Issue 52

C. Caselle et alii, Frattura ed Integrità Strutturale, 52 (2020) 247-255; DOI: 10.3221/IGF-ESIS.52.19 248 The stability assessment of quarry environments, both during the active exploitation or in abandoned conditions, requires an attentive investigation of mechanical response (e.g. [2–6]). The mechanical behaviour of gypsum rock also affects several frameworks of deep excavations, as inter-bed layer in evaporitic rock bodies (e.g.[7]) or as a caprock in oil reservoirs. The investigation of mechanical response of gypsum rock is complicated by the high heterogeneity in terms of porosity, grain size, mineralogical composition or anisotropy orientation [8–11]. The microstructural mechanisms that control the strain accommodation in gypsum have for long time interested the scientific world and are still not completely understood [12–14]. The coalescence of cracks in synthetic and natural gypsum samples have been successfully investigated by means of the visual analysis and comparison of photographic sequences [15– 19]. However, this methodology cannot be directly applied to more complex loading conditions, when the sample is inserted in a visual opaque structure for the application of a confining pressure. A largely used methodology for the investigation of samples interested by triaxial loading conditions is the analysis of the microstructures on thin sections. Zucali et al. [20] proposed a description of the microstructural changes induced to gypsum rock under large strains, Brantut et al. [21] investigated the mechanisms involved in the strain of gypsum with microstructural analysis and acoustic emission, deepening the field of the transition between brittle and ductile regimes. Both concluded that the deformation mechanism of gypsum may involve the coexistence of plastic and brittle microstructures. The present paper proposes an experimental investigation of gypsum response under uniaxial and triaxial compression. The mechanical data were associated to a multiscale analysis of the effects induced on the material by the compression. Digital Image Correlation (DIC) analysis and microstructural investigation techniques were adopted. The study aims to propose a description of the mechanisms involved in the strain accommodation and crack coalescence of natural gypsum and of their relation with the increase of confining pressure, with a specific focus on the influence of the rock texture and layering on the mechanical response. In the paper, the rock mechanics sign convention is adopted, with compressive stress and strains considered as positive. M ATERIAL AND METHODS he tested material is a gypsum rock facies deposed in the Monferrato area (NW Italy) during the Messinian Salinity Crisis, a period of anomalous paleo-oceanographic conditions that brought to the deposition of enormous volumes of gypsum, anhydrite and halite all over the Mediterranean basin. The gypsum rocks deposed during the Crisis have a large variability of facies, with big differences in terms of grain size, porosity and mineralogical composition. However, each facies may have a large distribution through the Mediterranean [22–24]. As instance, the branching selenite facies (object of the present experimental investigation) have been recognized and described in the same stratigraphic position in several areas of the Mediterranean (i.e. Spain, Cyprus, Central Italy). It consists of nodules and irregular lenses of white gypsum crystals (mean size 1-2 mm) surrounded by laminae of finer material, containing gypsum, calcite, dolomite, quartz, feldspar and clay minerals. Despite the availability of data about the stratigraphic features of this rock facies [23,25,26], a lack in mechanical data was recognized in the scientific literature. In order to investigate the micromechanical processes involved in the deformation and failure of the rock, different techniques were applied. The failure coalescence in uniaxial stress conditions was investigated by means of DIC analysis. The uniaxial test was performed on a prismatic sample to facilitate the video acquisition and processing. The axial compression (displacement rate of 0.03 mm/min) was applied with a GDS Instruments Medium Pressure Triaxial Apparatus (MPTA) adapted to work without the oil cell, applying only the axial compression and leaving the sample visible. The evolution of axial stresses and axial strains during the test was calculated considering the initial area and axial length of the sample as reference. The in-continuous measurements of loads (from the load-cell) and axial displacements (from the piston movement) complete the set of data used to retrieve the axial stress-strain paths. The sample face was filmed throughout the test with a digital camera. The final video was then processed with Ncorr v.1.2.2 software [27], as described in [17]. For the present application, the elaboration was started at a strain of 0.8% (i.e. in correspondence of the onset point), to avoid the noise produced by the elastic deformation phase and focalize the analysis on the failure coalescence. The DIC procedure is not suitable for the analysis of samples in triaxial stress conditions, because the presence of the oil cell inhibits the acquisition of photographic images during the test. For this reason, the analysis of the strain accommodation mechanisms under triaxial compression was performed by microstructural analysis at the end of the test. The tests were performed with the same GDS MPTA machine used for the uniaxial compression. An isotropic phase brought to the desired conditions of pressure in the oil cell (i.e. confining pressures of 4, 6, 8 MPa). The axial stress was then increased with a constant displacement rate of 0.03 mm/min. In addition to the measures of deviatoric stress and axial strain (analogous to the uniaxial test), the volumetric strain path was retrieved considering the changes of volume of the oil in the cell with T