What Sub-Grade Is
The sub-grade is the soil beneath the aggregate base of a parking lot. It’s the foundation of the entire pavement system. Even the strongest asphalt installed over a failed sub-grade will fail. Most pavement failures that look like ‘asphalt problems’ are actually sub-grade problems showing through.
What Sub-Grade Failure Looks Like
Visible signs include: alligator cracking that follows traffic patterns, persistent potholes at the same location, water and fines pumping up through cracks, settlement (one section sinks relative to surrounding pavement), rutting in drive lanes, drainage failures that appear after the lot is built. The pattern: surface-only fixes don’t last, and the same problems return.
Common Causes of Sub-Grade Failure
Saturation. The biggest cause. Water entering the sub-grade reduces its bearing capacity and creates conditions for failure under load. Sources: failed surface drainage, broken underground pipes, leaking water service connections, downspout drainage directly into the sub-grade. Insufficient initial design. Sub-grade was not properly compacted or wasn’t suitable soil for the loads imposed. Increased traffic loads. Pavement was designed for one use (e.g., light retail) but is now being used for another (e.g., distribution). Heavy axle loads exceed the original design. Tree root intrusion. Roots displace soil and create voids. Underground utility issues. Failed pipes leak water into sub-grade or sinkholes form when pipes collapse.
How to Prevent Sub-Grade Failure
Design for the actual use. Heavy industrial = 5 inches asphalt over 12 inches base, often with geotextile fabric. Light commercial = 3 inches asphalt over 6 inches base. Don’t under-design. Maintain drainage. Catch basins, surface grading, and crack sealing keep water from reaching the sub-grade. Address surface cracks before they let water in. Crack sealing is the front-line defense for sub-grade preservation. Inspect underground utilities. Camera inspection of storm and sanitary lines every 5 to 10 years catches deterioration before it triggers sub-grade washout.
How to Repair Sub-Grade Failure
Saw-cut and excavate the failed area down through asphalt and base to expose the failed sub-grade. Repair the sub-grade — typically by removing failed material and replacing with engineered fill, or by installing geotextile fabric to bridge over saturated soil. Compact in lifts to 95% standard Proctor density. Install fresh aggregate base, compacted in lifts. Pave back full-depth asphalt tied into surrounding lot. The fix is expensive but lasting — typically 20+ years.
Geotextile Fabric Reinforcement
For sites with chronically wet or weak sub-grade, geotextile fabric is laid between the sub-grade and the aggregate base. The fabric distributes loads, separates fine soils from base aggregate, and provides drainage. Common in industrial and intermodal applications. Adds a few cents per square foot to construction cost but dramatically extends pavement life on poor soils.
When the Whole Lot Has Sub-Grade Failure
If sub-grade failure is widespread (more than 25 to 30% of the lot), full-depth reconstruction is the right call. Spot reconstruction of multiple failed sections is usually more expensive than reconstructing the whole lot — and the rest of the lot is probably aging too. Math depends on lot size, but full reconstruction often makes more economic sense.
Sub-Grade and Building Code
Most municipal building codes specify sub-grade preparation for new commercial construction — compaction percentages, soil suitability, plate-load testing, and so on. New construction inspections verify sub-grade meets code. The problem is that 30-year-old lots have rarely had any sub-grade attention since original construction. Periodic assessment catches problems before they become reconstructions.
Get a Free Commercial Paving Estimate
Need help with a commercial paving project? Every estimate is free, on-site, and itemized. Most on-site estimate within 48 hours, with on-site estimate within 48 hours. Use the form on this page or call (630) 555-PAVE.
Get a Free Commercial Paving Estimate
Need help with a commercial paving project? Every estimate is free, on-site, and itemized. Use the form on this page or call (630) 555-PAVE.