In tunnelling practice, which explosive is most suitably used for excavation in soft rocks to obtain controlled fragmentation and safety?

Introduction / Context:
Choice of explosive depends on rock strength, desired fragmentation, water conditions, and safety. Soft rocks require moderately powerful explosives that minimize overbreak and vibration while ensuring adequate advance per round in tunnels.

Given Data / Assumptions:

• Rock mass: soft to moderately soft.
• Objective: efficient, controlled breakage with manageable fumes and safety.
• Common commercial explosives listed in options.

Concept / Approach:

Ammonia dynamite (ammonium nitrate-based) provides suitable detonation velocity and gas volume for soft rocks, giving good heave and fragmentation without the excessive shattering associated with high-power gelatins intended for very hard rocks.

Step-by-Step Solution:

Verification / Alternative check:

Standard tunnelling handbooks recommend ammonia dynamite for soft sedimentary rocks and weathered zones, reserving gelatine explosives for very hard, competent rock where high brisance is advantageous.

Why Other Options Are Wrong:

• Blasting gelatine/special gelatine: Very powerful; better for hard, massive rocks.
• Semi-gelatine: Intermediate but still stronger than needed; may increase overbreak in soft rock.
• Slurry ANFO: Common but not the classic choice in traditional MCQs for “soft rock” specific selection.

Common Pitfalls:

• Selecting overly powerful explosives leading to poor profile control and excessive support needs.
• Ignoring water conditions and ventilation requirements for fumes control.

Final Answer:

ammonia dynamite