When a bearing-ratio test is made of a compacted sample, you will use the 6-inch-diameter CBR mold with a 2 1/2-inch spacer disk in the mold beneath the sample. The use of the spacer reduces the depth of the sample to 4 1/2 inches. The use of another size spacer will result in volume and compactive effort changes that may not meet ASTM or other recognized standards for the CBR test.

The saturated condition is attained by soaking the sample. First, place the sample in the mold and compact it. The compactive effort used and the number of compacted samples required depend upon the soil type, weight and type of field compaction equipment, and other job conditions. Normally, compactive efforts of 12, 26, and 55 blows per layer (for five layers) are used in each of three successive compaction tests. The 10-pound tamper is used for compacting the samples. After compacting the sample, trim it and remove the base plate and spacer disk. Then place a piece of filter paper over the trimmed or struck-off top of the sample and place the base plate over this top. Turn the mold over and set it in a bucket on the base plate. The bottom of the sample, which was next to the spacer disk during compaction, is now uppermost. Apply the appropriate

number of surcharge weights needed to approximate the expected in-place weight of the pavement and base. One 5-pound surcharge weight is equivalent to 3 inches of overlying material. Then set in place the tripod attachment, dial, and swell plate, as shown in figure 13-10.

Immerse the mold and the sample in water in the bucket and leave them to soak for about 4 days. An initial reading of the tripod dial is made when the sample is first placed in the water. Then, at the end of the soaking period, the dial is read again to determine the amount of swell. A swell in excess of 3 percent of the initial height of the specimen is considered to be excessive. After making the final reading of the dial, you remove the sample and mold from the water and allow them to drain for about 15 minutes before conducting the penetration test.

In the penetration test, the bearing capacity of a soil is determined by measuring the extent to which the sample, placed in a mold, is penetrated by a penetration piston. The sample (in the CBR mold) is placed in the loading press, as shown in figure 13-9. The piston is placed on top of the material, and a proving ring is placed between the top of the piston and the top of the loading press.

As the jack is cranked upward, the dial in the center of the proving ring records the pressure being applied to

Figure 13-11A.Front of data sheet (DD Form 1212) for California bearing ratio test.

the piston. The penetration dial (fig. 13-9) measures the extent to which the piston penetrates the material in the CBR mold. To better understand the test procedure, lets study figure 13-11A. This figure shows an example of the data for a CBR test. In this example, figure 13-11A indicates that the sample was compacted in five layers with the 10-pound tamper, 55 blows per layer. A surcharge weight of 25 pounds is listed. That means that a 6-inch-diameter circular section of, in this case, the surface and base course of the airfield pavement is expected to weigh 25 pounds. As previously described, this weight was simulated using surcharge weights totaling 25 pounds.

Figure 13-11B.Stress-penetration curves (reverse of CBR test data sheet DD Form 1212).

In the penetration data portion of the data sheet, you that the first column lists standard penetrations starting at 0.025 inch and increasing to 0.500 inch. The second column lists standard unit loads. The test was carried out by cranking the jack until the penetration dial reaches the standard penetration, then reading the load for that penetration on the proving-ring dial. Notice that for each dial reading there is a corrected dial reading that is 0.003 inch less that the uncorrected reading. This

Figure 13-12.Adjusting the zero point of the curve.

indicates that the proving-ring dial used for this test contained a previously determined index error of 0.003 inch. An error of this kind develops as a result of repeated compressions of the proving ring during testing.

The figures under total load are the results obtained by multiplying the corrected dial reading by the proving-ring constant. Each unit load was obtained by dividing the total load by 3.

Figure 13-11B is the reverse side of the CBR test data sheet. This side of the data sheet is used to plot the unit loads against the penetration depths to determine if a correction to the unit load is necessary.

In figure 13-11B you see the curve for sample SF-PI-4 that we have been discussing. The convex shape of this curve indicates that no corrections were necessary. Sometimes, however, surface irregularities in the soil sample or disturbances during the test will result in curves having an initially concave shape, such as shown in figure 13-11B for samples SF-PI-5 and SF-PI-6. This shape indicates that a correction is necessary to obtain the true or corrected load. In this case, you must first adjust the zero point of the curve. Figure 13-12 illustrates the procedure you should use to adjust the zero point. First, draw a line that is tangent to the steepest portion of the concave curve and extend the tangent to the zero base line. The point of intersection of the tangent and the base line is the new zero-penetration point. The distance of the new zero point from the original zero point (distance X) is the distance that the 0.1 and 0.2 points are moved to the right to establish the corrected unit load.

Finally, lets look again at figure 13-11A and discuss how the CBR values are determined. To compute the CBR values, you divide the unit load (or corrected unit load) at 0.1 and 0.2 inch by the standard unit loads of 1,000 and 1,500 psi, respectively. Each result is then multiplied by 100 to obtain the CBR in percent. The CBR is usually selected at 0.1 inch; however, when the CBR at 0.2 inch is greater, you should first rerun the test and then if the check tests give similar results at 0.2-inch penetration, the CBR at 0.2 inch should be used.

Figure 13-13.Apparatus for grain size distribution by hydrometer analysis.