Impact of the stress state on the dynamic response of unsaturated sand containing fines

• Applicants: Dr.-Ing. Meisam Goudarzy; Professor Dr.-Ing. Torsten Wichtmann
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 276183850

Strain-dependent stiffness and damping of soils, described by the small-strain shear modulus, the modulus reduction curve and the damping curve are the key parameters to evaluate the ground motion characteristics under dynamic loadings, which are caused by earthquakes, machine foundations or traffic loading. This research project was originally planned to experimentally investigate and describe the strain-dependent dynamic characteristics of dry, saturated and unsaturated granular soils containing fines under isotropic and anisotropic stress states. The project was divided into two phases. In the first phase the main objective was to assess the effect of fines and stress anisotropy. The results revealed the significant effect of both parameters on the dynamic characteristics of granular soils at small to intermediate strains. Empirical relationships were extended to describe these influences. The concept of equivalent granular void ratio was successfully applied for both fine-in-coarse and coarse-in-fine mixtures. The first phase was successfully completed and the results were published in high-quality journals.The current proposal refers to the second phase of the project, where the main focus is laid on unsaturated states, considering that most soils between the ground surface and the ground water table are in such state. Due to the presence of both air and water in the pore space, the stiffness and damping of an unsaturated soil differ from the respective values in the dry or water-saturated state. Therefore, the following objectives are formulated for the second phase:i) Experimental investigation on the effect of the degree of saturation or suction, respectively, and suction hysteresis on the stiffness and damping of unsaturated sand with fines at small to intermediate strains under isotropic and anisotropic external stress states.ii) Description of stiffness and damping of unsaturated sand with fines by empirical equations.iii) Quantification of the influence of the unsaturated state under seismic loading in a ground response analysis.A mixture of 70 % Hostun sand with 30 % Kaolin by weight will be adopted as test material. The specimens will be prepared by proctor compaction at optimum water content outside the resonant column (RC) device and will experience suction along wetting or drying paths either starting from as prepared conditions or from saturated state. Afterwards, the specimens will be installed in the RC device, in which suction is applied using the Axis Translation Technique. Specimens will remain under these conditions for equalization. Finally, RC tests will be conducted to measure stiffness and damping at small to intermediate strains. Empirical relationships will be modified to describe the dynamic characteristics of unsaturated specimens, before being applied in the ground response analysis.

• DFG Programme: Research Grants