ABSTRACT:

A salt cavern worked by dissolution in north-eastern France (54), was instrumented in 2004 prior its collapse. The multi-parameter high resolution monitoring system consisted of a microseismic network, coupled to automatic levelling measurement system. This enabled the monitoring of the overburden response to changes in the cavern, on a daily basis, up until its collapse in February 2009. In particular, all the data highlights the major role of a competent layer, vertically above the cavern at a depth of 120 meters. The analysis of the microseismicity shows that from the spring of 2008, this layer alone provided the stability of the site, while no significant sign of instability was detected by the levelling measurements. Its general failure, nearly a year later, was followed a few hours afterwards by the formation of a crater with abrupt edges, nearly 150m in diameter. If these preliminary results clearly highlight the exceptional character of this experiment and of the information drawn from it from an operational point of view, they also show the necessity to determine accurately the various failure mechanisms in play. In particular the microseismic ‘burst’ events mechanism, detected during the main peaks of the microseismic activity, must be identified, in order to understand and analyse the data in their entirety.

1 INTRODUCTION

The presence of natural or anthropogenic caverns may cause soil movements by localised or generalized collapses. One solution, to reduce the vulnerability of concerned areas, is the management of the risk by geotechnical and/or geophysical monitoring. The important geotechnical and geophysical instrumentation was designed to improve:

  1. the detection and identification of the precursory signs of the collapse;

  2. the knowledge of the failure dynamics and its associated mechanisms;

  3. the design and exploitation of early warning systems, specifically for the management of gravitational instability hazards.

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