Squeezing Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 93

In this chapter we will see how to predict the radial displacements of the tunnel contour
in the case of squeezing rock conditions more realistically by using the elastic-viscoplas-
tic stress-strain law described in Chapter 3. With the aid of the finite element method
(FEM), treated in Chapter 10, uncertainties with respect to construction time and costs
can be considerably reduced compared with the above-mentioned approaches.

4.2 Phenomenon

After excavation of a tunnel the self-weight of the overlying rock mass must be trans-
ferred around the opening. Due to stress redistributions, an arch is formed in the rock
mass near to the opening. To stabilize the tunnel it is normally sufficient to support the
tunnel's contour by a shotcrete membrane of 15 to 30 cm thickness.

In the case of squeezing rock the rock mass strength is considerably smaller than the
stress resulting from the self-weight of the overlying rock. Thus, a high radial support-
ing pressure p r0 would be required to stabilize the tunnel's contour by a stiff internal
lining (Fig. 4.3, above) which is not economic or even feasible.

4.3 Yielding Support

In practice, a yielding support is installed that allows radial displacements of the
tunnel contour to occur in a controlled manner. The radial displacements lead to
stress redistributions in the rock mass that forms an enlarged arch around the tunnel
(Fig. 4.3, lower left). After certain displacements have taken place, the supporting
ring is closed and the full load-bearing capacity of the support is activated. A con-
siderably reduced radial supporting pressure of p r, re d is then sufficient to stabilize the
tunnel (Fig. 4.3, lower).
The overburden usually increases with increasing distance to the portals. Squeezing
rock conditions normally prevail at relatively large overburden and thus at a long
distance from the portals. To account for the radial displacements that are allowed
to take place in the squeezing rock sections, a tunnel with a constant clearance must
be excavated with an overprofile. Therefore the cross-section to be excavated must be
varied in the longitudinal direction. Since tunnel boring machines (Chapter 21) up
to now have no or only limited capabilities to vary the excavated cross-section during
heading (Wittke et al. 2006), tunnels in squeezing rock should normally be excavated
according to the conventional method (Chapter 20), which is more adaptable with
respect to both the excavated cross-section and the amount and means of support
(Wittke et al. 2002). Furthermore, it is still quite difficult to predict the required radial
displacements.
One option of a yielding support is a shotcrete membrane that is slotted in the lon-
gitudinal direction (Fig. 4.4). The slots between the shotcrete segments must be held
open until the required radial displacements have taken place. In this case a systemat-
ic anchoring is required that ensures the bond between the shotcrete segments and the