We often get calls after the first big rain. The asphalt looks perfect, but the subgrade underneath is mush. Austin's blackland prairie clay is notorious for this. It shrinks hard in the summer drought, then swells violently when the rain returns. If you design your flexible pavement section without a proper grain-size analysis and Atterberg limits test on that clay subgrade, you are building on a sponge. The pavement fails from the bottom up. We work with developers in East Austin up to the Hill Country to make sure the base course and asphalt concrete layers are specified to handle these volume changes. The goal is a pavement structure where every layer works together, not against each other.
Austin's expansive clay subgrades will destroy a pavement that wasn't designed with them in mind.
Methodology and scope
Local considerations
The Texas Blackland Prairies eco-region puts Austin right on top of some of the most expansive soils in the nation. The Taylor Marl and Eagle Ford formations are filled with smectite clays that can swell 10% or more in volume. A flexible pavement section designed for normal soil will tear apart in two seasons here. We see it manifest as longitudinal cracking along the wheel path and deep alligator cracking in parking lots. The danger isn't just surface distress. Water gets into those cracks, saturates the base, and the whole structural section loses its load-carrying capacity. A CBR test on intact soil gives us a baseline, but it's the combination with swell potential that really tells the story in Austin.
Applicable standards
TxDOT Pavement Design Guide (Flexible Pavement), AASHTO Guide for Design of Pavement Structures 1993, ASTM D4318 (Atterberg Limits), ASTM D1883 (CBR), ASTM D6927 (Marshall Stability), ASTM D4867 (Moisture Susceptibility)
Associated technical services
Subgrade Evaluation
We run full soil classification suites including grain size, Atterberg limits, and sulfate testing on your subgrade. This determines the design CBR or resilient modulus value and potential for swell.
Asphalt Mix Design Verification
We validate your hot mix asphalt against TxDOT specifications. We test for volumetric properties, Marshall stability, and flow, and can run Hamburg wheel-track tests for rutting susceptibility.
Field Compaction Control
We perform nuclear density and sand cone tests during construction. We verify that your base course and each asphalt lift hit the required density before you place the next layer.
Typical parameters
Frequently asked questions
What is the best way to handle expansive clay under a flexible pavement in Austin?
The most reliable method is to stabilize the subgrade. We typically recommend treating the top 6 to 12 inches of expansive clay with lime or cement. This chemically changes the clay's affinity for water, reducing its swell potential to near zero. The exact percentage of stabilizer is determined by our lab through a pH test and a strength gain series. Over that treated layer, you place your standard flexible base and asphalt.
How much does a flexible pavement design package cost for an Austin project?
A full pavement design package, including subgrade evaluation, mix design verification, and thickness recommendations, generally runs from US$1,670 to US$5,200. The scope depends on the number of soil samples, the size of the project, and the level of traffic data analysis required.
Which asphalt binder grade should we use for an Austin parking lot?
For almost all Austin applications, we specify a PG 70-22 binder as a minimum. The upper temperature grade of 70°C is critical to resist rutting in our July and August heat. For high-stress areas like intersections or bus stops, we often bump that up to a PG 76-22 to handle the slow, heavy loads without deforming.