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The California Bearing Ratio (CBR) is a standard test used to evaluate the strength of soil specimens by measuring their ability to resist stress relative to a standard sample. The test involves compacting a soil specimen into a mold, soaking it in water, and then applying a load using a loading machine that penetrates through the soil at a controlled
rate. The amount of penetration is measured using a dial gauge, while the corresponding load is recorded on a proving ring. By analyzing the relationship between stress and penetration, the CBR value can be calculated. The California Bearing Ratio test is typically performed in a laboratory setting, where the required apparatus includes a loading
machine, a penetration piston, sieves, molds, and mixing tools. The equipment must be capable of applying compressive forces at a controlled rate to ensure accurate measurements. The CBR value represents the percentage of stress that a soil specimen can resist for a certain amount of penetration relative to a standard soil's ability to resist stress.
In essence, it serves as an indicator of the soil's strength and resilience. Sieves Two sieves are required: ¾ in (19 mm) and #4 (4.75 mm). Mold preparation involves creating a rigid metal cylinder with an inside diameter of 6 ± 0.026 inch (152.4 ± 0.66 mm) and a height of 7 ± 0.018 inches (177.8 ± 0.46 mm). The mold must be attached to a base
plate with a metal extension collar at least 2.0 inches (50.8 mm) in height. Spacer Disk A circular metal spacer disc with a minimum outside diameter of 5 15/16 inches (150.8 mm) and a height of 2.416 ± 0.005 inches (61.37 ± 0.127 mm) is required. Additional equipment includes mixing tools such as a mixing pan, spoon, trowel, spatual, etc., as well
as a rammer for compaction, balance, filter paper, drying oven, and soaking tank. CBR Test Procedure Specimen preparation involves sieving the soil specimen through ¾ in (19 mm) sieve. If all material passes through, it can be used; otherwise, replace retained material with equivalent amount of passing material. Mix sufficient water to maintain
optimum water content. Three sample specimens are prepared: one compacted with about 10 blows, another with 30 blows, and the last with 56 blows for varying maximum dry density percentages. Compaction procedure involves: - Attaching mold to base plate - Measuring weight - Placing spacer disk with filter paper on top - Filling soil in three
layers (e.g., 10 blows per layer) - Water content determination before and after compaction The process is repeated for the remaining two specimens. Soaking procedure: - Applying surcharge weight (typically 4.54 kg) on base plate - Soaking specimen in water tank for around 4 days (96 hrs.) - Measuring height before and after soaking to determine
swell percentage Expansion measurement equipment may be used. Final step involves loading the mold under a penetration piston of a compressing machine with the same surcharge weight. The machine applies load at a constant rate, allowing for: - Proving ring indicator - Dial gauge indicating penetration The California Bearing Ratio (CBR) test is
used to evaluate the stability of soil subgrade and other flexible pavement materials. To perform this test, a proving ring reading is taken for various penetrations, and then multiplied by the machine constant to determine the piston load. From this, the penetration stress is calculated. A stress-strain curve is drawn from the test values, and if it's
concave upward near the origin, adjustments are made according to guidelines. The CBR value is determined using equations that relate stress of the soil specimen for specific penetrations (0.1 in. and 0.2 in.) to a standard pressure. Generally, the CBR value at 0.1 in penetration (CBR0.1) is considered as the CBR value for the material. However, if
there's a significant difference between CBR0.1 and CBR0.2 values, the test may need to be repeated. Additionally, this article discusses designing bearing ratios for specified dry densities of soil specimens using graphs with dry density vs. corresponding CBR values. A downloadable PDF version (CBR Test pdf) is also provided for easier study. The
ASTM designation for this procedure is D1883-07. References to this test include a video and specific standards for testing, including descriptions of the laboratory apparatus used. A crucial test for evaluating the strength of sub-grade soil is the California Bearing Ratio (CBR) test. The process involves repeating the test at a penetration depth of 5
mm. If the results are consistent, the higher value obtained is reported as the CBR value of the material. Occasionally, a curve with an initial upward concavity may be observed; in such cases, it should be corrected. The soaked CBR value of the soil sub-grade is then evaluated, and an appropriate design curve is chosen based on anticipated traffic or
wheel load considerations. This enables calculation of the total thickness of flexible pavement required to cover the sub-grade with a known CBR value. To determine the construction thickness over the sub-base, the thickness of the sub-base course can be calculated by subtracting the thickness over the sub-base from the total thickness. Similarly,
the thickness of the base course and wearing course can be found using corresponding CBR values. Several formulas are used to facilitate this process, including: 1. Thickness calculation: P = Wheel load in Kg, CBR = California Bearing ratio in percent 2. Design traffic estimation: A = no. of vehicles per day for design, r = annual rate of increase of
vehicles, m = Life period (10 years for major roads), n = no. of yrs between the last count & the completion year of construction A suitable design curve can be chosen from a table in the design chart according to the estimated design traffic. The CBR test is a widely used method for evaluating the strength of sub-grade soil, and its results are
employed along with empirical curves to determine pavement thickness and component layers.