Question

A flexible pavement consists of surface layer (asphalt concrete), granular base layer and subbase layer. Please determine the thickness of each layer using AASHTO (1993) design method using the Design Chart

• Design ESAL = 2 million
• It takes approximately one week for water to be drained from within the pavement, and the pavement structure will be exposed to moisture levels approaching saturation for 40 percent of the time
• Resilient modulus of subgrade is 9000 lb/inch/inch; Resilient modulus of base is 31,000 lb/inch/inch; Resilient modulus of subbase is 13,500 lb/inch/inch
• Reliability level = 99%
• Standard deviation = 0.5
• Initial serviceability index, pi = 4.5; Terminal serviceability index, pi = 2.5
• a1 = 0.44; a2 = 0.14; a3 = 0.10

Answer 1

Step 1:

ESAL=2x106

PSI=4.5-2.5=2

Mr=31000 psi for Base

Mr=13500 psi for sub base

Mr=9000 psi for subgrade soil

Step 2:

From AASHTO 1993 CHARTS

SN1=2.6

SN1=a1D1

2.6=0.44xD1

D1=5.9 in, Provide 6 in

SN1*=0.44x6=2.64

Step 3:

m2=0.8, for the given saturation time and drainage conditions

D2=SN2-SN1/(a2m2)

D2=3.85-2.64/(0.14x0.8)

D2>10.8 in, provide 12 in, for base layer

SN2*=2.64+(0.14x0.8x12)=3.984

Step 4:

D3=SN3-SN2/(a3m3)

D3=4.4-(3.984)/(0.10x0.8)

D3=5.2 in

Use 6 in for sub base layer, as it is the minimum requirement

Procedure : From given ESAL S.D and Reliability, project a line, to The Effective road bed modulus, if it is for asphalt layer, then do it for base, and if it is for base , do it for sub base and project the respective line to PSI, from there we get the respective SN for asphalt layer, sub base and base