Index

Tunnel Infrastructure

Figure 1. Relative locations of road tunnels, exit ramps and weakness zones Z1 to Z6 from 3DEC model

Recently, Itasca Consulting Group, Inc. has conducted several 3D stability analyses of complex geometry tunnel infrastructure for road as well as rail tunnels by applying both the continuum code FLAC3D and discontinuum code 3DEC. The analyses addressed various objectives such as:

• Evaluation of alternative designs with respect to excavation stability
• Influence of new tunnel infrastructure on the stability of existing near proximity tunnels
• Determining the affects of pervasive chlorite schist zones on tunnel deformations and stability
• Estimating the influence of multiple large-scale shear zones/faults on overall and local tunnel stability
• Determining the influence of local joint structure on local tunnel stability
• Determination of ground support response (shotcrete and grouted rock bolts) and its affect on stability

The investigations benefited from good information about ground conditions from local site characterization studies. The site information included ranges in ground conditions, the effects of which were also evaluated in the 3D models. Innovative analysis techniques were developed to demonstrate that the total utilization of all rock bolts is quite low with respect to characteristic strength and strain capacities. In one case 1740 rock bolts were used with 9 bolts predicted to yield. The yielding bolts were located across a weakness zone.

Figure 2. Left: Excavation geometry evaluated for stability in 3DEC model. Sequence of excavation considered as well as a locally jointed region. Right: Contours of major principal stress (Pa) (most compressive) in shotcrete liner for supported section of rail tunnels (from 3DEC model). Stress pattern affected by block displacements in the jointed region.