In the present research a highway twin tunnel project completed in Japan in 1998
is used as a case study to verify results of numerical analyses with measurement data.
Each of the tunnels had approximately 1.1 km of length. For this project a wide geometry
of approximately 18.0 m was selected by the designers to facilitate three lanes per tunnel.
A sequential tunneling technique known in Japan as the “TBM pilot and enlargement
method” was used along with NATM principles. The tunnel was used as a reference
project, involving performance testing and extensive monitoring in order to verify and
standardize support requirements for other tunnels excavated under similar geologic
conditions in the Tomei II expressway.
The tunnel was excavated in a region consisting mainly of soft sedimentary rocks,
such as locally weathered sandstone, underlain by interbedded sandstone and mudstone.
Due to observed non symmetric deformations and loads in the tunnel, the distinct element and
the convergence-confinement methods were used during the numerical simulations.
A parametric analysis was performed initially in a pseudo-continuum approach to
study the behavior of the wide tunnel geometry under various conditions. The effects of
rock mass elastic modulus, in situ Ko ratio and boundary conditions are discussed.
More complex parametric studies were performed in a stochastically generated
model by using joint spatial data from geotechnical investigations. The Barton-Bandis
constitutive law was assumed for the behavior of the joints. The sensitivity of the ground
“characteristic curves” was examined by statistical variation of the joint shear strength
parameters. A final simulation using the code UDEC and the convergence-confinement
method yields interesting results which are comparable to the monitored data.
