Targeting and mapping expansive soils (Loiret, France): geometrical analysis of laboratory soil spectra in the short-wave infrared domain (1100–2500 nm)
Résumé
Short-wave infrared (SWIR: 1100–2500 nm) reflectance spectra of soil samples, acquired under laboratory-controlled conditions, were used to estimate the swelling potentials of the soils and to use SWIR spectroscopy to improve a part of the swelling-risk map of France. A total of 332 samples were collected to the west of Orléans (France) in various geological formations and swelling-risk areas and along two (eastern and western) transects with different samples spacings. Comparisons between the swelling potentials of the soils and the swelling-risk areas of the map exhibit good correlation in the south of the sampling area; however, there are several inconsistencies in the north of the study area that highlight the necessity of locally redrawing the accepted swelling-risk map of France. The sampling interval (approximately 260 m) along the eastern transect was too sparse and does not appear to have effectively captured the smallest and/or isolated lithologies. The sampling interval along the western transect (approximately 50 m) revealed the presence of an unmapped swelling-potential zone, which was confirmed by several soil samples. The sample interval along the western transect appears to be more suitable for mapping the smallest lithologies. The presence of several samples in close proximity that exhibit the same swelling potential is a robust indication of the presence of a zone with constant swelling potential. A new swelling-risk map of the sampling area was produced based on the soil samples. The map produced by interpolation did not permit the representation of discrete lithological units, introduced spurious swelling-risk zones that however could be representative of tillage in agricultural zones. More samples are therefore needed to produce a reliable map on the scale of the sampling area. According to swelling potential uncertainty related to soil sampling and soil treatment, spectroscopy-based approach proposed here cannot be used to replace the existing swelling-risk map of France. This method permits however the rapid and low-cost estimation of the swell potentials of a large number of samples, which could be used at regional-scale to target areas where doubt remains or where infrastructure damages attributed to swelling behavior are known. At local scale, soil samples need to be properly and laboratory treated to accurately produce local revised and detailed swelling-risk maps.