In situ monitoring of tunnel deformation evolutions from auxiliary tunnel in deep mine

Deformation caused by excavation is of great effects on tunnel stability and tunnel support design. Monitoring using instruments that are installed in place within the examined tunnel, cannot record a large amount of surrounding rock deformation that usually takes place ahead and close to the tunnel face. For this important reason, the overall deformation process was observed using instruments installed prior to excavation from auxiliary tunnel in a 900 m deep coal mine tunnel. Two monitoring methods, i.e. multipoint extensometers and wave velocity tomography were used at two monitoring sections with different rock lithology. The results showed that a large amount of deformation at both monitoring sections had already been occurred when the excavation face passed the monitoring sections at a very small distance. The size of the plastic zone at monitoring section 1 is identified as 1.8 m (0.36 D, D denotes the diameter of the examined tunnel) and the velocities within this zone decreased from 4.5 km/s to 2 km/s. The primary deformation was generated in a distance from − 0.8 D (ahead the excavation face) to 2-3 D (behind the excavation face). The induced deformation was also observed after the primary deformation period, and some additional deformation was still occurred in the plastic zone although the size of plastic zone remains constant. The displacements and p-wave velocities at the two monitoring sections showed little differences.