Onset of the Pullout Motion of Spherical Object from Columns of Sand*

The problem of pulling out an object from a certain depth below the surface of granular media is an interesting physical granular failure phenomenon at the onset of the object motion. In this study, the sand is confined by rigid cylindrical tubes (diameters of 5.2 to 20 cm). The object was a sphere (3.5cm in diameter and 173g). In the experiment, the object was pulled by an adjustable weight. The two were connected through a string and guided by two pulleys. The cylinder was filled with sand to different heights by pouring gradually from above. At each realization, the pulling force was slowly increased to the failure point of sand. The experiments show that the pulling force at failure was same for all sand depths less than the object diameter measured from the object center. However, it then sharply increased for the narrowest cylinder, unlike other ones. A sharp increase in failure force was observed in larger sand heights for the cylinder of 6.1 cm in diameter. This observation implies a critical role of the confining boundary in changing the force network which boosts the resistance against the pullout. Parallel experiments on a 2D setup will be presented by Zhang et al.