Several disposal concepts involve disposal galleries with a diameter about 2 metre. However, smaller diameter disposal galleries can be considered also and have some advantages:

The most common method to place lining behind a micro-tunnel machine is pipe-jacking. This method consists in pushing the lining elements with hydraulic jacks, one after another, from the main gallery. For our purpose, tunnelling in clays at more than 200 metres depth, this method has a limited applicability: the length of the micro-galleries should be limited to 30 metres; otherwise the friction between the lining and the host rock would become too high.

The PRETEL project deals with the development of a new method to install the lining behind a micro-tunnel machine, avoiding the friction between the lining and the massif.

The aim of the experiment is to design and test such a new lining system allowing micro-tunnelling in cohesive soils like Boom Clay.

During this phase, the PRETEL lining was designed, tested on-surface, adapted and tested in situ. It was patented by SCKCEN (n° 95201867.9). The lining consists of expanded tube elements sectioned longitudinally; the external diameter is 63 cm. They ensure the stability of the micro-tunnel during excavation. This is realised, using steel lining tube segments sectioned in the length and bent with an overlapping, so that the outer diameter of the plied tube is smaller than the inner diameter of the lining already in place. This allows the passage of the elements to be installed to the front of the micro-tunnel (figure 1). After the complete excavation of the micro-tunnel, the final lining is placed inside the existing one to ensure long term stability.The in situ test was installed at the western sidewall (ring 21) of the URL in order to test the stability of such a lining element. The PRETEL lining element was fixed (punctual welds) to a cutting head (length: 75 cm; diameter: 635/675 mm) which was pushed in a borehole by means of a hydraulic jack. Once in place, the element was de-welded and expanded using a custom designed system with hydraulic jacks (figure 2).

This phase is carried out in cooperation with SMET Boring and is co-sponsored by IWT. A micro-tunneller was being designed in combination with the PRETEL lining system as designed during phase I. However, during the construction of the connecting gallery (2001-2002), useful experience was gathered and important changes were made to the design of the lining system. A new concept (using segmental, concrete lining) has been designed to overcome the difficulties of transporting and placing the lining segments in small tunnels. However, the main principle remains unchanged: a lining which can be transported as a whole through the already placed part of the tunnel and which can than be expanded/installed easily behind a micro-tunneller. This system consists of a lorry on which all segments (are connected to each other by means of hinges) are loaded and transported (figure 4). In “loaded” condition, the lorry can pass trough the already constructed part of the tunnel. The design is made in a way that the ducts and cables necessary for the tunnelling machine at the front can remain in place. Once the lorry is positioned just behind the tunnelling shield, the system ensures the placement of the lining using cables and hydraulic jacks. The lorry and hinges can be reused afterwards for a next lining section. The prototype was patented (tunneller: n° EP 1 094 198 A2, lining system: application in progress).

The lining system has only been tested on-surface; no instrumentation was used.

The results of the project up to now suggest that micro-tunnelling using another technique than pipe jacking will be feasible. A first design proved to be satisfactory at that time but lessons learned during the construction of the connecting gallery led to an important re-design of the lining system. On-surface tests with the re-designed lining are promising and a prototype (combining excavation and lining) will be built and tested in the future.

Some HLW disposal concepts include micro-tunnels. However, micro-tunnelling using the pipe jacking technique is not feasible in plastic clays at large depths (>200m) due to the high amount of friction between the pipes and the host rock. A new technique for the construction of small diameter galleries is being developed and shows promising results. Hence, it will probably become technically feasible to construct small diameter disposal galleries in a clay host rock at depths over 200m.

Supplementary (new) experiment(s) expected.
In situ demonstration of the re-designed lining system in combination with a micro-tunnelling machine.