Dynamic image analysis of Etna Sand in one-dimensional compression

The paper presents the results of a physical and mechanical characterization of a compressible and crushable volcanic ash collected during a recent eruptive event of Mount Etna. The characterization of this complex natural material required appropriate equipment, to carry out a comprehensive experimental programme, and a rigorous scientific approach to critically analyze the experimental results and put them in the theoretical framework of soil mechanics and of critical state theory. The experimental activities involved X-ray diffractometry, electron scanning microscopy (SEM) and one-dimensional compression tests carried out on both wet and dry soil specimens characterized by different initial densities. To examine the effects of loading on the evolution of the micro-structure, both the particle size distribution and the particle shape, before and after the tests, were analyzed through a dynamic image analysis using several size and shape parameters. The experimental results show that grain crushing becomes evident at vertical pressures larger than 1 MPa whereas it seems to be progressively inhibited for pressures larger than 10 MPa. Furthermore, regardless the initial void ratio of the soil specimens, at high stresses the obtained non-linear e =log s’v relationship converges toward a unique limiting compression curve.