The red clay in Cary North Carolina doesn’t forgive generic pavement sections. Summer humidity sits above 80% for weeks, winter brings freeze-thaw cycles into the subgrade, and the saprolite underneath swells and shrinks with every season. A flexible pavement design here has to account for moisture sensitivity first, structural number second. We’ve pulled cores across western Wake County where the weathered rock line jumps three feet in twenty meters — that’s normal for the Piedmont. The CBR road testing gives us a direct read on subgrade strength under soaked conditions, and when the profile gets erratic we cross-check with in-situ permeability to see where water is actually moving through the profile.
Cary’s saprolite subgrade can lose 60% of its stiffness when moisture climbs 3% above optimum — the structural number means nothing if drainage isn’t solved first.
Methodology and scope
The stone matrix matters too. We specify dense-graded aggregate base over open-graded only after checking the drainage path — trapped water at the base-subgrade interface destroys a pavement from the bottom up, and we’ve seen it happen on Tryon Road within five years of construction. Our lab runs gradation on the base aggregate per ASTM D6913 and modified Proctor per ASTM D1557 to lock in compaction targets before the first truck of stone ever arrives on site.
Local considerations
The soils east of US-1 toward Apex tend to be deeper saprolite with higher silt content — decent drainage if graded right. West toward the Jordan Lake arm, you hit more shallow rock and perched water tables that saturate the subgrade fast. A pavement section that performs fine near Crossroads can rut badly out near Green Level within two winters, same design, same traffic. The difference is moisture. We’ve pulled forensic cores where the base aggregate was fully saturated six inches below the asphalt because the subgrade was acting as a bathtub. That’s why we tie every flexible pavement design to a site-specific drainage analysis — cross-slope, underdrain location, and the permeability contrast between the subgrade and the base course. Ignore that contrast and you’re essentially building a reservoir under your asphalt.
Applicable standards
AASHTO 1993 Guide for Design of Pavement Structures, ASTM D1557 (Modified Proctor), ASTM D1883 / AASHTO T-193 (Soaked CBR), ASTM D6913 (Gradation of base aggregate), NCDOT Standard Specifications Section 520 & 620
Associated technical services
Subgrade Investigation & CBR Testing
Field-soaked CBR at formation level, dynamic cone penetrometer profiles, and moisture-density curves to establish the design subgrade strength under worst-case moisture conditions.
Pavement Structural Design (AASHTO 93 / MEPDG)
Layer thickness, material specs, and structural number calculations for collectors, arterials, and commercial parking lots — traffic-loaded per TM-5 or site-specific counts.
Base & Subbase Material Qualification
Gradation, Proctor compaction, LA abrasion, and permeability testing on aggregate sources before placement — NCDOT Section 520 compliance verified in our lab.
Construction QC & Field Density
Nuclear gauge density testing on each lift, proof rolling documentation, and subgrade stiffness verification during construction — we catch layer problems before the asphalt goes down.
Typical parameters
Frequently asked questions
What’s the typical flexible pavement section for a commercial parking lot in Cary?
For most commercial lots with moderate car and occasional truck traffic, we design around 3.5 to 4.5 inches of asphalt concrete over 8 to 10 inches of dense-graded aggregate base. The exact numbers depend on the soaked CBR at subgrade level — if we’re seeing values below 5%, the base gets thicker or we stabilize the subgrade. Every design runs through AASHTO 93 with a 20-year structural number check.
How do you handle the expansive clay subgrade common in Cary?
We test the Atterberg limits and the percent passing #200 sieve from the top 18 inches of subgrade. If the plasticity index exceeds 15 and the fines are above 50%, we typically specify a lime or cement stabilization layer — 6 to 8 inches compacted — before placing the aggregate base. This cuts the swell potential and provides a stiff working platform. On some sites near the lake zone we’ve also used geogrid reinforcement between the stabilized layer and the base course.
What’s the cost range for a flexible pavement design in Cary?
A full pavement design package — subgrade investigation, lab testing, structural design, and construction QC — typically falls between US$1,720 and US$4,520. The spread depends on the site area, number of test pits or borings, and whether we’re doing MEPDG modeling or a standard AASHTO 93 design. Smaller parking lots land on the lower end; roadway designs with multiple sections and drainage analysis push toward the upper range.
How long does the design and testing phase take before we can break ground?
Field investigation takes one to two days on site. Lab work — soaked CBR, Proctor, gradation — runs about seven to ten business days from sample receipt. The structural design report follows within three to five business days after lab results are in. Realistically, you’re looking at three to four weeks from notice-to-proceed to having a stamped design in hand, assuming weather doesn’t delay the fieldwork.
Do you provide the pavement design stamp for Cary permit submittals?
Yes. The design report is sealed by a licensed Professional Engineer registered in North Carolina, and it includes all the backup data — subgrade logs, lab reports, traffic calculations, and layer specifications — formatted to meet Town of Cary and NCDOT review requirements for commercial and subdivision submittals.