Robotic Air Pluviation for Large-Scale Sand Specimens

Robotic automated specimen preparation (RASP) by singlenozzle air pluviation was originally developed for conventional laboratory specimens with diameters of approximately 100 mm. The present study investigates its transfer to larger cylindrical sand specimens intended for large-scale triaxial and oedometer testing while maintaining reproducible preparation conditions. During this scale-up, however, a center high deposition surface developed and then part of the experimental program aimed to examine its formation and to identify a suitable method for reducing it. For this purpose, washed Karlsruhe Sand (KS) was pluviated into a single ring cylindrical mold with a nominal internal diameter D = 469 mm. The path diameter Dp and path topology (serpentine or concentric) were varied, whereas other influencing parameters, including the nozzle diameter and fall height, were kept constant. The results show that changing the path topology alone was insufficient to eliminate the central crown, as the concentric path still produced a center high surface. By contrast, extending the serpentine path beyond the mold internal diameter to Dp/D ≈ 1.05 produced a planar surface. Further path enlargement and the addition of a peripheral cut-off wall maintained surface planarity but increased material loss. However, the cut-off wall clearly separated retained from lost sand, enabling maintenance of a constant fall height. In a separate test series, vertical homogeneity was evaluated using unwashed KS, filling a mold composed of four stacked rings and calculating the mean dry density of the sand in each ring. The two repeated tests produced consistent ring averaged dry densities over the specimen height and the bottom ring was systematically looser. Overall, the results demonstrate that RASP can be transferred to large cylindrical specimens, provided that the trajectory coverage is adapted to the effective sand rain footprint and that boundary induced effects are considered.