DOUGLAS E. SMITH
        Smith Custom Builders

     There are many different types of foundation systems used in the tfouston area, such as Slab-on-grade, Pier and Beam, Void-Box, Crawl Space with or without Piers, Post- Tensioned and Driven Pile. But do not be confused, the decision on which to use will come from the soil report, structural engineer and your budget.
  Just as important as choosing the proper foundation is the evaluation and preparation of the site before the foundation work begins.
  First, one should contact a geo- technical company to evaluate subsurface soil conditions. To compile the information for their "soil report," the geotechnical company must perform one or more borings.
  A boring is a 2- to 3-inch diameter, 15- to 20 foot deep soil sample that the geotechnical company will take back to the lab to evaluate. The company will then write the soil report based on their findings. The evaluation develops recommendations to the structural engineering firm to determine and design the foundation system best suited for the lot.
  Next, the engineer reviews the flood plain maps provided by FEMA. This is necessary to determine the top of slab or finished floor elevation. The pad site is the area of select fill dirt that is built up beneath the slab.
  Pad thickness will determine the final top of slab (or floor) elevation. Select fill is dirt that has a very low percentage of clay and that can be compacted to desired percentage.
  Builders now have the select fill placed in "lifts," meaning 8 inches of select fill is gathered and compacted to 6 inches. This process is repeated until the desired pad height is reached. Compaction tests are done throughout this process by the geotechnical com- pany, measuring the density of the soil after it is compacted.

  - Post Tensioned Foundations are very commonly used. They utilize high-tension cables, which are hydraulically tightened after the concrete has hardened, giving the slab its ability to hold together under most ground movement. This is usually the most economical type available.
   - A Pier-and-Beam Foundation System uses drilled piers at a depth determined by the soil report. The pier is a concrete-filled cylindrical hole, at least 12 inches in diameter, usually with a 36-inch belled bottom. Grade beams are placed on top of these piers, and in turn support a slab -typi- cally 4 inches thick.
  - A structural (void-box) slab is an enhanced version of the pier-and-beam foundation, and has very thick, wax-coated, cardboard spacers between the grade beams under the slab. This is called the "crush zone," because uplift from swelling caused by in- creased moisture content in the clay-based soil crushes the void boxes, but without lifting the slab off the grade beams and piers.
 - Crawl Space Foundation can be detailed in different ways. Depending on the soil report, you can have a combination of drilled piers and grade beams, or just one of these types alone. I prefer to use the combination.
  On top of piers or grade beams you can have blocks, or the pier can extend above the ground level to the desired height. Above this level, beams (stringers), trusses or 2-by-12 joists can be used to support the finished floor. This is a typical system throughout the Historical Heights area.
  The last test required is the concrete strength test. During the foundation pour, plastic cylinders are filled with concrete. These cylinders are sent to the geotechnical company to break at 7 and 28 days. These tests show if the concrete mixture used falls within the engineered guidelines, usually a minimum of 3,000 pounds per square inch in 28 days.
  

  Following these tests, the surveying company determines the actual corner points of the foundation. They also set the benchmark, a marker that is used to establish the final foundation and slab elevation, relative to existing grade.
  Two very critical inspections must be done before concrete is poured. One is the pier hole inspection; another is the complete foundation makeup.
  These inspections are usually preformed by the city inspector along with a representative from the engineering company.
  Most builders obtain a letter from the engineering firm, stating that the builder has assembled the components accord- ing to plans and specs.
  In city municipalities, permission to pour concrete is not usually given until the builder provides the city inspectors with copies of compaction tests and the engineer's inspection letters.
  The most common types of foundations used in the Houston areas are:
 - Slab On Grade can be built with grade beams or without. If grade beams are not used, the slab is usually poured much thicker than the usual 4 inches. Grade beams are usually 24- to 36-inch-deep, and 12- to 16-inch- wide trenches in the ground, with reinforcing bar {rebar) running the length of the beams.
  Rebar is the steel in the concrete, which bonds it together and helps the concrete achieve its strength. Grade beams give the slab its strength and stiffness. If beams are not used, the slab can break easier with ground move- ment. The engineering firm determines grade beam height and width, as well as reinforcement.