Setting out a sewer trench: For a trench width of 2.0 m, at what typical distance from the center line is the offset line pegged parallel to it for setting out?

Introduction / Context:Setting out sewer trenches requires offset pegs/lines placed away from excavation to preserve reference after digging. The offset must clear the trench edge and provide safe working space for instruments and sighting rods.

Given Data / Assumptions:

• Trench width = 2.0 m (measured at ground level).
• Offset line is parallel to the sewer center line.
• Allowance is needed beyond half trench width for safety and sighting clearance.

Concept / Approach:

Half trench width is 1.0 m. A practical additional clearance (0.5–0.6 m) is typically added to place offset pegs beyond collapse or interference zones. Thus, many field practices adopt ≈ 1.6 m from the centre line for a 2.0 m trench, ensuring stability and clear line-of-sight after excavation.

Step-by-Step Solution:

Verification / Alternative check:

Agency manuals often prescribe offset distances based on trench widths, soil conditions, and anticipated side slopes; values near 1.5–1.6 m are common for a 2.0 m trench.

Why Other Options Are Wrong:

100–140 cm may be too close for safe preservation of pegs; 180 cm is workable but not the typical value indicated in many standard MCQs.

Common Pitfalls:

Placing offsets too close leading to loss during excavation; not recording offsets and references in the field book.

Final Answer:

160 cm