چكيده لاتين
The soft argillaceous-marly rocks of northwestern Iran—particularly the units belonging to the Baghmishe and Upper Red formations, which constitute the bedrock underlying much of Tabriz—are among the most challenging materials in engineering geology. Their high susceptibility to weathering, creep deformation, and progressive failure poses serious issues for the long-term stability of foundations, underground excavations, and both natural and man-made slopes. Despite their extensive occurrence within Neogene basins of Iran, the governing mechanisms of time-dependent deformation in these hard-soil/soft-rock materials remain inadequately understood due to heterogeneous mineralogy, variable carbonate content, and complex microstructures.
This research represents the first integrated geological, physicochemical, mineralogical, and rheological investigation of Tabriz marls, with a primary focus on creep-inducing mechanisms and the development of accurate predictive tools for engineering applications. Following comprehensive field investigations and mapping, 51 samples were collected from various parts of Tabriz and subjected to a full suite of laboratory analyses, including X-ray diffraction (XRD), calcimetry, SEM imaging, petrographic studies, and standard geotechnical and mechanical tests. The results indicate that the degree of cementation, calcium carbonate content, the type and amount of clay minerals—particularly montmorillonite and illite–montmorillonite mixed layers—as well as microstructural heterogeneity, play the central role in controlling strength, durability, and creep susceptibility.
Long-term uniaxial creep tests conducted at stresses exceeding the yield stress revealed that all specimens exhibit the three classical creep stages, with failure times ranging from 3 to 12 days. The green and yellow argillaceous–marly units demonstrated more accelerated creep behaviour, whereas the grey marls, despite lower strain accumulation rates, reached complete failure at smaller strains—reflecting their anisotropy and internal structure. To examine deformation mechanisms at the microstructural scale, a particle image velocimetry system (Geo-PIV) was employed during creep tests. The PIV analyses revealed internal displacement fields, crack initiation patterns, and zones of progressive stress concentration that were not detectable in macroscopic observations, thereby establishing a clear link between mineralogical–microstructural characteristics and time-dependent mechanical behaviour.
Based on these findings, a dedicated rheological model for soft argillceouse–marly rocks—termed the Argillaceous Creep Model (ACM)—was developed. The model explicitly incorporates swelling-clay content, cohesion, and friction angle within its framework, and accurately reproduces elastic, viscoelastic, and viscoplastic components of behaviour, enabling mechanism-based predictions of long-term deformation.
Furthermore, integration of mineralogical, microstructural, mechanical, and durability data led to the formulation of a new engineering-geological classification system for the argillaceous sequence of Tabriz. This framework introduces two quantitative indices—the Strength Index (STI) and the Degradation Index (DEI)—allowing effective differentiation between stable and highly unstable facies. The results show that the conventional term “marl” does not adequately reflect the true variability or engineering behaviour of these materials: the yellow and olive-coloured units fall outside the marl category, while the grey marls are subdivided into more precise engineering subgroups.
