Squeezing Rock - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 99

Using the convergence-confinement method the following effects remain unconsidered:
- increase of the in-situ stress state with depth,
- anisotropy of the in-situ stress state,
- anisotropy of the deformability of the rock mass,
- anisotropy of rock mass strength resulting, for example, from discontinuities,
- influence of time dependency on the radial displacements,
- influence of the time of closing the supporting ring,
- influence of time on the supporting pressure and
- deviation from a circular cross-sectional shape of the tunnel.
As a result, the convergence-confinement method can be considered as a method that
oversimplifies ground conditions, stress-strain behavior of the rock mass, construction
process and geometry. The convergence-confinement method therefore is unsuitable for
a reliable prediction of the radial displacements of the tunnel contour that are essential
for the design of tunnels in squeezing rock.

4.5 Example of a Tunnel in Squeezing Rock

4.5.1 Statement of Problem, Analysis Model, Parameters and
Analyzed Cases
A circular tunnel with a diameter of 10 m and an overburden of 400 m is considered,
which is located in argillaceous rock. Assuming a unit weight of

= 25 kN/m3 of the
rock mass, the vertical stress at the level of the tunnel's roof is 10 MPa.
The two-dimensional FE-mesh is 510 m high and 300 m wide (Fig. 4.10). These
relatively large dimensions of the computation section turned out to be required
because of the large plastic zone, which results from failure of the rock mass and
subsequent stress redistributions after excavation of the tunnel. The vertical plane
through the tunnel axis is assumed to form a plane of symmetry. Therefore only one
half of the tunnel's cross-section is modeled. The upper boundary of the FE-mesh
is loaded by a surcharge of p o = 3.75 MPa simulating the self-weight of the rock
mass overlying the computation section.

γ

The analysis results are represented for the section indicated as “detail” in Fig. 4.10. The
discretization and the dimensions of this section are shown in Fig. 4.11.
The rock mechanical parameters are selected in accordance with those of argillaceous
rocks. Elastic constants of the rock mass of E = 2000 MPa and ν = 0.3 as well as shear

parameters of the intact rock of c IR = 1 MPa and φ IR = 25°, adopting Mohr-Coulomb's

failure criterion are assumed. The residual strength is described by a cohesion c IR * that
is selected as 0.2 or 0.5 MPa. The dilatancy angle of the intact rock is assumed as
ψ IR = 12.5° (Fig. 4.10).