Project 1 - Geometric Design Parameter- Prerequisites

• For the geometric road design first and for most thing we required for design is to select what type of road (classification of road) we want to design and on what basis we will design based on material, location & function, traffic volume, economy, traffic type, rigidity, topography.

After the classification of road we need factor affecting the geometry design it plays the major role in safe geometry,aesthetics etc. those are

• Design Speed-Its imp factor and it has link directly with sight distance, horizontal curve, vertical curve, also it defines the highest speed a vehicle can travel on road at smooth condition.
• Topography-It represents the features of the land in the form values and its basically 3 dimensional representation, area of country having cross slope varying for Plain terrain:-0-10%,Rolling Terrain:-10-25%, Mountainous terrain-25-60% & steep terrain-> 60%.
• Traffic-It’s also imp factor as we need to consider peak traffic flow of the road.
• Design vehicle-this consist of standard weight, dimension & operating characteristics, & also selection of terrain condition, economic justification & importance of road.
• Environment-will planning the road in forest area we need to plan such a way that we shouldn’t disturb the environment and other restricted zone.

Other factor are due to human as they are going to use road & reaction of the driver, Economy play role in deciding the geometry and also match with the fund, also aesthetics its plays major role in design.

After the factor affecting we are having Cross-elements of road is also major input before we design it consist of

• Camber-its transverse slope provided with respect to horizontal, it’s provided 2% (1.5-2%) for rigid pavement & 2% (2-2.5%) for flexible pavement.
• Width of the carriageway-it depends on traffic density, capacity, present, future growth, economy, and mainly depends on width and clearance of vehicle.
• Kerb-it’s provided at edges of pavement provided on median & footpath.
• Road margin- it’s beyond the carriageway on roadway for single lane-3.5m,two lane-7m,intermediate lane-5.5m, & multilane-3.5m x no of lanes.
• Shoulder-it’s provided on edges of pavement for stopped vehicle also for emergency and lateral support for layer of pavement.
• Median-it’s generally provided to divide the carriageway like 4 lane, 6 lane, or multilane roads.
• Pavement surface characteristics like friction between wheel and road surface is imp factor in deciding the safe operational speed and also curve design, Light reflection plays important in night time, drainage should be provided proper to prevent seepage, unevenness of surface it should be better riding surface and effects operating cost speed and riding comfort etc.

After that we have to go through typical cross-section from SP-73(2018).

Then sight distance is also important because it plays imp role to driver for visibility of stationary or moving objects on the carriageway and ensure that when can vehicle stop safely.

Sight distance depends on reaction time of driver (t=2.5 Sec) (PIEV theory), speed of the vehicles higher speed more time will be required to stop the vehicle, vehicle condition & its efficiency of brakes it depends on vehicle characteristics & age of the vehicle, frictional resistance and gradient of road its depends on resistance between tyre 7 road surface & its effects the sight distance respectively.

Types of sight distance are

a.Stopping sight distance (SDD)-Minimum distance available on a highway to stop the vehicle without collision.its sum of lag distance & braking distance.

b.Intermediate sight distance(ISD)- Genrally its twice of SSD.

c.Head light sight distance(HLSD)-This opreation comes into picture due to illumination during night time.

d.Overtaking sight distance(OSD)-This type sight distance is required for two directional traffic.

Factor effecting OSD-spacing b/w the vehicle, speed of vehicle, gradient of speed, acceleration rate of overtaking vehicle, driver skill, velocities, reaction time driver.

e.Safe sight distance-Its basically for roads having intersection were more than 2 roads meet.

Codal reference for sight distance-IRC-66-1976-Sight distance on rural highway.

Horizontal curve design its imp for smooth & safe driving proper horizontal curve design essential when traffic is moving at high speed.

It depends on Alignment, topography, environmental & ecological qualities of area, site of structures, pressure of movements & worship place, availability of land & if not improper alignment it may led to accidents, reduce capacity and increase cost.

Terrain, vehicle characteristics, driver limitation, design speed, sight distance, drainage conditions, economy, environment effects the road design

Types of horizontal curve

1. Simple curve-it’s an arc of a circle and its commonly used type radius of circle determines the sharpness or flatness of the curve.
2. Compound curve-it consist of two simple curve curving in same direction.
3. Reverse curve-it consists of two simple curves joined together, but curving in opposite direction.
4. Spiral curve-radius is varying which is used railroads & some Morden highway.

Degree of curve-it’s simply expressed in length of radius of curve

Super elevation- necessity of providing the super elevation is to counteract centrifugal force due to friction and to prevent skidding on road surface an inward tilt which is known as super elevation.

