Ask the Builder: The challenges of building on solid rock
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 enormous granite plutons, the Meredith Porphyritic Granite. A giant piece of this solid bedrock is right next to my driveway. What a perfect place for me to live, considering my college major was geology.
This granite formed 340 million years ago about 80 miles beneath the surface of the planet along the edge of a tectonic plate. Giant blobs of this molten rock slowly floated up to the surface over millions of years, cooling ever so slowly during its journey. A pluton resembles a hot air balloon both in shape and movement. Many geologists consider the Meredith Porphyritic Granite to be the most beautiful rock in all of New England because of its massive feldspar crystals and mica inclusions.
About a month ago I was eating breakfast and I thought I heard a sonic boom. Just north of where I live military planes routinely practice flying over the White Mountains, and I thought one had gone astray and was making a fast tight turn to get back to where it should have been.
It turns what I had heard was preparations for a new home site near my house. A blasting company was using hundreds of pounds of dynamite in the process of pulverizing tens of millions of pounds of Meredith Porphyritic Granite, shaking my house every other day for three weeks.
You have to want to build badly if you decide to go to all this trouble to blast solid rock. First and foremost, blasting is expensive. Two previous owners of the lot, which sits above mine, went bankrupt trying to prepare the site for a house. They ran out of money and sold the land to the next person in line. 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 smaller lighthouse on solid rock, but it’s quite another to build a typical residential home on top of solid bedrock. You may be thinking of buying land to build your dream home. If so, 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.
Building on bedrock makes some tasks that are normally simple painfully difficult. Let’s run down the list.
Digging any hole or trench requires blasting. In normal deep clay soil, an excavator can dig a 10-foot deep 2-foot-wide trench for a sewer line in an hour or two. Forget about doing this in solid rock. There’s no way you’ll discover city sewers in most places where you have solid rock just inches beneath the surface. It’s just 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 the leach field that is perched above the solid rock? This is something you want to talk about with a septic designer before you make an offer on the building lot.
Did you want underground electricity into your new home? The National Electrical Code is quite specific about how deep these lines must be buried. Will you have to blast that trench too?