We saw a warehouse loop road off I-35 fail in 18 months because the subgrade was tested in July — bone dry — and nobody accounted for the 30% swell those San Antonio clays hit after the first wet winter. That kind of failure is avoidable. Flexible pavement design here isn't just about layer coefficients and ESALs; it's about understanding the moisture cycle of the Blackland Prairie and the edge loading from construction trucks that contractors never mention. We combine CBR field and lab testing with an Atterberg limits profile to lock in a design resilient modulus that reflects what the soil actually does, not what a textbook says. The city's average annual rainfall of 32 inches and summer temperatures topping 100°F create a pavement environment that punishes generic designs.
A pavement section designed without the moisture-conditioned resilient modulus of the subgrade is just a bet on good weather.
Methodology and scope
Local ground factors
The most common mistake we see on San Antonio projects is treating the subgrade CBR as a fixed number from one boring log and ignoring the seasonal low. A contractor runs a single test in August, gets a CBR of 8, and designs a thin section. Then the rainy season hits, the clay saturates, the CBR drops below 2, and the pavement deflects until it crocodile-cracks. We've seen this on Loop 1604 frontage roads and smaller commercial lots alike. The fix is simple: condition the CBR to the equilibrium moisture content, or run a resilient modulus test on a representative sample. Another risk is skipping the drainage analysis: flat grades on San Antonio's south side hold water against the pavement edge, softening the subgrade from the outside in. Without an edge drain or a daylighted base, the section life drops by half.
Regulatory framework
AASHTO Guide for Design of Pavement Structures (1993, with Texas supplements), TxDOT Pavement Design Guide (2021), ASTM D1883 (CBR), ASTM D4318 (Atterberg), ASTM D1557 (Proctor), ASTM D5311 (Cyclic Triaxial for resilient modulus), AASHTO MEPDG (Mechanistic-Empirical Pavement Design Guide)
Associated technical services
Subgrade Characterization for Pavement
We drill and sample the top 5 feet of subgrade, run moisture-conditioned CBR, Atterberg limits, and Proctor tests, and deliver a design resilient modulus per AASHTO T307. For expansive clays, we include swell potential and lime demand.
Full Pavement Section Design
Using local traffic data, subgrade modulus, and Texas climate files, we design the asphalt concrete, granular base, and any stabilization layers. We provide layer coefficients, thicknesses, and a construction sequence that works for San Antonio contractors.
Typical parameters
Frequently asked questions
What does a flexible pavement design package include for a San Antonio project?
It starts with a subgrade investigation: borings to 5 feet, CBR tests at natural and soaked conditions, Atterberg limits, and Proctor curves. We then run traffic projections in ESALs and build a pavement section with hot-mix asphalt thickness, base course type and depth, and any stabilization needed. You get a stamped report with construction recommendations.
How much does a flexible pavement design study cost in San Antonio?
For a typical commercial lot or small roadway, the geotechnical investigation and pavement design runs between US$1,660 and US$5,350, depending on the number of borings, lab tests, and traffic analysis depth. A larger arterial or industrial yard with multiple subgrade zones will be at the upper end.
Do you use the AASHTO 1993 method or the MEPDG for San Antonio pavements?
We use both, depending on the project. For standard commercial work, AASHTO 1993 with Texas-calibrated inputs is fast and reliable. For high-traffic arterials or when the client wants a performance-based design, we run the MEPDG with local climate files and calibrated distress models.
How do you handle the expansive clays common in San Antonio?
We test the plasticity index and sulfate content first. For PI above 25, we usually specify lime stabilization — typically 4% to 6% by weight — mixed into the top 6 to 8 inches of subgrade. We also mellow the lime for 48 hours before final compaction. If sulfates are high, we switch to a Type V cement or a fly-ash blend to avoid ettringite swelling.