Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite mediumAir exhaust shaftThe general exhaust air of the repository, blasting fumes and if necessary fire smoke, are extractedthrough this shaft. Its useful diameter of about 10 metres can be adapted according to the repositoryprocess management choices (simultaneous construction of several repository levels, maintaining theinstallations open and ventilated).• Shaft constructionThe conventional method for constructing a large diameter shaft in the 6-12 metre range is to sink theshaft from the surface by drilling and blasting the rock and gradually installing the rock support andshaft liner as excavation progresses. At this stage this method which is being well-suited to theconsidered diameters and with a large feedback, is adopted as the reference solution.However mechanised excavation methods, developed in the mining industry (such as raise-boring orshaft-drilling) could be considered. For example at the end of the chapter, the option of the "raiseboring"method to create a 2-4-metre diameter broken muck ejection chimney , followed by traditionalsinking to enlarge the final shaft diameter, is presented.The first two shafts are sunk simultaneously so that they can be linked at the bottom as soon aspossible to create a ventilation circuit.These shafts are constructed in the following stages:- preparatory detailed survey of the formations to be intersected;- platform and foreshaft construction;- sinking then lining the shaft.Detailed surveyIn and near the shaft axis, boreholes are drilled to survey the geological column to be intersected andto appraise the amount of possible water arrival.This survey ensures notably that the rock is conducive to the construction of underground stations atthe planned depths and, when necessary, informs about necessary that injections to be carried out inorder to limit any water arrival of water between these points and the surface.The foreshaftAfter possibly scraping surface-altered granite and making a reinforced concrete slab, the foreshaft issunk to a depth of several tens of metres (crossing the decompressed and generally permeable uppergranite formation). Sinking entails a number of successive drilling and blasting phases, andreinforcement with bolts and mesh (with shotcreting if necessary).Then the shaft wall is totally lined with formed concrete, making a leak-tight lining associated withpossible ground injections.Shaft-sinkingShaft-sinking is a cycle of operations. Its sequence is common to all drilling and blasting works:- blast holes are bored then filled with explosives (2-4 metres depending on the shaft diameter);- blasting, extracting the blast gases and inspecting the walls (dislodging unstable blocks ifnecessary: purging);- broken muck removal and geological survey of the walls;- installing temporary ground support (bolts, shotcrete, etc.)Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY64/228