Cyclic behaviour of clean and silty sand under drained conditions

Abstract

Soil is a natural granular material consisting of solid grains and fluid filled voids. Unlike ordinary solids, granular soils (e.g. sand and gravel) exhibit many interesting features when subjected to loading, such as dilatancy, anisotropy, density, and pressure dependence. Adding to the complexity, their mechanical behaviour is highly dependent on grain characteristics, loading history and drainage conditions. The present study deals with a problem that is of both practical and academic interest in soil mechanics and geotechnical engineering – strain accumulation due to drained cyclic loading. Strain accumulation is a phenomenon that when soils are subjected to cyclic loading with small strain amplitudes, the residual strain increases with the number of cycles and the rate of strain accumulation does not vanish completely. Sustainable offshore wind power turbines are likely to experience such cyclic loading cycles due to winds and waves. Reproducing such soil-structure interaction in the design process is a technical challenge but incredibly important from a fatigue point of view. Comprehensive triaxial test programs were accomplished, covering different soil properties, stress conditions and cyclic loading characteristics. Literature database was also compiled, regarding the critical state lines and strain accumulation resistance of silty sands, to enable further characterization and analysis. Six typical deformation accumulation patterns were identified in which the causes of different deformation patterns were further discussed. These deformation patterns are firstly summarized in detail based on extensive laboratory tests. Among these deformation patterns, the drained flow type caused large deformation and severe failure. Increasing void ratios and cyclic strain amplitudes lead to increasing strain accumulation rates. Within the framework of critical state soil mechanics, the relation between the accumulated strain and the state parameter that collectively accounts for the influences of void ratio and confining pressure is investigated, which sheds some light on constitutive modeling of cyclic strain accumulation. An influence of the cyclic loading frequency on the accumulation behaviour is not detected in the range of 0.01 to 1 Hz. The resilient behaviour confirmed the applicability of elastic constitutive laws for description of the dynamic soil behaviour. The elastic material parameters are found to be independent from the number of load cycles. The effects of saturation and loading sequence were also investigated with careful volume measurement and void ratio determination. The strain accumulation resistance increases with decreasing degree of saturation. And the Miner’s rule and strain hardening method were considered for the influence of loading sequence. Test results of long term cyclic triaxial tests were used for validation of the proposed accumulation model, and to improve the assessment of the permanent deformations and bearing capacity of the soil-foundation systems. An explicit formula was proposed to predict ε^acc as a function of N and 9 model parameters. The procedures of parameter identification and model validation were described. With the experimental data and calibrated parameters in this study, the model was able to predict the long-term permanent strain of silty sands.