Vertical Alignment and Gradients Questions and Answers

Answer: B

The maximum grade compensation is 75/R per cent.

On a horizontal curve, extra tractive effort is needed to overcome curve resistance, so IRC requires the gradient to be eased. The grade compensation is (30 + R)/R per cent, subject to a maximum of 75/R per cent, where R is the radius in metres. Compensation is not needed for gradients flatter than 4 per cent.

50/R, 100/R and 125/R are not the IRC limit.

Answer: C

The IRC ruling gradient in plain terrain is 1 in 30.

IRC recommends, for plain and rolling terrain, a ruling gradient of 3.3 per cent (1 in 30), a limiting gradient of 5 per cent (1 in 20) and an exceptional gradient of 6.7 per cent (1 in 15). The ruling gradient is the one adopted for design wherever possible.

1 in 20 and 1 in 15 are the limiting and exceptional values for plains, and 1 in 45 is flatter than any IRC design category.

Answer: B

The deviation angle is 11/600.

The deviation angle is the algebraic difference of the gradients. An ascending gradient is positive and a descending one negative:

\[ \begin{aligned} N &= \frac{1}{100} - \left(-\frac{1}{120}\right) \\ &= \frac{6 + 5}{600} = \frac{11}{600} \end{aligned} \]

Subtracting the magnitudes instead gives \(\frac{1}{600}\), the trap for forgetting the sign. \(\frac{1}{12000}\) is the product of the two gradients, which has no meaning here.