Subgrade is the natural or improved soil on which a pavement is built, beneath the base course and the surface layer of asphalt or concrete.
A pavement only spreads load; the subgrade carries it. If the soil is weak, wet or poorly compacted, the pavement above it will rut, crack and fail early no matter how good the surface is. Pavement thickness is designed from the subgrade’s measured strength and the expected traffic. See bituminous materials.
Preparing subgrade for a parking lot
- Investigation. A geotechnical report with soil borings and strength values, such as CBR or R-value.
- Removal and replacement. Unsuitable soils, organics and old fill are removed or treated.
- Compaction and proof rolling. The subgrade is compacted to specification and proof rolled with a loaded truck to find soft spots.
- Stabilization. Lime, cement or geotextiles can improve weak soils.
- Drainage. Water in the subgrade is the main cause of failure, so slopes, edge drains and frost protection matter.
Traffic in parking lots
- Parking stalls see light loads, but drive aisles, entrances, dumpster pads and bus or truck routes need thicker sections over the same subgrade.
The Parkonomics view
Most early pavement failures in parking lots start below the surface. Spending on investigation, compaction and drainage is cheaper than paying to rebuild the lot.
Related terms: Bituminous Materials, Foundation Systems, Stormwater Detention, Life Cycle Cost