Temporal behavior of a ventilated claystone at the Tournemire URL: Cross-spectral analyses focused on daily harmonics

• The temporal behavior of the URL is analyzed consecutively to the S1 atmospheric tide forcing.
• A procedure of “Spectral Tracking” based on cross-spectral analyses is developed.
• S1 atmospheric tide leads to gallery air temperature, relative and absolute air humidity fluctuations.
• The fluctuations of atmospheric parameters lead to the claystone desaturation and its damage.
• Rock hydraulic conductivity is evaluated using cross-spectral analyses on pore-water content probes.

The main topic of this communication is the presentation of study, auscultation and supervision procedures of deep geological radioactive waste storage repositories using natural harmonic forcings. In this paper, the effects of natural ventilation on the macroscopic behavior of a clayrock are investigated by means of time series recorded underground over a period of two years in the eastern part of Gallery 1996 at the Tournemire Underground Research Laboratory (URL). This study is based on time series acquired in the atmosphere, at the gallery wall surface, and inside the rock mass. It includes measured signals from 6 thermo-hygrometers, 5 crack-meters (measuring the displacement of 2 shrinkage cracks and 3 tectonic fractures), and a 1 meter-FDR (Frequency Domain Reflectometry) profile probe equipped with 6 sensors for measuring the volumetric pore-water content into the rock mass. Auto-spectral and cross-spectral analyses using the concept of Singular Spectrum Harmonics (SSHs and cross-SSHs) are developed. Our analyses and interpretations focus here mainly on the solar diurnal atmospheric tide (denoted S1). This tide corresponds to the insolation cycle of the Earth atmosphere during a mean Solar Day (24 h 00 min). This component is tracked throughout the various measured signals (“Spectral Tracking” of tide fluctuations across signals). This is equivalent, in a way, to analyzing the temporal behavior of the URL during a “Mean Solar Day on Earth”. Results indicate that the daily natural forcing caused mainly by a combination of barometric and temperature related fluctuations, is the most important effect overall on our various signals. The daily harmonic induces the fluctuations of gallery air temperature, relative and absolute air humidity, and it leads to desaturation of the claystone, which in turn leads to the claystone deformation and damage. The effects of the annual harmonic SA may also be significant (it was fully analyzed in the more complete version of this work); however the SA results are not presented here because the spectral resolution on the annual harmonic is insufficient at this stage with only 2 years and 2 months of clean recorded data. Focusing the present study on daily fluctuations, the cross-spectral time shifts were obtained from the phase spectrum, for the daily component S1, for various pairs of pore-water content sensors located at different distances from the gallery wall. These time shifts were then used to quantify a “spectral velocity” which is found to be on the same order than the hydraulic conductivities deduced previously from pneumatic tests performed in the Excavation Damaged Zone of Gallery 1996.

The temporal behavior of the Tournemire Underground Research Laboratory is studied during One Mean Solar Day on Earth.Figure optionsDownload as PowerPoint slide