Anisotropy in the Spatial Distribution of Roller-Measured Soil Stiffness

Abstract

The geostatistical analysis of roller-measured soil properties (from continuous compaction control and intelligent compaction) is required for advanced quality control/quality assurance of earthwork and asphalt compaction. This paper explores the existence of anisotropy in the spatial distribution of roller-measured soil stiffness and the effect of anisotropy on kriging. Field testing was conducted to collect roller measurement value (MV) data over typical roadway embankment evaluation areas and on a large square area to enable a robust investigation of anisotropy. The semivariogram analysis of the field data clearly indicates that range anisotropy exists. The spatial distributions of roller MV data are different in the longitudinal x-direction than in the transverse y-direction. Magnitudes of range anisotropy (x-range/y-range) varied from 2.4 to over 5. The observed range anisotropy is not due to the roller measurement system; rather, it is likely due to the directional nature of earthwork construction activities and to alignment geometry. The influence of anisotropy on kriging was found to be significant when considering the use of kriged data in earthwork specifications. The error introduced by not accounting for anisotropy in kriging varied from 5 to 17% when considering pass to pass or layer to layer map analysis. The anisotropy in the spatial distribution of roller MV data should be factored into kriging and other geostatistical analyses. For typical earthwork area geometries, the roller mapping procedure requires a slight modification to determine the y-range and anisotropy ratio.