Factors affecting Pavement design pdf

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factors to be considered for flexible pavement design
Factors affecting Pavement design pdf

Essential Difference between Rigid and Flexible Pavements

Flexible Pavement 

Rigid Pavement 

A load of any magnitude may be dissipated by carrying it deep into ground through layers of granular materials. Design is based on providing flexural strength in a structural slab to resist destructive action of wheel loads. 
The intensity of load diminishes in geometric proportions as it is transmitted downwards from the surface by virtue of spreading it over an increasingly larger area. Rigid pavement because of its rigidity and high modulus of elasticity tends to distribute the load over a relatively wider area of soil. 
Strength of each layer could be reduced with increased depth. 

Above 5 points are given to differentiate between flexible pavement and rigid pavement


Issues 

Flexible 

Rigid 

Flexural Strength Low or negligible Very high 
Design Principle Layering system concept Slab action 
Stress Compressive stress Tensile stress and temperature increases. 
Transfer of stress Grain to grain transfer and deformation on top is reflected on the bottom layer No such transfer and deformation, if any is not reflected on lower surface 
Material Granular material Load spreading ability depends upon the type of material and thickness. Portland Cement Concrete 40 kg/cm2 Capable of transmitting load stress through a wider area below. 
Distribute the load in the form of a truncated cone 
Design Practice Based on empirical design charts and equations, even though semi-empirical and theoretical design methods are available. Critical condition is maximum stress occuring in the slab. Design based on elastic theory. 

Factors Affecting Pavement Design

Factors to be considered in flexible pavement design and rigid pavement design

i. Pavement Design consists of two parts

  • Mix design of materials to be used in each pavement component layer
  • Thickness design of pavement and component layers.

ii. Factors to be considered in the design of a road or highway pavement

  • Design wheel load
  • Subgrade soil
  • Climatic factors
  • Pavement component materials
  • Environmental factors
  • Special factors in design of different types of pavements.

(a) Design wheel load in pavement

Thickness of pavement primarily depends upon the design wheel load. Higher the wheel load, higher will be the thickness, provided other design factors are the same. As speed increases, the rate of application of stress increase.

Also Read - Magnetic declination formula in surveying 

Elements of design wheel load

  • Static load on each wheel/dual/dual tandem wheel
  • Contact pressure
  • Load repetition and dynamic effect of transient loads.
  • Repetition of loads.

(b) Subgrade soil

  • Decides thickness requirement of the pavement.
  • Variations in moisture content affect the stability.
  • Stress-strain behaviour under varying loads is of great significance.

(c) Climatic factor

  • Rainfall affects moisture content and in turn the stability of subgrade.
  • Variation in temperature has significance in choice of bituminous binder, design and performance of rigid and bituminous pavements.
  • Freezing temperature results in frost action in subgrade.

(d) Pavement materials

  • Fatigue behaviour, durability and stress distribution characteristics of pavements depend upon the materials.

(e) Environmental factors

  • Height of embankment
  • Depth of cutting
  • Foundation details
  • Depth of sub-surface water table
  • Land use

(f) Special factors

Cross Sectional Elements

The cross sectional elements such as 'right of way', 'carriage way' and status of the road definitely has an impact on thickness of the pavement.

Traffic Characteristics

Physical, static and dynamic characters of traffic have greater bearing on pavement design. The traffic characteristics such as composition of vehicles, volume, speed, acceleration, have direct influence on the thickness of the pavement.

Special factors in the design of different types of pavements

Formation of shrinkage cracks, the crack pattern and the mode of propagation, and fatigue behaviour under such adverse conditions of hair cracks are to be studied before arriving at a rational method of design.

Design of wheel load

i. Maximum Wheel Load

The way in which a loads of a given vehicle is applied on a pavement surface depends on the wheel configuration of the vehicle. Therefore, it is important to know the wheel configuration of vehicles. Figure 3.2 shows typical wheel load configuration of tractor trailer unit of a heavy duty vehicle.

maximum equivalent single wheel load as per irc
wheel configuration of tractor

factors affecting pavement design pdf
wheel configuration of trailer

Maximum equivalent single wheel load as per IRC

The Indian Roads Congress has specified the maximum legal axle load as 8170 kg. with a maximum equivalent single wheel load of 4085 kg. 

The total load decides the pavement thickness.

ii. Contact Pressure

The tyre pressure is very high on upper layers of pavements. The tyre pressure diminishes in proportion to the depth of a pavement. Therefore, tyre pressure of high magnitude require high quality surface course. The tyre pressure is constant. Hence, the stress on the subgrade depends on the total load.

iii. Equivalent Single Wheel Load (ESWL) with Formula

equivalent single wheel load formula
Concept of equivalent single wheel load

Dual wheel assembly to rear axles of vehicles improve the carrying capacity of vehicles. It also helps to maintain maximum wheel load within the specified limit. However, the effect of dual carriage way on the pavement is not equal to two times the load on any one wheel. The effect is in between the load carried by a single wheel and that by a dual wheel. Fig.3.3 explains this concept.

Assumption - Load dispersion is at an angle of 45 degree

Let
  • d = clear gap between two wheels
  • s = spacing between centres of wheels 
  • P =the wheel load
  • a = radius of the circular contact area
  • s = (d+2a)

At a depth 2s and above, stresses induced are due to the effect of both wheels as the area of overlap is considerable. Therefore, at any depth greater than 2s the stresses due to dual wheels are considered to be equivalent to a single wheel load of magnitude 2P.

iv. Determination of ESWL

  1. Based on equivalent deflection
  2. Based on equivalent stress criterion

v. Deflection Criterion

The ESWL is that single wheel load having the same contact pressure and producing the same value of stress at a depth equal to the thickness of the pavement.

vi. Stress Criterion

The ESWL is the single wheel load producing the same value of maximum stress at a depth equal to the thickness of the pavement.

vii. Repetition of Loads

Pavement or subgrade may deform little a on a single application of wheel load. However, if that load is repeatedly applied, then the elastic and plastic deformations and so also accumulated un-recovered deformation increases. This may even result in failure of the pavement. Equivalent wheel load is a single load equivalent to the repeated applications of any particular wheel load on a pavement which requires the same thickness and strength of pavements.

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