Monumentations of control points for the measurement of soil vertical movements and their interactions with ground water contents

Historically, one of the techniques used in the study of vertical movements of soil is that of high precision spirit levelling. Nowadays, this technique is often accomplished by satellite interferometric synthetic aperture radar (SAR) analysis that requires a calibration phase, i.e. through the connection to a spirit levelling network or by means of a long time series acquired by global navigation satellite systems permanent stations in order to transform relative velocities derived by SAR in absolute terms. An important aspect of this process consists of the materialization of the object used as a control point, while its selection may depend on the geological context. Typically, due to the inevitably high cost of installation, very accurate monumentation is reserved only for a limited density of points on the territory; therefore, a type of negative correlation exists, in terms of reliability in the monumentation of the control points, with respect to their density in a real distribution within a specific monitoring technique. For example, in levelling networks, a density of one benchmark every 700–1000 m of line is often desired in order to reach a compromise among costs, practical operative requirements and precision. Levelling benchmarks are usually fixed on existing structures (i.e. buildings or concrete structures). This aspect is even more evident in the case of SAR interferometric analysis, where the persistent scatterers (PS) are identified based on the coherence in the radar response. Therefore, the PS display movements are measured by a variety of different structures, characterized by foundations fixed at different depths. Starting from repeated levelling measurements, we verified the order of magnitude of movements of control points characterized by shallow foundations in cohesive soils. In practice, we observed their behaviour in relation to the depth of the foundation under simple and very common conditions, such as the presence of periods of drought or rainfall. The results indicate movements in the order of 3–7 mm in the first metre of depth that occurred in a week during the transition between the period of summer drought and the first rains. The magnitude of the total uplift observed between the end of the summer drought and the beginning of successive springtime (30/08/2012–18/04/2013) exceeded 10 cm of movement in very shallow layers of the soil surface for the same test field.