Incidence of saturation and fabric on the physical and mechanical behaviour of soft carbonate rocks

Very soft carbonate rocks belonging to the Calcarenite di Gravina Formation (upper Pliocene-lower Pleistocene) crop out extensively in the Apulia region (Southern Italy) and, as such, are involved in a huge amount of engineering boundary value problems regarding building foundations, underground caves and sea cliffs. These calcarenites are poorly graded and dense, irregularly cemented, light yellow coloured and mainly composed of bioclasts. Throughout the region, these rock materials exhibit spatial (both horizontal and vertical) variability as a consequence of a complex depositional mechanisms so that their overall behaviour is strongly conditioned by depositional fabric and diagenetic processes. At the rock mass scale, post-depositional tectonic episodes and groundwater circulation have to be considered. In order to explore all the factors influencing the physical and mechanical response of these materials at the sample scale, a large set of data on different facies was collected and compared among them. Therefore, petrographic analysis on thin sections and, water imbibition, uniaxial compression and indirect tension (Brazilian) tests were carried out on samples retrieved from several sites of the region. In particular, the mechanical behaviour of these calcarenites results to be strongly affected by the presence of water in the pores, with strength and stiffness that significantly reduce in the transition from dry to saturated conditions; times and modes of saturation processes are strictly linked to the fabric features of the different facies. The post-peak behaviour of the rock samples was investigated by using a servo-controlled testing machine and a clear brittleness was generally observed for most of the samples, even though strength decay heavily reduces under saturation conditions especially for those samples characterized by an extremely variable distribution of cement.