Reliability of displacement measurements in two cases of viaduct-extremely slow landslide interactions

During the monitoring of extremely slow landslide movements great attention should be paid to the precision and accuracy of measurements, because they are usually affected by random and systematic errors with the same order of magni-tude of the displacements. This paper describes the errors recognized for incli-nometer and Total Station measurements carried out in two cases of viaduct-large extremely slow landslide interactions. Systematic errors in the inclinometer displacements were recognized by collecting four-groove measurements, and were reduced by integrating the displacements locally, where the sliding surfaces were identified. Systematic errors in the Total Station displacements were attributed to the changes of the refraction index depending on the atmospheric conditions, and were reduced by calculating only the average rate of displacements. For both the inclinometer and Total Station measurements systematic errors were found to be of the same order of magnitude of the displacements, while precision was at least one order less. After correcting the systematic errors, the redundancy between the two types of measurements resulted very good and then it was possible to estimate a rate of movement less than 10 mm/year by using only 2 or 3 years of measurements.