Microstructure development in lacustrine, fine-grained sediments traced by in situ and laboratory testing

Soil microstructure, often defined as the combination of particle arrangement and bonding, can strongly influence the stiffness and strength of sedimentary deposits. As undisturbed soil sampling is a challenging task in fine-grained soils, seismic in situ testing is becoming increasingly more important to investigate its mechanical behaviour. The aim of this article is to evaluate the influences of sediment depositional age and structure-forming processes on the degree of soil structure development in Alpine deposits. Seismic in situ and laboratory testing comprising X-ray diffraction (XRD), scanning electron microscopy (SEM), pore water chemical analysis and hydrochemical modelling were executed at three Austrian test sites, namely Lokalbahn Salzburg, Rhesi and water reservoir Raggal. Based on the comparison of in situ shear wave velocities (V_S,SDMT) with bender element results (V_S,BE), executed on reconstituted soil specimens, it is shown that the Pleisto–Holocene-aged deposits are characterized by a V_S,BE/V_S,SDMT ratio of < 1, indicating the presence of microstructure. However, the youngest sediments (< 50 years) exhibit the weakest microstructure (V_S,BE/V_S,SDMT ≈ 1). The increase of soil structure with sediment age is represented by the updated normalized small-strain rigidity index, K^*_G, which is situated at the proposed transition between structured and unstructured soils (K^*_G = 330), ranging between 250 and 350, at all test sites. The development of microstructure can be attributed to the precipitation of calcite (CaCO₃) cements in open pores, which strengthens the interparticle bonding between detrital quartz, feldspar, clay minerals and carbonate grains, subsequently reducing the soil`s porosity with increasing sediment age.