Laboratory CBR Testing for Pavement Design in Orlando, Florida

The Florida peninsula presents a unique geotechnical profile for pavement engineers, and Orlando sits squarely on the sandy ridges of the Central Florida Highlands. Beneath the asphalt of I-4 and the sprawling subdivisions of Orange County lies a mix of Pleistocene-age sands, silty sands, and occasional clay lenses that can vary drastically within a single project site. The water table here often sits just three to six feet below the surface, which means subgrade strength measured at construction may not reflect conditions once the pavement has seen a few wet seasons. A laboratory CBR test, conducted in strict accordance with ASTM D1883, gives us the soaked bearing capacity values that are essential for designing flexible and rigid pavements that will survive Central Florida's intense summer rainfall without rutting or cracking. When we prepare samples at optimum moisture from a Proctor test and then soak them for 96 hours, we simulate the worst-case scenario the subgrade will face after years of service. This soaked CBR value becomes the cornerstone of the structural design, dictating everything from base course thickness to the need for lime or cement stabilization before placing the asphalt.

A soaked CBR value under 3% in Central Florida's sandy silts signals that subgrade stabilization is not optional — it is a prerequisite for any pavement that will carry truck traffic.

Method and coverage

In Orlando, we frequently see contractors surprised when a sandy fill that looks excellent in the field yields a soaked CBR below 5%. The culprit is almost always fines content that went unnoticed during visual classification. Our laboratory procedure begins with remolding the sample at the target density and moisture content determined by the Proctor test, then submerging it in water with a surcharge load that simulates the weight of the pavement structure. Over four days of soaking, we record swell measurements and then penetrate the sample with a 1,954-square-inch piston at 0.05 inches per minute. The load readings at 0.1 and 0.2 inches of penetration are compared against the standard crushed stone reference to calculate the CBR percentage. For soils with significant gravel content, we often run a companion grain size analysis to verify the material meets FDOT specifications for select fill or limerock base. What makes Orlando different from coastal Florida is the presence of sandy soils with just enough silt to create capillary action that keeps the subgrade saturated long after the rain stops. We see this most clearly in the Winter Park and Pine Hills areas, where the fine sand matrix holds water against gravity and can reduce effective CBR by 30 to 40 percent compared to the unsoaked value.

Regional considerations

Orlando's growth pattern since the 1970s has pushed residential and commercial development into areas that were once citrus groves or low-lying wetlands. Many of these sites contain layers of organic silt or muck that were stripped and replaced with compacted fill, but the replacement materials were not always tested for CBR at the time of placement. A pavement designed on assumed CBR values from a county soil survey, without project-specific laboratory testing, inevitably develops alligator cracking and potholes within the first three to five years. The risk compounds when commercial parking lots or warehouse access roads are built with minimal asphalt thickness under the assumption that Florida soils drain freely. The reality in Orlando is that the flat topography and high water table create perched groundwater conditions that keep the subgrade in a near-saturated state for months at a time. Performing a laboratory CBR test on undisturbed or remolded samples from the actual subgrade depth provides the only reliable basis for predicting long-term pavement performance and avoiding costly post-construction repairs.

Technical parameters

Parameter	Typical value
Applicable Standard	ASTM D1883-21
Sample Preparation Method	Static or dynamic compaction at target moisture-density
Soaking Period	96 hours under surcharge load
Penetration Rate	0.05 in/min (1.27 mm/min)
Piston Area	3 in² (1,935 mm²)
Swell Measurement	Recorded every 24 hours during soaking
Typical FDOT Subgrade CBR Threshold	≥ 20% for base course; ≥ 6% for subgrade (varies by design)

Complementary services

Soaked and Unsoaked CBR Testing

Full ASTM D1883 procedure with 96-hour soak, swell monitoring, and load-penetration curve analysis. We report CBR at 0.1 and 0.2 inches of penetration, corrected for concavity when required, and provide compaction curve overlays to help contractors adjust field density targets.

Subgrade Evaluation Package

Combined Proctor compaction testing, laboratory CBR, and Atterberg limits on the same soil sample to deliver a complete subgrade characterization. We include moisture-sensitivity commentary for soils with PI above 10, which are common in the southern portions of the Orlando metro area.

Standards that apply

ASTM D1883-21: Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils, ASTM D1557-12e1: Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort, FDOT Standard Specifications for Road and Bridge Construction, Section 160, AASHTO T 193: Standard Method of Test for the California Bearing Ratio