During tunnel excavation, when should concrete lining be provided concurrently with the driving operation to ensure ground stability and safety?

Introduction / Context:
The timing of lining installation is driven by ground behavior. In competent rock, a lag between excavation and permanent lining is often acceptable. In weak or unstable soils—especially running ground that flows into the heading—immediate or concurrent lining is vital to prevent collapses and excessive deformation.

Given Data / Assumptions:

• “Running soil” denotes cohesionless, unstable material prone to ravel or flow.
• Objective: identify the condition requiring concurrent lining with driving.
• Concrete lining provides rapid, rigid support and containment.

Concept / Approach:

In running ground, the unsupported stand-up time is negligible. Therefore, excavation and lining must progress in close sequence—often with immediate shuttering and concreting or with preformed segments—to stabilize the face and prevent inflow or ground loss that could damage surface structures.

Step-by-Step Solution:

Verification / Alternative check:

Historical failures in loose, water-bearing soils underline the necessity of immediate support. Modern shield/TBM segmental linings play the same role in such formations.

Why Other Options Are Wrong:

• Rock terrain/soft rock: May allow delayed lining with temporary support.
• None/dense granular soil (low water): Often stable enough for staged support.

Common Pitfalls:

• Underestimating water pressure; ignore it and even soft soils can “run.”
• Delaying lining due to logistics, increasing collapse risk.

Final Answer:

Running soil