Foundations
By: Amanda Campbell
Updated: August 13, 2010
For our example, let's look at a Project House in Dallas, Texas, a part of the country that’s famous for its expansive clay soil that really punishes standard concrete slab foundations. When the Texas summer hits, the soil shrinks. When it rains, the soil swells, and this seasonal up-and-down motion, over years, can cause concrete slab foundations to buckle, crack, and in many cases, completely fail.
How expansive is the soil? Before we broke ground on the Project House, we had experts run tests. I highly recommend investing a few hundred dollars to have soil testing done before beginning construction on a house in a region with variable soils.
In this location, our tests revealed a potential expansion of 4.4 inches per ten feet of soil. In places, the soil under the Project House goes to a depth of almost 30 feet before reaching bedrock, giving a potential rise and fall of over one foot—more than enough to wreak havoc with a standard concrete slab foundation.
Because of this, and because we wanted to build a house that would stand for generations, we told our foundation engineers, Strand Engineering, to design a foundation that could withstand even the most adverse conditions.
Pier and Post-Tension "Mat" Foundation
DrillingStrand came up with a plan that calls for a "pier and post-tension" mat foundation.
The first step is to create 128 reinforced concrete piers, evenly spaced, in rows, supporting the surface slab. The piers are drilled down to, and one foot into, the blue shale substrate, at depths ranging from 20 to 37 feet. Each pier is 12 inches in diameter. The piers alone required 100 yards of concrete.
WaffleOnce the piers are in place, the foundation crew digs trenches that will create forms for concrete beams, 28 inches deep and one foot wide.
These beams will rest atop the piers, forming a "mat" that looks much like a waffle. This "mat" of concrete beams, resting atop piers sunk into bedrock, acts much like a snowshoe, distributing the weight of the house evenly throughout the foundation slab.
Laying CablesBefore the concrete is poured, steel cables, loosely sheathed in plastic, are run at even intervals in a grid throughout the area of the slab, extending out to the edges. The day after the pouring of the slab, a crew will come in and pull these cables tight, creating a compression, or "tension" in the surface of the slab that will significantly increase its strength. A week later, the crew will come back and pull the cables again, creating total of 7,500 pounds of cable tension pressure. This "post-tension" process creates a slab that is tightly bound together, with incredible tensile strength.
The Concrete
Pouring ConcreteThe concrete we’re using in this foundation is a "five-sack" mix—five standard sacks of cement per cubic yard of concrete. Our cement mix contains 80 per cent cement and 20 per cent "fly ash" which is ash waste emitted by coal-burning power plants. Straight cement "cures" after the pour, continually gaining strength for about 56 days. The fly ash works to slow the curing process, so that the cement gains strength for a much longer period, even up to ten years, resulting in much stronger concrete. Concrete made by mixing cement product with water and coarse and fine aggregates in a blend. There are hundreds of different ways to blend the materials to produce different kinds of cement.
For the Project House, we’re using a blend wherein one cubic yard of concrete contains:
1,850 pounds of coarse, one-inch rock
1,471 pounds of fine sand
376 pounds of cement
94 pounds of fly ash
251 pounds of water (30 gallons)
This foundation will take 350 cubic yards of concrete, or about 35 truckloads.
How expansive is the soil? Before we broke ground on the Project House, we had experts run tests. I highly recommend investing a few hundred dollars to have soil testing done before beginning construction on a house in a region with variable soils.
In this location, our tests revealed a potential expansion of 4.4 inches per ten feet of soil. In places, the soil under the Project House goes to a depth of almost 30 feet before reaching bedrock, giving a potential rise and fall of over one foot—more than enough to wreak havoc with a standard concrete slab foundation.
Because of this, and because we wanted to build a house that would stand for generations, we told our foundation engineers, Strand Engineering, to design a foundation that could withstand even the most adverse conditions.
Pier and Post-Tension "Mat" Foundation
DrillingStrand came up with a plan that calls for a "pier and post-tension" mat foundation.
The first step is to create 128 reinforced concrete piers, evenly spaced, in rows, supporting the surface slab. The piers are drilled down to, and one foot into, the blue shale substrate, at depths ranging from 20 to 37 feet. Each pier is 12 inches in diameter. The piers alone required 100 yards of concrete.
WaffleOnce the piers are in place, the foundation crew digs trenches that will create forms for concrete beams, 28 inches deep and one foot wide.
These beams will rest atop the piers, forming a "mat" that looks much like a waffle. This "mat" of concrete beams, resting atop piers sunk into bedrock, acts much like a snowshoe, distributing the weight of the house evenly throughout the foundation slab.
Laying CablesBefore the concrete is poured, steel cables, loosely sheathed in plastic, are run at even intervals in a grid throughout the area of the slab, extending out to the edges. The day after the pouring of the slab, a crew will come in and pull these cables tight, creating a compression, or "tension" in the surface of the slab that will significantly increase its strength. A week later, the crew will come back and pull the cables again, creating total of 7,500 pounds of cable tension pressure. This "post-tension" process creates a slab that is tightly bound together, with incredible tensile strength.
The Concrete
Pouring ConcreteThe concrete we’re using in this foundation is a "five-sack" mix—five standard sacks of cement per cubic yard of concrete. Our cement mix contains 80 per cent cement and 20 per cent "fly ash" which is ash waste emitted by coal-burning power plants. Straight cement "cures" after the pour, continually gaining strength for about 56 days. The fly ash works to slow the curing process, so that the cement gains strength for a much longer period, even up to ten years, resulting in much stronger concrete. Concrete made by mixing cement product with water and coarse and fine aggregates in a blend. There are hundreds of different ways to blend the materials to produce different kinds of cement.
For the Project House, we’re using a blend wherein one cubic yard of concrete contains:
1,850 pounds of coarse, one-inch rock
1,471 pounds of fine sand
376 pounds of cement
94 pounds of fly ash
251 pounds of water (30 gallons)
This foundation will take 350 cubic yards of concrete, or about 35 truckloads.
