The effects of destruction are quite visible above the ground, but our information about how it affects the underlying area is severely limited. Say a large meteor or a nuke just hit the ground, what changed beneath the surface? This is the question that physicists at Duke University are trying to answer.
The team has developed a technique to use artificial soils in the lab and stimulate high speed impacts on them. They then intend to observe what happens beneath the soil in super slow motion. The results have been quite informative.
The experiments have shown that such impacts tend to make the soil stronger, which increases with the level of impact. Their study also helped explain why ground penetrating missiles had little impact; their effects could not be widespread to cause much damage.
The harder the missile strikes the more resistance it would meet and the sooner it would stop. Not very surprisingly, the research has been funded by the Defense Threat Reduction Agency, to help build better ground penetrating missiles. The agency plans to use them to destroy weapons hidden underground.
The team used metals with rounded tips and dropped them from a height of 7 ft into beads.
The kinetic energy of the dropped object pushes the beads together. The physicists were also able to identify the areas of greatest stress, by using beads of plastic that transmit light differently. This helped them observe the transfer of energy, in the form of light, moving from one bead to another.
Even though, the impact took place at a mere 15 miles per hour, the beads generated pulses between 67-670 miles per hour, depending on their initial hardness. To view this experiment with the naked eye was not hence possible.
The researchers took the help of a high speed camera, with up to 40,000 frames a second, to record the experiment and watch it frame by frame. The study co-author, Abram Clark, used an analogy of a crowded room to explain the effect.
If you try to move out of the crowd faster than people can re-arrange, then you will face a lot of resistance from the crowd. This is what happens in a high speed impact with soil.