Monotonic Behavior of Pompeii Pyroclastic Soil Under Unsaturated Conditions

A study has been conducted at University of Napoli Federico II (Italy) to provide an initial assessment of the stability conditions of the walls that surround the worldwide known ancient city of Pompei. Geotechnical investigations were carried out in situ in an easily accessible area on the man-made embankment behind the wall. The soil was identified as a well-compacted pyroclastic silty sand in unsaturated conditions. A matric suction of about 30 kPa was measured by a tensiometer on an undisturbed specimen retrieved at 3-3.5m depth. To investigate the mechanical behaviour of the soil under in situ conditions, some monotonic triaxial tests have been performed in unsaturated conditions in a Bishop & Wesley triaxial cell specifically designed for unsaturated soils. Specimens have been reconstituted at in-situ void ratio using the moist tamping method. Drained triaxial tests have been performed with a matric suction of 30 kPa (in situ value) and under three different net stress levels (p-ua = 50, 100, 150 kPa). The experimental results have been compared with those achieved in saturated conditions, indicating that the shear strength angle in critical state conditions is equal to 37° regardless of the degree of saturation. Conversely, the CSL in the p’–e plane is a function of matric suction. These results allowed an estimate of the stability conditions of this part of the wall system, highlighting the critical influence of matric suction.