Flexible Pavement Design:-

The design procedure is based on 1993 AASHTO which involves 12 steps:

step 1: Reliability

Reliability level (R) can be found in (Table-1) is the probability that a purposed pavement will last for the entire period of service life without undergoing failure. Higher R value will mean higher performance but lower economy since thicker sections will be designed and greater amount of material will be used. R value depends on the intended function of the roadway and whether it is used for urban or rural areas. The table below shows the recommended values of R according to 1993 AASHTO.

                           Table-1: Reliability


Step 2: Overall standard deviation (So)

Generally So varies from 0.4 to 0.5 and it is selected based on the experience of highway engineer.

Step 3: Cumulative Equivalent Single Axle Load (W18)

The standard axle load used is 18 Kip

Cumulative ESAL per year=(vehicles/day)(lane distribution factor)(days/year)(EALF)

W18 =Cumulative ESAL per year x no.of years

EALF(equivalent axle laod factors) can be determined from the tables below.

               Table-2: Equivalent axle laod factors (EALF) for Single Axles

                        Table-3: Equivalent axle laod factors (EALF) for Tandem Axles

                        Table-4: Equivalent axle laod factors (EALF) for Triple Axles

                                     Table-5: Lane Distribution Factor

Step 4: Effective roadbed soil resilient modulus (Mr)

Mr can determined in the laboratory based on AASHTO T307 method. This value varies depending on the moisture content, therefore, different values of Mr are obtained during different times of the year. A critical value is chosen to represent the whole year.

In order to determine the effective value of Mr , Mr must be determined for each month and then from the scale below the relative damage (uf) is determined and recorded in the table. The average of uf is taken and used to determine the effective Mr from the scale.

Fig-1:(soil resilient modulus) Mr

Step 5:Resilient Moduli of Pavement Layers

Mr is then determined for the surface layer, base course and subbase using laboratory testing as discussed in the previous step.

Step 6: Serviceability Loss

Serviceability index can be determined by the difference between initial and final serviceability index.


Po is usually 4.6 or 4.5 and the values for Pt are 3.0 for major roads and 2.5 for intermediate roads and 2.0 for secondary roads.

Step 7: structural numbers (SN)

Is an index that is related to layer thicknesses, structural layer coefficients and drainage coefficients. They can be determined by using the figure below. The structural numbers must be determined for the subgrade, subbase and base course layer.

Fig-2: SN

Step 8: structural layer coefficients

These coefficients represent the structural performance of each layer (surface layer, base and subbase). These can be determined by AASHTO road test or can be determined by some derived correlations represented in the figures below.

Fig-3: correlations for surface layer

Fig-4: correlations for base layer

Fig-5: correlations for subbase layer

Step 9: Drainage Coefficients

Drainage coefficients represent the quality of drainage for bases and subbases (m2 and m3). for untreated bases and subbases, they can be determined the table below.

                  Table-6: Drainage coefficients


Step 10: Layer thicknesses

When the structural numbers are determined (SN1, SN2, and SN3) it is possible to find the thicknesses of surface layer, base course and subbase (D1, D2, and D3 respectively) from the equations below. When D1 is found from the first equation it is rounded to the nearest 0.5 in then is used to find D2 in the second equation. finally D1 and D2 can be used in the third equation to find D3.

SN1 ≤ a1D1

SN2 ≤ a1D1 + a2D2m2

SN3 ≤ a1D1 + a2D2m2 + a3D3m3

Step 11: Freeze or thaw and swelling

In case the pavement is located in a place that is susceptible to freeze and thaw, then AASTHO recommends the reduction of service life.

Step 12: Life cycle cost

the pavement designer often uses different design periods in order to determine the most economic design strategy. All the costs must be considered such as initial construction, maintenance, rehabilitation.


Design the pavement for an expressway consisting of an asphalt concrete surface, a crushed-stone base, and a granular subbase using the 1993 AASHTO design chart.


single axle load vehicles= 22 Kips


Lane distribution factor=0.8

design period=15 years

10% of the time the moisture level is approaching saturation

The effective roadbed soil resilient modulus is 7 ksi

subbase CBR value= 80

resilient modulus of the base is 40 Lb

resilient modulus of asphalt concrete is 4.5x105 psi

reliability level =95%



Step 1

Reliability level=95% (Given)

Step 2

overall standard deviation=0.45 (Given)

step 3

EALF=(1/2.18)=0.4587 From Fig 6

Cumulative ESAL per year=(vehicles/day)(lane distribution factor)(days/year)(EALF)


W18 =Cumulative ESAL per year x no.of years=267880.8x15=4018212

Step 4

Effective road-bed soil resilient modulus=7 ksi  (Given)  

Step 5
Resilient modulus of subbase=20 ksi (Figure 10)
Resilient modulus of base=40 ksi (Given)
Resilient modulus of asphalt concrete surface=450 ksi (Given)

Step 6

Assume initial serviceability index Po =4.6
Assume terminal serviceability index Pt = 3.0

Step 7

SN3=7.5 from Fig 11

SN2=4.5 from Fig 11

SN1=3.2 from Fig 11

Step 8

a3=0.14 (Figure 12)
a2=0.17 (Figure13)
a1=0.44 (Figure 14)

Step 9

Drainage coefficients=m2=m3=1.1

Step 10


D1=7.27 will be rounded to 7.5in

4.5≤0.44x7.5 + 0.17xD2x1.1

D2=6.4 will be rounded to 6.5in

7.5≤0.44x7.5 + 0.17x6.5x1.1 + 0.14xD3x1.1

D3=19.37 will be rounded to 19.5in


D1=7.5 in

D2=6.5 in

D3=19.5 in


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Mohammad Barzan

BSc- Geotechnical Engineering- Koya University.

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