Asphalt Institute design: gravel road upgrade, 0.8×10³ ESA

Given data and assumptions

• Two-lane single carriageway gravel road upgraded to bituminous; present traffic 0.8 x 10^3 (800) ESA/day (both directions), growth 6.5 %, design period 10 years, construction period 1 year.
• 87.5th percentile subgrade CBR = 5 %; moduli: asphalt concrete 2000 MPa, crushed stone base 350 MPa, granular sub-base 250 MPa.
• Two lanes: design lane carries 50 percent of the total (Asphalt Institute). Minimum asphalt concrete 50 mm.
• Method: Asphalt Institute (AI) thickness design as taught for IOE (and used in the Nepal DoR Pavement Design Guidelines); design reliability 90 percent, terminal serviceability 2.5.

Step 1 - Subgrade strength

The design (87.5th percentile) CBR of the subgrade is given as 5 %.

Resilient modulus of subgrade: $M_R = 10\times CBR = 10\times 5.0 = 50$ MPa.

Step 2 - Design traffic (ESAL)

Design-lane ESA/day at opening $=800\times0.5\times(1.065)^1=426.0$.

$$ESAL=365\times426.0\times\frac{(1.065)^{10}-1}{0.065}=365\times426.0\times13.494=2.098\times10^6$$

Design ESAL = 2.098 $\times 10^6$

Step 3 - Thickness required over each layer

The AI charts give the thickness of asphalt concrete (full depth) needed to protect a layer of a given modulus for the design ESAL. The same requirement is expressed here as a structural number $SN$ (AASHTO-93 relation with R = 90 percent, $S_0$ = 0.45, $\Delta PSI$ = 1.7), which closely approximates the AI chart readings (if the chart is supplied in the exam, read the thickness from it and use the same layer-coefficient steps below).

Step 4 - Layer coefficients

Using $a_1=0.323\log_{10}E-1.42$, $a_2=0.249\log_{10}E-0.977$, $a_3=0.227\log_{10}E-0.839$ with $E$ in psi:

• Asphalt concrete (2000 MPa): $a_1 = 0.344$
• crushed stone base (350 MPa): $a_2 = 0.195$
• granular sub-base (250 MPa): $a_3 = 0.196$

Step 5 - Layer thicknesses

• Asphalt concrete: $D_1 \ge SN_{base}/a_1 = 1.87/0.344=138$ mm, so provide 140 mm (minimum 50 mm). $SN_1 = 1.90$.
• Crushed stone base: $D_2 \ge (SN_{sub}-SN_1)/a_2 = (2.14-1.90)/0.195 = 32$ mm, so provide 100 mm. $SN_2 = 0.77$.
• Granular sub-base: $D_3 \ge (SN_{sg}-SN_1-SN_2)/a_3 = (3.96-1.90-0.77)/0.196 = 168$ mm, so provide 170 mm. $SN_3 = 1.31$.

Check: provided $SN = 1.90+0.77+1.31 = 3.98 \ge 3.96$ (required) - safe.

Answer: asphalt concrete 140 mm, crushed stone base 100 mm, granular sub-base 170 mm; total pavement thickness = 410 mm.

   +-------------------------------------+
   | Asphalt concrete             140 mm |
   +-------------------------------------+
   | Crushed stone base           100 mm |
   +-------------------------------------+
   | Granular sub-base            170 mm |
   +-------------------------------------+
   | Subgrade, CBR 5.0 %                 |
   +-------------------------------------+