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Highest and lowest points on vertical curves

Short answer

The highest point on a summit curve (zero gradient) gives maximum elevation for earthwork and setting out; drainage falls away from it. The lowest point on a valley curve (zero gradient) collects water, so drainage inlets are placed there with minimum gradient on both sides; its level checks bridge clearance and provides earthwork levels. Both points are needed for vertical alignment levels and longitudinal drainage design.

What is the significance of locating the highest and lowest points on vertical curves?

  • Highest point of a summit curve is the point of zero gradient on the curve. Its position and level give the maximum elevation of the road on the curve, which is needed for the earthwork levels and for setting out. Surface water drains away from it in both directions, so the side drains are graded away from this point.
  • Lowest point of a valley curve is also a point of zero gradient, and water collects there. The drainage inlet, cross-drain or culvert is placed at this point, and a minimum gradient is ensured on both sides so that the water drains away. Its level is also used to check the clearance under bridges and to get the levels required for earthwork and setting out.
  • In both cases, the position is required to calculate the vertical alignment levels and to provide proper longitudinal drainage.

✓ Answer checked by Claude Opus 5.5 (high effort)

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