dr. p. nanjundaswamy department of civil engineering s j ...€¦ · dr. p. nanjundaswamy...

Dr. P. NANJUNDASWAMYDepartment of Civil Engineering

S J College of EngineeringMysore – 570 006

pnswamy@yahoo.com

Pavement Design Parameters

• Traffic

• Climatic Factors

• Road Geometry

• Subgrade

• Material Properties

• Environment

Design Parameters – Traffic

• Maximum Wheel load

• Contact Pressure

• Multiple Wheel Loads

• Repetition of Loads

• Position

• Impact of wheels

• Iron-tyred vehicles

Traffic Loading - Approaches

Approaches

Fixed traffic• Single load, no load repetitions

Fixed vehicle• No. of repetitions of a std. axle load• Equivalent axle load factor

Variable traffic and vehicle• Stresses, strains and deflections under

each load group separately

Wheel Load and Contact Pressure

Contact Pressure

The influence of contact pressure on stress levels in base, subbaseand subgrade layers are marginal

Contact Pressure

The magnitude of contact pressure determines the quality and thickness of wearing and binder course

Wheel Load

The influence of the magnitude of the wheel load on stress levels in base, sub-base and subgrade layers is significant

Wheel Load

Total thickness of the pavement is mainly determined by the magnitude of the load and not the contact pressure

Axle Configurations and Loads

19t)

Axle Configurations

2 Axle Truck – 16t

3 Axle Truck – 24t

4 Axle Semi Articulated – 34t

4 Axle Articulated – 34t

5 Axle Truck – 40t

LCV

Axle Configurations

Axle Configurations

Axle Configurations

Axle Configurations

Design Vehicle ?

Effect of Wheel Configuration

The effect of axles 1, 2 and 3 on stresses and strains within pavement layers are considered independently

Effect of Wheel Configuration

Within a group of axles, each axle is not considered as independent

Effect of Wheel Configuration

In flexible pavement design by layer theory, only the wheels on one sideare considered

Effect of Wheel Configuration

In rigid pavement design by plate theory, the wheels on both sides are usually considered (even when distance > 1.8 m)

Notice that cars are insignificant and thus usually ignored in pavement design.

1.35

1.85

5.11

0.100.00070

1

2

3

4

5

6

Car Delivery Truck Loaded 18-Wheeler Loaded 40' Bus Loaded 60'

Articulated Bus

ES

AL

s p

er

Ve

hic

le

Shape of Contact area

The true shape of contact area is elliptical

In flexible pavement analysis, it is approximated to circular shape for the ease of calculations

Radius of contact area,

p

Pa

Shape of Contact area

In rigid pavement analysis, circular shape approximation leads to significant error

For the convenience of calculations, the elliptical shape is approximated by a rectangle and two semicircles

Shape of Contact area

0.3 L

0.3 L

0.4 L L

0.6 L

The contact area

In FEM analysis of rigid pavement, equivalent rectangular area of 0.8172 L x 0.6 L is assumed

Multiple Wheel Loads

Equivalent Single Wheel Load (ESWL)

Is a single wheel load having same contact pressure which produces same value of • Maximum stress• Deflection• Tensile stress• Contact pressure

at the desired depth

Graphical Solution

Equivalent Single Wheel Load (ESWL)

Boyd and Foster methodSemi-rational method

Assumptions• Equalancy concept is based on equal

vertical stress• Contact area is circular• Influence angle is 45o

• Soil medium is elastic, homogeneous and isotropic half space

Graphical Solution . . . .

Equivalent Single Wheel Load (ESWL)

Graphical Solution . . . .

Equivalent Single Wheel Load (ESWL)

Graphical Solution . . . .

Equivalent Single Wheel Load (ESWL)

Depth z (Log Scale)

ESW

L (L

og

Scal

e)

P

d/2 2 S

2P

Z1

P1

A

B

Graphical Solution . . . .

