Microscopic and mineralogical characteristics behind the engineering properties of a compacted andesitic volcanic soil

Most of the volcanic soils found around the world are concentrated in regions of high demographic and economic growth. Research shows that the drying process to which volcanic soil samples are subjected to as part of the preparation process for laboratory testing can affect soil engineering behaviour, inadvertently leading to undesirable results. Previous studies suggest that the key to the understanding of the behaviour of volcanic soils lies in its mineralogy. Thus, this paper proposes an experimental investigation on an andesitic volcanic soil from Popayan, Colombia, to investigate the extent to which changes in drying affect the engineering behaviour of the soil. Mineralogical tests indicated the presence of halloysite in the soil. From the geotechnical, hydromechanical and structural tests it was possible to recognise that the drying procedure influenced: compaction behaviour; consistency limits; particle size distribution; retention behaviour; compressibility and pore size distribution (>1 μm), while not having great effect on: specific surface; pore size distribution (<1 μm). Combined analyses of the laboratory tests performed suggested that the dehydration of halloysite at mineral scale could then explain the distinct change in engineering behaviour observed when this soil is subjected to drying.