Fine sand in motion: the influence of interstitial air

Sand is a granular material, and therefore it consists of individual grains arranged in a packing. The pores in-between the grains are usually filled with a fluid, in this case air. Now, is this interstitial air able to influence the behavior of the sand bed as a whole? When a ball impacts on fine, very loose sand, the interstitial air causes a pronounced change. If the bed is loose enough the ball is able to penetrate the sand and surprisingly, the final depth decreases if there is less air present in-between the grains. While the ball penetrates the bed, sand around the ball is compactified and the air in this region is compressed. Since air needs a finite amount of time to flow away it is temporarily trapped and will locally increase the pressure. If an object moves through the bed very slowly, air has ample time to move out of this compactified region and this quasi-static situation should be similar to that without air. The drag force experienced by the object for very low penetration velocities is indeed equal to the measured force at reduced ambient pressure, and much larger than the force needed to push the ball at higher velocities. The pressurized trapped air around the ball effectively reduces the drag of the sand.