AASHTO 1993 still drives most flexible pavement design in California, but the method is only as good as the subgrade input. In Concord, the subgrade is rarely uniform. We encounter stiff alluvial clay on one block and silty fill on the next. That variability demands more than a textbook structural number. Our design process starts with site-specific CBR and R-value testing to anchor the pavement section in measured stiffness. Where the upper soils are too soft to support the base course, we evaluate vibrocompaction or aggregate piers as subgrade improvement before the pavement goes in. The goal is the same every time: a structural section that handles ESALs without rutting, fatigue cracking, or shear failure at the subgrade-base interface.
A flexible pavement is not just asphalt thickness. It is a layered system where the subgrade, base, and binder work together to distribute load without deforming.
Common questions
How much does flexible pavement design cost for a typical project in Concord?
For a site-specific flexible pavement design with subgrade evaluation and a stamped report, the fee generally ranges from US$1,600 to US$4,520. The spread depends on project size, number of borings, and whether we are designing for a parking lot or a public road with high ESALs.
What is the difference between the AASHTO 93 method and the newer mechanistic-empirical approach?
AASHTO 93 is an empirical method based on road test data from the 1960s. It uses the structural number and serviceability loss as main inputs. The Mechanistic-Empirical Pavement Design Guide (MEPDG) models stresses, strains, and damage accumulation through climate and load spectra. For most local roads in Concord, AASHTO 93 is still the accepted standard; MEPDG is reserved for high-volume corridors where distress prediction must be more precise.
How do you handle expansive clay subgrade under a flexible pavement?
We typically excavate the upper 12 to 24 inches of expansive clay and replace it with a non-expansive select fill compacted to 95 percent of modified Proctor. A geotextile separator between the fill and the aggregate base prevents fines migration. In areas with extreme volume change potential, we may also increase the asphalt thickness or specify a lime-treated subbase.
What design ESAL value do you recommend for a commercial parking lot in Concord?
We base the ESAL estimate on the specific tenant and delivery pattern. A typical retail plaza with light truck traffic might need 50,000 to 150,000 ESALs over 20 years. A warehouse with daily heavy freight could exceed 500,000. We always verify with the owner's traffic forecast before selecting the structural number.
