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Published February 22, 2017
Ensuring your home's strong foundation
Project manager Brian Egloff consults a precision altimeter to insure the foundation is level. Photo Cathy Dausman

There are old-fashioned house-raising projects, and then there are the types that Jim and Janet Hoffman recently experienced: The Lafayette couple saw right away a "little droop" in the front room floor of the 1955-built ranch house they intended to purchase last April.
The room was an addition, approximately 20 feet by 30 feet composed of 1-and-a-half to 2-inch thick concrete subflooring set on poorly graded earth. No rebar tied the addition into the existing house. The first-time homeowners went through with their purchase but knew they had to "do the right thing" and fix what might be a fatal flaw before investing in any above ground remodeling, so the Hoffmans recently completed a crash course on underpinning to straighten up their floor.
Underpinning is done to prop up or support from below, thus strengthening by reinforcing a foundation - a professional drill, fill and patch operation. Although it sounds simple when those actions apply to tabletop woodworking projects, it is much more difficult when the object in question is a concrete house floor.
Remedial work on a pier-and-post wooden floor is a bit easier to conceptualize, but concrete underpinning, while slightly more costly and labor intensive, can also be done. A concrete floor is thought to be an immovable object, but add enough helical anchor/piles, steel resistance piers and special non-stick polyurethane foam to fill and level the voids under the slab, and voilÍ!
Much of the initial work is preparation, like cutting holes large enough to drive 3-inch galvanized steel piers one 3-foot section at a time, into the soil approximately 20 feet. The work necessitates a piering crew, a poly(urethane) crew, a helicut crew and a concrete crew.
After completion, "if you X-rayed the (repaired area of the) house (floor), it would look like it was up on stilts," explained project manager John Mahoney of Bay Area Underpinning.
At that depth the piers have likely plunged through a layer of clay subsoil and into a substance called the Orinda Formation, says Mason Walters, senior principal at the San Francisco-based Forell/Elsesser Engineers.
A geotechnical engineer explained the Orinda Formation as a geologically young sedimentary bedrock layer composed of weakly cemented sand and clay particles. When weathered, this layer tends to disintegrate into a soil-like material, which is prone to landslides.
Walters applauded the Hoffmans for correcting their relatively minor (inch to inch and a half) problem before it got out of hand. He says ignoring foundation problems "can be a good way to wreck (a home ownership) investment."
In addition to sinking the piers, workers drilled a series of small holes through the existing concrete. In each hole they injected quick-setting polyurethane foam, which helped level the slab. A chemical reaction causes the foam's two ingredients to heat to 700 degrees Fahrenheit when mixed and just as quickly cool down. The foam insertion at 37 psi sounds a bit like a working espresso machine.
"It only takes 11 psi to lift a 1-by-1-by-1-foot block of concrete," explained crew member Blake Stokke.
The concrete slab was then raised and leveled using hydraulic lifts starting from the room's perimeter and working toward the center. Measurements were taken using a Ziplevel (r) precision altimeter, accurate to 1/8 an inch. A slope of one inch or less over 20 feet meets current code, said BAU's Ron Hobbs. Cleanup day found the crew sealing access holes and installing carbon fiber staples across a crack in the length of the floor to keep the crack from widening.
Hobbs estimated this particular job rated about a difficulty 7 on a scale of 1 to 10.
While the work crew spent two weeks on site, Hobbs said the actual lifting likely took only an hour. After final cleanup, there would be additional coats of concrete applied to smooth and finish the subflooring. Project expenses were approximately $30,000, Hoffman said.
Thirteen holes and seven piers later, the remodel decisions once again rest in the homeowner's hands. "It's been a great motivation," Jim Hoffman said.
"It's great that they're doing this," added Walters.

Polyurethane foam is injected into small holes drilled into the concrete slab.
Injected foam oozes out between the concrete slab and the earth. Photos Cathy Dausman
One of several hydraulic lifts used to raise the slab. Photos Cathy Dausman

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