Surveying plays a crucial role in the construction and development of highways, railways, and other infrastructure projects. One of the essential aspects of surveying is the implementation of curves to ensure a smooth and safe transition between different alignments. Curves are necessary where changes in direction or elevation occur to provide comfortable movement for vehicles and pedestrians. This blog explores the types of curves in surveying, their significance, and their applications.
What Is a Curve?
A curve is a smoothly flowing line that alters the alignment of a route, ensuring a gradual and safe change in direction. Curves are essential in roadways and railways where two points are at different levels or directions, allowing for smooth navigation without abrupt turns.
Curves are introduced at intersections of two straight paths to facilitate safe movement. These curves can be in a horizontal or vertical plane, leading to their classification into two primary types:
- Horizontal Curves
- Vertical Curves
1. Horizontal Curves
A horizontal curve is applied when two straight paths intersect on a horizontal plane. These curves ensure smooth movement and gradual direction changes without sharp turns.
Types of Horizontal Curves
a) Simple Curve
A simple curve consists of a single circular arc that connects two straight paths tangentially. It is the most basic type of horizontal curve. Key elements include:
- Back Tangent: The tangent line that extends before the beginning of the curve.
- Forward Tangent: The tangent line that continues after the curve ends.
- Point of Intersection (PI): The junction where two tangents meet.
- Intersection Angle: The angle formed between the back tangent and the forward tangent.
- Angle of Deflection: The angle by which the forward tangent deviates from the back tangent.
- Point of Curvature (PC): The starting point of the curve where the tangent transitions into a curve.
- Point of Tangency (PT): The endpoint of the curve where it transitions back into a tangent line.
- Tangent Distance: The length between the point of curvature and the point of intersection.
- Length of Curve (L): The total curved distance from the point of curvature to the point of tangency.
- Long Chord: A straight line connecting the point of curvature and the point of tangency.
- Normal Chord: A chord connecting two consecutive pegs along the curve.
- Sub-Chord: A chord that is shorter than the normal chord.
- Mid-Ordinate: The perpendicular distance between the midpoint of the curve and the midpoint of the long chord.
- External Distance: The straight-line distance from the point of intersection to the midpoint of the curve.
b) Compound Curve
A compound curve consists of two or more circular arcs with different radii but having their centres on the same side of the common tangent. These curves are useful in confined spaces where a single arc may not fit.
c) Reverse Curve
Also known as an S-curve, a reverse curve consists of two circular arcs with opposite curvatures connected by a common tangent. These curves are primarily used in railway and road design, especially in hilly terrains.
d) Transition Curve
A transition curve has a varying radius, gradually increasing or decreasing to connect straight paths with circular curves. These curves are introduced to prevent sudden directional changes, improving comfort and reducing vehicle wear and tear.
e) Combined Curve
A combined curve integrates both a circular curve and a transition curve, ensuring a smoother transition from straight sections to curved paths. These curves are commonly used in highways and railways.
2. Vertical Curves
Vertical curves are necessary when a route experiences elevation changes. These curves provide a smooth transition between different levels, ensuring vehicles and trains move without sudden jerks.
Types of Vertical Curves
a) Summit Curve
A summit curve is convex in shape, allowing a smooth transition over a peak. It is required in the following cases:
- When an upward slope is followed by a downward slope.
- When a steep upgrade transitions into a milder upgrade.
- When a gentle downgrade leads to a steeper upgrade.
Summit curves ensure safety by providing visibility and reducing abrupt elevation changes.
b) Sag Curve (Valley Curve)
A sag curve, or valley curve, is concave in shape, ensuring a smooth transition through a valley. It is necessary in the following situations:
- When a downward slope is followed by an upward slope.
- When a steep downgrade leads to a milder upgrade.
- When a gentle upgrade transitions into a steep upgrade.
Valley curves are critical for comfort and drainage, ensuring water runoff and preventing vehicle suspension damage.Importance of Curves in Surveying
- Ensures Safety: Smooth transitions reduce the chances of accidents by preventing sudden direction and elevation changes.
- Enhances Comfort: Gradual alignment changes provide a better travel experience, reducing discomfort for passengers and drivers.
- Improves Visibility: Proper curve alignment enhances visibility for drivers, preventing unexpected obstacles.
- Reduces Vehicle Wear and Tear: Transition and compound curves help reduce tyre friction and mechanical strain on vehicles.
Is codes for surveying
In India, surveying practices are governed by various IS (Indian Standard) codes issued by the Bureau of Indian Standards (BIS). Here are some important IS codes related to surveying:
- IS 1498:1970 – Classification and identification of soils for general engineering purposes.
- IS 6962:1989 – Recommendations for field measurements in river surveying.
- IS 16472:2017 – Guidelines for land surveying using modern instruments.
- IS 11233:1985 – Guidelines for precision surveys in construction projects.
- IS 11927:1986 – Code of practice for setting out of buildings and survey work.
- IS 3764:1992 – Safety code for excavation work, including survey techniques.
- IS 8835:1978 – Method of mapping and surveying in remote sensing applications.
- IS 12894:2002 – Standards for GIS and digital surveying methodologies.
- IS 7338:1974 – Methods for topographical surveying.
- IS 7453:1974 – Guide for contour surveying and mapping.
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Conclusion🎯
Curves in surveying are essential for the efficient design of roads, railways, and other transport infrastructure. Horizontal and vertical curves help in achieving smooth transitions, reducing risks, and enhancing travel comfort. By understanding the types of curves and their applications, engineers and surveyors can design safer and more efficient routes.