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

Figure 13-11B.Stress-penetration curves (reverse of CBR test data sheet DD Form 1212). - 14070_280

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Figure 13-12.—Adjusting the zero point of the curve. indicates that the proving-ring dial used for this test necessary to obtain the true or corrected load. In this 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 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, let’s 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. 13-15

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