An overview on the nonlinear behavior of intermediate soils under seismic conditions

Risk-sensitive areas are predominantly located in a wide variety of natural depositional environments, such as alluvial plains, where intermediate soils (silty sands, silts, and sandy silts) are commonly found. Additionally, intermediate soils are common in man-made geotechnical structures such as riverbanks, hydraulic fills, dredging sediments and mine tailings. Due to the complexity of their behavior and consequent variability of their main physical and mechanical properties, the geotechnical modelling of intermediate soils is still relatively poorly understood. Existing characterization and interpretation approaches, typically developed for markedly cohesive-behavior soils (clays) or cohesionless-behavior soils (sands), show severe limitations for geotechnical engineering applications that involve intermediate soils under dynamic conditions. Most of the recent studies, both experimental and theoretical, have been carried out on clean and reconstituted sands, while considerable effort is still devoted to understanding the behavior of natural soils with different grading and plasticity. An emblematic case history that revealed this need is, for instance, the city of Adapazari (Turkey), where liquefaction was observed after the 1999 Kocaeli earthquake of magnitude Mw = 7.4. Hundreds of buildings settled as much as 1.5 m, or tilted, in part, due to liquefaction-induced softening and shear strength loss of the foundation soils. Punching of buildings into cyclically softened foundation soils caused occasional bulging of sidewalks, while lateral translations of buildings were observed in the same cases. The soils investigated in Adapazari consist of silt and clay of variable proportions, with cyclic triaxial tests correlating the results with the observed ground failure-induced damage. Moreover, the classification of liquefiable soils based on current criteria does not successfully identify liquefaction triggering, highlighting the peculiarities of these challenging soils and the need for further studies. This paper proposes an overview of the nonlinear behavior of intermediate soils under seismic conditions, which is based on the most recent literature on this demanding topic. The outcomes of this collection are used to discuss the adequacy of currently available methods and testing techniques for the characterization and modelling of intermediate soils and define deficiencies where the research should concentrate in the next near future.