Engineering properties and characterisation of sediments at the Belgrade Waterfront test site

The Belgrade Waterfront (BW) is one of the largest ongoing construction sites in the Republic of Serbia. It is at the confluence of two major rivers, the Danube and Sava, where natural soil conditions vary from soft clays, silts, and sands to heavily overconsolidated marls underlined by limestone. The geological conditions encountered on the BW site can be considered a benchmark test site for other sites in Belgrade with similar ground conditions. Various in situ tests were performed to characterize ground conditions. The most important tests used were: The bi-directional pile static load test (BD test), seismic dilatometer test (SDMT), piezocone penetration test (CPT), and Menard pressuremeter test (MPT). We focused on the ground investigation performed at two plots. Plot 12 was closer to the confluence, where marl was thinner and softer compared to plot 19, where marl was thicker, with preserved microstructure, and was less influenced by water from alluvial deposits lying above. Our intention of this research was to address some of the most important factors that influence soil behavior and their significance for the interpretation of in situ tests. Those factors are partial consolidation effects (rate effect), microstructure, and stress history. Having a good understanding of geological conditions at the site, the applicability of common correlations used to derive stress history, stiffness, and strength parameters from in situ tests were assessed for various soil types. It was shown that partial consolidation around the dilatometer blade can develop in the lower parts of floodplain sediments. Marls behave undrained for standard rates of penetration and testing. However, microstructure influences the interpretation of in situ tests, particularly CPT, in terms of soil type. Following the basic CPT classification, marl is described as a coarse-grained soil. However, the CPT also indicated high measured pore pressure at the piezo element, suggesting the presence of fine-grained material. According to previous research, this behavior is common for soils with a microstructure. It was shown that OCR in alluvial silty clay and marls can be overpredicted 3–4 times from common CPT correlations integrated in commercial software. This uncertainty may be attributed to unusual soil conditions in terms of microstructure and partial drainage effects. The interpretation of MPT results indicated that marls have distinct strain-softening behavior, while macrostrain measurements from the BD test indicate nonlinear softening of skin friction in limestone.