Basin-Scale 3D Sedimentary Modelling: An Approach to Subdivide Baltic Sea Onshore Sediments for Land use and Construction

Understanding the local stratigraphy and geometry of sediment units is necessary for successful 3D modelling and the prediction of ground behaviour and engineering-geological properties for urban land use and construction. A detailed 3D model of the main sediment units for the Rastaala basin in southern coastal Finland was constructed, combining a conceptual geological model and information from local drill-hole logs and geotechnical drilling. The 3D modelling was based on a cross-section approach, where sediment units were defined on the basis of lithological boundaries that are of major geotechnical importance for land use and construction. The conceptual geological model was based on a sequence of events that characterize the late- and postglacial evolution of the Rastaala basin. The present study combines the allostratigraphy and lithostratigraphy of unconsolidated glacial, late-and postglacial sediment units in southern coastal Finland. It also investigates the appearance and distribution of sediment units in the Rastaala basin and compares modelling and visualization results for unit surfaces and solids using two independent modelling software packages. Finally, information on the geotechnical properties of different sediment units in the Rastaala basin is provided so that they can be applied to other onshore regions with similar geological settings in the coastal Baltic Sea Basin.

Regional-scale susceptibility modelling of shallow landslides involving the weathered and fractured sub-surface bedrock

<p>Landsliding is a complex phenomenon and its modelling aimed at predicting where the processes are most likely to occur is a tricky issue to be performed. Apart the chosen modelling approach, for both data-driven and physically-based models, paying adequate attention to the predisposing and triggering factors, as well as the input parameters is no less important. Generally, shallow landslides mobilize relatively small volumes of material sliding along a nearly planar rupture surface which is assumed to be roughly parallel to the ground surface. In the literature it is also widely accepted that shallow landslides involve only unconsolidated slope deposits (i.e., the colluvium), then the rupture surface corresponds to the discontinuity between the bedrock and the overlying loose soil. In this work, based on systematic field observations, we highlight that shallow landslides often involve also portions of the sub-surface bedrock showing different levels of weathering and fracturing. Then, we show that the engineering geological properties of slope deposits, as well as those related to the underlying bedrock, must be considered to obtain more reliable shallow landslides susceptibility assessment. As a first task, a multi-temporal shallow landslide inventory was built by photointerpretation of aerial orthoimages. Then, a new fieldwork-based method is proposed and implemented to acquire, process and spatialize the engineering geological properties of both slope deposits and bedrock. To support the regional scale approach, field observations were collected within, in the neighbour and far from the shallow landslide areas. Finally, both physically-based and data-driven methods were implemented to assess and compare shallow landslide susceptibility at regional scale, as well as to analyse the role of spatial distribution of rock mass quality for shallow slope failure development. The results highlight that, according to geology, structural setting and morphometric conditions, bedrock properties spatially change, defining clusters influencing both the distribution and characters of shallow landslides. As a consequence, the physically-based modelling provides better prediction accuracy when two possible rupture surfaces are analysed, the shallower one located at the slope deposit / bedrock discontinuity, and the deeper one located at the bottom of the fractured and weathered bedrock horizon. Even though the physically-based and data-driven models provide similar results in terms of ROC curves, the resulting susceptibility maps highlight quite substantial differences.</p>

Experimental investigation into non-collapsible loess-like soil's engineering geological properties under the influence of freeze–thaw cycles

Recurrent freeze–thaw cycling is a powerful weathering process that subjects soil to frost heave and thaw settlement. The cycling significantly changes the soil structure and leads to particle recombination, ultimately causing deterioration of the soil's properties. Herein, we investigated the effect of freeze–thaw cycles on loess-like soil in seasonally frozen regions; specifically with respect to the engineering geological properties, such as changes in grain-size distribution, liquid–plastic limit and collapse behaviour. The results indicate that over the course of 20 freeze–thaw cycles, the soil samples’ dry density decreased, the particle diameter of each soil fraction changed in a bi-directional manner (large particles were broken into finer sizes and fine particles agglomerated into larger particles), and the liquid limit and plasticity index first increased and then decreased. An increase in the initial water content resulted in more significant changes. In contrast to the undisturbed soil samples, the freeze–thaw cycles caused non-collapsible remoulded loess-like soil to collapse. Furthermore, microscopic evaluation allowed us to determine the collapse mechanism. Essentially, after the freeze–thaw cycles, the quantity of micropores in the soil decreased, whereas that of medium pores and macropores significantly increased. In addition, a trellis structure, similar to the typical loess structure, was generated.

