Dualism of geology and geotechnical engineering: two parallel approaches to define lake water infiltration into an interlobate esker

This case study is a modern example of how geologists and engineers have worked together to reconcile groundwater abstraction from the Pispalanharju aquifer and urban planning of the shore of Lake Näsijärvi in the vicinity of the Hyhky pumping station, located in the Tampere region of southern Finland. The authorities have been concerned about the potential impact of urban development on the important aquifer system to which the lake shore region is possibly connected. Before the study, it was estimated by the authorities that the infiltration rate into the esker from Lake Näsijärvi could be up to 1000 m 3 /day, forming a significant source of groundwater. Two parallel studies were conducted to examine water infiltration from the lake to the esker. The effects of a proposed land reclamation project on the shore of Lake Näsijärvi were modeled with the finite element code Plaxis 2D. Coupled analysis was conducted to analyze both the flow conditions and deformations in these conditions caused by the fill. An essential part of the geotechnical modeling was the creation of the present state of the shore complex, which already includes manmade fills and structures and a low permeability lake bottom next to the esker. Combining observations with geological and geotechnical data was highly important in the task. The amount of water flow to the esker was estimated from the analysis, and the values were compared with the results of hydrogeological analysis. For the hydrogeological studies, thousands of soil investigations were interpreted by different methods to fit a geological concept that formed the basis of geological cross-section construction work (2.5D), which was then used for groundwater flow modeling by GMS. Both GMS and Plaxis modeling simulations predicted that, due to the low conductance of silt and clay materials found on the shore of Lake Näsijärvi, the amount of water infiltrating in the direction of the esker is very limited, being only 60 to 155 m 3 /day. Hence, the dualism of hydrogeology and geotechnical engineering hypothesized in this case study was confirmed by modeling results.