Cbr Value Calculator
Calculate California Bearing Ratio for subgrade strength assessment in road design. Enter values for instant results with step-by-step formulas.
Formula
CBR = (Test Load / Standard Load) x 100
Where CBR is California Bearing Ratio as a percentage, Test Load is the measured force at a specific penetration depth, and Standard Load is the force required to achieve the same penetration in standard crushed rock (13.24 kN at 2.54 mm, 19.96 kN at 5.08 mm). The higher value at either penetration depth governs.
Worked Examples
Example 1: Highway Subgrade Assessment
Problem: A soil sample tested in the lab shows a load of 35 kN at 2.54 mm penetration. The standard load is 13.24 kN. Calculate the CBR value and determine soil suitability.
Solution: CBR at 2.54 mm = (Test Load / Standard Load) x 100\nCBR = (35 / 13.24) x 100 = 264.4%\n\nEstimated CBR at 5.08 mm = (35 x 1.35 / 19.96) x 100 = 236.7%\n\nGoverning CBR = max(264.4, 236.7) = 264.4%\nClassification: Excellent - suitable as base course material
Result: CBR = 264.4% | Classification: Excellent | Resilient Modulus = 396,600 psi
Example 2: Weak Clay Subgrade Evaluation
Problem: A saturated clay sample shows a test load of only 2.5 kN at 2.54 mm penetration with 18% moisture content. Evaluate subgrade quality.
Solution: CBR at 2.54 mm = (2.5 / 13.24) x 100 = 18.9%\nEstimated CBR at 5.08 mm = (2.5 x 1.35 / 19.96) x 100 = 16.9%\n\nGoverning CBR = 18.9%\nMoisture correction factor (>15%) = 0.85\nCorrected CBR = 18.9 x 0.85 = 16.1%\nClassification: Fair - needs moderate pavement thickness
Result: CBR = 18.9% | Corrected CBR = 16.1% | Required pavement ~8.5 inches
Frequently Asked Questions
How is the CBR test performed in the laboratory?
The laboratory CBR test follows ASTM D1883 or BS 1377 Part 4 standards. A soil sample is compacted in a cylindrical mold at its optimum moisture content, then soaked in water for 96 hours to simulate the worst-case field conditions. After soaking, a standard plunger of 49.6 mm diameter is pushed into the soil at a constant rate of 1.27 mm per minute. The load readings are recorded at penetration depths of 0.64, 1.27, 1.91, 2.54, 3.81, 5.08, 7.62, 10.16, and 12.7 mm. The CBR is calculated by comparing the test load to the standard load at 2.54 mm and 5.08 mm penetration depths.
What is the difference between soaked and unsoaked CBR values?
Soaked CBR values are determined after the soil sample has been submerged in water for four days, simulating the worst possible moisture conditions in the field such as prolonged rainfall or high water table situations. Unsoaked CBR values are tested at the natural moisture content or optimum moisture content without soaking. Soaked CBR values are typically 30 to 50 percent lower than unsoaked values depending on the soil type. For pavement design, the soaked CBR is always used because it represents the weakest condition the subgrade might experience during the pavement service life. Sandy and gravelly soils show less difference between soaked and unsoaked values compared to clay soils.
How does CBR relate to pavement thickness design?
The CBR value directly influences the required pavement thickness through empirical design methods like the AASHTO and Corps of Engineers methods. A soil with a CBR of 3 percent might require a total pavement thickness of 500 mm or more, while a soil with CBR of 30 percent might only need 200 mm of pavement. The relationship is not linear but follows a power curve, meaning that improving a very weak soil even slightly can dramatically reduce the required pavement thickness. Most highway agencies specify minimum CBR values for different pavement layers, typically requiring CBR of 80 or more for base course, 30 or more for subbase, and accepting whatever the natural subgrade provides.
What factors affect CBR test results in the field?
Several factors significantly influence field CBR values including soil type, moisture content, degree of compaction, and surcharge loading. Clay soils are highly moisture-sensitive and can show CBR values ranging from 2 to 15 percent depending on water content. Sandy and gravelly soils are less affected by moisture and typically show CBR values between 10 and 80 percent. The degree of compaction relative to maximum dry density is critical because even a small reduction in compaction can significantly lower the CBR. Temperature can also affect results in cohesive soils, and the presence of organic matter generally reduces CBR values substantially.
What are typical CBR values for different soil types?
Different soil types exhibit characteristic CBR ranges that engineers use for preliminary design. Well-graded gravels and crushed stone typically have CBR values of 60 to 100 percent or higher, making them excellent base course materials. Sandy gravels range from 20 to 60 percent and work well as subbase layers. Clean sands show CBR values of 10 to 40 percent depending on gradation and density. Silty soils generally range from 5 to 15 percent and are considered fair subgrade materials. Clay soils have the lowest values, typically 2 to 8 percent when saturated, and highly plastic clays can drop below 2 percent making them very poor subgrade materials.
How is CBR used to estimate resilient modulus for AASHTO design?
The AASHTO Mechanistic-Empirical Pavement Design Guide uses resilient modulus as the primary soil strength parameter, and CBR values are commonly converted using the empirical correlation Mr equals 1500 times CBR in psi units. This equation was developed from laboratory correlations and is widely accepted for preliminary design when resilient modulus testing is not available. For more accurate conversions, the NCHRP 1-28A study recommends using Mr equals 2555 times CBR raised to the power 0.64 for fine-grained soils. The converted resilient modulus values are then used in the structural number equation or layered elastic analysis to determine required pavement layer thicknesses.