Fullerton sits on a mix of older alluvial fans and younger floodplain deposits, so the subgrade you find on one block can be completely different from the one two streets over. We have tested subgrade conditions across the city, from the gentle slopes near Coyote Hills down to the flat industrial corridors along the 91 freeway. Many design teams assume a uniform soil profile and skip the site-specific work. That rarely ends well here. A proper road subgrade design has to account for the variable clay content, the seasonal moisture changes, and the old utility trenches that crisscross most rights-of-way. Before we recommend a pavement section, we always run a full suite of index tests and at least one proof-roll to catch soft spots. For projects where expansive clay is suspected, we combine that with a suelos expansivos analysis to avoid the heave damage that cracks many Fullerton streets within the first five years.
A CBR value that drops from 8 to 4 can double the required base thickness. Verify before you pave.
Methodology and scope
Local considerations
Fullerton recorded a magnitude 5.1 earthquake in 1933 (the Long Beach event), and the city lies within the influence zone of the Whittier and Elsinore faults. That seismic context matters for road subgrade design because loose saturated sands can liquefy beneath a pavement section during strong shaking. We have mapped several areas near the Santa Ana River channel where the groundwater table sits less than 10 feet deep, increasing that risk. A standard subgrade design that ignores cyclic softening will fail when the next moderate earthquake hits. We integrate respuesta-sismica analyses to evaluate site amplification and recommend mitigation measures like deep soil mixing or drainage improvements before placing the pavement.
Explanatory video
Applicable standards
ASTM D1883 (CBR), ASTM D698 (Standard Proctor), Caltrans HDM Chapter 600, AASHTO T-99 / T-180, ASCE 7-16 seismic site class
Associated technical services
Subgrade Investigation & Sampling
Boreholes, test pits, and hand augers to characterize soil layering, moisture content, and in-situ density across the project alignment.
CBR & Proctor Testing
Laboratory compaction curves and soaked/unsoaked CBR values calibrated to Fullerton's typical alluvial clays and sands.
Pavement Structural Design
AASHTO 1993 and Caltrans HDM-based analysis to determine required base thickness, asphalt layer depth, and subgrade improvement recommendations.
Typical parameters
Frequently asked questions
What is the typical CBR value for subgrade soils in Fullerton?
In our experience, soaked CBR values range from 4 in clay-rich zones near downtown to 15 in sandy areas closer to the Coyote Hills. We always recommend site-specific testing because the variability is high.
How does seasonal moisture affect road subgrade design in Fullerton?
Fullerton's dry summers and wet winters can cause significant moisture fluctuation. We run soaked CBR tests to simulate worst-case winter conditions and adjust the pavement thickness accordingly. Ignoring seasonal swelling can lead to premature cracking.
Do I need a seismic site class analysis for a local street project?
Yes, if the project falls under the California Building Code or requires Caltrans approval. We determine site class per ASCE 7-16 using shear wave velocity or SPT N-values. Liquefaction potential is evaluated near the Santa Ana River channels.
How much does a road subgrade design study cost in Fullerton?
A typical study for a residential street runs between US$990 and US$3.540 depending on the number of test locations, laboratory work, and the complexity of the analysis. We provide a fixed quote after reviewing the project scope.