Design of Flexible Pavements for Low Volume Rural Roads

D.V .Bhavanna Rao M.Tech., F.I.E.,

Retired R&B Chief Engineer

E-mail: [email protected]

Phone: +919494440202

Egis-india consulting engineers pvt ltd.

for all my presentations, please visit

http://aproadbuildqa.blogspot.com

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Sub-Grade (300mm)

Sub base Base

Embankment

shoulder shoulder

Typical Pavement Cross Section

Earlier Design Approach as per IRC:SP:20-2002

As per IRC: SP:20-2002, traffic is

evaluated in Commercial Vehicles Per Day

grouping together all types of vehicles

weighing 3.0t or more.

The percentage of laden, un-laden or over

loaded vehicles was not considered.

sub-Grade strength is evaluated in terms

of 4-day soaked CBR values except in areas

with annual rainfall less than 500mm and where

the water table is too deep.

Pavement thickness chart as per IRC: SP 20 2002

Pavement thickness as per IRC: SP 20 2002

CBR Crust 0 to 15

CVPD

15 to 45

CVPD

45 to 150

CVPD

150 to 450

CVPD

2%

Base 150 150 225 225

Sub base 275 365 370 455

3%

Base 150 150 150 225

Sub base 200 265 330 320

4%

Base 150 150 150 150

Sub base 125 200 260 315

5%

Base 150 150 150 150

Sub base 100 165 210 260

6%

Base 150 150 150 150

Sub base 60 115 150 175

Pavement thickness as per IRC: SP 20 2002

CBR Crust 0 to 15

CVPD

15 to 45

CVPD

45 to 150

CVPD

150 to

450

CVPD

7

Base 150 150 150 150

Sub base 60 115 150 175

10%

Base 150 150 150 150

Sub base 30 70 85 125

15%

Base 150 150 150 150

Sub base nil 30 50 75

20%

Base 150 150 150 150

Sub base nil nil 30 50

Recommended Design Approach

as per IRC:SP:72-2007

Low volume roads are divided into three

categories

1) Gravel/Aggregate –surfaced (Un-paved) roads

2) Flexible Pavements (Paved roads)

3) Rigid Pavements

note: AASHTO guide for Design of Pavement

structures was considered in this publication.

Recommended Design features in

IRC:SP:72-2007

Pavement design for new roads as well as for the up

gradation/rehabilitation of existing roads have been included.

Maximising the use of localised materials.

Simple procedures for traffic counts, computing

ADT(Average Daily Traffic) and ESAL (Equivalent Standard Axle

Loads) in design life, selected as 10 years.

5 classes of sub-grade strength and 7 ranges of design

traffic.

Warrants for providing bituminous surface treatment.

Long term performance monitoring by systematic

condition surveys at regular intervals.

Design Traffic parameter

It has been expressed in terms of

cumulative 80kN (8.16 tonnes) ESAL

(Equivalent Standard Axle Loads) applications

during the design life (10 years).

Seasonal variations by way of enhanced

traffic during the harvesting seasons have

also been considered.

Traffic growth rate shall be assessed by

conducting traffic survey at least for 3 days

each during peak and non peak seasons. In

the absence of details, a traffic growth rate

of 6% may be taken.

Composition of rural traffic

It is not only the traffic volume but also its

composition that plays an important role in

determining the pavement thickness and its

composition.

Heavy Commercial Vehicles (HCV) like full sized

trucks and buses and the Medium-heavy commercial

Vehicles (MCV) with a gross laden weight 3.0 tonnes

and above shall be considered.

Procedure has also been suggested to evaluate

and consider the effect of Solid Wheeled Vehicles

(SWC/iron-rimmed) in computing the design traffic.

