1. Highway Alignment and Geometric design.
1. History of highway development
Roman's are pioneers in road construction. At Rome 29 roads were meeting.
Indian Roads:
* Indus valley civilisation towns are planned with grid pattern.
* Mouryans (4000 BC to 2000 BC) Chandra Gupta Mourya constructed 2,400 km long road from Pataliputra (Patna) to Takshashila (now in Pakistan).
* First British Road: Grand Trunk road from Calcutta to Delhi.
Treasagat construction(France) (1716-1796).
Met Calf(England) (1717-1810)
Telford construction (1757-1834):
* Subgrade is kept horizontal and hence subgrade drainage was not proper.
* Heavy foundation stones, to total thickness of the order of 35 cm at edge to 41 cm at centre.
Macadem construction (1756-1856)
* The importance of subgrade drainage and compaction was recognised and cross slope of 1 in 36 was proposed from sub grade level itself.
* The first method based on scientific thinking. It was realised that the stresses due to wheel loads of traffic gets decreased and the lower layers of the pavement and therefore it is not required to provide large boulder and stones or solving course at the lower layer of the pavement and therefore it is not required to provide large boulder and stones or solving course at the lower layer of the pavement.
2. Functional Classification of Highways
A. National Highway (NH)
B. State Highway (SH)
C. Major District Roads (MDR)
D. Other District Roads (ODR)
E. Village Roads (VR)
A. National Highways
Main name on which entire road communication is based, highest specification.
A National Highway connects state capitals, Industrial tours, neighbouring countries etc.
Important National Highways:
NH-1: Delhi - Amritsar Via Ambala
NH-3: Bombay - Agra
NH-4: Madras - Banglore - Pune (Length : 1,235 km)
NH-5: Madras - Calcutta
NH-7: Varanasi - Kanyakumari (Longest National Highway)
NH-9: Bombay - Pune - Hyderabad - Vijayawada
B. State Highways
Specifications similar to that of NH.
MDR: Roughly to be same specifications of SH.
ODR: Some what lower specification. However, these roads should be maintained to function in all weathers to carry the traffic.
VR: Lower specifications.
Details of various Road Networks in India
Indian road network of 33 lakh km is second largest in the world and consists of
* Express ways 200 Km.
* National Highways 70,548 Km.
* State Highways 1,31,899 Km.
* Major District roads 4,67,763 Km.
* Rural roads and other roads 26,50,000 Km.
3. Road patterns
1. Rectangular or clock pattern:
Adopted in Chandigarh, from traffic operation point this is not considered convenient.
2. Star and grid pattern:
Nagpur road plan formative were prepared assuming star and grid pattern.
* Provides inter communication facilities to each of the villages, towns, district head quarters, state capitals, etc.,
* Radial and circular pattern:
Connaught place, New Delhi.
4. Master Plan:
It is the final road development plan for the area under study which may be a district, state or the whole country.
5. Concept of saturation system (or) Maximum utility system:
Useful in arriving at the best road system out of the alternative proposals. In this system optimum road length is calculated for the area, based on maximum utility per unit length of road.
Factors considering are
a. Population
b. Agricultural and Industrial productivity
Highway Alignment
Alignment:
The position or layout of the centre line of the highway on the ground is called Alignment. Basic requirement of an alignment are-
A. Short B. Easy C. Safe D. Economical
Factors controlling highway Alignment:
* Obligatory points: There are two control points through which alignment has to pass and not to pass.
* Traffic: Origin and destination studies are conducted not preferred and desire lines are drawn.
* Geometric design: Gradient, Radius, Sight distance etc.,
* Economy: Balance in cutting and filling.
* Other considerations: Drainage, hydrology, political and Topography.
Engineering Surveys
Engineering surveys for highway Alignment:
A. Map study
B. Reconnaissance
C. Preliminary survey
D. Final location and detailed survey
A. Map study: Gives a rough guidance of the routes to be further study in the field.
B. Reconnaissance: To examine general characteristics of the area with a view to select possible alternate alignment. Instruments used are simple in nature like Abney level, Tangent clinometer, Barometer etc.,
C. Preliminary Survey: Aim is to finalize the most suitable alignment out of various alternative alignments. All modern equipment like Theodolite are used.
The primary survey is involved with the following steps:
i. Primary traversing
ii. Topographical features
iii. Levelling works
iv. Drainage studies and hydrological
v. Data soil survey
vi. Material survey
vii. Traffic survey
viii. Determination of final centre line
D. Final location and detailed survey:
* Alignment finalized in the design office during primary survey is to be first located on the field.
* All detailed topographical, hydrological and geo technical data are collected. Longitudinal and cross sections are drawn.
