By Dan Van Roosendaal, PE, and David Crouthamel, GE
This Just Answers continues the overview of essential environmental risk factors to consider when choosing an excavation method. Part one looked at high groundwater flow; this article examines adverse ground conditions and provides some general guidelines for choosing an excavation method.
The primary geologic risk factors associated with adverse rock mass conditions are limited standup time and squeezing ground. Limited standup time can be caused by block instability (wedge failure), raveling, fast raveling, or flowing ground. Use of a TBM incompatible with these conditions can result in immobilization of the machine. Significant ground loss ahead or around the TBM can result in ground loads acting directly on the machine, causing it to become trapped. Ground load magnitude is primarily dependent on ground quality characteristics, rock structure, cover above the tunnel, tunnel size, and ground disturbance caused by excavation.
Squeezing ground is a time-dependent convergence of weak rock, occurring at the face and along the tunnel periphery. Ground displacements as little as 4 inches (100 mm) can negatively impact TBM equipment. Displacements at the face can jam the cutterhead; and along the tunnel periphery can jam the shield, cause damage to initial support, or even require rescue of the TBM by conventional excavation. TBM technology can be configured to mitigate mild effects of squeezing ground; however, under severe and frequent occurrences, TBMs are insufficient.
Drill-and-blast (D&B) and associated conventional excavation methods are more able to manage severe and changing ground conditions. Full access to the face and tunnel periphery allows the contractor to clearly identify ground conditions and modify excavation procedures and support methods. Pre-support methods can be used to maintain ground support at the face, tunnel crown, and tunnel walls. Weak rock conditions in the invert can be controlled through the installation of invert struts and concrete. In squeezing ground, initial support can be enhanced to mitigate the effects of the ground loads and adverse ground behavior. Likewise, if initial support is overloaded, the tunnel can be re-mined.
The choice between TBM and D&B methods should be carefully evaluated for rock tunnels in difficult ground conditions or challenging groundwater regimes. The key factor is understanding the TBM threshold capacity relative to properly identified and understood geologic and groundwater conditions. TBM technology continues to improve. However, a TBM not properly matched to difficult ground and groundwater conditions can result in significant economic impacts and risk, even project failure. D&B excavation production rates are more clearly proven and management of risks more clearly understood, leaving the threshold capacity of drill-and-blast methods less uncertain.
Drill-and-blast should be seriously considered when significant ground treatment is required to control groundwater inflow, squeezing ground is prevalent, variable ground support measures may benefit production, or when ground conditions cannot be fully characterized.
Dan is a principal in the New York office and is the East Coast Regional Manager. Dave is a principal based in the San Francisco office.