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Figure  13-11B.Stress-penetration  curves  (reverse  of  CBR  test  data  sheet  DD  Form  1212). - 14070_280
HYDROMETER  ANALYSIS - 14070_282

Engineering Aid 1 - Advanced Structural engineering guide book
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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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