Current safety requirements for both construction and operation of tunnels demand reliable monitoring and data processing methods. The conventional techniques of convergence measurements are characterized by intermittent recordings, frequently interrupting construction procedures and are not applicable to survey tunnel stability upon its completion. The new extensometric method is aimed to override those difficulties by means of installing sets of electric resistance strain gages (or fiber optics sensors) on the steel arches of any tunnel support system, thus providing continuous recordings of deformations, even beyond the construction phase. A description of laboratory and field tests there were performed to validate the process, as well as the mathematical models formulated to implement it, are presented in association with examples of application. Comparison of field results with those provided by conventional techniques indicated good agreement in terms of obtained convergence values.