Factors affecting pavement design

A pavement must carry traffic loads for its design life without excessive cracking, rutting or roughness. The factors affecting the design (IRC:37-2018 for flexible and IRC:58-2015 for rigid pavements; Nepal: DoR Pavement Design Guidelines / Nepal Road Standard 2070, based on CBR charts) are:

1. Design wheel/axle load

• Magnitude of load and axle configuration (single, tandem, tridem). The standard axle is an 80 kN (8.16 t) single axle with dual wheels.
• Tyre pressure and contact area govern the surface stress; a contact pressure of about 0.56 MPa is used in IRC:37.
• Repetition of loads: damage increases with the number of passes.

2. Traffic

• Present commercial traffic (vehicles of more than 3 t laden weight) in each direction, expressed as CVPD.
• Growth rate $r$ and design life $n$ (15 years for NH/SH in IRC:37-2012; at least 20 years in IRC:37-2018, and longer for expressways).
• Vehicle damage factor (VDF), the equivalent standard axles per commercial vehicle, and the lane distribution factor (LDF).
• Cumulative standard axles:

$$N_s = \frac{365\left[(1+r)^n - 1\right]}{r} \times A \times D \times F$$

where $A$ = initial traffic at opening, $D$ = lane distribution factor and $F$ = VDF. The pavement is designed for $N_s$ in million standard axles (msa).

3. Subgrade strength

• Soaked CBR of the subgrade (at the expected field density and moisture), or the resilient modulus $M_R$ (IRC:37: $M_R = 10\,CBR$ MPa for CBR $\le$ 5 % and $17.6\,CBR^{0.64}$ MPa for CBR > 5 %); for rigid pavements, the modulus of subgrade reaction $k$.
• Soil type, plasticity, compaction, depth of water table and swelling potential.

4. Pavement materials

• Strength and stiffness of the granular sub-base, base (WBM/WMM), bituminous layers (modulus, fatigue, rutting) or cement concrete (flexural strength, modulus of elasticity).
• Availability and cost of local materials.

5. Climate and environment

• Temperature (bitumen stiffness, thermal stresses in concrete), rainfall and moisture variation of the subgrade; in the hills, frost action and landslides.
• Drainage and ground-water level: poor drainage greatly reduces the subgrade strength.

6. Other factors

• Design period, required reliability (80-90 percent) and failure criteria (fatigue cracking, rutting).
• Construction quality, method and maintenance level.
• Economy: initial cost and life-cycle cost; stage construction.
• In rigid pavements: joints and load transfer, slab thickness and the temperature differential.