Q. We're building a home on a site with difficult access, and we need to remove a lot of boulders without using heavy equipment. Is there a way to break the boulders into smaller chunks without having to deal with dynamite and the associated permitting?
A. Dave Crosby, an excavation contractor in Santa Fe, N.M., responds: Expansive demolition agents are a good alternative to explosives for quietly and safely breaking boulders into smaller pieces. They're shipped as a powder that you mix with water on site, and they have an expansive strength many times greater than that of rock or structural concrete. Several brands are available; I've used Dexpan (Archer Co., 866/272-4378, www.archerusa.com) with consistently good results. The procedure recommended by manufacturers is simple: With a rotary hammer, you drill holes of roughly 1 1/2 inches in diameter in a pattern and depth suitable for the material and desired result; then you place the mixed agent into the holes according to the manufacturer's instructions and leave the site. The next day, when you return, the rock should be broken.
In practice, I've found that drilling a 1 1/2-inch-diameter hole 24 inches deep into hard rock or heavily reinforced concrete is a lot easier said than done. When access or budget prevents the use of bigger tools, I drill 11/4-inch-diameter holes on 12-inch centers about 18 inches deep, then drill 3/4-inch-diameter holes to the same depth — or deeper — every 6 inches between the larger holes. This weakens the material for crack propagation. If the rock or concrete is really tough, I cut a 3-inch-deep kerf along the desired fracture line with a 14-inch demolition saw, which weakens it still further.
Even though expansive demolition agents aren't explosive, they're still very powerful: If you're working on a foundation or anywhere near a building, make sure that you've thought very carefully about how the forces would act and that you've provided a safe route for the expansion. It would be fairly easy to accidentally destroy a foundation with this stuff.
