Finite Element Method (FEM) - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 275

nized by the decrease of the normal stresses σ z computed in the horizontal section

above the roof. This indicates that an unloading takes place above the tunnel.

Figure 10.50 Normal and shear stresses along horizontal and vertical sections
and the vault (Wittke 2000b)

The stress resultants in the shotcrete membrane calculated for cases 1 and 2 shows the
large influence of a temporary support of the vault's invert (Fig. 10.51). Considerably
smaller normal thrusts and bending moments result in both cases compared with the
advancing vault excavation with open invert (cf. Figs. 10.47 and 10.51).
Thus, the installation of a temporary shotcrete support of the invert is the most effec-
tive measure to increase the stability of an advancing vault excavation. This measure
considerably improves the load-bearing capacity of the shotcrete membrane and limits
the vertical displacements. However, the installation of a temporary invert and its sub-
sequent removal when the benches are excavated are additional steps of construction
and are disadvantageous with regard to costs and time. Therefore contractors try to
avoid such a solution as far as possible.