Sewer setting-out with sight rails and boning rods: A sewer runs from manhole A to manhole B, 250 m apart, on a gradient of 1 in 125 falling from A to B. If the reduced level (RL) of the invert at A is 205.75 m and the boning rod height is 3.00 m, what is the RL of the sight rail at B?

Introduction / Context:For sewer construction, sight rails and boning rods establish a straight line parallel to the designed invert. The sight-rail level must be set to account for gradient and the boning-rod height so that when the rod top aligns with the sight line, the invert is at the correct RL.

Given Data / Assumptions:

• Horizontal distance A→B = 250 m.
• Gradient = 1 in 125, falling from A to B.
• RL(invert at A) = 205.75 m.
• Boning-rod height = 3.00 m.

Concept / Approach:A fall of 1 in 125 means a drop of 1 m per 125 m. Over 250 m, the invert fall is 250 / 125 = 2.00 m. Therefore RL(invert at B) = 205.75 − 2.00 = 203.75 m. The sight rail at B is set above the invert by the boning-rod height so that the rod top aligns with the sight string: RL(sight rail at B) = RL(invert at B) + 3.00 m.

Step-by-Step Solution:

Verification / Alternative check:If instead the gradient rose toward B, the invert would be higher and the sight rail RL would increase accordingly; here it decreases then adds the rod height.

Why Other Options Are Wrong:

• 208.75 m / 211.75 m: Overestimate; do not reflect the 2 m fall correctly.
• 202.75 m: Below even the invert at B; ignores the added boning-rod height.

Common Pitfalls:Confusing invert fall direction; forgetting to add the boning-rod height to get the sight-rail level.

Final Answer:206.75 m