Equivalency Factors for different axle loads

Axle l Load Equivalency

factors tonnes kN

3.0 29.4 0.02

4.0 39.2 0.06

5.0 49.1 0.14

6.0 58.8 0.29

7.0 68.7 0.54

8.0 78.5 0.92

9.0 88.3 1.48

10.0 98.1 2.25

11.0 107.9 3.30

12.0 117.7 4.70

13.0 127.5 6.40

14.0 137.3 8.66

15.0 147.1 11.42

Axle equivalency

factor

= (W/Ws)4

W is single axle

load in kN

Ws is Standard

Axle Load of

80kN(8.16t)

Vehicle Damage Factor

1) Fully loaded Heavy Vehicle (HCV):

Rear Axle=10.2t and Front axle=5.0t.

VDF= 2.44+0.14=2.58

2) Un-laden/partially loaded HCV:


VDF=0.29+0.02= 0.31

3) Overloaded (20%) HCV:


VDF= 5.06+0.29= 5.35

4) Unladen/partially loaded MCV


VDF=0.018+0.001= 0.019

Equivalent Standard Axle Load

applications

N=T0×365×𝟏+𝟎.𝟎𝒍𝒓 𝒏−𝟏

𝟎.𝟎𝒍𝒓 ×L

Where T0 = ESAL per day = number of commercial

vehicles per day in the year of opening ×VDF

𝒍𝒓 = annual traffic growth rate(6%)

L = lane Distribution Factor:

L = 1 for single lane or intermediate lane

L = 0.75 for two-lane roads

n = design life in years(10 years)

Seasonal Variations in Rural Traffic

Average Annual Daily

Traffic (AADT) in CVPD

Correction Factor for

Solid-Wheeled (iron

rimmed)Cart Traffic

100 1.70

150 1.25

200 1.20

300 1.15

400 1.10

500 1.07

In the Equivalent Standard Axle Load calculations, the above

correction factor Is to be applied for Sold-Wheeled Carts if

there Is significant volume of SWC(Iron Rimmed Vehicles).

Damaging effect to the road surface by SWC is twice the

damaging effect of a laden 6-8 t MCV.

Annual Rain fall

Type of Surfacing

Traffic category

T1

ADT<

100

T2

ADT=

100to150

T3

ADT=

150to200

T4

ADT>

200

Over 1500mm/ year Gravel BT BT BT

1000-1500mm/ year Gravel Gravel BT BT

Less than 1000mm/

year Gravel Gravel Gravel BT

Table 4 of SP:62 Guidelines for providing a

Bituminous Surface Treatment

Sub Grade Strength

4-day soaked CBR values for new roads can be

assessed by one of the following.

1) Based on Soil Classification and table 1 of

SP:72

2) Using a Nomo graph (appendix C of SP:62)

3) using 2 sets of equations, based on

classification test data, one for plastic soils and

other for non plastic soils (appendix D of SP:62)

4) By conducting actual CBR tests in the

laboratory.

Table 1 of SP:62: Typical Presumptive Design CBR values

Description of

sub grade soil

IS Soil

Classification

Typical Soaked

CBR Values %

Highly Plastic

Clays and Silts CH, MH 2 – 3*

Silty clays and

sandy clays ML, MI 4 - 5

Clayey sands and

Silty sands SC, SM 6 - 10

*Expansive soils like BC soil may have a soaked CBR of less

than 2%. Free Swelling Index test as per IS:2720 – part 40

should be conducted.

Quick Estimation of CBR (appendix D)

Plastic Soil

CBR = 75/(1+0.728WPI)

Where WPI = Weighted Plasticity Index = P075× PI

PI = Plasticity Index of the soil in %

P075 = % passing 0.075mm sieve in decimal.

Non-Plastic soil

CBR = 28.091(D60)0.3581

Where D60 = diameter in mm of the grain size

corresponding to 60% finer.

Expansive Soil Sub Grade

Expansive soil should be compacted to

95% Standard Compaction at a moisture

content 1 to 2% higher than optimum.

When the CBR of subgrade is less than

2%, the economic feasibility of replacing

300mm sub grade with suitable soil and CBR

of improved sub grade shall be considered

for design.

