Squeezing Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 102

Table 4.1 Back analyzed rock viscosities

Project Location Rock type References [MPa·s]

10 5

Gotthard
Road Tunnel

Urseren-Garvere
(Switzerland)

phyllite Lombardi (1977)

Wittke (1990)

8 ⋅

10 7

Repository
Konrad

Konrad mine,
Salzgitter
(Germany)

mudstone WBI (1991)

3 ⋅

1 ⋅ 10 7

mudstone

2.5 ⋅ 10 8

limestone and marl WBI (1994a)

10 8

mudstone and marl WBI (1993)

WBI (1994b)

1 ⋅

In squeezing rock, the viscosity of intact rock η IR , which controls time dependency

of viscoplastic deformations as described in Section 3.2.3, is of particular impor-
tance. From back analyses of measured displacements that have taken place after
excavation of some tunnels in squeezing rock, viscosity values between 1·10 6 and
1·10 8 MPa·s were derived (Wittke 1990, WBI 1991, WBI 1993, WBI 1994a, WBI
1994b) that are specified in Table 4.1. η IR = 10 6 MPa·s was selected for the analyses

(Fig. 4.10).
The analyses were carried out using the finite element program system FEST03 of
WBI, described in Section 10.7.1. The analyzed cases are compiled in Table 4.2.
In case 1, representing the reference case, the deformability and strength of the
rock mass are assumed to be isotropic and equal to those of the intact rock, that
is, no discontinuities are considered. The residual cohesion is assumed to be half
of the peak cohesion (c IR * = 0.5 MPa) and no support is accounted for. A period
of 300 days after excavation is simulated. To check the influence of the residual
strength in case 2 the residual strength of the intact rock is reduced to 20% of the
peak strength (c IR * = 0.2 MPa). Case 3 considers the influence of discontinuities
assuming a vertical joint set J and horizontal bedding B. The selected shear param-
eters and dilatancy angles are specified in Fig. 4.10. In case 4 a supporting pressure
of p r = 0.6 MPa is applied to the tunnel contour immediately after simulating the
excavation while in case 5 a yielding support is simulated by applying this pressure
30 days after excavation.

The section for the representation of the subsequently described analysis results is illus-
trated in Fig. 4.11.