Drawings required
A. Key Map: Shows the proposed and existing roads and important places to be connected.
B. Index map: shows the general topography of the formation level of road should be 0.6m above H.F.L (High Flood Level).
Highway Geometric design - basic elements
Design speed
Most important factor controlling the geometric design elements of highways.
* Factors considering in India for deciding design speed are–
i. Importance of road
ii. Terrain or Topography
* For rural highways two types of Design speeds.
* Ruling Design speed: The speed that should generally be considered as guiding extension for correlating the various design features.
* Minimum Design speed: The speed that should be adopted in section where site conditions or economics do not permit a design based on the ruling design speed.
Highway cross section elements
A. Pavement surface characteristics
1. Friction or Skid resistance
Determines operating speed, distance requirements in stopping accelerating.
* For calculation of SSD, IRC has recommended longitudinal frictional coefficient 0.35 to 0.4.
* In the case of horizontal curve design, lateral coefficient of 0.15 is recommended.
* Coefficient of friction is less when pavement surface is smooth and wet.
* Also decrease with increase in temperature, type pressure and load.
* New tyres are more dependable than smooth and old ones in adverse surface and conditions like wet.
* Smooth and worn out tyres offer higher friction factors on dry pavement than new types with threads.
Longitudinal Coefficient of friction (f) as per IRC pavement unevenness:
Measured by unevenness index which the cumulative vertical undulations of the pavement surface per unit horizontal length of road (Cm/ Km) it should below-
For good pavement = 150 Cm / Km
* 250 Cm/ Km is satisfactory for design speed of 100 Kmph.
* Value greater than 320 Cm/ Km is very uncommon table even at speed above 55 Kmph.
Important Instruments:
Roughometer, Bump Indicator.
B. Camber or Cross slope
Functions: To drain off rain water from the road surface.
Better surface drainage is important from consideration of-
i. Prevention of entry of surface water into sulograde soil through pavement.
ii. To make the surface dry soon after the rain so that skid resistance does not reduce.
Designation: As '1' in 'n'
(1 v ; n + 1) or as percentage (%)
Factors:
i. Type of pavement surface
ii. Amount of rainfall
Types of camber based on shape.
a. Parabolic or elliptic shape
Equation; Y = 2 X2 /nW
b. Straight line camber
c. Combination of straight and parabolic slope
This is particularly useful to increase the area of contact of the abeel and thus decrease the contact straight pressure in case of animal drawn vehicles with steel tyres occupying different lateral positions of the pavement.
C. Width of pavement or carriage way
The maximum of width of vehicle as per IRC is 2.44 m.
* The number of lanes depends on
a) Predicted traffic volume and
b) Traffic capacity of each lane.
Width of Carriage way
Traffic Separators or Medians
Main function: To prevent head on collision.
* IRC recommends a minimum desirable width of 5.0 m for medians of rural highways.
* On long bridges it may be upto 1.2m
Road margins
Shoulders:
* Serve as emergency lane. Also act as service lanes for vehicles that have broken down.
* The Surface of shoulder should be rough than the traffic lanes so that vehicles are discouraged to used the shoulder as a regular traffic lane.
* The minimum shoulder width recommended by IRC is 2.5m.
Frontage Roads
Are provided to gave access to properties along an important highway with
controlled access as express way or free way.
Drive ways: Concepts the high way with commercial establishments like fuel station, service stations etc.,
Guard Rails: When embankment height is more than 3m.
Highway Geometric design- Gradients
Vertical Alignment- Longitudinal Gradients-
Gradient is the rate of rise or fall along the length of the road with respect to horizontal.
* Expressed as '1' vertical unit to 'N' horizontal units.
a. Types of Gradients
i. Ruling Gradient: Also known as Design Gradient.
* During normal course, the gradient on road should never exceed this value.
* The gradient is fixed in such a way that speed of the vehicles should not decrease while moving on the gradient.
* The gradient is also fixed in such a way that maximum power developed by the engine is equal to the power required to overcome the resistance to motion on the grade at design speeds.
ii. Limiting Gradients:
A gradient steeper than the ruling gradient which may be used in restricted length, where keeping with in the ruling gradient is not possible due to topography.
iii. Exceptional Gradient:
A Gradient steeper than the limiting gradient which may be used in short stretches only in exceptional situations.
iv. Minimum Gradient: On the basis of satisfactory road drainage measurements generally about 0.5% or 1 in 200 in concrete drains and 1% or 1 in 100 in Kutcha open drains.
b. Gradients for roads in different terrains
c. Grade Compensation
As per limitation IRC grade compensation is not necessary for gradient flatter than 4% and there fore when applying grade compensation correction gradient needs not be eased beyond 4%.
