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The New Zealand Railways Magazine, Volume 6, Issue 1 (May 1, 1931)

Location and Setting out of Curves

Location and Setting out of Curves.

In railway engineering the location and setting out of curves enters into surveying practice. All true curves bear certain relationships according to their magnitude or radii to the angle of intersection of the straight lines which they join up, i.e., the length of the starting and finishing points of the curve from such intersection. The length of the curve is also known, the distance in a straight line across the curve and joining the curve ends, and the angle a chord of any length makes with the straights when drawn across any portion of the curve from the starting point or end of a curve (see figure No. 14). This information is given in books of tables for any curve for any angle of intersection of the straights and the calculations are reproduced on the ground with a theodolite.

There are several types of curves as shown in figure No. 15 and their characteristics are as follows:—

A true curve is one of even curvature throughout its length. The radius is decided upon to meet, economically, the configuration of the country with a minimum of construction work in the placing or the removal of earthwork. The present allowable minimum curvature is 7 ½ chains radius, but this is only adopted in very rough country and easier curves are provided if at all practicable, even with increased construction costs.

Reverse curves are provided in rough country to meet local conditions, but are not good practice as the opposing elevations of the outer rails of each curve give a rolling motion to vehicles at the junction of the curves. A compound curve is a curve made up of two or more true radii page 42 and is sometimes used in following along a river bank with a limited location between the river and the adjacent hill. The entrance to sharp curves gives rough transit to rolling stock and necessitates reduction in speed. To overcome to some extent, this disability, the ends of the true curve are flattened by easy approaches running from the straight track with gradually increasing curvature, until the true curve is reached. This is known as a transition curve. At the intersection of steep grades vertical curves are provided to eliminate discomfort in running from one grade to another.