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The Hammonds' foundation required extensive blasting to clear a rocky ledge. |
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The Hammonds considered all the angles when they planned for their foundation. They had already renovated two existing barns on site, so they had no need for added storage. Besides, they reasoned, a full basement would add cost and time to the construction schedule. With the help of the designers at Acorn, the Hammonds settled on a partial or crawl space foundation.
Then, the site plan revealed a problem with the grade. To build on the existing site would mean using a pump station for waste removal. Since the owners preferred a gravity system, they decided to blast the underlying foundation to accommodate their waste disposal plan. The Hammonds soon realized, however, that a few more feet of blasting would allow for concrete footings and a more secure tie-down on a wind-blown coast. Those few feet would also mean space for a utility basement with access to plumbing, electrical, heat, and hot water service. The floor would be filled with crushed stone, perforated pipe, filtration mat, and more crushed stone on top for drainage and moisture control.
The Blast
The Hammonds served as their own general contractor, so the hiring of blasting, excavation, and foundation contractors was left to them. They selected Bartlett & Steadman of Marblehead for the blasting and excavation work.
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Here, the foundation walls and additional footings have been poured and the wooden forms removed. |
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Blasting impacts more than earth and stone, and can fracture water mains or lines anywhere near the blasting area. In fact, most municipalities require mapping and permitting prior to the laying of charges. DigSafe is the agency that surveys existing pipes, conduits, and underground utilities. They also videotape all structures and their contents within 300 feet of the detonation site. Once their map and approval have been issued, it's safe to lay the charges.
To prepare for blasting, Bartlett & Steadman used a pneumatic drill to make shafts in the ledge to house the dynamite. The charges were placed in the shafts, wire nets were drawn across the area to contain flying debris, and the blasting zone was cleared for detonation. The end result was a stone bowl or basin that now provides drainage for the new home.
Forming the Footprint
To tie down the foundation walls, 5/8-inch iron rods or rebar were drilled into the footings and underlying ledge. Horizontal rods complete the fence or quill, and reinforce the walls once poured. Wooden forms are customized on site to frame window and door openings, and ultimately hold the concrete in place until cured.
Once the pour is complete, vibrators are used to eliminate air pockets and settle the concrete. Forms may be removed the next day, but concrete must cure before backfilling the foundation. It's hard to wait, but a strong foundation is the first giant step in any successful building project.
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