The Dirt Whisperer

Say you’re a civil engineer, and you’re looking for a good place to put a bridge. Wouldn’t it be nice to be able to ask the ground how stable it is before you start digging, like some sort of dirt whisperer?  Or how close an older bridge is to failing, after erosion has affected the ground around it for several years?

And wouldn’t it be really nifty if you could “talk” to fault lines to determine which areas along the fault are under more strain, and where earthquakes may be more likely to occur?

NC State physicist Karen Daniels thinks so. She’s looking at ways to help scientists and engineers see –and hear – the forces at play in the ground by studying force chains in granular materials.

Simply defined, a force chain describes the way in which individual grains within a material bear weight and distribute stresses throughout the material as a whole.  In other words, let’s say you’ve got a pile of dirt.  Not every grain in the pile bears weight equally, so when you compress the pile the energy isn’t distributed equally.  It travels through certain pathways, or force chains.  Understanding this distribution – how the pile of dirt is being compressed – is key to understanding how stable it is.

Daniels has created a simple way to see force chains in the lab, using special “grains” made out of plastic that transmits light when stress is applied to it. These grains are placed in a box and covered with sheets of polarized material, so that the transmitted light is visible to the human eye.  When compressed, the force chains appear like streaks of lightning through the material, telling you how the forces are distributed.  It’s really cool, but unfortunately you can’t do that with real dirt.

That’s where sound waves come in. Daniels and graduate student Eli Owens wanted to see if sound waves would follow those same force chains, because acoustical probes are already used to look at underground objects. If  the sound waves and the force chains matched up, it could lead to a new technology that would help everyone from civil engineers to seismologists in the field.

So Daniels and Owens created some “smart particles” in their box of grains by embedding electric sensors in them to trace the movement of acoustic waves, then they “talked” to the grains by shooting acoustic waves through the material.  Lo and behold, the sound followed the same pathways.

The research is still in its early stages, but if the theory holds up, we may one day be able to ask the ground, “Which way do your force chains go?” and actually get an answer.