Incorporating the Excavation Disturbed Zone (EDZ), into the Design of the NIREX Rock Characterization Facility (RCF)

Abstract

United Kingdom Nirex Limited ('Nirex') has applied for permission to construct a Rock Characterisation Facility (RCF) within the Potential Repository Zone ('PRZ') at Sellafield in Cumbria (UK). The purpose of the RCF is to enable Nirex to decide whether or not to propose the development of a repository at Sellafield and, subsequently, to develop an evaluation of post- closure safety that will enable a decision to be made on the authorisation of disposal of wastes when, and if, a repository is constructed. The facility will be constructed in three phases. Phase 1 comprises construction of two shafts, and underground gallery development. Phase 2 involves construction of three 150m-long galleries and an intensive programme of scientific investigation. Phase 3 is an extension to Phase 2, providing greater coverage. All three phases integrate a range of scientific activities with the construction work. This paper describes the rock mechanical data acquisition to be undertaken within the proposed RCF, and outlines how these investigations will influence the design and construction programme of the facility. The design of the geotechnical data acquisition programme will take cognisance of the practical experience gained in underground laboratories and in the ZEDEX project, in which Nirex is a participant. During Phase 1 temporary suspension of shaft sinking will be programmed to allow profile, convergence and stress measurements, rock mass response measurements, and to monitor the development of the Excavation Disturbed Zone. These measurements and their implications for shaft design and programme, are described. During Phases 2 and 3, two experiments are currently planned to measure the extent and magnitude of the Disturbed Zone; an EDZ experiment, to study the development of the EDZ, and a "mine-through experiment" which studies the mechanical and hydraulic response of a fault as it is mined through. Measurements before, during and after excavation will be made with the construction process operating on a "go-slow" basis to allow progressive deformation to be measured. These types of experiments have important effects on the construction of the RCF.