Replacing a Beach-House Foundation - Continued
The main additive was the fly ash, a waste product that comes
from the dust collectors in coal-burning power plants. A fine
powder consisting mostly of glass particles, fly ash can be
substituted for some of the cement in a concrete mix. Our local
concrete plant has been doing this for years because fly ash is
cheaper than cement and has properties that make for a better
finished product. As the owner of a "green" building company, I
like fly ash because using it reduces the amount of cement I
need; cement production is a major source of the carbon dioxide
that contributes to global warming. Also, fly ash typically
ends up in landfills, so using it for concrete diverts it from
the waste stream.
Thus, to minimize shrinkage, we replaced 35 percent of the
cement with fly ash. And to reduce cracking, we used Degussa's
Polyheed 997, a water-reducing admixture (less water means less
cracking), and Pozzolith 300R, a water-reducing plasticizer
(Degussa Admixtures, 800/628-9990,
www.degussa.com). With these additives, we
could limit the amount of water in the mix yet still have
concrete flowable enough to pump and place.
As an extra precaution, we added fiber to reduce cracking still
further and used a corrosion-inhibiting admixture —
another Degussa product — called Rheocrete CNI.
Pouring the Foundation
Since there wouldn't be room to bring in a drill rig until the
pile of excavated soil was used to backfill the grid, we
planned to pour the house foundation separately from the deck
piers. A couple of days before the pour, the structural
engineer and the city inspector inspected the rebar and forms.
On the day of the pour, a special inspector from an independent
testing company showed up to observe; he verified that we
followed proper procedures and saw to it that test cylinders
were made from the concrete mix.
It took eight trucks all morning to deliver the 77 yards of
concrete needed to pour the grid and house columns (Figure 6,
previous page). The design strength of the mix was 3,000 psi,
but when the cylinders were tested at 28 days, we found that
the concrete was already at 6,000 psi and would likely continue
to gain strength for years.
Figure 6.Access was poor, so the crew used a pump
to place concrete for the grid (top) and columns
Pouring the columns tight to the bottom of the column caps
wouldn't have been practical, because when we vibrated them,
the concrete would consolidate and separate from the caps. So
we poured the columns an inch or so short, then grouted them
tight to the bottom of the brackets with high-strength grout
once the concrete had set (Figure 7).
Figure 7.The threaded rod coming down from this
column cap (left) is attached to a large nut and washer that
will be embedded in the concrete. The Sonotube was lifted to
within an inch or two of the beam before the concrete was
poured, and the gap was later packed with grout. Because the
column received a stuccolike waterproof coating, it's not
possible to see the joint between the concrete and the grout
After about a week of cure time, the foundation was strong
enough to support the cottage, so the house mover came back and
removed the cribbing. We all breathed a sigh of relief once the
cottage was firmly bolted to the new foundation. With the
cribbing out of the way, we were able to bury the grid with the
excavated material and begin working on the individual piers
that would support the decks (Figure 8).
Figure 8.In the photo at left, the grid and
columns are both visible. Once the foundation was backfilled,
only the columns could be seen (right).
Friction Piers for the Deck
It had always been part of the plan to rebuild the rear deck on
new concrete piers. The old piers and piles weren't worth
saving, and with no obstructions overhead it would be simple to
get a drill rig into position.
But end-bearing piers — what you get when you bury a
Sonotube and fill it with concrete — weren't practical in
this situation. With the high water table, we couldn't pour an
end-bearing pier in such iffy soil without first inspecting the
bottom of the hole. And even if the hole were dry, it would
need to be large enough for someone to climb inside and inspect
the bottom. In addition, the low bearing capacity of the soil
would require large-diameter end-bearing piers to provide the
We decided to use friction piers instead. A friction pier is
"gripped" by the surrounding soil, so its bearing capacity
depends on how much of it is in the ground — the deeper
the pier, the more it will carry. The soils engineer determined
that the friction capacity of the soil was 300 psf. Given the
local wind and seismic conditions, and knowing we wanted to use
16-inch piers, the structural engineer designed them to go 12
to 15 feet below grade.
