Evaluation of Spatial Variability for Site Characterization Based on Standard Penetration Test Results-Paper

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The study evaluates whether variograms can reliably characterize the spatial variability of standard penetration test (SPT) blow counts, a commonly used measure in geotechnical site investigations.<br /><br />The analysis used SPT results from 75 boreholes at a relatively flat site with a broadly homogeneous soil profile. For the competent foundation layer, characteristic blow counts ranged from 50 to 100, with a mean of 92.6, standard deviation of 11.76, and coefficient of variation of 12%. The data showed no clear trend across the site, but were neither normally distributed nor continuous: SPT blow counts are discrete measurements. Experimental and model variograms were calculated using the borehole locations and SPT values.<br /><br />The variogram showed an almost pure nugget effect, which conventionally indicates strong spatial variability and little or no correlation between measurements—even across the site. This result conflicted with the site’s relatively low variability, dense borehole coverage, and geotechnical assessment that an average SPT value could reasonably represent the foundation layer.<br /><br />The authors attribute the misleading result to a key limitation: variogram analysis assumes continuous data that change gradually across space, whereas SPT blow counts are discrete. They conclude that, in this case, variograms were unsuitable for assessing spatial variability from SPT data and falsely suggested extensive variability. Care is therefore needed when applying geostatistical tools to discrete geotechnical measurements.

Keywords

variogram analysis

spatial variability

SPT blow counts

geotechnical site investigation

borehole data

nugget effect

soil profile

discrete measurements

geostatistics

foundation layer