In the drift (pilot heading) method of tunnelling, where may the initial drift be excavated relative to the full tunnel section?

Introduction / Context:
The drift (or pilot heading) method advances a smaller opening to explore ground conditions, provide ventilation, and create access for subsequent enlargement to full section. The position of the drift within the final profile is a strategic choice influenced by geology, support strategy, and construction logistics.

Given Data / Assumptions:

• A reduced-size heading is driven first.
• Goal: facilitate ground assessment, drainage, and controlled enlargement.
• Drift location options include centre, crown, invert, or side positions.

Concept / Approach:
Central drifts offer symmetric enlargement; crown drifts help control roof behavior and allow immediate installation of supports; invert/bottom drifts aid drainage and establish a stable base; side drifts address asymmetric geology or staging constraints. Hence, multiple positions are valid depending on project priorities (e.g., settlement control, water handling, or staging around existing structures).

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