Laboratory CBR Testing for Pavement Design in Knoxville

The red clay subgrades of Bearden need a completely different pavement section than the dolomite residuum in West Hills. Same city, two miles apart, but the bearing capacity can change by a factor of four. Knoxville sits at the junction of the Tennessee River and the Ridge-and-Valley physiographic province, where residual soils derived from limestone, dolomite, and shale create highly variable subgrade conditions. A standard laboratory CBR test removes the guesswork. Our lab runs soaked CBR per ASTM D1883 on remolded specimens compacted to target density, giving you the design number that dictates base course thickness. For projects near the Holston River floodplain, we often pair soaked CBR with grain-size analysis to quantify fines migration potential before finalizing the pavement structural number.

Soaked CBR values below 3% in Knoxville's shale-derived clays can require full subgrade stabilization before pavement construction.

Process overview

ASTM D1883-21 and AASHTO T-193 are the governing standards, and in a city where winter groundwater can saturate shallow subgrades for weeks, the 96-hour soaking requirement is not optional. Knoxville averages 48 inches of annual rainfall, and perched water tables are common where shale bedrock acts as an aquitard just 3 to 6 feet below grade. The laboratory procedure uses a 3-inch diameter mold, a 4.54-kg surcharge weight to simulate overburden, and a penetration piston advancing at 0.05 inch per minute. We read resistance at 0.1-inch and 0.2-inch penetration and express CBR as a percentage of the standard crushed stone reference. The value you get is only as reliable as the field density it references, so we recommend running the lab CBR alongside in-situ sand-cone-density testing on the same lift to correlate compaction effort with bearing response.

Local context

The elevation drop from Sharp's Ridge to First Creek creates drainage patterns that saturate subgrades in South Knoxville while soils on the ridge crest remain dry. A CBR test run without soaking on a summer-compacted sample can read 12% or higher, but that same soil after winter saturation may drop below 4%. That difference is the gap between a 6-inch aggregate base and a 14-inch section requiring chemical stabilization. The Tennessee Department of Transportation specifies minimum soaked CBR values for subgrade acceptance: below 6% demands undercut and replacement, and below 3% requires lime or cement treatment. We have seen projects where ignoring the soaked condition led to rutting within two freeze-thaw cycles. For commercial developments near the I-640 corridor, combining laboratory CBR with proctor-tests defines the moisture-density envelope that contractors must maintain during compaction.

Parameter	Typical value
Standard	ASTM D1883 / AASHTO T-193
Mold diameter	6 in (152.4 mm)
Penetration rate	0.05 in/min (1.27 mm/min)
Soaking period	96 hours submerged
Surcharge weight	4.54 kg (10 lb) annular
Reported values	CBR at 0.1 in and 0.2 in penetration
Typical specimen	Remolded at Proctor optimum moisture

Parameter

Typical value

Standard

ASTM D1883 / AASHTO T-193

Mold diameter

6 in (152.4 mm)

Penetration rate

0.05 in/min (1.27 mm/min)

Soaking period

96 hours submerged

Surcharge weight

4.54 kg (10 lb) annular

Reported values

CBR at 0.1 in and 0.2 in penetration

Typical specimen

Remolded at Proctor optimum moisture

Additional services

Soaked CBR (ASTM D1883)

96-hour submerged test on remolded specimens at target density. Standard for TDOT and municipal pavement design.

Unsoaked CBR

Immediate penetration test for temporary haul roads and construction platform evaluation where saturation is not anticipated.

Proctor Compaction Correlation

Standard or modified Proctor to establish the moisture-density relationship before CBR specimen preparation.

Subgrade Resilient Modulus Estimation

Conversion of CBR to M_r using AASHTO mechanistic-empirical correlations for flexible pavement structural design.

Common questions

What is the typical turnaround time for a laboratory CBR test in Knoxville?

Standard reporting is 5 to 7 business days from sample delivery. The 96-hour soaking period is the controlling factor. Expedited unsoaked CBR results can be provided in 2 business days when project schedules require rapid subgrade approval.

What CBR value does TDOT require for subgrade acceptance?

TDOT Standard Specifications Section 207 requires a minimum soaked CBR of 6% for subgrade. Values between 3% and 6% require undercut and replacement with select material. Values below 3% mandate chemical stabilization with lime or cement before pavement construction can proceed.

How much does a laboratory CBR test cost?

A single-point soaked CBR test ranges from US$130 to US$230 depending on whether Proctor compaction data is provided by the client or performed in our laboratory. Three-point CBR curves for moisture sensitivity analysis are priced proportionally.

Can you test samples from any part of the Knoxville metropolitan area?

We accept samples from all Knox County and surrounding areas including Farragut, Powell, Halls, and Maryville. Disturbed bulk samples should be shipped in sealed bags with chain-of-custody documentation. Undisturbed Shelby tube samples from deeper formations can also be tested for CBR after remolding.

Laboratory CBR Testing for Pavement Design in Knoxville

Process overview

Local context

Need a geotechnical assessment?

Technical parameters