ASTM D4429 CBR test apparatus for Method of California Bearing Ratio (CBR)  in Place

On site CBR testing is used to evaluate and design flexible pavement components, such as base and sub base layers and roadbeds, as well as other applications where CBR is a required strength parameter (such as unpaved roads). If the on-site CBR is directly used for evaluation or design without considering changes in moisture content, the test should be conducted under one of the following conditions: (a) when the saturation (percentage of filled voids) water is 80% or higher, (b) when the material is coarse and non cohesive, and therefore not significantly affected by changes in moisture content, or (c) when the soil has not changed due to construction activities within two years before testing. In the latter case, the water content does not actually become constant, but typically fluctuates within a relatively narrow range. Therefore, on-site test data can effectively demonstrate the average bearing capacity. Any construction activities carried out after the bearing ratio test, such as leveling or compaction, may render the test results invalid. Soil and flexible pavement components at the same location may exhibit significantly different load deflection relationships. 

Due to the variability of soil and flexible pavement component materials, there is currently no method to evaluate the accuracy of a set of non repetitive plate load tests. Note 18212: On site tests are used to determine the relative strength of soil, subbase, and certain substrates under the conditions in which they are tested. These results can be directly applied to experimental section work and some emergency construction, military or similar operations. In addition, as shown in 4.1, on-site testing can be used for design under conditions of nominal stability, density, and general characteristics of the tested material in water. However, any significant processing, interference, handling, compaction, or changes in water can affect soil strength and render the pre-test measurements no longer applicable, requiring retesting and reanalysis. Note 28212; Although this standard contains statements regarding accuracy and bias: the accuracy of this testing method depends on the ability of the personnel performing the method and the suitability of the equipment and facilities used. Organizations that comply with Practice D 3740 standards are generally considered capable of conducting qualified and objective testing. Users using this method should note that following Practice D 3740 alone does not guarantee the reliability of the test. Reliable testing depends on many factors; Practice D 3740 provides a method for evaluating some of these factors.

ASTM D4429 testing method covers determining the California Bearing Ratio (CBR) of soil for in-situ testing through the penetration load of soil. This test method covers the evaluation of the relative quality of subgrade soil, but is applicable to subbase and some base materials. This testing method aims to test on-site materials and corresponds to Test Method D 1883.

Applications:

Pavement design: the CBR value is used to estimate the strength of the subgrade and base layers of a pavement structure, which helps in determining the thickness and type of pavement required.

Foundation design: the CBR test can be used to assess the suitability of soils for foundation construction.

Soil classification: the CBR test can be used as a secondary classification criterion for soils.

A mold is filled with the soil specimen. It is compacted into the mold with a rammer. Then the mold is soaked in water for a certain period of time. Then a loading machine is used to apply load on a plunger. This will penetrate through the soil mold. The machine will penetrate through the soil by increasing the load gradually. There are one proving ring and one dial gauge attached to the machine. The dial gauge indicates the penetration amount. The proving ring indicates the amount of load machine is applying to the surface. For certain amounts of penetrations, corresponding load values have to be recorded. Later stress vs. penetration curve is drawn by using these values.

From that curve, both for 1 in (2.54 mm) & 2 in (5.08 mm) penetration, corresponding stress value is determined. These values are used in the equation mentioned above to calculate the CBR value.

Laboratory California Bearing Ratio Test Apparatus

The followings are the apparatus required to perform the CBR test of soil.

Loading Machine

Penetration Piston

Sieves

Mold

Spacer Disk

Mixing Tools

Load Test

Place the mold under the penetration piston of the compressing machine. The same amount of surcharge weight (4.54 kg) shall be placed on top of the mold.

Then the compressing machine shall be started to apply load with a constant penetration rate of 0.05 in. (1.27 mm)/min. The piston will start to penetrate through the soil for the loading.

The machine has two indicators. One is a proving ring and another is dial gauge. The dial gauge indicates the penetration and the proving ring will indicate the amount of load is applied to gain that penetration.

See the table below, column 2 shall be filled in with corresponding proving ring readings for the penetrations specified in column 1.

Proving readings shall be multiplied with machine constant to find the piston load.

Then penetration stress shall be determined from piston load.