A flexible pavement was designed for the following traffic w

A flexible pavement was designed for the following traffic with a 12-ycar design life: The highway was designed with 4-in hot-mix asphalt wearing surface, a 4-in hot-mix asphaltic base, and an 8-in crushed stone sub-base. The reliability was 70%, overall standard deviation was 0.5, APSI was 2.0 (with a TSI of 2.5), and all drainage coefficients were 1.0. What was the soil resilient modulus of the subgrade used in design?

Solution

Solution:

D₁ = 4 inch of HMA;               a₁= 0.44

D₂ = 4 inch HMA;                    a₂ = 0.40

D₃ = 8 inch crushed stone;    a₃ = 0.11

Reliability = 70%

            S_o = 0.5                                    PSI = 2

            TSI = 2.5                      M₁ = M₂ = M₃ = 1

Calculations:

            SN = a₁D₁ + a₂D₂M₂ + a₃D₃M₃             ------------ (1)    

Equation (1) gives:

            SN = 4.24

Axle Loads ---------

            Single 8-kips = 0.03932

            Tandem 15-kips = 0.04308

            Single 40-kips = 22.164

            Tandem 40-kips = 2.042

Calculating 18-kip ESALs:

            Daily W₁₈ = 1300(0.03932) + 900(0.04308) + 20(22.164) + 200(2.042) = 941.568

So;

            Total W₁₈ = 941.568 x 365 x 12

                             = 4124067.84

            Z_r = -0.524

Now, from design equation of AASHTO

            M_r = 5250.31 psi