It's a hard question: How big does a rock have to be to stop a truck?

Posted at 4:18 PM, Jan 06, 2015
and last updated 2015-01-07 10:24:42-05

The question from the Bureau of Diplomatic Security at the Department of State was simple enough: How big does a rock have to be to stop a 6½-ton truck trying to ram into the walls of a facility at 30 miles an hour?

Coming up with an answer was not simple. Researchers at Penn State University used equations that would make your brain hurt just to look at them, built computer models and finally crash-tested trucks to come up with a blueprint for a boulder that’s not too small, but not so big that it’s impractical. It was paid for in part with a $6.9 million State Department grant to develop new types of crash barriers.

“You can design all kinds of systems, but that system has to be deployable overseas,’’ said Tong Qiu, an assistant professor of civil engineering who led the research team. “You could put a 100-ton boulder out there and that would stop most anything, but you want to minimize the size so it’s not too intrusive or impractical to move,’’ Qiu added.

Besides boulder size, the team took into account what kind of soil is holding the rock (sand or gravel) and the speed of the truck in modeling a rock’s stopping power. The key performance measure was how much a boulder would move as the result of an impact at a certain speed.

The team then confirmed the accuracy of the model with full-scale truck crashes carried out at Penn State’s Larson Transportation Institute test track. Using a special towing system, they ran trucks weighing about 15,000 pounds and moving about 30 miles an hour into two granite boulders, one just over 5 tons and sunk about four feet deep, the other more than 25 tons, and buried seven feet. The impacts were recorded with high speed cameras.

Not surprisingly, the larger boulder moved only slightly and stopped the truck as the model predicted; the smaller one dislodged from the soil and rolled over when struck, allowing the vehicle to move slightly more than the model had foreseen.

Qiu and his colleagues, reporting their findings in the January issue of the International Journal of Impact Engineering, say that even though the model did not account for every outcome, it already has been shared with at least one embassy.

The boulder project is only one of several efforts to test and evaluate “vehicle anti-ram barriers” under the grant from the State Department, which runs through July.

[Also by Lee Bowman: Congress passes bill to better understand sudden infant deaths]

Want to keep up with all the latest DecodeDC stories and podcasts? Sign up for our weekly newsletter at