Flexible Pavement Design Example

Flexible Pavement Design Example

Solution using nomograph

Selection of Layer Thicknesses

The reliability level to be used is based on the functional classification of the pavement

Nomograph Solution (continued)

Minimum Recommended Thicknesses for Surface and Base Layers

Chart for estimation of a1 for asphalt concrete

Chart for estimation of subbase layer coefficient (a3) based on strength parameters

Chart for estimation of base layer coefficient (a2) based on strength parameters

Recommended mi values for modifying layer coefficients of untreated base and subbase materials

Assume that the following pavement materials are to be used: • Asphalt concrete: EAC = 400,000 psi, a1 = 0.42 • Granular base: EBS = 30,000 psi, a2 =0.14 • Granular subbase: ESB = 11,000 psi, a3= 0.08 • Assume drainage coefficients, m2 = m3 = 0.9

Procedure for determining thicknesses of layers using a layered analysis approach

Determine the required SN1 and SN2 (using the elastic modulus of the base and subbase as subgrade modulus)

Computation of layer thicknesses SN1 = 2.7 D*1 = SN1 /a1 = 2.7 / 0.42 = 6.4 inches Use D*1 = 7 inches SN*1 = 0.42 X 7 = 2.94

SN2 = 3.8 D*2 = (SN2 – SN*1) / m2 a2 = (3.8 – 2.94) / (0.9 X 0.14) = 6.8 inches Use D*2 = 7 inches SN*2 = 0.14 X 7 X 0.9 = 0.88

Computation of required subbase thickness: SN3 =5 D*3 = (SN3 – SN*1 - SN*2)/ (m3 a3) = (5 – 2.94 –0.88) / ( 0.9 X 0.08) = 16.4 inches Use D*3 = 17 inches

Summary of Design Asphalt Concrete

7 inches

Granular Base

7 inches

Granular Subbase

17 inches