Greensboro sits squarely in the Piedmont Triad, where the red clay isn’t just a nuisance—it’s a fundamental design parameter. Most pavement failures we investigate originate in the subgrade, not the asphalt layer. The silty clay residuals derived from weathered felsic rock compact well at optimum moisture but swell when wet and shrink dramatically during drought. A pavement section designed without this cyclic volume change in mind will develop alligator cracking within three years. Our lab runs soaked CBR tests on compacted Shelby tube samples because the in-service moisture condition after a Guilford County summer thunderstorm is what governs performance, not the dry-weather strength. For commercial lots over residual soils, we often recommend a CBR pavement design that ties the structural number directly to the soaked bearing capacity measured in our triaxial cells.
In Greensboro’s Piedmont clay, the soaked CBR value is the single most important number in a flexible pavement design—everything else flows from it.
Our approach and scope
Local context
What we observe in the field around Greensboro: the transition zone between cut and fill sections is the single most failure-prone location in any flexible pavement. The natural ground has been stiff and overconsolidated for millennia; the adjacent fill, even when compacted to 95% of standard Proctor, settles differentially. We’ve measured a 3-inch elevation difference across a 20-foot transition in a shopping center on West Wendover Avenue. The design response is not just a thicker pavement—it’s a geotextile separator at the cut-fill contact and a tapered transition wedge in the aggregate base. Another risk is perched water in the upper weathered zone. Spring seepage saturates the subgrade from below, reducing the resilient modulus to a fraction of its design value. Our designs always include a subsurface drainage analysis: we calculate the time-to-drain for the base layer using the Casagrande & Shannon equation and specify edge drains when the time exceeds two hours.
Regulatory framework
AASHTO Guide for Design of Pavement Structures (1993), NCDOT Pavement Design Manual (current edition), ASTM D1883 – Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils, ASTM D6951 – Standard Test Method for Use of the Dynamic Cone Penetrometer in Shallow Pavement Applications, ASTM D4318 – Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils
Related services
Subgrade Investigation & CBR Testing
We extract Shelby tube samples from the top 3 feet of subgrade, run moisture-density relations per ASTM D698, then soak the compacted specimens for 96 hours before punching the CBR. The result is a design CBR that reflects the actual post-construction moisture equilibrium in Greensboro’s clay, not an optimistic dry-weather number.
Layered Elastic Analysis & Section Design
Using the AASHTO 93 structural number equation, we design the asphalt and base thicknesses for the traffic loading (ESALs) and subgrade strength. We check tensile strain at the bottom of the asphalt layer and compressive strain on top of the subgrade—both fatigue and rutting criteria must pass. Deliverables include a cross-section drawing, layer coefficients, and a construction QC testing plan.
Typical parameters
FAQ
How much does a flexible pavement design cost for a project in Greensboro?
For a standard commercial parking lot or access road in Greensboro, the pavement design portion—including subgrade sampling, soaked CBR testing, and the AASHTO 93 structural analysis—typically ranges from US$1,690 to US$5,970 depending on the number of subgrade zones, the required number of CBR points, and whether a drainage analysis is included. We deliver a fixed-price proposal after reviewing the site plan and soil survey.
Why is the soaked CBR test so critical for Greensboro’s clay soils?
Greensboro’s Piedmont residual clays have a high plasticity index—typically 15 to 30—and they absorb moisture over time after construction. A dry CBR test might show 15 or 20, but the soaked CBR after 96 hours of immersion often drops to 3 or 4. The pavement design must be based on the soaked value because that’s the condition the subgrade will reach after a few wet seasons. Designing on the dry strength guarantees premature rutting and fatigue cracking.
What’s the difference between AASHTO 93 and the mechanistic-empirical (MEPDG) approach for pavement design?
AASHTO 93 is an empirical method derived from the AASHO Road Test data; it uses the structural number concept and is the standard referenced by NCDOT for most projects. The MEPDG approach uses layered elastic analysis to compute stresses and strains, then predicts distresses—rutting, fatigue cracking, thermal cracking—over the design life using transfer functions calibrated to local conditions. For complex projects in Greensboro with unusual traffic spectra or staged construction, we can run a Level 2 MEPDG analysis using site-specific resilient modulus values from our triaxial cell, but for most commercial developments the AASHTO 93 method backed by soaked CBR is the practical and approved choice.