The unprecedented time span of concern and consequent need to predict future geological evolution, coupled with the potential consequences of failure, dictate that geological characterizations prepared for nuclear waste repository evaluation be immensely complex. The complexity of such investigations can, however, be used to optimize the knowledge achieved by a synergistic interaction between the individual geological disciplines. Additionally, we view the use of "back analysis" of geological structures as an essential tool of the characterisation process, for example for the assessment of constitutive relationships applicable during long periods of time.

Considering a sedimentary basin as an example of a geologic system to be characterised, the role of the stratigraphy/lithology and structural geology/tectonics disciplines are reviewed. Comments are included on the role of vertical crustal movements in intraplate environments, as a directly measurable and therefore readily appreciable facet of the evolution of geologic systems. The significance of vertical crustal movement upon strain energy changes is noted. The roles of state of stress and geomorphology/palaeoclimate are introduced and used to illustrate the use of interaction between disciplines to optimize the geological characterisation.

INTRODUCTION

Emplacement in a geological system is a favored solution to the alternative proposals for isolation of high level nuclear waste. Selection of a suitable site, repository design and safety assessment depend upon a characterisation of the geological system within which the waste is to be stored. It must be assumed that the quality and relevance of geological characterisations prepared for repository development will therefore be directly reflected in the subsequent performance of the repository. In general, all activities involved in the isolation of waste in the earth's crust depend upon the geological characterizations prepared for this purpose.

Because of the potential consequences of failure of a waste isolation facility, it is assumed that the investment in geological exploration will be substantial and thus the evaluation of geological conditions will be detailed. This in itself means that such investigations will be complex. Additionally, however, the comprehension of the geological system which must be achieved by such investigations is unprecedented. The lack of precedence arises largely as a result of the vastly increased time scale of concern. Previous investigations for potentially hazardous facilities have been related to engineering lives of generally less than 102 years; the nature of the nuclear waste to be isolated is such that time spans of the order of 106 years are of concern. Thus, the geologist is rather abruptly faced with the change of perspective determined by a time scale which is increased by approximately 104 years. This means that the geologist must consider evolution of the geologic system during the life of the waste repository. Other reasons which influence the level of the characterisation to be achieved, and the difficulties in achieving such, include the need to minimize disturbance of the rock mass, to account for groundwater flow at a regional scale and to account for thermomechanical effects. However, it is the time scale of concern and the consequence of failure which primarily determine the complexity of the geologic characterisation, and the need to optimize investigation.

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