Design of Flexible Pavements for Low Volume Rural Roads

BySri P. Ganeswara Rao,

M. Tech.(Structures),

Chief Engineer, PR, Vig & QC

Pre requisites for suitable and economical designs

The following various points to be considered

• Use of locally available materials to the extent possible in the larger interest of economy.

• The design life to be taken for the pavement design should not be neither too short nor too long.

• Maintenance consideration should be built into the design to minimize subsequent maintenance requirements eq., provision of adequate drainage, adequate lateral support from shoulders, etc.

• The designs shall be performance based. The pavement should not fall below the minimum acceptable level during the design life.

Design Parameters

• Sub Grade Strength and traffic of commercial vehicles (heavy & medium) are very important parameters which decide crust thickness & type of pavement requirement.

Design As per IRC: SP:20-2002

• The sub grade strength parameter is evaluated in terms of 4 day soaked CBR values except where annual rainfall is less than 500mm and where water table is too deep

• A set of pavement design curves A,B,C,D for traffic categories 0-15, 15-45, 45-150 and 150-450 CVPD have been provided

• The minimum base course thickness of 150mm for curves A&B and 225mm for C & D curves are considered.

• The thicknesses of sub base course have been arrived at by subtracting the minimum base course thickness from the total pavement thickness requirement obtained from the pavement design curves.

• As per this design, almost all the roads constructed in our country are black topped.

Fig.1 CBR curves for Flexible Pavement Design

Recommended Design Approach as per IRC:SP:72-2007

• For the evaluation of sub grade strength for new roads in the selection of moisture content has been dealt with scientifically instead of always insisting on 4 day soaked CBR values.

• For the rehabilitation or upgradation of existing rural roads, the use of DCP (mm/blow) (Dynamic Cone Penetrometer) has been recommended for insitu sub grade strength evaluation.

• The design traffic parameter has been expressed in terms of cumulative ESAL (80 KN or 8.16 Tonnes) applications during the design life of 10 years.

• The sub grade strength is classified into 5 classes and the traffic into 7 ranges has been simplified the presentation of design catalogues for both gravel roads and flexible pavements

SUB GRADE STRENGTHS

It is divided into 5 Categories

Quality of Class Range Sub Grade (CBR %)

Very Poor S1 2

Poor S2 3 - 4

Fair S3 5 - 6

Good S4 7 - 9

Very Good S5 10 - 15

• Where the CBR of Sub grade soil is less than 2The economical feasibility replacing 300mm sub grade with suitable soil needs to be explored if found feasible.

(or)Stabilization of soils using various techniques is to be done depend upon suitability of the soils and cost (Mechanical Stabilization, Lime fly ash stabilization, Bitumen stabilization, Two stage stabilization).

• The pavement should then be designed based on the CBR value of the improved sub grade.

• Alternatively a caping layer of thickness not less than 100mm of modified soil (with CBR not less than 10) should be provided through out the total width of the road.

• The top of the sub grade preferably not less than 300mm above the GL and not less than 600mm above the highest GWT or existing water levels. Similarly high embankments should not be provided if the site condition does not requires it.

Fig.2 Extended Modified Soil (CBR not less than 10) OR Fig.2 Extended Modified Soil (CBR not less than 10) OR Extended Sub-baseExtended Sub-base

Fig.3 ROAD CROSS-SECTION IN A FILLFig.3 ROAD CROSS-SECTION IN A FILL

Fig.4 ROAD CROSS-SECTION IN A CUTFig.4 ROAD CROSS-SECTION IN A CUT

Typical Presumptive Design CBR values

Sl. Description of IS Soil Typical SoakedNo. Sub grade soil Classification CBR value (%)

1. Highly plastic clays and silts CH, MH 2 - 3

2. Silty clays and Sandy clays ML, MI, CL, CI 4 - 5

3. Clayey sands and Silty sands SC, SM 6 – 10

Note: Expansive clays like BC soil may have soaked CBR of less than

2%

Quick Estimation of CBR for Preliminary Report Preparation

Plastic Soil• CBR =75/(1+0.728 WPI), R2 = 0.67

• Where WPI = Weighted Plastic Index = P0.075X PI

• P0.075 = % passing 0.075mm sieve in decimal

• PI = Plastic Index of the soil in decimal

Non Plastic Soil• CBR= 28.091 (D60)

0.3581

• Where D60 = Diameter in mm of the grain size

corresponding to 60% finer

PAVEMENT COMPOSITION

Sub Base Course:

For Granular sub base, the materials generally used are natural sand, moorum, gravel, crushed stone, crushed slag, brick metal, kankar or combination thereof depending upon the grading required as per clause 401 of the MORD specifications. The CBR is not less than 20, in exceptional case may be relaxed to 15. The thickness of sub base shall not be less than 100mm.

