Mineralogical and geotechnical characterization of a clay unit that underlies the unstable flanks of Mount Etna – Sicily

The mineralogical and geomechanical properties of a distinctive early Quaternary marine clay unit that underlies the eastern and southern flanks of Mt Etna, which may play a role in gov-erning the style of instability on the edifice, have been determined. Preliminary clay mineral identification revealed the presence of interlayered illite-smectite and disordered kaolinite. The clay is heavily over consolidated (OCR≥32) and is a very stiff, extremely high undrained strength closely fissured, dark yellowish grey silty calcareous CLAY. Direct shear and ring shear tests measured peak shear strength parameters of c΄ = 51 kN/m2 and φ΄ = 20˚ and re-sidual shear strength parameters c΄r = 0 to 3.0 kN/m2 and φ΄r = 6.4 to 9.7˚. The mass strength of the clay will be influenced by the polished fissures, which are close to φ΄r, and may be a controlling factor in the style and rate of movement of the observed flank instability depend-ing on fissure orientation.