November 21, 2017
I went to a small K-12 grade school — there were 63 people in my class of 1964. Nearly half of the boys-turned-men in my graduating class served in the military. The U.S. was in the midst of the Vietnam War, and I suspect most of my classmates in the military, myself included, served in Vietnam. My closest friend in college also served in Vietnam, although I never saw him or my high school buddies while “in country.”
One of the first things I noticed after arriving in Vietnam (after the horrible smell) was the prevalence of sandbags. They were placed everywhere to protect people, buildings, and equipment from bullets and shrapnel from mortars and rocket-propelled grenades.
Sandbags were by no means unique to Vietnam. They are known to have been used as long ago as the American Revolutionary War by loyalists defending Fort Ninety-Six (aka Star Fort) in South Carolina. The ubiquitous sandbag has been used in nearly every war since.
However, sandbags are perhaps most famous for the role they played in the trench warfare of World War I. The top two or three feet of the front and rear sides of the trench consisted of a thick line of sandbags to absorb any bullets or shell fragments. Millions of sandbags found their way to the Western Front; 150 million from Scottish factories during just one two-week period in 1915.
Research by the British Army determined that a typical bullet used in World War I would only penetrate a sandbag by 15 inches. So how can sand stop a projectile?
A sandbag works somewhat like a non-Newtonian fluid, which behaves like both a liquid and a solid. Think about Oobleck, non-Newtonian fluid consisting of a mixture of two parts cornstarch and one part water. If you slowly poke your finger into a ball of Oobleck, the cornstarch particles can move out of the way. But if you try to quickly jab your finger into Oobleck, the particles jam up against one another, resisting the penetration. (Caution: If you make Oobleck, don’t wash it down the drain. It can plug up your plumbing.)
Likewise, a sandbag is filled with many small grains of sand that can move out of each other’s way when the sandbag is slowly poked with a stick, but when a bullet strikes a sandbag, the grains of sand can’t get out of each other’s way fast enough. The impact also results in an extreme frictional force as the sand grains dissipate the pressure while the projectile compacts the sand along its path of penetration. These forces combine to stop the penetration of the bullet.
This is a pretty simple explanation for a pretty simple piece of military equipment. But when you are a-standin’ up to the sandbags, it’s funny the thoughts wot come.