Physical and structural properties of a pyroclastic soft rock

The subsoil of a large area surrounding the city of Rome (Italy) consists mainly of pyroclastic deposits originated from the explosive activity of the Colli Albani volcanic complex. This paper focuses on a type of pyroclastic soft rock, locally known as Pozzolana Nera. The structural features of the deposit are presented in the paper. These are characterised by a marked heterogeneity in terms of grading, nature of grains, and inter-granular bonds. With reference to unconfined compression strength the Pozzolana Nera can be classified as a weak rock. The mineralogy and microstructure of the Pozzolana were examined by means of X-ray diffractometry and optical and electron scanning microscopy (SEM). New techniques were developed to obtain cylindrical laboratory samples of the Pozzolana in its natural state; minor effects of the sampling procedures on the microstructure were assessed by comparing the velocity of propagation of compression waves through the soft rock in its natural state and after sample preparation.