As per IRC-38-1988(for camber values)

Methods of providing super elevation

1. Elimination of crown of the cambered section
2. Rotation of pavement to attain full super elevation.

Different methods employed for attainting the super elevation

1. Revolving pavement about the centreline.
2. Revolving pavement about the inner edge.
3. Revolving pavement about the outer edge.

Minimum curve radius=`Rmin=V^2/(127(emax+fmax)`

Extra widening is provided when vehicle negotiate the curve the rear wheels generally do not follow the same track as that of front wheel. Its addition of mechanical & psychological widening.

1.Mechanical widening-its accounted for off tracking due to rigidity of wheel base.

1. 2.Psychological widening-at horizontal curve drivers have to maintain greater clearance b/w the vehicles than on straight starches of road

Requirement of transition curve

Radius of curvature should gradually decrease from infinity, rate of change of centrifugal acceleration should not be caused discomfort.

Types of transition curve-

1. Lemniscate-when radius of curvature does not decrease directly with length of the curve. For deflection greater than 30 degree, the rate of changes of radius decrease& is not uniform
2. Cubic parabola-it’s suitable for transition but this curve has no transitional properties for polar deflection angels greater 9 degree.
3. Spiral-its general use of the transition curve as the setting out in the field spiral. And it satisfy ideally the requirement of the transition curve.

Minimum length of transition length of transition curve be greater than the values derived from considered of

1. Rate of change of centrifugal acceleration.

It shouldn’t cause discomfort to drivers

`Ls=(0.0215V^3)/(CR)`

Ls=lenght of transition cruve in 'm'.

V=Speed in Kmph

R=Radius of circular cruve in 'm'

• Therefore length of transition

2. Rate of change of super elevation or runoff.

It shouldn’t cause discomfort to drivers or to make the road appear unsightly.

rate of change shouldnt be steeper then 0.66% for plain & rolling terrian & 1.66% for mountainous or steep terrain

`Ls=(2.7V^2)/R`

V=Speed in Kmph

R=Radius of circular cruve in 'm'

Setback distance-available across the inside of the horizontal curve.

`m=R-(R-n)cosθ`

were `θ=R/(2(R-n)` in radians

Two basic components of vertical alignment

1. Gradient-rate of rise or fall along the length of the road with respect to the horizontal its expressed as ratio of 1 in x

Depends on

1. Terrain type
2. Design speed
3. Pulling power of vehicle in upward direction
4. Length of the grade depends on change in speed get effected with respect to the length of road.

Types of gradient-

1. Ruling/Design gradient-max gradient that is used for design.
2. Limiting gradient-it may be used where the topography of places necessitates the steeper gradient than ruling gradient.
3. Exceptional gradient-it’s provided in extreme situation.
4. Minimum gradient-it’s provided to ensure the drainage of water & no stagnation of water on the road. Generally 0.5% is provide for minimum gradient.

Gradient as per IRC-SP-23

Deviation angle (N):- `N=n1-n2`

Grade compensation (%)=`(30+R)/R`

were R=the radius of curve in 'm`

2.Vertical curve-it’s provided for gradual change from one tangent grade to another, so that vehicles may smoothly navigate changes in grade as they travel the highway.

Types of vertical curve

1. Summit /crest curve- it’s a vertical curve with upward gradient & simple parabola is used as curve.
2. Valley/sag curve - it’s a vertical curve with downward gradient.

summit curve depends on

1. Deviation angle
2. required sight distance which may be OSD,ISD or the minimum requirement of SSD

Calculation

1. Two cases are considered for OSD & ISD

i.Length of the curve exceeding the required sight distance

`L=(NS^2)/(9.6)`

ii.Length of the curve less than the required sight distance

`L=2S−9.6/N`

2. Two cases are considered for SSD

i.Length of the curve exceeding the required sight distance

`L=(NS^2)/(4.4)`

ii.Length of the curve less than the required sight distance

`L=2S−4.4/N`

Were S=ISD or OSD or SSD and N=deviation angle

sag curve depends on

1. Headlight distance
2. Rider comfort

Calculation

1. Two cases are considered for HLSD

i.Length of the curve exceeding the required sight distance

`L=(NS^2)/(1.50+0.035S)`

ii.Length of the curve less than the required sight distance

`L=2S−(1.50+0.035S)/N`

2. for rider comfort total lengh of valley curve

`L=2((NV^3)/C)^(1/2)=0.38(NV^3)^(1/2)`

Were S=HLSD, N=deviation angle, L=Length of valley curve, V=design speed