Forepoling (driving spiles ahead of the face) is generally adopted for tunnelling in which type of ground condition to maintain face stability?

Introduction / Context:
Forepoling installs short, closely spaced spiles or boards ahead of the advancing face to create a protective canopy. This technique is part of the observational method in weak or unstable soils and prevents sudden inflow or collapse at the heading.

Given Data / Assumptions:

• Question seeks the ground type where forepoling is “generally adopted.”
• Running ground denotes cohesionless, unstable, or water-bearing soils with negligible stand-up time.
• Objective: select the most critical condition warranting forepoling.

Concept / Approach:

Forepoling is most associated with running ground where advance without a canopy leads to face sloughing or runs. By pre-supporting the face and crown, forepoling creates a controlled arch, allowing incremental excavation beneath the spiles with immediate temporary support and subsequent lining.

Step-by-Step Solution:

Verification / Alternative check:

Modern analogues include pipe-roof canopies and jet-grouted umbrellas used in similar ground, reinforcing that forepoling targets the most unstable soft ground conditions.

Why Other Options Are Wrong:

• Soft ground (general) is broad; forepoling is specifically for the “running” subset.
• Firm ground or sound rock usually does not require such pre-support.
• “None” contradicts established practice.

Common Pitfalls:

• Insufficient overlap of spiles reduces canopy effectiveness.
• Ignoring groundwater pressure that can still destabilize the face.

Final Answer:

Running ground