ABSTRACT
While pushing a finger slowly into the sand is a common beach experience, the penetration of an object through a dense granular medium also presents an interesting physics problem associated with the jamming of grains in front of the penetrating object (Albert, 1999; Wieghardt, 1982). Furthermore, this problem is closely related to issues of practical importance in soil science where penetration testing is a standard technique (Mahmoud, 2000; Vaz, 2001; Yu, 1998; Wright, 1983; Lee, 1968; Peterson, 1991; Parkin, 1982; Randolph, 2000). The propagation of stress in granular media presents a complex problem, since forces are not transmitted uniformly through a granular sample but are localized along directional force chains (Geng, 2001; Liu, 1995; Da Silva, 2000; Reydellet, 2001; Bouchaud, 1995; Mueggenburg, 2002). An applied stress results in the development of a rigid internal structure of grains along which the force chains are concentrated, known as a “jammed” state (Liu, 1998; O’Hern, 2001). A solid object pushed slowly through the grains creates and then destroys locally jammed states as it stresses and then displaces the material immediately in front of it (Kolb, 2004). While previous studies have probed the resulting drag force (Albert, 1999; Wieghardt, 1982), there has been little study of the effective size of the threedimensional grain volume perturbed by the penetration through the grains, i.e. the jammed region which is progressively created and destroyed by the object’s motion. Indirect evidence indicates that the presence of boundaries parallel to the motion affects the fluctuation spectrum but not the average value of the resistance force (Albert, 2000; Albert, 2001a). There
have been no previous studies, however, of the penetration resistance of an object moving directly toward the boundary of a granular medium. In this case, the range of granular reorganization should profoundly impact the penetration process, since the boundary of the medium places a well-defined limit on the extent of the jammed state.
