Modal and thermal analysis of les arches unstable rock column (vercors massif, french alps)

Abstract

A potentially unstable limestone column (~1000 m3, Vercors, French Alps) delineated by anopen rear fracture was continuously instrumented with two three-component seismic sensorsfrom mid-May 2009 to mid-October 2011. Spectral analysis of seismic noise allowed severalresonance frequencies to be determined, ranging from 6 to 21 Hz. The frequency domaindecomposition (FDD) technique was applied to the ambient vibrations recorded on the topof the rock column. Three vibration modes were identified at 6, 7.5 and 9 Hz, describing theupper part of corresponding modal shapes. Finite element numerical modelling of the columndynamic response confirmed that the first two modes are bending modes perpendicular andparallel to the fracture, respectively, while the third one corresponds to torsion. Seismic noisemonitoring also pointed out that resonance frequencies fluctuate with time, under thermomechanicalcontrol. For seasonal cycles, changes in frequency are due to the variations of thebulk elastic properties with temperature. At daily scale, increase in fundamental frequencywith temperature has been interpreted as resulting from the rock expansion inducing a closureof the rear fracture rock bridges, hence stiffening the contact between the column and the rockmass. Conversely, the rock contraction induces a fracture opening and a decrease in resonancefrequency. In winter, when the temperature drops below 0 °C, a dramatic increase in fundamentalfrequency is observed from 6 Hz to more than 25 Hz, resulting from ice formation inthe fracture. During spring, the resonance frequency gradually diminishes with ice melting toreach the value measured before winter.© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.