Drilled cased piers. In firmer soil,
we could have drilled the holes, dropped in the steel, and gone
from there. But with loose soil and a high water table, we had
to drill and case the piers. Casing is a steel pipe that
prevents a hole from caving in. It's big enough for the drill
bit to fit through and is pushed into the ground as the hole is
being drilled; it's pulled back out as the concrete is placed.
Removing the casing allows the concrete to "key" into the soil,
thereby creating the necessary friction. If the casing is not
removed, the pier will have less load-bearing capacity.
At each pier location, the drilling contractor drilled several
feet down, retracted the bit, and used the rig to push a 4-foot
casing most of the way into the hole (Figure 9). He tack-welded
a second section of casing onto the first, reinserted the bit,
and drilled deeper before pushing both casings farther into the
ground. This continued until the holes were 12 to 15 feet deep
and contained multiple sections of casing, with the top one
projecting a few inches above grade. While the casing kept the
hole from caving in, it didn't prevent ground water from coming
halfway up inside.
Figure 9.Here, the drilling contractor prepares to
drill the first pier for the rear deck structure. The metal
cylinders in the left foreground were used to case the
Pouring the piers. As with the
cottage columns, we had rebar cages for the piers fabricated
and hot-dip galvanized off site. On the day of the pour, the
drilling contractor redrilled the bottom of each hole to clean
it out, placed the rebar cage, and began filling the hole with
concrete (Figure 10). Because the holes were half filled with
water, he put a tremie pipe on the concrete pump hose, inserted
it into the hole, and pumped from the bottom up, making sure to
keep the nozzle below the surface of the liquid concrete.
Figure 10.Before pouring a pier, the drilling
contractor redrilled the bottom of the hole to make sure it
hadn't filled up with silt (top). Like the columns, the piers
were heavily reinforced with galvanized rebar cages
Since concrete is denser than water, it forced the water and
silt up and out of the hole without diluting or mixing with the
concrete. As each hole was being filled, the drilling
contractor vibrated the concrete and pulled out the casing, one
section at a time (Figure 11). When the process was complete,
the rebar cages projected several feet into the air and were
encased in concrete up to grade.
Figure 11.The crew pumped from the bottom up to
prevent concrete from mixing with the water in the hole (top).
The casing was pulled out as the concrete was poured. Bottom
left, a welder cuts off a section that had just come out of the
ground. The silt pouring out of the top indicates that this
hole was nearly filled with concrete (bottom
Finishing up above grade. It's
impractical to pour the above-grade and below-grade portions of
a friction pier in a single operation. While the lower end
requires no form because it's surrounded by soil, the upper end
requires a Sonotube, but there's no way to brace it into
position while the drilling crew is working on nearby piers. So
we had to pour the piers in two separate operations.
After the lower sections had set, our crew braced the deck
beams into position and tacked stainless steel column caps onto
them (Figure 12). Then all the drilling contractor had to do
was apply a bonding agent, Rezi-Weld 1000 (W.R. Meadows,
800/342-5976, www.wrmeadows.com), to the top of the pier,
flex the rebar off to one side, slide a Sonotube over it, and
finish the pier by pumping the Sonotube full of concrete. That
way, there was no possibility the piers would not line up with
the beams or that the mounting brackets would be set at the
Figure 12.Once the piers were up to grade, members
of the crew braced the deck support beams into position and
installed the column caps (left). Then they formed up to the
beam with Sonotubes and poured the rest of the way
Once the concrete was set, we stripped the Sonotubes and
grouted up to the column caps before bolting them permanently
in place. As an added defense against the elements and to give
the exposed portions of the foundation a uniform appearance, we
coated the columns, piers, and grout with Super Blockade
(Merlex Stucco, 714/637-1700, www.merlex.com), a cementitious waterproof
coating. After that, finishing the cottage was a regular
Jeff Morosois an owner of Moroso Construction Inc.
in Pacifica, Calif.