Influence of particle characteristics on the liquefaction resistance and the dynamic properties of sands containing non-plastic fines

• Applicants: Dr.-Ing. Meisam Goudarzy; Professor Dr.-Ing. Torsten Wichtmann
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 466544626

A major part of the damage in urban areas caused by earthquakes results from a liquefaction of the subsoil. Many liquefaction-induced catastrophic incidents have occurred in deposits of silty sands, i.e. sands containing non-plastic fines. Consequently, numerous experimental studies on mixtures of sand and fines are documented in the literature. Most previous investigations concentrated on the influence of the amount of fines on various soil properties like shear strength, critical state parameters, liquefaction resistance or small-strain stiffness. However, the conclusions regarding the tendencies for a certain soil property with increasing fines content are not unique between the different studies. The discrepancies may be due to differences in the particle characteristics (size, shape and mineral composition of the grains) of both the host sand and the fines used by the different authors. Considering the lack of respective investigations in the literature, a systematic study on the combined influence of the amount, shape and mineralogy of the coarse and the fine particles is a main objective of the proposed project. The investigation concentrates on quantities being relevant for seismic events, in particular the liquefaction resistance and the dynamic soil properties, i.e. stiffness and damping at small to intermediate strain levels, determining shear wave propagation. Four materials with significantly different grain shape or mineralogy, namely round glass beads, natural sand, crushed glass and crushed limestone are used for both the coarse and the fine particles. The four coarse and four fine materials are mixed in 16 different combinations, each being tested with different fines contents. While the first phase of the project will concentrate on mixtures below the threshold fines content (fines-in-sand structure, in total 52 materials), the research will be extended to mixtures above that transition value (sand-in-fines structure) in the second phase. For each mixture, undrained monotonic and cyclic triaxial tests as well as resonant column tests are performed. The influence of the amount of fines and the particle characteristics of the host sand and the fines on the critical state lines (CSL), the liquefaction resistance and the dynamic soil properties is evaluated. A thorough inspection of the applicability of the concept of equivalent granular void ratio e* is another objective of the project. The parameters of the e* concept are determined based on the CSL data for the various mixtures and formulated as functions of fines content and particle characteristics. The applicability of the e* concept for the interpretation of the measured liquefaction resistance and the dynamic soil properties is examined. Furthermore, it is inspected if constitutive models developed for clean sand applied with e* describe the behaviour of silty sands with sufficient accuracy, in particular under undrained cyclic loading conditions.

• DFG Programme: Research Grants