The average person has no sense of the history of the design and construction of residential Houston area foundations. The same thing is true of TREC inspectors, real estate agents, and Professional Engineers.
Consider the definition of performance in the TREC SOP
Achievement of an operation, function or configuration relative to accepted industry standard practices with consideration of age and normal wear and tear from ordinary use.
I like this definition. In my opinion, it is much better than the old standard: performing or not performing as intended.
The new definition requires the TREC inspector to take into account age and normal wear and tear. There is no way an inspector or engineer can do this competently without a basic understanding of the history of Houston residential design and construction. Modern slab-on-ground foundations are very different from slab foundations built as recently as the 1990’s
The original Houston house foundations were made of wood
The first homes built in Houston were undoubtedly built with wood foundations. Wood was the only material readily available.
The major drawback to using wood as foundation material is that it will inevitably be destroyed by moisture, wood-destroying insects, or fire. That is why so few such houses still exist. Those that do still exist, do so only because they are carefully maintained.
The house at the top of this page is called: The Old Place. It is the oldest house in Harris County. Notice the wood posts being used to elevate the house above the ground. Such houses provided very little protection against insects such as mosquitoes, roaches, etc. There was no central heat or air conditioning and no electric power. There was no need for electric power as there were neither light bulbs nor electric appliances.
Block and pad foundations
The Kellum-Noble House, built in 1847, is the oldest building in Houston. Surprisingly, the foundation is original. The house retains its original brick walls. The bricks were made with mud from Nathaniel Kellum’s brickyard on the banks of Buffalo Bayou.
Since block and pad foundations are founded on the ground surface, they are almost always on unstable soil. Block and pad foundations are not suitable for use on expansive soils.
Pier and beam foundations
Pier and beam foundations are really not much different than block and pad foundations. In a block and pad foundation, the pad was a rectangular piece of concrete that was simply set on top of the ground. In a pier and beam foundation, a pier is used in place of a pad. The main difference is that a pier is not set on the ground surface; it is instead set at some depth, usually fairly shallow at 2 to 4 feet below the ground surface.
This type of foundation was extensively used between the early part of the 20th century and WWII. Variations of this can be found in many of the older residential areas near downtown.
Traditional pier and beam foundations are not suitable for use in expansive soil areas. The piers are founded at too shallow a depth to be founded on stable soil.
Slab-on-ground foundations – 1950 to 1980
When soldiers returned home after WWII, they almost immediately began to marry and start a families. They also began to buy homes. There soon developed a huge housing shortage partly due to price controls and partly because there had been very little new home construction since the late 1920s. It was clear to everyone that if you could find a way to build single-family homes quickly and at low cost, there was a lot of money to be made.
One person who seized the opportunity was a man named William Levitt. His family had been in the custom home building business in the Long Island, New York area since 1929. When WWII broke out, Levitt joined the Navy where he became an expert in mass-producing housing for the military. The buildings were small and of similar design. This allowed the use of uniform, interchangeable parts, and modules essential for rapid, low-cost construction.
By July 1948, only a year after they started, they were producing 30 houses a day. They cut costs wherever possible using, for example, nonunion labor so the work could be organized in the most efficient way and pre-cut lumber to minimize the amount of site labor. Of particular interest is that the foundations were slab foundations. Until then, concrete slab foundations had not been allowed by code. Partly due to Levitt’s insistence and also due to the extreme shortage of housing, the local authorities relented and allowed the use of concrete slab foundations.
Slab foundations were an important key to cutting costs in both money and time. Up until the end of WWII, very few slab foundations were built anywhere for three reasons: concrete was very expensive because it had to be mixed in small batches on site, the quality of site-mixed concrete was sketchy, to say the least, and reinforcing steel was expensive so long as the war continued.
The big change was the introduction of ready-mix concrete trucks. These trucks made site-mixed concrete uncompetitive almost overnight as the trucks entered new markets. By around 1950, ready-mix concrete trucks were in Houston and other large cities in the United States.
Ready-mixed concrete was not only cost competitive as compared to site-mixed concrete, but the concrete was also a higher and more consistent quality.
In addition to these savings, slab foundations have another cost advantage: the surface of the slab takes the place of a traditional elevated wood frame floor. Thus, there was no need for expensive skilled carpenters to build a floor frame for single story houses.
Modern slab-on-ground foundations – 1980 to today
In spite of the cost and schedule advantages of slab-on-ground foundation construction, there was a problem that remained to be solved. In some areas, these foundations did not work very well. Over time, it became obvious that the areas where slab foundations did not work as well as we would like were expansive soil areas: areas with clay soils that shrink when they dry and swell as they absorb moisture.
The Federal Housing Administration was underwriting many of the loans for new houses, so they became involved in trying to find a solution to the expansive soil problem with slab-on-ground foundations.
Independent Professional Engineers began to experiment with different designs, mostly by looking at successful and faulty designs and trying to figure out the underlying reason for the difference.
One such engineer was Walter Snowden, PE. Snowden is the best known Texas engineer who worked for many years experimenting to find a way to design slab-on-ground foundations that would perform adequately and be no more costly than necessary to build.
Snowden and other engineers developed their design methodologies mainly by trying out different ideas and then seeing what worked and what didn’t. Many designs worked adequately, but some did not. The foundations that fell short in performance were studied and changes made in the hope of getting better performance without excessive cost.
He worked mainly in the Austin and San Antonio areas. In Houston, slab-on-ground foundations that were designed using his methods were and are sometimes referred to as San Antonio slabs. The name can from the fact that the San Antonio office of the FHA accepted foundations that were designed using his method. Snowden’s design methodology is still code approved and used to some extent.
Pier-supported slab-on-ground foundations
One solution to the slab-on-ground foundation problem was to support the slab with bell-bottom piers. This is essentially the same foundation design as a modern slab-on-ground foundation designed for both load-bearing and stiffness with the exception that the foundation is supported by both the surface soils and by bell-bottom piers that are supported by the soil beneath the active soil zone.
The advantage of a pier-supported slab-on-ground foundation is that it is less likely to develop significant foundation related issues. In spite of that, there are limitations with a pier-supported slab. The field area of the slab will eventually sag as the ground underneath inevitably settles. The piers can settle up to ½ inch in part because it is very difficult to make sure that the pier holes are clean with no loose soil.
Structural slab foundations
Another solution was to design the slab as a structural slab. This is similar to a pier-supported slab-on-ground foundation with the exception that the field area of the slab is designed and constructed so that it does not rely on ground-support. This foundation has the least risk of future foundation related issues.
Like the other options, there is some residual risk of future foundation issues. It is very difficult to make sure the pier holes are free of debris and loose soil. It is not unusual for some of the piers to settle up to ½ inch. The stiffening beams are likely to sag over time to some degree between the piers and the structural slab field area between the stiffening beams will also sag over time. Levelness issues, minor door problems, drywall cracks, and brick veneer cracks may be reduced, but there is no engineering basis for believing that these issues will go away.