Project 1 - Geometric Design Parameter- Prerequisites

ANSWER:

The geometric road design has the following prerequisite and can be done considering the following points below: -

• CLASSIFICATION OF ROAD: The geometric road design addresses the driver's and the vehicle's requirements for safety and comfort. The geometric design of a road depends on various factors. The very first step before starting the design of the alignment and profile is to classify the road. A road can be majorly classified as a Rural or Urban Road. Various type of Rural roads is

1. Expressways - more than 120 km/h
2. Highways - more than 100 km/h
3. State Highways - 80 to 100 km/h
4. Major District Roads - 80 km/h
5. Village Road - 40 km/h

And the various types of Urban Roads are

1. City Roads - 40 km/h
2. Artesian Roads/Outer Ring Road - 60 km/h
3. Service Road
4. Grade Separated Road
5. Intersections/Junctions

• Once the road is classified as per the above-mentioned types, the most important geometric factor i.e. the design speed for that particular road is fixed. Other than this we also estimate the design speed using the 85th percentile design speed. A particular stretch of road can have one design speed. In other words, a single road can have multiple design speeds throughout its stretch.
• Other factors include

1. material i.e. earthen roads, gravel roads, murram roads, concrete roads, and bituminous roads.
2. traffic volume i.e. high, medium, or low traffic volume
3. traffic type i.e. pedestrian way, cycle tracks, motorways.
4. topography i.e. plain or rolling terrain.

Terrain	Slope	Speed (km/h)
Plain	0-10%	100-80
Rolling	10-25%	80-65
Mountainous	25-60%	65-40
Steep	60 and above	40-30

• Now for example considering the classification and other factors; one road can be an expressway with concrete pavement. This expressway may pass through a plain or a rolling terrain. Then based on the traffic calculation, it can be a high-volume road. Hence, we can say that'' a single road will be a concrete expressway through a rolling terrain with heavy traffic. This statement includes all types of classification.

• CLASSIFICATION OF VEHICLE: Now coming to the vehicle's requirements, the dimension, weight, and characteristics of the vehicle influence the gradient, turning radius, width of the carriageway, and other geometric features. The types of vehicles recognized by the code are

1. Single unit
2. Semi-trailer
3. Truck trailer combination

• ECONOMY AND AESTHETICS: This also plays an important role in defining the geometry of the road. Road design should be as economical as possible and should match the funds planned. Road aesthetics helps in defining the importance of an area.

• GEOMETRIC DESIGN FACTORS: Following are the prerequisite that should be considered, before starting the geometric design of pavement.

1) Pavement Surface Characteristics-

a) Friction- It supports the movement of the vehicle. As per IRC the value of the coefficient longitudinal friction is between 0.35 to 0.4. The value of the coefficient of lateral friction is 0.15.

b) Light reflection - White roads provide better visibility at night whereas black roads provide better visibility during the day time.

c) Unevenness - the road should be smooth to provide a better riding experience. this can be measured by using Bump Indicator.

2) Cros-sectional Elements -

a) Camber- It is a transverse slope provided on the road to facilitate proper drainage of water from the road surface. This is done by rising the centreline of the road. There can be straight, parabolic, or combined camber. As per IRC camber for different types of roads following are the camber values

Types of Pavements	for Heavy Rain	for Light Rain
Cement Road/ High Bituminous Road	2%	1.7%
Thin Bituminous	2.5%	2%
Gravel Road	3%	2.5%
Earthen Road	4%	3%

For the shoulder, the camber value is 0.5% more than that of the carriageway.

b) Kerb - It indicates the edge of the carriageway and also aids in channelizing the rainwater to the drains.

c) Width of the carriageway-

Single Lane	3.75m
Double Lane without kerb	7m
Double Lane with kerb	7.5m
Intermediate lane	5.5m
Multiple lanes	3.5m each lane width.

d) Shoulder - It acts as an emergency lane and provides lateral support to other pavement layers. Generally, there can be paved or earthen shoulders.