Base Course:

For base course, the materials generally used are WBM, WMM or Crusher Run Macadam Base (CRMB) as per clauses 405, 406 and 411 of MORD specifications. The CBR is not less than 100.

Surfacing: 20mm premix carpet is recommended as per MORD

specifications.

DRAINAGE AND SHOULDERS

DRAINAGE DESIGN:• A scientifically worked out drainage design is a vital part of the design of the

pavement system as a whole.

• Proper cross slopes are to be provided over the carriage way and the road side shoulder to shed off the rain water quickly.

• Top of the sub grade or improved sub grade preferably not less than 300mm above the GL and not less than 600mm above the highest GWT.

• Adequately designed road side drains are to be provided.

• All cross drainage structures are to be provided as per requirements.

• In case of sub base, it is to be provided over sub grade of relatively low permeability i.e., a clay sub grade, it is necessary that atleast that half the sub base layer thickness or a minimum of 100mm should be extended across the shoulders if no separate drainage layer is provided. In the sub base material the percentage passing 75 micron sieve must not exceed 5%.

SHOULDERS:

• Shoulders are to be provided to give adequate lateral support to the pavement.

• The shoulder material should normally be of sub base quality compacted to a thickness of 100mm.

• If the anticipated traffic on the shoulders is high, gravel shoulders need to be provided over 1.00 m width from the edge of the carriageway as per the MORD specification 407.

DESIGN LIFE OF THE PAVEMENT

• A design life of 10 years is recommended for flexible pavement.

• At the end of the design life, the pavement will not have to be reconstructed all over again but needs to be strengthened.

TRAFFIC DESIGN

• For the purpose of pavement design, the large no.of bicycles, motor cycles and pneumatic tyred animal drawn carts are of little consequence and only the motorized commercial vehicles of gross laden weight of 3 tonnes and above (i.e., HCV & MCV) are to be considered.

• The effect Solid Wheeled Carts (SWC) is to be considered (One SWC is approx. equal to two MCVS of laden weight of 6 - 8 tons each).

• For new roads to estimate the amount of traffic, traffic counts are to be carried out in the vicinity with similar conditions and the amount of traffic expected to ply on the new proposed road can suitably worked out.

TRAFFIC GROWTH RATE

Tn = To (1+r)n

n = No. of Years

To = Traffic in Zero year

Tn = Traffic in nth year

r = Annual rate of growth of the traffic in decimals

(r = 6% over the designed life may be adopted)

Computation Of Design Traffic For Upgradation Of Existing Road

• Traffic census should be conducted over a period of atleast 3days both during peak harvesting season and also during the lean season for various vehicle types both motorized as well as non motorized. The number of laden, unladen and overloaded commercial vehicles also to be recorded during traffic counts.

• Average Daily Traffic (ADT) for 24 hours should be computed for each vehicle type, both during the peak harvesting season and also during the lean season.

• Total traffic during the year can be computed and consequently the Average Annual Daily Traffic (AADT) can be computed for each vehicle type.

Fig.5

• The above figure typically takes about 40% of the duration of a harvesting season (t) to build up the traffic from lean season level ‘T’ to the peak. The peak traffic may continue for about 20% of the duration of harvesting period before coming down to the lean season traffic level over a period of time again about 40% of the total duration of the harvesting season.

• The total number of repetitions (N) of a given vehicle type during the year

N = T x 365 + 2n (0.6 t)T

AADT = T +1.2nTt

365 Local enquiries play an important role in assessment of many

parameters under traffic studies.

Determination of ESAL Applications

• The traffic parameter is evaluated in terms of Standard Axle Load of 80 KN and the cumulative repetitions of Equivalent Standard Axle Load (ESAL) are calculated over the design life.

• Rural vehicles with single axle loads different from 80 KN can be converted into standard axles using the Axle Equivalency factor.

• Axle equivalency Factor = w ws

W = Single axle load (in KN) of the rural vehicle in question

Ws = Standard Axle load of 80 KN

4

Equivalency Factors for Different Axle Loads

Axle Load Equivalency Factor

(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

Vehicle Damage Factor (VDF)

• It is defined as equivalent number of standard axles per commercial vehicle. It is useful to convert the number of commercial vehicles of different axle loads into number of standard axle load repetitions.

