The California Bearing Ratio (CBR) is an empirical index used to evaluate the strength and load-bearing capacity of subgrade soils, subbases, and base course materials in geotechnical engineering. This measurement is a key element in the design of flexible pavements for roads, highways, and airfields. The CBR test estimates the mechanical strength of a soil by comparing its resistance to penetration against the resistance of a standard, high-quality crushed stone material. This ratio, expressed as a percentage, provides a quantified estimate of a material’s suitability to support traffic loads. The process is standardized internationally, with the American Society for Testing and Materials (ASTM) standard D1883 guiding laboratory procedures.
Sample Preparation and Apparatus Setup
The process of determining the California Bearing Ratio begins with preparing a representative soil sample by compacting the material in a standardized cylindrical mold. The specimen is compacted at a specific moisture content and dry density, often corresponding to the desired field compaction density, such as a percentage of the Modified Proctor density. The material used for testing must pass a three-quarter inch (19 mm) sieve to ensure applicability within the standardized apparatus.
A crucial step is the four-day soaking period, particularly for the commonly used “soaked CBR” test, which simulates worst-case conditions. The compacted specimen is immersed in water for 96 hours to allow for potential soil swelling and saturation, reflecting adverse moisture changes a subgrade might experience. Before and during soaking, a surcharge weight is placed on the sample surface to simulate the weight of the overlying pavement structure.
The apparatus setup includes the soil-filled mold, the perforated base plate, and the extension collar, which are assembled and placed into the loading machine. The surcharge weights, typically annular plates weighing around 10 pounds (4.54 kg) total, are positioned on top of the sample. This assembly is aligned beneath the penetration piston, ensuring proper seating before the test begins.
The Load Penetration Test Procedure
With the prepared and conditioned sample in place, the physical testing involves forcing a standardized circular penetration piston, approximately 1.95 inches (50 mm) in diameter, into the soil. The piston is driven into the material at a fixed, slow rate of 0.05 inches (1.27 mm) per minute. Maintaining this constant rate of strain ensures consistent and comparable results.
Raw data is collected by simultaneously recording the total load required to push the piston and the corresponding depth of penetration. Readings are taken at regular intervals, such as every 0.025 inches (0.64 mm), up to a total penetration of 0.500 inches (12.7 mm). These data points are then used to plot the Load versus Penetration curve.
Before calculation, the load-penetration curve may require correction. The initial part of the curve can be concave upwards due to surface irregularities or improper seating, leading to an artificially low initial load reading. To compensate, a tangent is drawn to the steepest part of the curve, and the origin is shifted to where this tangent intersects the penetration axis, establishing a corrected zero point.
Deriving the California Bearing Ratio
The California Bearing Ratio is mathematically determined by comparing the measured test load to standard reference loads derived from a high-quality crushed stone material. The formula expresses the CBR as a percentage: CBR = (Test Load / Standard Load) x 100. The test load is taken from the corrected load-penetration curve at specific penetration depths.
The calculation uses two specific depths: 0.1 inches (2.5 mm) and 0.2 inches (5.0 mm). The standard reference loads are 1,000 pounds (4.45 kN) for the 0.1-inch penetration and 1,500 pounds (6.67 kN) for the 0.2-inch penetration.
The test load measured at 0.1 inches is divided by 1,000 pounds to find the first CBR value, and the load at 0.2 inches is divided by 1,500 pounds for the second value. The higher of the two percentages is generally reported as the final CBR. If the 0.2-inch value is significantly greater, the test may be repeated, as this outcome can indicate an issue with the initial setup or material.
Practical Interpretation of CBR Values
The final California Bearing Ratio value serves as a direct indicator of the material’s structural strength for pavement design. A material with a low CBR percentage, typically between 0% and 5%, is classified as a weak subgrade soil. Such soils have low bearing capacity and require significant stabilization or the construction of thicker pavement layers to support traffic loads.
In contrast, a high CBR value, ranging from 50% to 100% or more, indicates an excellent material, such as high-quality crushed stone. These materials are suitable for use in the base or subbase layers of a pavement structure, offering substantial support. A value between 15% and 30% generally represents a good strength soil suitable for subbase layers under lighter loads.
Engineers use the determined CBR value to calculate the required thickness of the entire pavement structure, including the subbase, base, and surfacing layers. A higher CBR value means the underlying soil is stronger, allowing for a reduction in the required thickness of the overlying pavement layers. This provides a quantifiable metric for classifying soil quality and ensuring the structural integrity of the final construction.