For highway projects in hilly terrain, tunnelling is generally preferred over an open cut when the required open-cut depth exceeds approximately what value?

Introduction / Context:
Choosing between an open cut and a tunnel depends on cost, environmental impact, stability, and aesthetics. In mountainous terrain, deep open cuts can trigger excessive excavation volumes, slope stabilization expenses, and visual impacts. Hence, beyond a certain depth, tunnelling becomes more economical and environmentally prudent for highways.

Given Data / Assumptions:

• Highway alignment through a hill or ridge.
• Comparison of alternatives: open cut vs. tunnel.
• Rule-of-thumb depth threshold where tunnelling is favored.

Concept / Approach:
Open cuts involve large excavation prisms that grow rapidly with depth due to stable slope requirements and benching. Support, drainage, and protection works escalate with depth. A practical threshold often adopted in planning guides is around 20 metres, beyond which the economics and impacts generally tip in favor of a tunnel. Exact numbers vary with geology and land constraints, but this benchmark aids preliminary decisions before detailed design.

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