Microscopic Structural Study on the Growth History of Granular Heaps Prepared by the Raining Method

Granular heaps are critical in both industrial applications and natural processes, exhibiting complex behaviors that have sparked significant research interest. The stress dip phenomenon observed beneath granular heaps continues to be a topic of significant debate. Current models based on force transmission often assume that the packing is near the isostatic point, overlooking the critical influence of internal structure and formation history on the mechanical properties of granular heaps. Consequently, these models fail to fully account for diverse observations. In this study, we experimentally explore the structural evolution of three dimensional (3D) granular heaps composed of monodisperse spherical particles prepared using the raining method. Our results reveal the presence of two distinct regions within the heaps, characterized by significant differences in structural properties such as packing fraction, contact number, and contact anisotropy. We attribute these structural variations to the differing formation mechanisms during heap growth. Our findings emphasize the substantial influence of the preparation protocols on the internal structure of granular heaps and provide valuable insights into stress distribution within granular materials. This research may contribute to the development of more accurate constitutive relations for granular materials by informing and refining future modeling approaches