Introduction / Context:
Shaping the tunnel profile is a critical step in underground excavation. After the initial blast that advances the face, the final, accurate tunnel outline must be produced to meet clearance, structural lining, and waterproofing requirements. This question tests your understanding of which holes or tools are specifically intended to achieve the precise finished contour of a tunnel section.
Given Data / Assumptions:
- Tunnel excavation typically uses a blast-hole pattern with cut holes, easers (relievers), lifters, and trimmers (also called contour or line holes).
- Objective: obtain the designed tunnel outline with minimal overbreak and damage to rock surrounding the future lining.
- Mechanical chisels may be used for local touch-up but are not the principal method for full-profile shaping in modern practice.
Concept / Approach:
The blast-hole pattern begins with cut holes to create initial void and relieve confinement at the face. Easer holes help the blast break to the periphery. The final and most precise element is the ring of trimmer (contour) holes along the design boundary, often lightly charged or decoupled to limit overbreak. Controlled blasting techniques (pre-splitting/smooth blasting) focus on these perimeter holes to deliver a smooth, undamaged excavation surface for the lining.
Step-by-Step Solution:
Identify roles: cut holes initiate the break; easers assist propagation; lifters clear floor; trimmers define the profile perimeter.Select the element whose primary purpose is accurate shaping at the boundary: trimmers.Note that chisels are occasional finishing tools and not the main profile-control method in systematic blasting.
Verification / Alternative check:
In smooth blasting, perimeter (trimmer) holes are closely spaced, lightly charged, and sometimes decoupled to control damage and overbreak—validating their shaping role.
Why Other Options Are Wrong:
Easers: relieve the face but do not set the final contour.Cut holes: create the initial opening, not the profile.Chisels: local hand/mechanical trimming only.Secondary lifters: floor control, not profile shaping.
Common Pitfalls:
Assuming any hole near the perimeter is a trimmer—true trimmers are specifically aligned on the design profile with controlled charge.Overlooking the importance of reduced charge in perimeter holes to avoid overbreak.
Final Answer:
Trimmers (contour/line holes)
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