Use of the tunnel face stability factor in unsupported and supported tunnel faces

The paper studies face stability in shallow and deep tunnels with unsupported and supported faces (using fiberglass nails, forepoling umbrella or a combination of them). Parametric 3D finite element analyses in elasto-plastic ground show that stability of unsupported tunnel faces is controlled by a dimensionless Tunnel Face Stability Factor (Λ_ο), defined in terms of ground strength and tunnel geometry. The parameter (Λ_ο) is correlated with a dimensionless average tunnel face extrusion (Ω_h0) which permits to define face instability. In supported tunnel faces, the numerical analyses calculate the reduced average tunnel face extrusion (Ω_h) due to the beneficial effect of the face support measures. This permits to calculate safety against face instability via the same (Λ_ο - Ω_h) empirical correlation, for both unsupported and supported tunnel faces. In shallow tunnels, the parameter (Ω_h) is also correlated with the excavation volume loss (V_L). Finally, the above dimensionless parameters are correlated with the 2D tunnel deconfinement ratio (λ) along the tunnel axis, which permits to calculate (λ) for use in 2D numerical analyses of tunnels with unsupported and supported faces.