2. What is Vertical Alignment?
The alignment is the route of the road, defined as
a series of horizontal tangents and curves. The
profile is the vertical aspect of the road, including
crest and sag curves, and the straight grade lines
connecting them.
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3. Basic components of Vertical
Alignment
The two basic elements of vertical alignment are
Grades and Vertical Curves.
Gradient
Vertical Curves
Gradient:
It is the rate of rise or fall along the length of the
road with respect to the horizontal. It is expressed
as a ratio of 1 in x (1 vertical unit to x horizontal
unit). Some times the gradient is also expressed as
a percentage i.e. n% (n in 100) 3
5. Vertical Curves
Vertical Curves are the second of the two
important transition elements in geometric design
for highways, the first being Horizontal Curves. A
vertical curve provides a transition between two
sloped roadways, allowing a vehicle to negotiate
the elevation rate change at a gradual rate rather
than a sharp cut.
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7. Categories of Gradients:
1. Ruling or Design Gradient:
• Maximum gradient, within that the vertical profile is
designed
• Difficult to fix because depends on
– Type of terrain
– The length of the grade ( Change in speed affected by the length)
– The design speed (classification of roads)
– Pulling power of the Vehicles
– Presence of horizontal curve ( provide flatter gradient)
3% for plain, 5% for Rolling, 7 % for Hilly
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8. 2. Limiting Gradient:
• Steeper than the ruling gradient
• Provide due to topographic constraints
• Extra care required
– Place a level stretch or easier grade between longer limiting grades
– 5% for Plain and Rolling, 7 % for Hilly,
3. Exceptional Gradient:
• Provided in extreme difficult situations
• Steeper than the limiting
• For only shorter stretches ( not > 60 m in one Km)
• 7% for Plain and Rolling, 8% for hilly
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9. 4. Minimum Gradient
• Provided to drain out the water along the side drains and
depends
– Surface of the drains ( earthen, R.C.C ….)
– Rainfall Run-off
– Type of soil
– Topography and site condition
• 0.2% for used generally
• 1% for earthen or open drains
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11. Vertical Curves
• Summit / crest curves:
Whose convexity upward Vertical curves at a crest or at the
top of a hill are called also called summit curves. Crest vertical
curves are used to connect two separate inclined sections.
• Valley / sag curves:
Whose convexity downward. Vertical curves at the bottom of a
hill are called sag curves. Sag vertical curves are used to
connect two descending grades which form an upside down
parabola, or a sag. Similar to crest vertical curves, the sight
distance is the primary parameter needed to find the length of
the curve.
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12. Shape of Summit Curve
1. Circular
Equal Sight distance at all points
Most Ideal
2. Parabola
Good riding comfort
Calculation of ordinates
Laying out on ground
Most preferred
For small deviation angles above shapes doesn’t
make substantial difference
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13. Design Parameters for Length
• Sight Distance
Stopping Sight Distance
Overtaking Sight Distance
• Centrifugal Force
Acts Upwards
Counteracted by weight of vehicle
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21. Design Parameters
• Daytime – No Problem
• SD reduces at night
SSD under head lights
• CF acts downwards
• W acts downwards
• From the above
Impact free movement of vehicles
Availability of SSD
• Transition curves – for safely introducing C.F (P)
• Cubic Parabola shape is preferred 21
