When evaluating the friction ratio for soil classification purposes, it is essential that the measured values of tip resistance and sleeve friction are shifted relative to one another due to the physical offset between cone and the friction sleeve (where applicable). The shift distance is determined through a statistical function called cross correlation function. It should be noted that many electric CPT equipment apply automatically this kind of correction on the measured values.
To perform the cross correlation function click on the Calc shift button. The following dialog will appear:
Cross coreallation evaluation dialog
In the above graph X axis is the lag distance where one lag is the physical distance between 2 CPT soundings. Y axis is the sample cross-correlation coefficient (red line). Positive values represent a down shift value of fs (relative to qc) and a negative value represents an up shift value of fs relative to qc. The software will locate the maximum absolute value of the cross-correlation coefficient and will visualize it with a red circle and its associated lag distance value above. Therefore, in the above graph the sleeve friction values should be shifted upwards 5 times the lag distance (e.g. if 1 lag equals 2 centimeters then shift distance should be 10 centimeters).
Note In order for the above method to give correct results the lag distance must be the same between all test points. |