Natural marine clays exhibit distinct dynamic behavior compared to remolded counterparts due to their inherent structural properties. Dynamic and static triaxial tests were conducted on both marine clay types to evaluate stress-strain behavior, double amplitude strains, pore water pressure, and dynamic elastic modulus, as well as post-cyclic strength attenuation. The results indicate that due to the structural properties, the effective stress path of undisturbed samples is more ductile than that of remolded samples. Also, there is a clear inflection point in the strain development curve of undisturbed samples. The structure exerts a certain degree of restraint on the strain development of the undisturbed samples, and has a distinct impact on the variation of pore water pressure at varying dynamic stress levels. Both marine clay types exhibited gradual reductions in dynamic elastic modulus and marked undrained strength attenuation. Critically, the attenuation of dynamic elastic modulus in undisturbed samples aligned with post-cyclic strength loss, while remolded samples exhibited greater dynamic elastic modulus loss relative to strength degradation. These findings clarify the role of soil structure in cyclic response and strength degradation, offering insights for the long-term stability assessment of structures and disaster mitigation in marine clay engineering.

Undrained residual strength, s(ur), often termed remolded or postcyclic strength, is a critical input into embankment dam numerical deformation analyses. There are multiple methods available to assess s(ur) for fine-grained soils, each with advantages and disadvantages. Field tests, such as the vane shear test and the cone penetration test, can provide reliable in situ measurements of s(ur). In the laboratory, s(ur) can be estimated by measuring the shear stress mobilized at high strains in monotonic tests such as direct simple shear or triaxial shear. s(ur) is also frequently determined from postcyclic monotonic testing; however, the postcyclic stress-strain curves can be difficult to interpret because of high excess pore water pressure existing at the start of monotonic shear due to the sample being previously subjected to cyclic loading. Such analyses often have a significant amount of uncertainty. The work described here presents two new methods developed to quantify s(ur) through lab testing, namely, analysis of stress paths from postcyclic monotonic tests and iterative strain-controlled cyclic loading. This paper introduces the new approaches and presents results from testing performed on five fine-grained soils from the foundations of embankment dams. Values of s(ur) from the new approaches are compared with those from VST and monotonic and postcyclic monotonic direct simple shear testing. The paper details the new approaches and presents results and conclusions from five fine-grained soils from various sites across the western United States.

Fine-grained soils containing diatom microfossils exhibit unique geotechnical behavior due to their biological origins, but their strength properties controlled jointly by diatom content (DC) and stress history remain to be revealed. In this study, reconstituted diatomaceous soil was prepared by mixing pure diatom and kaolin powders in different proportions. These mixtures were subjected to undrained consolidated triaxial shear tests performed using the Stress History and Normalized Soil Engineering Properties (SHANSEP) procedures, revealing how the DC and stress history affect the soil strength. Adding diatoms improved the mixture strength, and a critical DC of approximately 20% was determined, beyond which the normalized undrained strength of the soil was considerably higher than that of common clay without diatoms. Also, a DC higher than 20% associates with the dilatancy of the studied soil with high OCR. Improving the strength of diatomaceous soil by adding diatoms differs essentially from the case of common clay because the plasticity index of the former remains almost unchanged. New formulas incorporating DC and OCR are proposed for predicting the strength of diatomaceous soil, and data for several well-studied soils confirm their validity. This study improves the understanding of fine-grained soils with biological origins and provides important data regarding the mechanical behavior of diatomaceous soil.

The typical undrained behaviour observed in brittle non-plastic soil is ruled by the combination of density and stress levels. Some specific silty and sandy mining waste with particles morphologies that generate high small-strain stiffness to strength ratios when increasing deviatoric stress (q) in stress paths that tend to decrease mean effective stress (p ') may drop down the deviatoric stress before reaching the frictional critical sate. The ratio between the peak (or yield) value (S-u=q/2) and the corresponding p ' is usually associated with a locus in q-p ' that is commonly associated to a straight Instability Line (IL) with a unique ratio (eta(IL)) and for an initial state parameter. However, this is not the case if an induced anisotropy is installed differently while the at rest stress ratio (hereby defined as initial, K-0) is achieved by continuous rate the principal stresses consolidated in lab prior to loading with distinct values. This fabric effect is decisive for design and in stability assessment of earth structures, like dams or piles of mine tailings where non-plastic fines are dominant, even if the prevailing stress-path is in compression. In a thorough and quite complete program varying these conditions on iron ore tailings from Minas Gerais state in Brazil, reconstituted in lab with differentiated state parameters, and its relation to the induced anisotropy effect.