Sandpile style: Poured or rained.

Two identical-looking piles of sand can differ in a curious way: One will exert the most pressure under its central peak, while the other's greatest load lies further out.

Scientists who study granular materials explain the difference as the result of grains forming arch-like structures within some piles. Like the flying buttresses of cathedrals, the structures steer forces away from the center (SN: 8/19/95, p. 127; 9/20/97, p. 186). However, why some piles develop the arches while others don't has remained a puzzle.

In a new experiment, researchers in France and North Carolina show that the way sand falls onto a pile can determine how its load gets distributed. Loic Vanel of the University of Pierre and Marie Curie in Paris and his colleagues measured pressures beneath piles made either by pouring sand through funnels or letting it rain through sieves.

In the November PHYSICAL REVIEW E, the researchers report that the funneled sand, which flows onto a small section of the mounting heap, consistently produced piles with off-center maximum pressure. Sieved sand, which fell evenly across the entire platform, built into mounds with on-center maximum loads.

Research team member Robert P. Behringer of Duke University in Durham, N.C., explains that funneled sand repeatedly avalanches down the pile's outwardly slanting slopes as it accumulates. Consequently, grains can settle into arches that shift weight away from the center.

By contrast in sieved mounds, the grains fall straight down, he says, so the maximum pressure lies under the heap's tallest part, its center, as one would expect.