What is the contribution of time-dependent deformation in tunnel convergence?

Tunnel excavation produces stress changes to the ground and strain to the support lining, leading to the closure (convergence) or instability of the excavated area. Convergence recorded after section excavation is assigned to: (i) strain resulting from the progressive tunnel front advance (face advance effect) and (ii) the time-dependent properties of the soil material (ground creep effect). In the present study, based on the geodetic monitoring records of two recent road tunnels in Greece, a simple methodology to estimate the contribution of each of the two effects is presented. Our analysis reveals that at least half of the total deformation of the examined tunnel sections is due to ground creep, indicating that the major portion of tunnel deformation is due to the time-dependent properties of the ground; a result supported by previous studies from other tunnels as well.     

Predictions and observations of convergence in shallow tunnels: case histories in Greece

Convergence of shallow tunnels (30–120 m overburden thickness) constructed in Greece in different types of rock masses has been assessed as a function of the Geological Strength Index (GSI classification). Predictions of maximum vertical and horizontal convergence, during or shortly after tunnel excavation, were made using Finite Element Modeling (FEM) and the ‘characteristic line’ theory, and were found to be in good agreement with geodetic observations of convergence collected during a period of approximately 2 months after the section excavation. The results from FEM were found to adequately and reliably predict the expected deformation during tunnel excavation. The theory of the ‘characteristic line’, on the other hand, seems to offer a realistic and reliable upper-bound estimate of the convergence.     

Ground subsidence,Quaternary faults and vulnerability of utilities and transportation networks in Thessaly,Greece

Ground fissures and subsidence have caused extensive damage in structures and the utility networks (roads, railroad line, Larisa NATO airport, telecommunication network) in the plain of Thessaly, central Greece, in the last 20 years. The fact that these fissures are parallel to Quaternary faults controlling the structural evolution of the Thessaly basin, marked by intense recent seismic activity, led some scientists to regard them as manifestation of tectonic creep, even premonitory phenomena of major future seismic events. A careful study of the geometric characteristics and of the kinematics of these ground fissures, however, revealed that they only reflect shear failure of the ground due to compaction of loose sediments following significant decline of the groundwater level due to over-pumping; i.e. a geotechnical and not a tectonic process. Furthermore, theoretical considerations and modeling predict surface fissuring along zones of anomalous basement, normal-fault controlled topography. This explains the correlation between ground fissures and active Quaternary normal faults.