Development of a Non-Contact Survey Method of Tunnel Excavation Face for DEM Modelling

The excavation of underground voids, both for mining or civil purpose, inevitably determine a variation of the acting state of stress and consequently of the safety condition. The results are: a general deformation state induced around the excavation and, possibly, localized displacements in correspondence with pre existing discontinuities. Moreover, during tunnel excavation, there is a constant need to verify the real fracture state of the rock mass since, during the design phase, the rock mass condition has been determined on the base of borehole or surface rock mass surveys with low reliable level. All these reasons lead to the common practice to monitor tunnel excavation with convergence measurements, face extrusion measurements and structural survey of the rock mass at the tunnel face. The structural survey can be a complex and dangerous operation when dealing with low qualitaty rock mass due to the possible unstable condition of the face due to the quoted reasons. Moreover in certain delicate cases the face must be rapidly consolidated and this makes the survey even more difficult to be realized. Finally the tunnel section can be some meters high making impossible to reach it all without a special lifting system. All these reasons led the authors to develop a method for the geostructure survey based on photogrammetry. The core of the method is the possibility to obtain all face information by taking just few photographs, reconstructing automatically a digital surface model (DSM) of the face and then applying a code developed by the authors called Rockscan for the determination of the discontinuity in terms of orientation and location on the face. The method has been developed trough the application to a real case at a railway tunnel face in the province of Parma (North Italy). The paper describes the method, the developed code and the application to the tunnel.