock and the subsequent falling of blocks out of the face. The gallery itself was excavated in less than 6 weeks. Except for the first and last few meters of the gallery, the minimal excavation rate 2m/day was always respected and sometimes even doubled.A feature that caused some problems was the presence of pyrite concretions. Lumps of up to 30 cm diameter were encountered and damaged the teeth of the roadheader since the head was designed to excavate soft rock only. The teeth had to be replaced several times. Implication(s) of the results |
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| Bibliography : | |||||||||||
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