Sewer construction sequence: select the correct order for laying a sewer in a trench—sight rails, invert pegs, transferring center line, and placing pipes.

Introduction / Context:
Accurate grade and alignment control are vital when laying sewers by gravity flow. Site procedures use sight rails, boning rods, and invert pegs to fix levels before the pipes are placed. This question checks practical field sequencing to ensure alignment and gradient are maintained.

Given Data / Assumptions:

• Steps: (2) set sight rails, (3) drive pegs to invert level, (1) transfer center line to trench bottom, (4) place the sewer pipes.
• Trench is already excavated to near formation level.
• Team has standard leveling equipment.

Concept / Approach:
Sight rails provide permanent reference for elevation above the trench. Invert pegs (or grade pegs) are set using these references to define the pipe invert level. The center line is then transferred to the trench bottom to guide alignment. Only after establishing level and line should the pipes be placed to avoid rework.

Step-by-Step Solution:
(2) Establish sight rails across the trench to control levels.(3) Drive and level pegs to the sewer invert line using boning rods/levels.(1) Transfer the sewer center line to the trench bottom (plumb bob/line marking).(4) Lay and joint the sewer pipes using the pegs and line as guides.

Verification / Alternative check:
Field manuals describe this order to minimize disturbance and ensure grade accuracy before pipe placement.

Why Other Options Are Wrong:

• Placing pipes before level/line control risks misalignment.
• Skipping pegs or transferring line too early can be disturbed by subsequent operations.

Common Pitfalls:
Failing to recheck pegs after rain or trench sloughing; not maintaining consistent boning rod length.

Final Answer:
2, 3, 1, 4