Owing to the presence of rock salt within a thrust fault on the site of a projected large (355 m high) dam, specific studies were required for assessment of the extension of the potential salt dissolution during and after construction of the dam. Previous work was reviewed, from which it appeared that assumptions made on input parameters such as hydraulic properties of the rocks and the rate of rising of the salt wedge within the fault in the foundations should be viewed with caution. A crucial assumption is that equilibrium is reached between the salt leaching rate and salt wedge rising rate. The authors therefore attempted to find out a best estimate of input parameters by means of site investigations. A large-scale pumping test allowed the determination of some key parameters for the dissolution process. A sensitivity analysis for the unknown parameters (such as salt rising rate) was conducted, in order to analyse possible scenarios. The results using these new parameters conclude that the dam project is feasible, provided that at least two main salt-dissolution mitigation measures can be implemented and properly maintained during the lifetime of the scheme.

INTRODUCTION

The present work has been developed in the frame of the feasibility study of a projected large dam in a mountainous area. According to the project, the dam is to be constructed in two stages, respectively some 145 m and 355 m height. The dam foundation presents peculiar features, due to an active, compressive, tectonic context, and notably a major regional reverse fault, dipping 70–80° in a downstream direction and crossing the river at the dam site.

Investigations initiated more than 40 years ago revealed within the fault the presence of evaporitic rock with major content of halite (rock salt), extruded from a deep evaporitic layer. This evaporitic rock contained in the fault will be referred to as the ‘salt wedge’. The fault is geologically active, since creep in the form of a relative movement between both walls at a rate of some 1.5 to 2 mm/a was evidenced.

Considering the absence of any other suitable location for such a dam elsewhere in the valley, the project needs to cope with such conditions, and an earth fill dam is projected, with an impervious clay core located downstream of the salt-containing fault.

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