A gradient is a rate at which a road goes up or down along its path. A road’s gradient rises or decreases with the road’s horizontal alignment. It’s the height difference between where you started and where you ended, expressed over the same length of time. Slopes and other undulations on the land’s surface are the norms rather than the exception. Errors may occur if a road is built there. The road’s gradient absorbs these fluctuations in altitude. To build highways, they must be included since they regulate the transitions between straight stretches. They answer problems like uneven terrain, flooding, high construction costs, and public discontent.
Purpose of Gradient of Roads:
Roads are built to connect two or more places, and the best way to build a road’s grade is to follow the land’s shape as closely as possible to avoid having to do a lot of cutting or filling. For example, in hilly areas, the slope of the road is designed and built to match the slope of the land, but efforts are made to meet the desired maximum slope. Because of this, most roads have 5% to 8% steeper grades.
In flat areas, like urban streets or highways, the road slopes are designed and built first to match the existing ground profile and then drain surface water with systems like pipe/gully or kerb drains. It’s important to remember that roads on completely flat land must have a minimum slope of 0.5% to drain properly.
Importance of Gradient of Roads:
- The most important part of building a road is the gradient. Along the length of the road’s alignment, it’s important to ensure the road has the right slope.
- Gradient lets the vehicle move smoothly around the corners.
- The slope also helps water drain off the roads after it rains.
- Gradients are very helpful on roads with sharp turns in flat areas and drainage problems.
- Before deciding on the road’s slope, it’s important to think about how much it will cost to build, how much it will cost to run cars on it, and what kind of problems might happen on the site.
Effects of Gradient of Roads:
- Gradients have a significant impact on the rate at which vehicles travel. Roads with a high concentration of heavy vehicles require special attention to this issue.
- The slower speeds of these large vehicles are typically enforced on uphill grades when visibility is reduced. Thus, vehicle running expenses rise, and road capacity must be decreased.
- Accidents are common in the grades because of the high speeds between large and light vehicles and between the uphill and downhill gradients.
The Factors for Selection of Gradient of Roads:
- Existing topography influences road grade. The planned road profile must be within the maximum gradient range if the existing topography is hilly.
- The road’s classification determines the road’s gradient. Maximum grades of 3% are recommended for high-speed highways, whereas 6% are recommended for all-purpose single-carriageways.
- Existing and projected drainage systems affect the gradient requirement. If the proposed road is designed where drainage attenuation is needed, it should be atop a barrier to allow a big attenuation pond.
- The bridge approach at both ends affects the grade of the approach road at both ends. In most cases, bridge surface water drainage is managed separately from road drainage; however, road and bridge gradients must be coordinated for a smooth transition.
Types of Gradient of Roads:
1. Ruling Gradient:
The ruling gradient, also called the design gradient, is the steepest angle with which the vertical profile of the road can be made. It depends on the terrain, the inclined length, the speed, the vehicle’s pulling power, and whether or not there is a horizontal curve. If the land is flat, it might be possible to make the slopes flat, but if the land is hilly, it could be more cost-effective and sometimes even possible. The designer picks the rule-of-thumb gradient by using a certain speed as the design speed and a standard-sized vehicle for the design.
2. Exceptional Gradient:
The exceptional gradient is the gradient that is steeper than the limiting gradient. This slope is usually only used in special situations where the roads are shorter than usual. Exceptional gradients are steep gradients used in situations that can’t be avoided, like mountainous areas and terrain. But a steep slope has a downside: it uses more fuel and causes more friction losses.
3. Limiting gradient:
This gradient is used when the cost of the building goes up significantly with the standard gradient. On rolling land or land with hills, it may be necessary to use a limiting gradient. But the length of the steepest parts of the road should be limited, and they should be surrounded by straight roads or roads with less steep grades.
4. Floating Gradient:
A “floating gradient” is a slope where a car moving at a constant speed keeps going down at the same speed without the driver applying power or the brakes.
5. Average Gradient:
For any two places along the alignment, the average gradient is calculated by dividing the vertical difference between them by the horizontal separation. The initial paper location or primary survey benefits from knowing the average grade because it approximates the length of the proposed alignment.
6. Minimum Gradient:
When surface drainage is important, the minimum gradient is used. The side drainage will be taken care of by the camber. But the drainage along the side drains needs a slope so water can flow smoothly. So, there is a minimum slope for drainage, which depends on how much rain falls, the type of soil, and other site conditions. A minimum of 1 in 500 may be enough for concrete drains, and 1 in 200 works well for loose soil drains.
