Jamming and static stress transmission in granular materials

We have recently developed some simple continuum models of static granular media which display "fragile" behavior: They predict that the medium is una ble to support certain types of infinitesimal load (which we call "incompat ible" loads) without plastic rearrangement. We argue that a fragile descrip tion may be appropriate when the mechanical integrity of the medium arises adaptively, in response to a load, through an internal jamming process. We hypothesize that a network of force chains (or "granular skeleton") evolves until it can just support the applied load, at which point it comes to res t; it then remains intact so long as no incompatible load is applied. Our f ragile models exhibits unusual mechanical responses involving hyperbolic eq uations for stress propagation along fixed characteristics through the mate rial. These characteristics represent force chains; their arrangement expre ssly depends on the construction history. Thus, for example, we predict a l arge difference in the stress pattern beneath two conical piles of sand, on e poured from a point source and one created by sieving. (C) 1999 American Institute of Physics. [S1054-1500(99)00303-1].