In tunnel construction methods, identify which technique does not require the use of temporary timbering (i.e., no time-dependent timber supports are needed for the excavation face and perimeter).

Introduction / Context:
Tunnel construction employs different excavation and support systems depending on ground conditions, groundwater, and logistics. A recurring safety requirement is immediate support of the newly excavated face and perimeter. Some methods rely on temporary timbering (timber sets, laggings, forepoling), while other methods provide built-in or mechanical support, eliminating the need for timber. This question tests recognition of the method that inherently dispenses with temporary timbering.

Given Data / Assumptions:

• The options include liner plate, shield, compressed air, and mechanical mole methods.
• ‘‘Time bearing’’ in the stem is interpreted as temporary timbering.
• Objective: choose the method that does not require timbering because support is otherwise ensured.

Concept / Approach:

The shield method advances a rigid steel shield that directly supports the excavation perimeter. As the shield moves forward, segmental linings (e.g., cast iron or concrete segments) are erected inside, creating a continuous, protective ring. Since the shield provides immediate confinement at the excavation front, temporary timbering is unnecessary. This core characteristic distinguishes it from methods that excavate first and then install support members.

Step-by-Step Solution:

Verification / Alternative check:

Historic river-crossing tunnels and many modern soft-ground metro drives use shields or TBMs whose shields protect the ground. Where shields are used, temporary timber sets are redundant for overall perimeter support, confirming the answer.

Why Other Options Are Wrong:

• Liner plate method: Usually requires temporary support until plates are erected; timbering or ribs may be used during staging.
• Compressed air method: Air pressure helps stabilize the face but does not by itself replace all temporary supports everywhere; timbering or other supports may still be required.
• Mechanical mole (early road-headers without full shields): May still require temporary supports depending on ground.
• All the these: Incorrect because the shield method specifically avoids timbering.

Common Pitfalls:

• Assuming compressed air alone eliminates all support needs.
• Confusing modern fully shielded TBMs with unshielded mechanical miners.

Final Answer:

Shield method