Architecture and management of a geological repository - Andra

5 – Repository ModulesThe term 'drilling' is used here to refer to any technique where the rotation of the boring tool is ensuredby a motor remaining in the access drift (on the drill) and where a central drill string transmits therotation and thrust to the boring tool. Inversely, the microtunneller rotation motor penetrates theexcavation and follows the bore path. The thrust is transmitted to the boring tool by a tube generallyleft in place as a lining.Drilling techniques seem to offer higher excavation rates than microtunneling (industrial excavationrates, not including tube connection - in the order of 5 m/h, as opposed to 2 m/h for microtunneling).The microtunneling technique allows better control of trajectory deviations. However, that criterion isnot relevant here since the position of the dead end is not very important (one-metre accuracy).The two methods are considered equivalent in terms of annular space created at the start. They offerthe same possibilities for adopting a drive shoe (left in place) or a retractable tool (removed afterexcavation). The equipment dimensions are comparable.The drilling method is privileged at this stage due to its greater speed of execution. Microtunneling isnevertheless a completely valid alternative.• Sleeve installation (during or after excavation)The sleeve can be installed inside the disposal cell and advanced as the excavation progresses. It isalso possible to first excavate the entire disposal cell and only install the sleeve afterwards.The perturbations induced in an excavated terrain generally tend to increase with the length of timeduring which it is left without support. In this respect, installing the sleeve at the same time as theexcavation work will contribute to minimising the instantaneous convergence of the terrain if theinitial annular space is sufficiently low.In addition, this method can only reduce the possible risks of losing the structure due to destabilisationof non-supported walls.On the other hand, it induces the following constraints:- technology allowing the boring tool to be withdrawn after completion of the drilling by retractingit into the tube left in place- a posteriori closure of the tube end- cleaning the sleeve after installationTechnical solutions are available to satisfy these three requirements.The insertion of the sleeve during excavation is therefore privileged in the description that follows.However, the solution consisting of installing the sleeve after the drilling work remains a possiblealternative.• Drilling technique (in-the-hole drill, or rotary drill with carbide teeth)Two air drilling techniques allow the boring tool to be retracted into the tube left in place:- In-the-hole drill with retractable bits 88 ,- Rotary drill with carbide teeth and pivotal bitsThese two techniques offer similar industrial excavation rates (in the order of 5 m/h, not includingtube connection). The in-the-hole drill technique seems to be slightly faster (in situ tests are requiredto improve the estimates). However, it requires the use of powerful compressors (70 m 3 /minute for1 MPa) and imposes handling the dust at the head of the cell.This analysis leads to the conclusion that both solutions can be considered for C waste disposal cellconstruction.88 Disposable drill bit tools are also available, but they are excluded for reasons of cost in favor of pivotal drills, which offer the advantageof complete recovery.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM218/495