In the needle-beam method of tunnelling, what is the usual overall length range of a needle beam used to support the excavation and lining installation?

Introduction / Context:
The needle-beam method is an older yet instructive tunnelling technique used for small to medium spans, often in soft ground. A stout beam (the “needle”) temporarily supports the roof while excavation proceeds and the lining is erected. Knowing the typical length of a needle beam helps in planning staging, support spacing, and safe working envelopes.

Given Data / Assumptions:

• Method: needle-beam tunnelling with temporary roof support.
• Parameter of interest: overall length of the needle beam.
• Span and working space constraints govern typical sizes.

Concept / Approach:
Needle beams must be long enough to span the excavation width with bearing onto sidewalls or temporary posts, while remaining manageable for installation and adjustment. Practical experience and handbook guidance place typical lengths in a moderate range that accommodates common heading widths and lining sequences without excessive weight or deflection concerns.

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