A method for estimating the accuracy of tunnel convergence measurements using tape distometers

A 60 km railway near Bolzano in Northern Italy has been re-opened after 15 years of disuse. The railway passes through three tunnels that were built at the beginning of the twentieth century and were consistently being repaired up until the 80s. Since 2004 convergence measurements using a tape distometer have been carried out once a year in those tunnels in order to check their stability. Because the movements were quite minimal, a method to assess the accuracy of the convergence measures was needed for recognizing the actual changes in length of the reference points. The accuracy was estimated as the total error calculated by introducing the measurement error, the calibration error and the frame error. When the frame error was calculated for each day of measurements, change in line length as little as 0.25 mm was possible to detect. This value is about one half of the change in length that can be measured by using other techniques such as the Total Station or the digital camera.