I live in central New Hampshire, aka the Granite State. They don’t call it that for nothing.
My own house sits directly on top of one of the state’s enormous granite plutons, the Meredith Porphyritic Granite. Many geologists consider it to be the most beautiful rock in all of New England because of its massive feldspar crystals and mica inclusions. A giant piece of this solid bedrock is right next to my driveway.
Recently I was eating breakfast and thought I heard a sonic boom. (Military planes routinely practice flying over the nearby White Mountains.) It turns out what I had heard was preparations for a new homesite near my house. A blasting company was using hundreds of pounds of dynamite to pulverize tens of millions of pounds of Meredith Porphyritic Granite. The process continued to shake my house for the next three weeks.
You have to want to build pretty badly if you choose to go to all that trouble to blast solid rock. First and foremost, blasting is expensive: Two previous owners of the lot ran out of funds trying to prepare it for a house. Apparently, the current landowner has much deeper pockets, as the blasting is complete and he’s about to pour a footing.
It’s one thing to build a lighthouse or other small structure on solid bedrock, but it’s quite another to build a typical residential home. If you are thinking of buying land to build your dream home, be sure to review the soil map to check the depth of the soil on the lot you want. These soil maps are usually available online at no cost.
Prepare for a blast
Building on bedrock makes some tasks that are normally simple painfully difficult. Among those is simply digging a hole or trench, which will require blasting.
In normal, deep clay soil, an excavator can dig a trench that’s 10 feet deep and 2 feet wide for a sewer line in an hour or two. Forget about doing this in solid rock. You won’t find city sewers in most places where you have solid rock just inches beneath the surface. It’s simply too expensive to install sewers in these conditions.
This means you’ll probably have to blast out an area for your septic tank. What kind of special septic design will be required for solid rock? This is something you should talk about with a septic designer before you make an offer on the building lot.
In addition, the National Electrical Code is quite specific about how deep underground electricity lines must be buried. Will you have to blast that trench, too?
Other challenging tasks to consider:
• How will the water line get from the well to your house? Will a trench have to be chiseled with a monster jackhammer on an excavator’s arm?
• How much fill dirt will you have to truck in, as well as topsoil to create enough soil to grow plants, bushes and trees?
What about radon?
Be sure to check the online map for the likelihood of radon. Blasting fractures rocks, which can funnel radon into your home. If you feel this might happen, you can deal with it fairly inexpensively using a passive radon collection and exhaust system.
An interconnected network of perforated 4-inch pipes needs to be placed within the gravel under your crawl space or basement slab. I’d surround the inside of the footing with this piping, and then place pipes across the floor every 6 or 8 feet that connect with the outer band of pipes. Be sure to install a high-performance vapor barrier over the pipes and gravel before pouring the concrete floor. This material should meet or exceed the standard specification of ASTM (American Society for Testing and Materials) E1745.
Strategically place one or two 4-inch pipes that connect to the under-slab pipes and then poke out of the slab. These will eventually be connected to 4-inch PVC pipes that run up through interior walls and out the roof. Flash these pipes as you would any plumbing vent pipe.
When the wind blows across the top of these pipes, it creates a vacuum and sucks the radon out of the ground below your home and vents it safely away.
Tim Carter has worked as a home improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.