Mineralogical and Micro structural Investigation into the Mechanical Behaviour of Tabriz marl and presenting a new comprehensive classification based on the main engineering geology aspects

The rapid urban development in eastern Tabriz, Iran, has highlighted the geotechnical challenges posed by widespread argillaceous-marly soft rocks, which exhibit complex mineralogy and weak mechanical behavior. These Neogene sediments, deposited in a shallow, low-energy lacustrine environment, contain gypsum, illite, muscovite, and fossil fragments. Their high porosity, low cementation, and heterogeneous microstructure contribute to limited strength and durability, characteristics that are inadequately captured by conventional rock mechanics tests. This study employs an integrated approach combining soil and rock mechanics methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), petrographic analysis, and standard mechanical testing, to systematically characterize the mineralogical, microstructural, and geomechanical properties of these transitional deposits. Results demonstrate that clay bonding, carbonate cementation, and microstructural heterogeneity critically control strength, stiffness, and failure behavior. To improve engineering assessment and material classification, two indices—the Strength Index (STI) and the Destruction Index (DEI)—are introduced. Materials with STI < 0.5 MPa and DEI > 30 % are highly prone to instability, whereas those with STI > 1 MPa and DEI < 10 % display favorable geomechanical performance. The proposed classification framework provides a robust tool for evaluating weak argillaceous rocks, supporting more reliable and cost-effective foundation design in similar geological settings.