IntroduetjonBasement rocks constitute 25% of the surface geology of France. Throughout the geological record, and particularly during the Early Cretaceous and Paleogene, these roeks underwent significant weathering to form alterites more than 50 m thick in places. Later erosion incised vaileys in the alterites, which is why, in schistose areas, one may find aggregate quarries very close to day workings (for brick manufacture), the fom'ier being in the valley bottoms (minimum weathering) and the latter on the plateaus (maximum weathering). The aim of this research work is to assess the mechanical properties of these surface alterites in order to draw up maps for use in civil engineering sectors, such as excavation work for major communicatjon routes (roads, etc.) and laying underground pipes and cables (gas, electricity, telecomniunjcatjon lines).
Theoritical basisThe basis of the study is the assumption that a relationship exists between the erosion thickness "dz" and the degree of weathering (figure 1), the parameter "dz" being the difference between the altitude of reconstructed paleosurfaces and that of the current digital elevation model (Wyns, 1991). It consists in establishing a correlation between "dz t' and the mechanical strength of alterites in order to geostatisticafly extrapolate a map of surface mechanical strength ( figure 2). An estimated confidence interval for the calculated mechanical strength is assigned to each map point. Furthermore, because penetrometer measurements are very expensive for large-scale surveys, we chose a less-expensive physical parameter -electrical resistivity -for correlating mechanical strength.
Continuous resistivity measurements and penetrometersAn electrical-resistivity survey was carried out over several tens of kilometres along characteristic weathering profiles, as complete as possible, working from the valley bottom (no weathering) towards the plateau (maximum weathering). The equipment used (figure 3) was a quadripole profihing device with six spacings of the potential dipole (capacitive electrode recording) relative to the current dipole (electrostatic injection). With this system it is possible to obtain continuous measurements of the six resistivities simultaneously (figure 4). A semi-automatic spatial correlation using differential GPS provided precision to the nearest few tens of centimetres for x, y and z. Mechanical strength was measured along the electrical profiles using a manual penetrometer and an average measuring interval of 5 m inheight (about 250 m in distance). The density of resistivity profiles is 1 km / 5 kin 2 . The measurement of the penetrometer used as the mechanical strenght is taken at 1.5 m.
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