Abstract

France now faces the closure of most of its mining sites. Surface instability risk may sometimes remain, especially where large underground voids exist. This paper illustrates the importance of failure mechanisms identification in order to define properly how to manage the residual risk.

RESUME

La France fait aujourd'hui face à la fermeture de ses exploitations minières. Des risques d'instabilites de surface peuvent persister, notamment lorsque des vides residuels existent au sein des travaux souterrains. L'article illustre l'importance que presente la nature des mecanismes de rupture potentiels dans le choix des mesures compensatoires à adopter pour gerer le risque residuel à long terme.

ZUSAMMENFASSUNG

In Frankreich stellt sich derzeit das Problem der definitiven Schliessung des meisten Bergwerke. In manchen fallen ist die Stabilitat der Oberflache nicht garantiert, speziell wenn unter Tage nicht verfuellte Hohlraume verblieben sind. Dieser Bericht zeigt die Wichtigkeit, bei der Auswahl geeigneter Langzeit-Risikomanagementmethoden, der verschiedenen Mechanismen mit denen ein eventueller Bruch an die Tagesoberflache geraten kann.

Introduction

After exploiting its underground mineral resources for centuries, France has witnessed the gradual closure of its mining sites. These closures have often taken place without any satisfactory analysis being performed to identify the possible consequences which could have long-term impact on persons, property and the natural environment.

Of the various risks inherent in a post-mining period, the surface instability may be particularly significant where large residual voids remain underground once mining has ended. This "ground movement" risk can be evaluated by combining the predictable damage and surface occupation. It is used to judge the need to envisage the implementation of appropriate compensatory measures.

Treatment such as backfilling or providing support in voids are indeed often technically and economically impossible because of the volume, depth and accessibility of the mining works concerned. Although priority is traditionally given to solutions for managing the surface occupation in sectors which are still uninhabited, when the risky zones already show surface instability, this solution is no longer adequate and moving populations would generate sociopolitical and financial problems which are difficult to solve.

When there is no other suitable solution in the context, monitoring potentially unstable zones may provide an interesting alternative. This solution is intended to provide risk management by detecting the signs of unfavourable evolution of the undermined land. In this way populations can be maintained in a "doubtful" zone while making it possible to evacuate them in the event of an alert judged sufficiently serious by the competent authorities.

Using a monitoring system requires the failure mechanism, particularly its dynamics, to be identified as accurately as possible, so that it can be determined whether it is possible to detect preliminary signs and whether they are adequate for operational risk management (trigger a warning).

In-depth investigation must therefore be conducted on the type and dynamics of failure mechanisms liable to affect mine working and to spread to the surface (gradual or sudden, smooth or discontinuous phenomena). In this context, INERIS and GEODERIS have recently developed techniques.

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