A 10.4km x 14.4m diameter tunnel is currently under construction in the Canadian city of Niagara Falls, Ontario. This tunnel, excavated by TBM, has descended through the entire stratigraphy of the Niagara Escarpment, including dolomites, limestones, sandstones, shales. and interbedded zones of these rock types. The tunnel is currently in the extensive Queenston Formation, where it will remain for almost 80% of the over all length. The tunnel will encounter many challenges such as mixed face conditions, blocky ground, swelling shale, topographic effects from close passage beneath a buried gorge, vertically variable hydrological conditions, parallel structure and high horizontal stresses.
This world class project provides the backdrop for a study of large scale heterogeneity and anisotropy in geology, material properties and in-situ stresses and the impact on excavation stability. Both discrete (UDEC) and finite (Phase2) element analyses have been used to simulate 3D tunnel advance in 2D to develop preliminary stability curves.
A series of tunnels, with diameters ranging from 8 to 16 meters, were modelled with variable lamination thicknesses ranging from 0.01 to 0.5 meters. Vertical to sub-vertical jointing were used with variations in continuity to provide block geometry. The model simulations were done with 20 to 500m meters of vertical cover and variable Ko values ranging from 0.5 - 4. The interaction with lamination planes, of ductile or brittle intact rock behaviour, is also examined.
By monitoring the vertical displacements and using stability criteria, determined through initial model performance and tunnel excavation experience, a series of preliminary design curves were established for lamination thickness versus span width, noting the stable and unstable regions. With further testing on other large scale excavations from around the world, these numerically based design curves could prove to be a useful tool in future feasibility studies for large scale excavation project in horizontally laminated ground.
