Question

A tunnel with vertical walls and arched crown is constructed through a set of sedimentary beds. The thickness of individual beds is significantly less than the wall-height of the tunnel. For which of the following conditions, one of the tunnel walls becomes unstable?

• A. The tunnel axis is horizontal and the beds are horizontal
• B. The tunnel axis is parallel to the strike of beds and the beds dip 45⁰ – 60⁰
• C. The tunnel axis is parallel to the strike of beds and the beds are vertical
• D. The tunnel axis is perpendicular to the strike of beds and the beds are vertical

Answer 1

The Correct Option is B

Instability of Tunnel Walls Due to Bed Orientation 

Understanding Tunnel Wall Instability

In this scenario, the tunnel wall becomes unstable when the tunnel axis is parallel to the strike of the beds, and the beds dip between 45° and 60°.

Step 1: Why Does Instability Occur?

• When the tunnel axis is parallel to the strike, the structural orientation of the rock layers plays a crucial role in stability.
• The dip of the beds (45°–60°) creates a situation where rock layers exert additional stress on the tunnel walls.
• This loading condition makes the tunnel walls susceptible to shear failure and slippage along the bedding planes.

Step 2: Stress Distribution and Failure Mechanism

The steep dip of the beds leads to a redistribution of stress, increasing the likelihood of failure. The unstable conditions occur because:

• The weight of overlying rock layers acts along the bedding planes, increasing the shear stress.
• Weak planes in the bedding may facilitate rock sliding along these planes.
• Increased lateral stress on the tunnel walls can lead to structural instability and collapse.

Conclusion:

When tunneling in dipping bedrock formations, special support measures are required to counteract these instability effects. In this case, tunnel failure is most likely when:

\[ \mathbf{Tunnel\ Axis \parallel Strike, \quad 45^\circ - 60^\circ \ Dip} \]

Proper reinforcement techniques, such as rock bolting and lining, are crucial to maintaining tunnel stability.