3) Sight Distance Considerations - The geometric design of the highway is done in such a way that at every point on the highway, there should be a sufficient length of view available to the driver such that the vehicle could be stopped at a safe distance. This is called sight distance. The sight distance depends upon PIEV theory, the speed of the vehicle, the efficiency of brakes, frictional resistance, and the gradient of the road. There are further four types of sight distance that we provide as per the situation.

a) Stopping sight Distance - This is the minimum distance provided along the line of sight on the highway such that when the driver sees the obstruction, he could stop the vehicle without collision. This is given by the equation:

`SSD = V*tr + (V^2)/(2gf)`

for upwards or downwards gradient - `SSD = V*tr + (V^2)/(2g(f+-0.01n)`

b) Overtaking Sight Distance - It is the minimum distance provided for the driver who is intending to overtake the slow-moving vehicle safely against the traffic coming from the opposite direction. This is given by the following equation:

`OSD = d1+d2+d3`

where d1= Vb*tr

`d2 = 2s+ Vb* sqrt((4s)/a)` where s = 0.2Vb + length of the vehicle and `T= sqrt((4s)/a)`

d3 = Vc*T

c) Intermediate Sight Distance - This is generally twice the stopping sight distance. The highways are designed at an ISD design at all times in a single direction.

This is given by the expression

ISD = 2x SSD

Headlight Sight Distance - The distance of the road available during the illumination of the headlight of the vehicle during the nighttime is called HLSD. As per IRC, HLSD is taken equal to SSD.

The minimum vertical sag curve depends on the HLSD for a safe and comfortable design.

e) Safe Sight Distance - this is the distance that is provided such that the drivers from each side at the intersection should be able to see the vehicle approaching from another side.

4) Horizontal Alignment - It is vital to design the horizontal alignment as it helps in countering the centrifugal force which is acting on the vehicle. Here the design speed plays an important role in the design of the horizontal curve. There are four types of horizontal curves i.e. Simple Curve, Compound Curve, Reverse Curve, and Spiral Curve To carry forward the horizontal design we need to take care of the following parameters.

a) Design of Superelevation- By providing the superelevation or transverse slope on the horizontal curve we can prevent the vehicle from overturning. This can be done by raising the outer edge of the pavement with respect to the inner edge. The rate of superelevation increases with the speed and sharper curves. The design superelevation for mixed traffic conditions is a very complex problem as every vehicle is running at some different speed. Hence, considering the maximum number of vehicles IRC has suggested for the design of superelevation as 75% of the design speed. Here, the coefficient of lateral friction is considered zero. This is done to increase efficiency. The equation for superelevation is

`e= (V^2)/(225R)` m/sec.

This should be less than the maximum value suggested by the code. As per IRC, the values for maximum superelevation are-

Terrain	Maximum superelevation	Minimum superelevation
Urban Area	4%	Equivalent to camber and is provided for drainage purposes.
Plain and Rolling Terrain	7%
Mountainous or Steep Terrain	10%

If the superelevation value exceeds the maximum value of superelevation, then proceed by considering e=emax, and consider 100% design speed. Then check the coefficient of lateral friction by assuming 100% design speed. So,

`e+f(lat)= (V^2)/(127R)` Here, assume e=emax

then, `f(lat)= (V^2)/(127R) -emax`

Here the value of the coefficient of lateral friction should not be greater than 0.15. However, if e and f both come to greater than the permissible then we need to restrict the speed and this can be done by the following expression

`Vmax= sqrt(127R(emax+f )`

Here, V is not the design speed but the speed of the vehicle running at that particular section.

b) Extra Widening - This is provided on the curve as the movement of the front wheels of a vehicle is different from that of the rear wheels. The rear wheels tend to move outwards with respect to the front wheels. The provision of extra widening is done to avoid off-tracking of the vehicle. This also helps in increasing the visibility at the curve. Extra widening depends on 2 factors- mechanical and psychological widening.

Hence, `We= Wm+Wp`

Where `Wm = (nL^2)/(2R)`

`Wp = (V)/(9.5*sqrtR )`

`We= (nL^2)/(2R) + V/(9.5*sqrtR)`

Extra widening can be provided based on the following conditions-

For 1 lane road-

a) If R>300m, then IRC suggests not considering any extra widening

b) If 50<R<300 m, then extra widening should be provided equally on both edges.

c) If R<50m, then it should be provided on the inner edge only.

