Analysis of sub-seasonal vertical deformations in shallow expansive soils of the northern coastal plain of the Netherlands

Deformations in the clay-rich coastal plain of the Netherlands are believed to be a primary driver of damage to buildings and infrastructure. Nevertheless, the driving processes responsible for vertical land motion in the upper meters of this clay area have been poorly investigated, since the research has been concentrated on deeper processes associated with gas extraction. This study investigates the mechanisms of shallow vertical deformations near Ezinge (Groningen) using high-precision measurements from a unique monitoring setup. Borehole extensometers anchored at three different depths and paired piezometers are used to analyze a one-year record of soil movement. Integrating open-access meteorological data, as well as geomechanical characterization data (CPTs), and implementing statistical analysis, the study quantifies the primary drivers of deformation across different intervals of the Holocene coastal sequence. The results identify the expansive clay in the upper sequence as the primary driver of sub-seasonal vertical deformations, exhibiting vertical variations of up to 12 mm. These movements respond to periods of rainfall and drought, manifesting as distinct swelling and shrinkage processes. Furthermore, the implementation of a numerical model successfully captures the magnitude of these deformations, but reveals the complexity of phenomena such as hysteresis and preferential flow through desiccation cracks, which influence the mechanical response of this layer in the field, and that conventional models struggle to fully replicate.