Spatial Interpolation of UAV Survey Data for Lift Thickness Determination during Earthwork Construction

The vertical lift thickness of a soil that is placed, spread, and compacted during embankment construction is a critical parameter that ensures effective compactor energy penetration and a successful outcome of the compaction process. Consequently, most quality assurance/quality control guidance documents specify a maximum “not to exceed” lift thickness. In practice, it can be difficult to monitor lift thickness for large fill or embankment construction projects without frequent surveying, which adds cost and in some cases time delays to a project. One possible emerging approach for lift thickness monitoring is the use of UAV surveying. In this study, compacted lift thickness was inferred from unmanned aerial vehicle (UAV) imagery collected during active construction of an earthen embankment, which was processed using photogrammetric analysis techniques. To assess the accuracy of UAV surveying for lift thickness applications, 82 discrete locations along the embankment were surveyed utilizing traditional survey equipment. Physical lift thicknesses were determined at each of the 82 discrete locations utilizing a caliper and steel ruler. Spatial interpolation using isotropic kriging was performed on the lift thickness data set that was determined from photogrammetric analysis, to infer UAV-determined lift thickness at each of the 82 surveyed data points. The results from this study indicated that compacted soil lift thicknesses along the embankment ranged from 10 cm to 70 cm. When compared against physical lift thicknesses, lift thickness measurements made by the UAV in conjunction with isotropic kriging had a mean absolute error of 1.1 cm. The results from this study provide valuable insight on the relative accuracy of UAV surveying for lift thickness determination during active earthwork construction.