The method developed so far has been applied to simulate the swelling behaviour of the ground around the exploratory adit of the Caneva-Stevena Quarry. The tunnel has a horseshoe shaped cross section, typical of mine tunnels excavated with the conventional method. As shown in Figure 1, the tunnel is located with the invert in clay and with the crown in a weak highly fractured limestone (locally called "Marmorino", which represents the mined ore), which can be given a Geological Strength Index - GSI according to Hoek & Brown (1997) - equal to 40-50. A 20 cm thick concrete lining was installed after excavation.
A close view of the plane strain finite difference mesh used in the analyses is shown in Figure 1.
Appropriate boundary conditions were applied at the boundaries. The effect of gravity was not taken
into account and the initial state of stress was considered to be isotropic and constant within the
model; as a crude approximation, initial pore pressure and full saturation were assumed to hold true in
the model.
The excavation of the adit was simulated under undrained conditions, while the approach shown
for the triaxial specimen was applied in order to simulate the swelling drained phase.
Present in the specimen at the end of the undrained shearing phase, and the total volumetric strain
(ε vol), measured at the end of the drained phase of the same test.
Numerical analyses were performed to
simulate swelling by generating a volumetric strain increment in a given zone of the model related to
the volumetric strain measured from laboratory tests.
Even though a detailed quantitative comparison
with the case study was not possible, it is believed that the method described has fulfilled the aim of
capturing the phenomenological aspects of the problem and represents a valid alternative to design
analysis methods presently available.
Special triaxial laboratory tests were used to obtain a relationship among the pore overpressure (Δ u), present in the specimen at the end of the undrained shearing phase, and the total volumetric strain (ε vol), measured at the end of the drained phase of the same test. Numerical analyses were performed to simulate swelling by generating a volumetric strain increment in a given zone of the model related to the volumetric strain measured from laboratory tests. Even though a detailed quantitative comparison with the case study was not possible, it is believed that the method described has fulfilled the aim of capturing the phenomenological aspects of the problem and represents a valid alternative to design analysis methods presently available.
REFERENCES:
- 1. Aristorenas, G. V. (1992). Time-dependent behaviour of tunnels excavated in shale. PhD Thesis. Massachusetts
Institute of Technology. Boston, USA.
- 2. Barla, M. (1999). Tunnels in Swelling Ground – Simulation of 3D stress paths by triaxial laboratory testing. Ph.D. Thesis in Geotechnical Engineering. Politecnico di Torino. Pp. 180.
- 3. Bellwald, P. (1990). A contribution to the design of tunnels in argillaceous rock. PhD Thesis. Massachusetts Institute of technology. Boston, USA.
- 2. Barla, M. (1999). Tunnels in Swelling Ground – Simulation of 3D stress paths by triaxial laboratory testing. Ph.D. Thesis in Geotechnical Engineering. Politecnico di Torino. Pp. 180.