Deformation and Failure Properties of Colli Albani Tuff

The Colli Albani are an explosive Quaternary volcanic district, which may pose a threat to the city of Rome, Italy. Recent seismic swarms and hydrothermal activity suggest that the main faults, affecting the volcanic complex are cyclically active. A 350 m scientific borehole was therefore drilled into this volcanic area to elucidate its inner structure for the first time (Mariucci et al., 2008). The main unit drilled, called Tufo Pisolitico di Trigoria unit, is a fine-grained, matrix-supported pyroclastic deposit with rare lithic lava clasts and sparse pumice. Elastic wave velocities (Vinciguerra et al., 2008) revealed how, within the same lithology, the different degree of lithification and presence of clasts can affect significantly physical property values. In this study we investigate the micromechanics leading to failure in Colli Albani tuff. Samples of 38 mm in length and 18 mm in diameter were cored in the vertical direction from a core at the Colli Albani site. Our block of tuff had a nominal connected porosity (measured by water saturation) of 32%. We performed a series of hydrostatic and conventional triaxial compression experiments on water saturated samples in drained conditions with 10 MPa pore pressure at room temperature. Under hydrostatic condition, the onset of grain crushing and pore collapse P* occurred at an effective pressure of 41 MPa. Intense microcracking was observed in the deformed sample which failed by cataclastic flow. Four samples were deformed at effective pressures between 5 and 30 MPa. Shear-enhanced compaction was observed at all pressure conditions. In the stress space, the resulting yield envelope mapped out an elliptical cap comparable to previous observations on porous sandstones and carbonate rocks. In the deformed samples we observed pore collapse and microcracking. A theoretical model is being developed for the micromechanics of compaction and failure in Colli Albani tuff. Results support the interpretation of the mechanisms of deformation monitored at the field scale.