Survey control for sewer construction: Which primary instrument is used to set out the alignment and grade while laying sewers on site?

Introduction / Context:
Laying sewers requires accurate alignment (direction) and grade (invert slope). Survey instruments provide the control needed to ensure gravity flow, avoid backfalls, and meet design levels. Two instruments are commonly involved: theodolite for alignment and level for elevations.

Given Data / Assumptions:

• Gravity sewer with designed longitudinal slope.
• Need for accurate direction between consecutive manholes.
• Site control points available.

Concept / Approach:

Theodolite precisely sets out straight lines and angles, establishing the sewer centerline between manholes. Elevations are then checked with a level (or total station). The question asks for the primary instrument used for laying out sewers—interpreted in practice as establishing alignment with a theodolite, followed by level checks for grade using boning rods or laser levels.

Step-by-Step Solution:

Verification / Alternative check:

Total stations and lasers integrate angle and distance measurement; however, conceptually theodolite remains the alignment instrument of reference, with a level for elevations.

Why Other Options Are Wrong:

Magnetic compass and plane table lack required precision; clinometer gives slopes but not alignment angles; a level alone cannot set horizontal direction.

Common Pitfalls:

Poor instrument centering; failure to protect reference pegs; neglecting temperature and refraction on long sights.

Final Answer:

Theodolite