2.3.4. Methods of horizontal curve setting

A curve may be set out either by (a) linear methods, where chain, tape or EDM is used, or (b) angular methods, where a theodolite with or without a tape or total station is used.

There are two main ways to set out curves for road construction: linear methods and angular methods. Linear methods involve using measuring tools like chains, tapes, or Electronic Distance Measurement (EDM) devices to find the right distance. Angular methods utilize a theodolite, which measures angles accurately. This ensures that the curve is laid out correctly in relation to the straight sections of the road.

Think of setting out a flower bed in your garden. You could use a measuring tape to determine how far apart to place the flowers (linear method), or you could adjust the angle of your planting to make sure they look evenly spaced from different viewing angles (angular method).

Before starting to setting out a curve by any method, the exact positions of the tangent points between which the curve would lie, is to be determined.

The first step in setting a curve is to accurately find the tangent points, which are the points where the straight lines of the road meet the curve. This is crucial because the entire layout of the curve depends on these points. Identifying them ensures the curve connects smoothly with the straight sections.

Imagine planning a road trip. You need to know your start point (where the road begins) and your end point (where the road curves) before mapping out your route. If you choose the wrong starting or ending point, you might end up on a completely different path!

Set up a theodolite at the intersection point B and measure the angle of intersection I, using repetition method of angle measurement. Determine the deflection angle  as (1800-I).

Next, the theodolite is placed at the intersection of the tangents. Once set up, the angle between those straight paths is measured. This angle is crucial as it informs how sharply the curve will turn. The deflection angle is calculated by subtracting this measured angle from 180 degrees, giving insight into the curvature needed.

Picture yourself trying to direct a friend to a new café. You might use a compass to find the right direction while ensuring that your friend starts at the right angle to proceed smoothly. This is similar to how the theodolite helps set the correct angle for the curve.

Calculate the tangent length lengthR tan ϕ.

Once the deflection angle is determined, the next step is to calculate how long the tangent will be. This is done using the curve's radius and the tangent of the deflection angle. The tangent length helps in determining how far back the curve starts from the intersection point.

When you're baking, you may need to measure ingredients in specific quantities. Just like correctly measuring out sugar for a cake helps it rise properly, accurately calculating tangent length ensures the curve is smooth and right.

From the intersection point B, measure the tangent length BT backward along the rear tangent BA to locate T1. Similarly, locate the position of T2 by measuring the same distance from B along the forward tangent BC.

After knowing how long the tangents are, you will measure those distances backward and forward from the intersection point to find where the curve begins and ends. This gives you the actual locations of the tangent points T1 and T2 on the ground.

Think about laying down a piece of tape in a curve on the floor. Once you know how long the straight pieces need to be, you can move from the center point outward to place the tape in its proper curve, marking the start and end points.

The chainages of tangent points T1 and T2 are determined. The chainage is the distance of point T1 and T2 with respect to reference point on the road with known chainage.

Chainages help keep track of how far along the road you are. For T1, you subtract the tangent length from the known distance at point B, and for T2, you add the arc length to T1's chainage. This system is key for navigation and construction purposes.

Imagine turning on a GPS during a road trip; it gives you the distance traveled from the last known point. Here, knowing the distances to T1 and T2 helps in future navigation and measuring progress along the road.

The pegs are then fixed at equal intervals (normally 30 m) on the curve.

After determining the tangent points, pegs are set at equal spacing along the curve to guide the construction. This helps ensure that the curve is built consistently without sharp variations, which can cause problems.

It's like putting flags at intervals when setting up a running track. The flags guide runners, ensuring they stay on the intended path. In the same way, placing pegs along the curve keeps the construction aligned.

The distances along the centre line of the curve are continuously measured from the point of beginning of the line up to the end.

Finally, continuous measurements are taken along the center line to ensure consistency and accuracy from the start to the end of the curve. This ensures each segment matches the planned layout of the road.

Imagine painting a mural. You want to make sure every section is well-aligned and proportional, so you frequently step back to see how it all looks together. Similarly, measuring along the curve ensures every section flows instinctively.

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