Tunnelling is an obviously three-dimensional process. Tunnelling in yielding ground also generates a 3D, bullet shaped zone of plasticity in soft rock, but also generates a bullet shaped fracture zones in the brittle rock masses. Longitudinal displacement profiles (LDP) characterize and describe the displacement history during tunnel excavation, including that occurring ahead of the tunnel face. Experimentally determined LDP provide a valuable database for validation of the tunnelling approaches. This study is presenting the in-situ measurements of two geotechnical instrumentation sections which includes MPBX extensometers and NATM pressure cells from the shallow depth M85 motorway tunnel which is excavated in soft ground and is situated beside Sopron city (Hungary).
The measurement results give us opportunity to validate the LDP and to verify the tunnel support interaction with the tunnel excavation. The results show good qualitative and quantitative agreement ahead of the tunnel face.
Due to the tunnelling one of the technical factors that controls the choice of which tunnelling method and support to be used is the stability of the tunnel face. When the stresses in the rock mass surrounding a tunnel exceed the strength of the rock mass a zone of failure or a "plastic" zone (in hard rock tunnel a fractured zone, mainly EDZs) is formed around the tunnel. When the radius of the "plastic" zone around a tunnel exceeds twice the radius of the tunnel, the zone of failure around the tunnel interacts with the failed rock ahead of the tunnel face to form a continuous bullet shaped "plastic" zone. This three-dimensional plastic zone becomes increasingly difficult to stabilize as the ratio of stress to available rock mass strength increases (Hoek et al. 2008).
For investigation of the mentioned Longitudinal Displacement Profiles (LDP) with convergence-confinement analysis (Duncan-Fama 1993, Panet 1995, Carranza Torres and Fairhurst 2000, Hoek 1999, Vlachopoulos and Diederichs 2009 and others) is a widely used tool for preliminary assessment of squeezing potential and support requirements for circular tunnels in a variety of geological conditions and stress states. An analytical plasticity solution such as that developed by Carranza-Torres and Fairhurst (2000) is applied to a circular opening in an isotropic stress field. An internal pressure, initially equal to the in-situ stress is applied on the inside of the excavation boundary. The pressure is incrementally relaxed until the excavation boundary condition is that of zero normal stress. The extent of plastic yielding (or fracturing) and thereby, the boundary deformation is calculated at each stage of the process. This developing plasticity or yielding zone (or fractured zone in brittle rock mass), combined with the elastic closure of the surrounding rock mass creates a wall displacement profile that is non-linear, develops partially before the advancing face and continues for a number of tunnel diameters before equilibrium conditions are achieved. This profile, known as the LDP is a function of tunnel radius and the extent of the ultimate plastic radius (tunnel radius plus thickness of yielded ground).
