The correct option is (c).
Explanation
Manholes are masonry or reinforced concrete chambers constructed on the alignment of a sewer to provide access for inspection, testing, cleaning, and removal of obstructions from the sewer line.
- Statement 1 is Incorrect: Manholes are typically located at intervals of 50 m to 60 m along the sewerage line. An interval of 500 m to 600 m is too large for standard maintenance equipment (such as cleaning rods) to function effectively.
- Statement 2 is Correct: Manholes are constructed at the junction of two or more sewers. This allows for the inspection of the confluence point where the flow merges and where blockages are statistically more likely to occur.
- Statement 3 is Correct: Manholes are mandatory at points where there is a change in direction, change in gradient, or change in the size of the sewer. Since sewer lines are generally laid in straight lines to facilitate cleaning, a manhole is required wherever a turn occurs.
Key Takeaway: Manholes are critical access points in a sewerage system, placed at regular short intervals (approx. 50–60 m), at junctions, and at changes in direction or gradient to facilitate maintenance.