You are planning the tunnel alignment for a subway project in a major metropolitan area with several active faults. What is the best orientation of the tunnel when it intersects the fault?

• A. Tangential to the fault
• B. Parallel to the fault
• C. Perpendicular to the fault
• D. At a 45-degree angle to the fault

Answer: (C)

Crossing an active fault at right angles minimizes the length of the tunnel that lies within the fault zone, which is the main reason this orientation is preferred. A fault zone is a highly fractured, weaker portion of rock that can experience sudden slip or rearrangement during seismic or other movements. If the tunnel were to run along or tangential to the fault, a longer segment would be exposed to that problematic zone, increasing risks of ground instability, irregular settlement, water inflow, and more complex, extensive lining requirements.

By intersecting the fault perpendicularly, the tunnel crosses the fault in the shortest possible distance along the fault plane, giving a cleaner transition from one side to the other. This arrangement makes it easier to design and install a robust lining that can bridge any potential fault displacement, and it reduces the likelihood of prolonged exposure to altered rock conditions along the tunnel axis. An oblique crossing, like a 45-degree angle, tends to combine along-fault and across-fault effects, complicating ground response and support design.