For Multiple lanes, then extra widening should be provided equally on both edges.

c) Length of Transition Curve- The transition curve is used to introduce superelevation gradually at the curve. It also aids in providing extra widening and avoiding sudden jerks. The transition curve can be of three types i.e. Lemniscate, Cubical parabola, and Spiral. The length of the transition curve is the maximum of the following-

i) rate of change of centrifugal acceleration which is given by the expression - `Lt = (V^3)/(R*C)`

ii) rate of introduction of superelevation- for plain and rolling terrain - 150x

for mountainous terrain - 60x

Here x can be worked out in 2 ways

a) for rotation about inner edge, `x = (W+We)*e`

b) for rotation about centre, `x = ((W+We)*e)/2`

iii) minimum length of transition curve- for plain and rolling terrain - `Lt = (2.7V^2)/R`

for mountainous terrain - `Lt = V^2/R`

d) Minimum Curve Radius - This is given by the expression, `Rmin = (V^2)/(127*(emax+fmax))`

e) Setback Distance - This is the distance from the centreline line of the road within which the obstructions should be cleared to ensure the needed visibility. The equation for setback distance is, `m = R-(R-n)cosO/`

5) Vertical Profile - Vertical curves are generally provided to facilitate gradual change from one tangent to another so that the vehicle can smoothly navigate the change in grade as it travels ahead. There are two basic components in a vertical profile

a) Gradient - It can be further subdivided into four types

i) Ruling gradient - this gradient is provided by the designer when the cost of the project is not the concern. It is the maximum gradient that the designer can provide depending on the topography.

ii) Limiting gradient - When the topography of a place necessitates a steeper gradient than the ruling gradient then a limiting gradient is provided.

iii) Exceptional gradient - This gradient is steeper than the limiting gradient. it is provided in extremely difficult situations.

iv) Minimum gradient - It is provided from a drainage point of view. Generally, 0.5% is provided as a minimum gradient.

Terrain	Ruling Gradient	Limiting Gradient	Exceptional Gradient
Plain or Rolling	3.3%	5%	6.7%
Mountainous Terrain	5%	6%	7%
Steep Terrain	6%	7%	8%

Grade compensation - This is the amount by which the gradient should be eased to reduce the extra traction involved in the horizontal curve. It is the minimum of - `(30+R)/R`and `75/R` in %.

b) Curves- These curves can be further subdivided into valley and summit curves.

i) Summit Curve - it is a vertical curve with convexity upwards and concavity downwards. The Summit curve has further four cases.

CASE 1 - when ascending gradient meets another ascending gradient. Then the change in gradient will be N= |n1-n2|%

CASE 2 - when ascending gradient meets flat gradient. Then change in gradient will be, N= |n1-0|= n1%

CASE 3 - when ascending gradient meets descending gradient. Then the change in gradient will be N= |n1+n2|%

CASE 4 - when descending gradient meets another descending gradient. Then the change in gradient will be N= |n2-n1|%

Length of Summit Curve - The length of the summit curve depends on the deviation angle. There are 2 cases for OSD/ISD and 2 for SSD.

For ODS/ISD, a) when the length of the curve exceeds the required sight distance then, `L=(NS^2)/9.6`

b) when the length of the curve is less than the required sight distance then, `L= 2S- 9.6/N`

For SSD, a) when the length of the curve exceeds the required sight distance, then, `L= (NS^2)/4.4`

b) when the length of the curve is less than the required sight distance then, `L= 2S-4.4/N`

where S= stopping sight distance, N= deviation angle.

ii) Valley curve - it is a vertical curve with convexity downwards and concavity upwards. The valley curve has further four cases.

CASE 1 - when descending gradient meets another descending gradient. Then the change in gradient will be N= |n2-n1|%

CASE 2 - when descending gradient meets flat gradient. Then change in gradient will be, N= |-n1-0|= -n1%

CASE 3 - when descending gradient meets ascending gradient. Then the change in gradient will be N= |-n1-n2|%

CASE 4 - when ascending gradient meets another ascending gradient. Then the change in gradient will be N= |n1-n2|%

Length of valley cure - It is determined by headlight distance and rider comfort.

As per Rider comfort- `Lv= 2*sqrt((NV^3)/C`

where N=deviation angle, V design speed, L total length of valley curve.

As per headlight sight distance- a) when the length of the valley curve exceeds the required sight distance then, `L=(NS^2)/(1.50+0.035S)`

b) when the length of the valley curve is less than the required sight distance then, `L=2S-(1.50+0.035S)/N`