Minero-petrographic characterisation of Palatine Hill’s Central Cryptoporticus foundation materials: implications for structural stability and conservation

The Central Cryptoporticus on Rome’s Palatine Hill is a key underground monument of the Imperial period whose accessibility and long-term conservation are increasingly constrained by localised deformation, differential settlement, and cavity-related instability. Because these hazards are strongly conditioned by the nature and evolution of the foundation interface, we investigated how stratigraphy, mineralogy, and microstructure interact to control present-day vulnerability. Lithofacies logging from three boreholes (S1, S2, 1MS) and a trench site was integrated with portable X-ray fluorescence (pXRF), X-ray diffraction (XRD), petrography, and soil micromorphology on representative foundation units: pedogenised floodplain deposits of the Aurelia formation (AEL), variably altered and cemented volcaniclastic deposits of the Villa Senni formation (VSN), and underlying fluvial sand-silt deposits of the Fosso del Torrino formation (FTR). Results show that AEL is best interpreted as a structured, compound palaeosol shaped by polyphase wetting-drying and pedogenic redistribution. Alluvial clay coatings, vertic fabrics/slickensides, Fe-Mn redox features, and carbonate nodules produce a centimetre-scale mosaic of contrasting porosity and hydraulic behaviour, implying strong sensitivity to moisture change and water routing. In VSN, two end-member petrofacies coexist: a friable, pedogenised ash-rich facies where vitric-derived material is extensively transformed into clays and secondary phases, and a more coherent lithoid facies strengthened by zeolite and calcite cementation. Although cementation increases cohesion locally, hydrous secondary phases and alteration pathways create moisture-sensitive fabrics that can promote microcracking and softening where saturation persists. Across the foundation profile, rainwater infiltration, percolation, and capillary rise couple AEL and VSN, driving shrink-swell deformation, alteration and cement redistribution, and spatially patchy CaCO₃ re-precipitation that is insufficient to offset ongoing weakening. Where these moisture-sensitive horizons intersect a dense network of anthropogenic voids, the likelihood of deformation and local collapse increases, making differential settlement and cavity-roof instability the dominant hazards for the monument. The findings support conservation priorities focused on surface- and groundwater control, targeted stabilisation of the most compressible or voided foundation sectors, and long-term hydro-geotechnical monitoring.