AASHTO R74 CBR test apparatus for Wet Preparation of Disturbed Soil Samples for Test

This method describes the wet preparation of soil samples, as received from the field, for mechanical analysis and the determination of soil constants.

Method A provides for drying the field samples at a temperature not exceeding 60°C (140°F), making a wet separation on the 0.425-mm (No. 40) sieve and drying to a temperature not exceeding 60°C (140°F). Method B provides that the field sample shall be kept at a moisture content equal to or greater than the natural moisture content. Samples of such soil should be shipped to the laboratory in sealed containers and not allowed to dry out before testing. If no procedure is specified, Method A shall govern.

Test procedure:

Soil preparation: the soil sample is compacted to a specific density and moisture content using a standard Proctor compaction test.

CBR determination: a cylindrical mold containing the compacted soil is placed under a load-testing machine. A standard penetrometer is then forced into the soil at a controlled rate. The load required to achieve a penetration of 0.5 inches (12.7 mm) is recorded.

CBR calculation: the CBR is calculated as the ratio of the load required to achieve 0.5 inches of penetration in the test soil to the load required to achieve the same penetration in a standard aggregate base material.

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.