The most recent round of mudslides in Southern California left a 30-foot boulder teetering on the edge of the Pacific Coast Highway last week. Before the road could be reopened, a crew from the California Department of Transportation had to destroy the enormous hunk of sandstone and conglomerate. One proposed method: "Inject the rock with a type of gel that would cause it to disintegrate from the inside." How does that process work?
The "gel" is actually a special kind of cement that expands as it hardens. (Most cement used in construction shrinks when it dries.) Contractors typically mix up some of this expansive mortar and then pour it into a series of holes they've drilled in a special pattern on the side of the boulder they want to destroy. As the mixture cools, it expands against the walls of the hole, exerting a large amount of pressure on the inside of the rock. Over a span of hours, this pressure would cause the boulder to fracture and fall apart.
It's more common to blow up a boulder the old-fashioned way. When a slightly smaller rock blocked off Topanga Canyon Boulevard last month, contractors drilled holes in its surface and set off explosives inside them. While this method can be very effective, the risk of "flyrock" makes it impossible to use in some situations. A couple of homes not far from the boulder over the Pacific Coast Highway made this form of demolition too risky.
If the workers had attempted to use dynamite, they could have used a blasting mat, which is draped over the boulder and prevents the bits of flyrock from doing any damage. Blasting mats are made from recycled truck tires, woven rope, or steel mesh.
You don't always have room to drill, so workers sometimes plant explosives on the surface of a boulder. In general, this form of demolition requires three times as much energy as planting charges in holes. It also increases the chance of flyrock. Covering the charge with a big pile of mud, or "mudcapping" it, mitigates this risk.
Other techniques have been proposed for destroying boulders, like using bursts of electricity from a high-voltage capacitor, slugs of water shot at high speeds, or steel pistons rammed into water-filled holes. But it wasn't dynamite, expanding cement, or any of these more exotic techniques that broke the rock in California, though. In the end, workers were able to use a mounted, super-sized jackhammer—informally known as a Ho-Ram—to knock the 1,200-ton rock apart.
Explainer thanks Jaak Daemen of the University of Nevada, Reno, Lewis Oriard of the International Society of Explosive Engineers, and Gustavo Ortega of the California Department of Transportation. Thanks also to reader Jim Keller for asking the question.