How Is a Pavement Section Designed for a Commercial Site?

Pavement section design sizes the surface, base, and subbase layers so traffic loads are distributed to the subgrade within its bearing capacity. The engineer uses the geotechnical report's subgrade strength, the projected traffic loadings over the design life, and methods like AASHTO or the Asphalt Institute to determine each layer's thickness. The concept is load spreading. A truck tire hits the pavement with thousands of pounds on a small contact patch; each layer spreads that load over a wider area, so by the time the stress reaches the subgrade it's a fraction of what it was at the surface. The design makes sure every layer is thick enough — and the subgrade strong enough — that nothing gets overstressed over the pavement's design life, typically 20 years.

Traffic characterization comes first: not just today's traffic but the 20-year projection, converted to equivalent single-axle loads (ESALs) — the standard currency of pavement design. One loaded delivery truck does the damage of thousands of cars, which is why the truck count dominates the design even on a retail site. Subgrade strength comes from the geotechnical report, and drainage conditions modify everything. A high water table or poor surface drainage weakens the subgrade and the base, so the pavement design and the site drainage design have to agree. In expansive soil areas, the section may need a thicker non-expansive base or moisture barriers to isolate the pavement from soil movement. The AASHTO or Asphalt Institute method then combines traffic, subgrade, reliability, and serviceability into layer thicknesses — a calculation, not a guess.

A well-designed section can still fail if construction or drainage lets it down. The design assumptions have to survive contact with the field. These are the failure points I watch.

• Poor drainage: saturated base and subgrade lose strength — edge drains and surface grading are structural
• Inadequate compaction: base rock at 90% compaction instead of 95% consolidates under traffic and ruts
• Wrong traffic assumptions: the section designed for cars now serves daily delivery trucks
• Thin spots: inconsistent base thickness during construction leaves weak points that fail first
• No maintenance: unsealed cracks let water into the base, and the deterioration accelerates from there