Question

A road bridge is proposed to be constructed across an alluvial stream with the estimated design discharge as 2500 m^3/s. It is decided to adopt a total effective waterway equal to 300 m for this bridge as against the regime width of 237.5 m due to afflux considerations. If the HFL at the site is at RL 214 m, then the RL of the open foundation can be suggested as _______

(a) 206.6 m

(b) 208.5 m

(c) 207.2 m

(d) 210.2 m

This question was addressed to me in an interview.

This intriguing question comes from Culverts Construction in chapter Culverts and Small Road Bridges Construction of Irrigation Engineering

Answer 1

Correct answer is (a) 206.6 m

Best explanation: Using Lacey’s regime formula for alluvial stream –

R = 1.35 (q^2/f)^1/3 = 1.35 (2500/300)^1/3 = 5.55 m

The foundation may be taken down below HFL by 1.33R or R + 1.2 m (whichever is more).

i. 1.33R = 7.38 m

ii. R + 1.2 = 6.75 m (<7.38 m)

Hence, foundation level = 214 – 7.38 = 206.6 m.