Alternatively, a capping layer of

thickness less than 100mm of modified soil

with CBR not less than10% should be

provided. CBR of 2% may be considered for

design

Cumulative

ESAL

applications

Bituminous

Surface

treated

WBM/CRMB

Base of

gravel,

WBM,

CRMB of

CBR not

less than

100%

Gravel Base

of CBR not

less than

80%

Granular

Sub Base of

CBR not

less than

20%

Modified

soil or sub

grade of

CBR not

less than

10%

10,000 to

30,000 Nil nil 200 nil 100

30,000 to

60,000 75 nil 150 nil 100

60,000 to

1,00,000 75 100 nil 100 100

1,00,000 to

2,00,000 75 100 nil 100 150

2,00,000 to

3,00,000 75 100 nil 150 150

3,00,000 to

6,00,000 75 100 nil 225 150

6,00,000

to10,00,000 75 150 nil 200 225

IRC: SP 72 – 2007, Pavement Design Catalogue. CBR = 2%

Cumulative

ESAL

applications

Bituminous

Surface

treated

WBM/CRMB

Base of

gravel,

WBM,

CRMB of

CBR not

less than

100%

Gravel Base

of CBR not

less than

80%

Granular

Sub Base of

CBR not

less than

20%

Modified

soil or sub

grade of

CBR not

less than

10%

10,000 to

30,000 Nil nil 200 nil nil

30,000 to

60,000 nil nil 275 nil nil

60,000 to

1,00,000 75 100 nil 150 nil

1,00,000 to

2,00,000 75 100 nil 100 100

2,00,000 to

3,00,000 75 100 nil 100 150

3,00,000 to

6,00,000 75 150 nil 100 150

6,00,000

to10,00,000 75 150 nil 150 150

IRC:SP :72 – 2007, Pavement Design Catalogue.CBR = 3 to 4%

Cumulative

ESAL

applications

Bituminous

Surface

treated

WBM/CRMB

Base of

gravel,

WBM,

CRMB of

CBR not

less than

100%

Gravel Base

of CBR not

less than

80%

Granular

Sub Base of

CBR not

less than

20%

Modified

soil or sub

grade of

CBR not

less than

10%

10,000 to


30,000 to


60,000 to

1,00,000 nil nil 275 nil nil

1,00,000 to

2,00,000 75 100 nil 125 nil

2,00,000 to

3,00,000 75 100 nil 150 nil

3,00,000 to

6,00,000 75 100 nil 100 100

6,00,000

to10,00,000 75 150 nil 100 100

IRC: SP 72 – 2007, Pavement Design Catalogue. CBR = 5 to 6%

Cumulative

ESAL

applications

Bituminous

Surface

treated

WBM/CRMB

Base of

gravel,

WBM,

CRMB of

CBR not

less than

100%

Gravel Base

of CBR not

less than

80%

Granular

Sub Base of

CBR not

less than

20%

Modified

soil or sub

grade of

CBR not

less than

10%

10,000 to


30,000 to


60,000 to


1,00,000 to

2,00,000 75 100 nil 100 nil

2,00,000 to

3,00,000 75 100 nil 125 nil

3,00,000 to

6,00,000 75 100 nil 150 nil

6,00,000

to10,00,000 75 150 nil 150 nil


Cumulative

ESAL

applications

Bituminous

Surface

treated

WBM/CRMB

Base of

gravel,

WBM,

CRMB of

CBR not

less than

100%

Gravel Base

of CBR not

less than

80%

Granular

Sub Base of

CBR not

less than

20%

Modified

soil or sub

grade of

CBR not

less than

10%

10,000 to


30,000 to


60,000 to


1,00,000 to

2,00,000 75 150 nil nil nil

2,00,000 to

3,00,000 75 100 nil 100 nil

3,00,000 to

6,00,000 75 100 nil 125 nil

6,00,000

to10,00,000 75 150 nil 125 nil


Design of Flexible Pavements for Low Volume Rural Roads

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