Multitemporal monitoring of rock walls using topographic methodologies and persistent scatterers interferometry

Purpose: Multitemporal monitoring of rock walls is aimed at checking a study area, located in the northern area of Vecchiano municipality (Pisa, Italy) which is characterized by a morphological slope with vertical rock walls, somewhere weathered and interested by fracturing systems, prone to gravitational instability phenomena. The geological hazard of the site is conditioned also by the presence of a municipal road at slope bottom which increases the risk for pedestrians. Methods: The rock walls were monitored by multitemporal topographic measurements carried out by Total Station (TS) using 34 prisms permanently installed on the rock walls. Among these prisms, 4 reference prisms were installed far from the walls, in areas considered to be stable over time, while the remaining 30 were located on the rock walls, in correspondence of discontinuities and on potentially unstable blocks. All the monitored prisms and targets were georeferenced by a GNSS survey carried out in static mode on 2 reference accessible points used to roto-translate the coordinates of all the targets and prisms under observation. These topographic measurements were coupled with a satellite radar imagery analysis carried out by utilizing Sentinel-1A and 1B data, in both ascending and descending orbit, lasting for 2 years period. The chosen methodology is the Persistent Scatterers Interferometry (PSI) which uses natural or artificial targets that are considered stable over time in terms of radiometric response. Theoretically, the PSI allows to evaluate quantitatively and qualitatively the possible variations in distance of individual features (pixels) along the Line of Sight (LOS) between the satellite sensor and the object on the ground. Given the morphological complexity of the site with heterogeneous vegetation distribution (shrubs and small bushes with scattered grassy areas), limestone outcrops, and the poor presence of man-made structures to be considered stable over time, three metal targets were designed and subsequently installed on the edge of the rock walls. The reason of this installation was to guarantee the presence of some artificial metal targets to be used as persistent scatterers. Results: The TS measurements were performed every 4 months, for 2 years, with the following settings: two face measurement, for 10 repeated times at each cycle, for every prism. None of the monitored prisms have recorded significant displacements both in terms of elevation and slope distance. PSI analysis showed that the area is generally stable downstream of the monitored wall with speed along the LOS generally within the tolerances of the technique (± 2mm/year). Conclusions: Topographic monitoring showed that the rock walls, in the monitored time span, was stable, with no movements suggesting an imminent detachment of material. On the other hand, some anomalies in the data have brought to light how the area, where the fixed base of the TS is located, is affected by the seasonal variations of the groundwater which determine subsidence and significant oscillations of the soil level up to 6 mm. The PSI analysis allowed to assess and confirm this subsidence most probably due to the TS location on the alluvial plain.