flexible pavement thickness design (aastho method)

8/13/2019 Flexible Pavement Thickness Design (Aastho method)

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FLEXIBLE PAVEMENT THICKNESS DESIGN

AASHTO METHOD

Example

Determine the thickness of the surface, base and subbase layers using the following

design parameters:

Equivalent Single Axle Load (ESAL) = 2 106

Initial Present Serviceability Index, PSIi= 4.5

Terminal Present Serviceability Index, PSIt= 2.5

Resilient modulus of asphalt concrete, MR1= 450,000 psi

CBR of crushed stone base = 100%

CBR of gravel subbase = 22%

CBR of subgrade = 6%

Exposure to moisture = 30% of the timeQuality of drainage: Surface layer Good

Base layer Fair

Subbase layer Fairly poor

Reliability, R = 99%

Standard deviation, So= 0.35

Solution

(1) Design Serviceability Loss, PSI

PSI = PSIt PSIi = 4.5 2.5 = 2.0

(2) Resilient Modulus

Subgrade : MR4= 6,800 psi (Figure 16-4 Appendix A-4, with CBR = 6%)

Subbase : MR3= 13,300 psi (Figure 16-16, Appendix A-10, with CBR = 22%)

Base : MR2= 30,000 psi (Figure 16-15, Appendix A-9, with CBR = 100%)

(3) Structural Number

(Use nomograph in Figure 16-11, Appendix A-5)

Surface : SN1= 3.2 (with R = 99%, S = 0.35, ESAL = 2 106, and

MR2= 30 103)

Base : SN2= 4.2 (with R = 99%, S = 0.35, ESAL = 2 106, and

MR3= 13 103)

Subbase : SN3= 5.2 (with R = 99%, S = 0.35, ESAL = 2 106, and

MR4= 6.8 103)


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(4) Structural Coefficient

Surface : a1= 0.44 (Figure 16-13, Appendix A-7 with MR1= 450,000 psi)

Base : a2= 0.14 (Figure 16-15, Appendix A-9, with CBR = 100%)

Subbase : a3= 0.098 (Figure 16-16, Appendix A-10, with CBR = 22%)

(5) Drainage Coefficient

(Use Table 16-5, Appendix A-11)

Surface : m1= 1.00 (with moisture exposure > 25% and drainage

quality = good)

Base : m2= 0.80 (with moisture exposure > 25% and drainage

quality = fair)

Subbase : m3= 0.70 (with moisture exposure > 25% and drainage

quality = fairly poor)

(6) SURFACE THICKNESS

27.7)00.1(44.0

2.3

ma

SND

11

11 === in 7.5 in

SN1* = a1D1m1 = 0.44 (7.5) (1.00) = 3.3

SN1* SN1 OK!

(7) BASE THICKNESS

=

=

=)80.0(14.0

3.32.4

ma

*SNSND

22

122 8.04 in 8.5 in

SN2* = a2D2m2 = 0.14 (8.5) (0.80) = 0.95

SN1* + SN2* = 3.3 + 0.95 = 4.25

SN1* + SN2* SN2 OK!


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(8) SUBBASE THICKNESS

=

=

=)70.0(098.0

3.395.02.5

ma

*SN*SNSND

33

1233 13.85 in 14 in

SN3* = a3D3m3 = 0.098 (14) (0.70) = 0.96

SN1* + SN2* + SN3* = 3.3 + 0.95 + 0.96 = 5.21

SN1* + SN2* + SN3* SN3 OK!

(9) PAVEMENT LAYER PROFILE

Surface a1= 0.44 , m1= 1.00 7.5 in

MR1= 450,000 psi

Base a2= 0.14 , m2= 0.80 8.5 in

MR2= 30,000 psi

Subbase a3= 0.098 , m3= 0.70 14 inMR3= 13,300 psi

Subgrade MR4= 6,800 psi

(1 inch = 25.4 mm , 1 psi = 6894.76 Pa)


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flexible pavement thickness design (aastho method)

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