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An Investigation of the increase in variability of standard penetration test results for gravelly soils

Pehlivan, Alper
Variability in the results of standard penetration tests (SPT) due to nonuniformity of particle size distribution in soil layers is a known issue in geotechnical applications. Such phenomenon may result in artificial increase in penetration resistance in silts, clays and sands due to encountering large particles. An analysis of SPT variability in such cases can be conducted by dividing the total penetration distance into two smaller segments. In this study, the difference between number of blow counts for the second 15 cm penetration and that for the third 15 cm blow counts is used for analyzing the variance. Furthermore, possible statistical limits for the range of reasonable differences are derived to enable the detection of any artificial increase in SPT blow counts due to the existence of gravel or larger sized particles. Determination of such limits allows a correction for the SPT blow count to be used for estimation of geotechnical parameters. In order to ensure that the analyses conducted on the SPT results are not biased by the possible errors due to equipment and procedural variabilities, all the samples investigated in this study are based on a dataset of tests conducted by the same equipment and operators.