Bakersfield sits at the southern end of the San Joaquin Valley, where summer pavement surface temperatures routinely exceed 140°F and winter nights drop near freezing. The underlying soil—much of it lean clay with moderate to high expansion potential—creates a punishing environment for asphalt layers. Standard empirical designs rarely hold up here beyond a few seasons without deep cracking and rutting. Our approach combines CBR road testing data with mechanistic-empirical analysis to define layer thicknesses that resist both thermal fatigue and subgrade volume change. The Kern River alluvium that underlies much of the city introduces additional variability: gravel lenses and sandy pockets within the clay matrix demand site-specific characterization before any structural number is assigned.
A well-designed flexible pavement in the southern San Joaquin Valley must manage three simultaneous threats: expansive subgrade movement, thermal oxidation of the binder, and poor subsurface drainage.
How we work
Local considerations
The AASHTO 93 flexible pavement design guide assumes drained conditions and uniform subgrade support—two assumptions that Bakersfield’s geology violates routinely. The city’s expansive Porterville-series clays swell with winter moisture and shrink during the dry summer, imposing cyclic vertical movement on the pavement structure that the structural number alone does not address. A pavement section designed solely from R-value without swell potential data risks premature longitudinal cracking and serviceability loss. Caltrans Section 608 requires moisture conditioning and swell testing for any subgrade with a Plasticity Index above 15, a threshold that much of Bakersfield’s near-surface soil exceeds. When the subgrade PI exceeds 25, we specify lime or cement stabilization to a depth of 12 inches and extend the design period analysis to capture the reduced layer coefficients. Construction timing matters here too: base compaction during the rainy months of January and February requires careful moisture control to avoid pumping and future rutting under traffic.
Video resource
Relevant standards
The design references include the AASHTO Guide for Design of Pavement Structures (1993 edition with 1998 supplement), Caltrans Highway Design Manual Chapter
Associated technical services
Subgrade Evaluation and CBR Testing Program
Field CBR testing at proposed subgrade elevation, supplemented by laboratory soaked CBR on Shelby tube samples. We map variability across the site because Bakersfield’s alluvial deposits can change bearing capacity within 100 lateral feet.
Structural Section Design and Life-Cycle Analysis
Mechanistic-empirical design using AASHTOWare Pavement ME or CalME for projects exceeding 1,000,000 ESALs. Includes layer coefficient optimization, drainage factor calibration, and binder grade selection matched to Bakersfield’s climate station data.
Typical parameters
Common questions
What is the typical cost range for a flexible pavement design study in Bakersfield?
How does Bakersfield’s expansive clay affect pavement design life?
The Porterville-series clays common across Bakersfield can swell up to 3% volumetrically with seasonal moisture changes. Without stabilization or a thickened base course, this movement induces longitudinal cracking within the first three to five years. Our designs address this through swell potential testing per ASTM D4546 and, when the expansion index exceeds 90, by specifying a lime-treated subgrade layer that chemically reduces the plasticity of the upper 12 inches.
Which binder grade should be specified for Bakersfield’s summer heat?
Based on Caltrans climate zone data and AASHTO M332, Bakersfield falls into the high-temperature band requiring a Performance Grade binder of at least PG 70-10. For projects in the hotter eastern edges of the city near the foothills, we often specify PG 76-16 to resist rutting during extended heat waves. The polymer modification also improves fatigue resistance during the 40°F overnight temperature drops that are typical in the valley.
What is the difference between AASHTO 93 and mechanistic-empirical design for local projects?
AASHTO 93 uses empirical equations derived from the AASHO Road Test in Illinois, which did not include Bakersfield’s expansive subgrade conditions or high-temperature binder aging. Mechanistic-empirical design—implemented through AASHTOWare Pavement ME or CalME—incorporates local climate data, actual traffic spectra, and material-specific resilient modulus values. For projects exceeding 1,000,000 ESALs or where pavement performance is critical, the ME approach provides a more reliable prediction of distress over the design life.