Tunnel monitoring is an important task in civil engineering that aims at determining the stability and safety of a structure by using information about its deformations. In this paper we present the development and the results of a fast method for displacement measurement based on digital images, which allows a deformation analysis along the cross-sections of a tunnel. This technique was created to overcome some drawbacks of most traditional sensors, especially in the case of underground lines, when time becomes an issue of primary importance and limits the number of cross-sections that it is possible to check.
The method was tested inside a laboratory and inside a real gallery, by using several digital cameras and different target configurations in order to determine the best compromise in terms of precision and survey time. The accuracy was checked by giving some displacements with a micrometric sledge capable of moving a reference target along prefixed directions. In addition, a control over a period of some months was carried out to test the stability of the method under different conditions in terms of temperature, humidity and illumination.
The experimental results demonstrated an accuracy better than ±1 mm for a tunnel wider than 12 meters, notwithstanding the shorter data acquisition time and the low cost of the cameras used.