VDF values for various vehicles

Laden Heavy Commercial Vehicles (HCV)Fully loaded HCV (comprising heavy trucks, full sized buses) have a Rear Axle Load of 10.2 tonnes and a Front Axle Load, about half the Rear Axle Load i.e., 5 tonnes. The VDF works out to 2.58 (=2.44 + 0.14)

Unladen/Partially Loaded Heavy Commercial VehiclesSince the extent of loading of commercial vehicles is difficult to determine, a Rear Axle Load of 6 tonnes and a front axle load of 3 tonnes may be assumed for an Unladen/Partially Loaded HCV. The VDF works out to 0.31 (=0.29 + 0.02)

Overloaded Heavy Commercial VehiclesThe extent of overloading may vary widely from one situation to the other. However, if an overload of 20% is there, the VDF goes upto 5.35 (=5.06 + 0.29). However, if only 10% of the laden HCV are overloaded to the extent of 20%, the VDF works out to 2.86 (=0.9 x 2.58 + 0.1 x 5.35)

Laden Medium-heavy Commercial Vehicles (MCV)Fully loaded MCV (mostly comprising Tractor-Trailers) have a Rear Axle Load of 6 tonnes and a Front Axle Load of 3 tonnes. The VDF works out to 0.31 (=0.29 + 0.02)

Unladen/Partially Loaded Medium-heavy Commercial VehiclesSince the extent of loading of commercial vehicles is difficult to determine, a Rear Axle Load of 3 tonnes and a Front Axle Load of 1.5 tonnes may be assumed. The VDF works out to 0.019 (=0.018 + 0.001)

Overloaded Medium –heavy Commercial VehiclesThe extent of overloading may vary widely from one situation to the other. However, if an overload of 20% is there, the VDF goes upto 0.65 (=0.61 + 0.04). If only 10% of the laden MCV are overloaded to the extent of 20%, the VDF = 0.344 (=0.1 x 0.65 + 0.9 x 0.31)

The VDF values of HCV & MCV are givenbelow

Vehicle Type Laden Unladen/Partially LadenHCV 2.86 0.31MCV 0.34 0.02

The cumulative ESAL applications (N) over the design life can be computed using the following formula.

N = To x 365 x (1+0.01r)n-1 x L 0.01r

r = traffic growth rate 6%n = Design life 10 yearsN = To x 4811 x L

To = ESAL per day = No.of commercial vehicles per day in the year of opening x VDF

L = Lane distribution factor, L = 1 for single lane and intermediate lane and L=0.75 for two lane roads.

Traffic CategoriesFor pavement design the traffic has been categorized into

seven categories as under.

Traffic Category Cumulative ESAL Application

T1 10,000-30,000

T2 30,000-60,000

T3 60,000-100,000

T4 100,000-200,000

T5 200,000-300,000

T6 300,000–600,000

T7 600,000–1,000,000

Pavement Design Catalogues

• Knowing the traffic category (T1 to T7) and sub grade strength (S1 to S5), the pavement can be designed using the pavement designed catalogues

Fig.6

ILLUSTRATIVE EXAMPLE FOR PAVEMENT DESIGN

Twenty four hour traffic counts over a period of 3 days taken on a single lane rural road during the peak harvesting season, yielded the following results for the average daily traffic.

Animal Drawn carts (pneumatic tyred) = 25

Bicycles = 450

Full sized Trucks (Trucks & Buses) = 18

Agriculture Tractor-Trailers and Jugads = 90

Cars and Jeeps = 15

Motor Cycles = 200

Total 798

• Average CBR of sub grade soil is 2.1

Design Calculations

Average Daily Traffic during non = 798 = 394harvesting season (T) 2 AADT = T + 1.2nTt

365 n = 1 t = 75 days AADT =394 + 1.2x1x394x75 = 492

365Before opening of the road to traffic i.e., 2 years after traffic study

AADT (Tn) = To (1+r)n

r = 6% = 0.06

n = 2 years

To = 492 AADT = 492 (1.06)2 = 553

Heavy Commercial Vehicles HCVAADT before opening of the traffic

= 18 + 1.2 x 18 x 75 x 1.062 = 13 2 2

____________ 365

(or)AADT = 553 x 18 =13

798 Medium Commercial Vehicles MCV

AADT before opening of the traffic =

90 + 1.2 x 90 x 75 x 1.062 = 63 2 2

____________ 375

(or)AADT = 553 x 90 = 63

798 Therefore, Commercial Vehicles per day CVPD = 13+63=76

• Since the traffic count data does not give the proportion of unladen and laden vehicles, it is assumed they are equal in number.

Vehicle Damage FactorFor HCV No./day VDF/Vehicle VDF

Laden 6.5 2.86 18.59Unladen 6.5 0.31 2.01

For MCVLaden 31.5 0.34 10.71Unladen 31.5 0.02 0.63

Total 31.94

Assuming traffic growth rate (r) of 6% over design life (n) of 10 years the cumulative ESAL applications (N)

= 4811 To where To = ESAL/day= 4811 x 31.94 = 1,53,663

Pavement Design

For N = 1,53,663 & Avg. CBR of sub grade is 2.1, referring to the traffic category (T4) in the range 1,00000 to 2,00000 and the sub grade category of (S1) CBR=2 from pavement design catalogue, crust thickness required is 425mm

• Existing sub base = 0

• Existing metal crust = 0

• Proposed sub base = 275mm

• Proposed base = 150mm(Grade II 75mm & Grade III 75mm thick)

• Proposed surface course = 20mm with Seal coat Type-A