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Communication Dans Un Congrès Année : 2018

Installation of a borehole vertical array in the Var valley, Nice, France

Résumé

Because of its geodynamic context, its extreme geography (transition from a deep basin to high mountains in less than 80 km), its concentration of stakes (human, urban centers, economic), combining to the increase of the coastal population density, the french Riviera forms an ideal laboratory to study natural hazards and risks. This is particularly true for seismic risk since the area is prone to the highest level of seismic activity in France's mainland. One of the main component of seismic hazard assessment is related to lithological site effect due to soft layers overlaying hard bedrock encountered for example in the Alpine valleys. The quantitative assessment of the ground motion effects associated with the local surface geology, is thus a major issue in seismic hazard and engineering seismology studies. Frequency dependent site amplifications are known to be mainly caused by reverberations and resonance effects of S-waves within the unconsolidated sediments that are found in fluviatil basin for instance. To better understand these site effects it seems interesting to record the interaction of the seismic waves with the soil materials by setting up verticals seismological arrays. Within the framework of the PORTE project, supported by the PACA region and the European ERDF funds, one of the objectives of which is to create a technical and innovation platform dedicated to environmental observation in order to obtain long-term recordings in-between and during crisis. We propose the establishment of a vertical seismological network in the lower Var valley in Nice. The valley is less than 1 km width but previous geophysical and geotechnical studies have shown that the bedrock depth could locally reach 150 m. We started by carrying out in-situ and laboratory measurements to characterize the future site of implantation. The geophysical measurements (MASW and AVA) allowed us to define a S-wave velocity profile while the laboratory measurements on core drillings samples informed us on the mechanical behaviour of the different sedimentary layers. Other in situ characterisation (pressiometers) coming from the literature where also used. On the studied site, the bedrock (Pliocene marl) is found close to 30 meters depth, the shear wave velocity measurements result in a Vs30 close to 370 m/s and the associated fundamental resonance frequency, derived from H/V spectral ratio on ambiant vibration, is found at around 2.5 Hz. The soil column is mainly composed of saturated sandy layers including pebbles and gravels. In a few months, an instrumentation combining accelerometric and velocimetric sensors will be set up at three depths (surface,-10 m and-31 m) and will continuously record the seismicity of the area. This instrumentation will be complemented by interstitial pressure measurements that will allow to record its variation related to different cyclic sollicitations.

Domaines

Géotechnique
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Dates et versions

hal-01844538 , version 1 (03-12-2020)

Identifiants

  • HAL Id : hal-01844538 , version 1

Citer

Nathalie Dufour, Jean-Baptiste Payeur, Etienne Bertrand, Diego Mercerat, Régnier Julie, et al.. Installation of a borehole vertical array in the Var valley, Nice, France. 16ECEE, Jun 2018, Thessalonique, Greece. ⟨hal-01844538⟩

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