Squeezing Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 103

Table 4.2 Analyzed cases

Case Residual strength Discontinuities Supporting pressure

c IR * = 0.5 MPa 1 no no

c IR * = 0.2 MPa 2 “ “

c IR * = 0.5 MPa 3 yes “

4 “ “ p r = 0.6 MPa

(immediately after excavation)

5 “ “ p r = 0.6 MPa

(30 days after excavation)

4.5.2 Reference Case
Fig. 4.12 shows the principal normal stresses and the extent of the plastic zone
300 days after excavation which are calculated for case 1. In addition, the vertical
normal stress component along the horizontal section through the tunnel axis is
also plotted. It can be seen that a plastic zone is formed in diagonal directions. The
elastic solution yields a vertical tangential stress of some 24 MPa at the unsupport-
ed tunnel contour. At 300 days after excavation, this stress reduces due to stress
redistributions and viscoplastic behavior to 1.8 MPa, corresponding to the residual
unconfined compressive strength specified for case 1. From the directions of the
calculated principal normal stresses the forming of an enlarged arch (cf. Fig. 4.3)
is clearly visible. The arching effect leads to high vertical stresses beyond the plastic
zone on both sides of the tunnel up to some 15 MPa.
Fig. 4.13 shows the displacements due to excavation of the tunnel that are calcu-
lated for case 1 at 300 days after excavation. In addition, the vertical and horizon-
tal displacements along the vertical and horizontal section through the tunnel axis
are also plotted. At the roof, the sidewalls and the invert the radial displacements
amount to more than 10 cm. On both sides of the tunnel, the horizontal displace-
ments directed towards the tunnel decrease, within one tunnel diameter, to zero and
change their direction at larger distances away from the tunnel. Above and under-
neath the tunnel, the vertical displacements decrease more slowly.