In tunnel construction safety and operations, which mechanical ventilation strategy is used to maintain breathable air at the face and along the heading?

Introduction / Context:
Tunnel works generate fumes, dust, and heat from drilling, blasting, mucking, and equipment. Mechanical ventilation is essential to supply oxygen, dilute contaminants, and keep visibility acceptable. Depending on the project and heading length, systems may blow, exhaust, or combine both to achieve target air quality at the working face.

Given Data / Assumptions:

• The choices list blowing, exhausting, and a combined (push–pull) approach.
• All are recognized mechanical ventilation strategies.
• Objective: identify which of these is used in tunnels.

Concept / Approach:

Blowing systems push fresh air to the face via ducts, improving oxygen and flushing dust forward. Exhaust systems extract foul air at the face to reduce backflow into the heading. Combined systems use both to maintain desired flow patterns and contaminant removal, especially after blasting when noxious gases peak.

Step-by-Step Solution:

Verification / Alternative check:

Construction method statements typically prescribe blowers for active headings and switch to exhaust or combined modes post-blast to clear fumes quickly, confirming the variety in practice.

Why Other Options Are Wrong:

• Any single method alone ignores the fact that all three are used across projects.
• “None of these” contradicts established tunnelling practice.

Common Pitfalls:

• Using inadequate duct diameter/length, reducing effective airflow.
• Delaying re-entry without achieving gas thresholds after blasting.

Final Answer:

All the above.