Download PDF version (894.5k) Log In or Register to view the full article as a PDF document.

 

Image
Image

Our company specializes in oceanfront construction and rehab in southeastern Massachusetts. We do our own foundation work because of the control it gives us over the schedule. In some parts of this area, digging down is likely to uncover a mushy mix of sand, peat, and clay, not to mention a high water table. Since these soil conditions won’t support a conventional concrete foundation, local homes have traditionally been built on wood pilings, which allow floodwaters to wash in and out freely without undermining the structure.

The job shown here involved replacing a rotted piling foundation. The house stands along Provincetown’s historic harbor, where it’s common for 19th century buildings to be separated by only a few feet. This crowding leaves little or no space for conventional excavation; besides, trenching in the loose sandy soil risks undermining neighboring houses.

So for this job we decided to use helical piers, which looked to be an ideal solution because they require no excavation and can be driven quickly even in tight quarters (see “Helical Pier Foundations for Problem Sites,” 5/04). We obtained certification in the Chance Helical Pier Foundation System (573/682-8414, www.abchance.com) and purchased the equipment needed for installation, which included a hydraulic auger adapted to our excavator’s boom and a computerized torque meter. Altogether, it cost around $8,000.

Engineered Design

To design the foundation, we called on structural engineer John Bologna of Coastal Engineering Co., a firm familiar with the local geology and structural requirements. His knowledge of surrounding properties led John to expect alternating layers of sand, peat, and clay, with water at 4 feet below grade.

Based on this information, he specified a reinforced concrete grade-beam footing supported on helical piers spaced about 9 feet apart and driven to a minimum depth of 22 feet. On top of the grade beam, a 10-inch-thick stem wall would support the new floor framing. Flood vents in the stem wall would allow high water to wash in and out, preventing the wall from collapsing in a surge.

  • Image
    Grouting adds surface area and lateral friction to the column, increasing its capacity. It also adds a layer of protection against corrosive soils.

House Overhead

A major consideration was how to install the pier-and-beam system beneath a structure that couldn’t be accessed from all sides or raised and moved aside. The neighborhood’s historic classification barred complete demolition and replacement of the house, certainly the easiest approach.

Instead, we raised the house 4 feet, lifting it by the first-floor ceiling joists. We then cut away the ground-floor framing and installed steel I-beams and cribbing to support the second story. This gave us 14 feet of clearance, just enough to maneuver the excavator boom and its hydraulic auger attachment over the pier locations.