Engineering-geological characteristic of the sediments of the loess-palaeosol sequence of the key section Lysohora (Podolian Upland)

The Lysohora key section is one of the most complete and interesting sections of the loess-palaeosol sequence of the eastern part of Podolian Upland. This is the area where the Pleistocene loess-palaeosol sequence was deposited directly on the rocks of Precambrian crystalline shield, its weathered part (eluvium). The value of engineering-geological research of the section is increasing due to the fact that it is located near the city of Vinnytsia, which has been intensively built up in recent years. We emphasize that in this section Upper Pleistocene loesses play a minor role. The value of the Middle Pleistocene loesses, which are generally poorly studied, is abruptly growing. The Lysohora section was sampled. Above 30 monoliths were taken with the purpose of studying the engineering-geological characteristics of the sediments. Among them are grain size distribution, natural moisture content, plasticity index, soil density, porosity coefficient, subsidence, angle of internal friction, specific cohesion, deformation modulus, chemical composition of aqueous extract, humus content, CaCO3 content, etc. The results of the exploration of the loess-palaeosol sequence properties are correlated with the corresponding stratigraphic horizons of the other sections of Podolian Upland. Besides the main section, which was described in a pit, two additional cuts were studied: one on the Korshiv palaeosol complex, another one on the ice wedge pseudomorphosis of Yarmolyntsi palaeocryogenic stage. It is revealed that loess and palaeosol horizons are very different in their engineering-geological properties. In particular, insignificant subsidence is a characteristic of only individual samples from the upper horizon of the Middle Pleistocene loesses. Palaeosol horizons usually have higher values of density and natural humidity. They have a heavier particle size distribution. According to the analysis of the water extract, the highly soluble salts are practically washed away from the loess-palaeosol series. The significant influence of palaeocryogenesis processes on the formation of engineering-geological properties of rocks is also described. Key words: loess-palaeosol sequence, Pleistocene, engineering-geological characteristic of sediments, palaeocryogenesis.

Engineering-geological characteristic of the sediments of the loess-soil series of the key sections Ternopil and Malyi Khodachkiv (Podolian Upland)

Pleistocene periglacial loess-soil series covers the watersheds, watershed slopes and high river terraces of the Volyn-Podolian Upland by the almost continuous mantle of different thickness. A detailed description of two key sections of the loess-soil series of Podillia – Ternopil and Malyi Khodachkiv is given. Both sections are located on the territory of the Ternopil Plateau. The monoliths were taken with the purpose of studying the engineering-geological characteristics of the sediments only from the Upper Pleistocene part of the sections of about 5 m thickness. The following engineering-geological characteristics are defined for the sediments of the described sections: grain size distribution, natural moisture content, plasticity index, soil density, porosity coefficient, subsidence, angle of internal friction, specific cohesion, deformation modulus, the chemical composition of aqueous extract, humus content, CaCO3 content, etc. Integrated analysis of the distribution of parameters of composition and properties of sediments of loess and palaeosol horizons is carried out. It demonstrates their significant difference between certain horizons caused by peculiarities of palaeogeographical conditions of their formation and diagenetic transformation. It is also revealed that the degree of contrast of parameters of composition and properties of loess and palaeosol horizons in Ternopil and Malyi Khodachkiv sections in comparison with key loess-soil sections of Volhynian Upland is much lower. Evidently, it may be due to the small thickness of stratigraphic horizons. It should be noted that palaeocryogenic processes, in particular, diluvial-solifluction deformations and frost wedging, had a significant influence on the formation of engineering-geological properties of the sediments of these key sections. It is important that the engineering-geological properties of the loess-soil series were studied according to the same method in licensed engineering-geological laboratories. This gave us the possibility of correlating the properties of sediments not only in particular sections but also on the regional scale. The conducted research has not only scientific but also important practical value as the loess-soil stratum everywhere is the object of human economic activity and the parent rock of modern soil, etc. Key words: loess-soil series, loess, palaeosol, engineering-geological characteristic of sediments, key section, Podolian Upland.