Example

Find ESWL at depths of 5 cm, 20 cm and 40 cm for adual wheel carrying 2044 kg each. The center to centertyre spacing is 20 cm and distance between walls of thetwo tyres is 10cm

Solution

For desired depth z1 = 5 cm, which is half the distancebetween the walls of tyre, ESWL = P = 2044

For z3 = 40 cm, which is twice the tyre spacing, ESWL =2P = 4088 kg

Graphical Solution . . . .

Equivalent Single Wheel Load (ESWL)

Depth z (Log Scale)

ESW

L (L

og

Scal

e)

P

d/2 2 S

2P

Z2

P2 = 3.5

A

BLog10(d/2) = 0.7 Log10(P) = 3.3

Log10(2S) = 1.6Log10(2P) = 3.6

Log10(Z2) = 1.3

P2 = Antilog (3.5)= 3162 kg

Graphical Solution . . . .

Equivalent Single Wheel Load (ESWL)

5.38log

4log301.03.3log

10

10210 P

kgP 316210 5.32

Equal Vertical Stress Criterion

From Boussinesq’s Theory

or

PS

ZA

Pd Pd

Z

1 32

Sd

Sd/2

σzs Maximum vertical stress at A

σzd Maximum of vertical stresses at 1, 2 and 3

Equal Vertical Deflection Criterion

Foster and Ahlvin (1958) PS

ZA

Pd Pd

Z

1 32

Sd

Sd/2

ws Maximum vertical deflection at A

wd Maximum of vertical deflections at 1, 2 and 3

and

Equal Vertical Deflection Criterion

Huang (1968)ESWL based on interfacedeflection of two layeredsystems

Other Criteria

• Equal Tensile Strain

• Equal Contact Pressure

• Equivalent Contact Radius

Equivalent Single Axle Load (ESAL)

Is the equivalent repetitions of Standard Axle during the design life of pavement

IRC terms this ESAL as Cumulative number of standard axles during the design life

The number of repetitions of different types of axles are converted into equivalent repetitions of standard axle by using Equivalent Axle Load Factors (EALF)

Equivalent Axle Load Factor (EALF)

Defines the damage per pass to a pavement by an axle relative to the damage per pass of a standard axle

Exact EALF can be worked out only by using distress models

Approximate EALF can be worked out using the fourth power rule

Standard Axle Load Single axle : 8160 kgTandam axle : 14968 kg

Vehicle Damage Factor (VDF)

Instead of converting each axle pass into equivalent standard axle passes, it will be convenient to convert one truck pass into equivalent standard axle passes

The factor that converts – VDF

VDF is the number of standard axles per truck

Determining VDF

Sample Axle Load Survey

Sample Axle Load Survey

Computation of VDF

Traffic on Design Lane

Need for Distribution Factors

Traffic on Design Lane

Worked out by finding the

Directional Distribution Factor (0.5 to 0.6)

Proportion of ADT of trucks occurring in the maximum direction

Lane Distribution Factor

Proportion of trucks occurring on the design lane which depends on

Number of lanes and Traffic volume

Factors Suggested by IRC

No. of Traffic lanes in two

directions

Percentage of trucks in

Design Lane

1 100

2 75

4 40

Undivided Roads (Single Carriageway)

Factors Suggested by IRC

No. of Traffic lanes in two

directions

Percentage of trucks in

Design Lane

1 100

2 75

3 60

4 45

Divided Roads (Dual Carriageway)

Design Period

Depends on

traffic volume growth rate capacity of road and possibility of augmentation

Flexible Pavement

15 years – NH, 20 years – Express ways & Urban Roads, 10 to 15 years – Other Roads

Rigid Pavement

30 years. When Accurate prediction not possible – 20 years

Design Traffic

N = Cumulative std. axle repetitions during design period (expressed in msa)

A = Initial traffic intensity (CVPD)D = Lane distribution factorF = Vehicle damage factorn = Design life (years)r = Annual rate of growth for commercial vehicles

Average annual growth rate – 7.5%

Thank you