A Sustainable Home Starts With An Engineered Foundation

The American Society of Civil Engineers estimates that one quarter of all homes in the United States have some damage caused by expansive soils. In a typical year in the United States they cause a greater financial loss to property owners than earthquakes, floods, hurricanes and tornadoes combined.

State building codes provide general guidelines, minimum footing requirements and presumptive soil-bearing values for various soil types, bearing capacities, material requirements and estimated lateral loads.  But a building code cannot accurately predict your building project’s specific needs. Savvy builders in Texas understand that with subsurface soil conditions ranging from solid rock to highly expansive clays (oftentimes in the same community), construction can be high-stakes and expensive if not handled correctly.  A structural engineer’s stamp of approval on a foundation plan is the only way to assure foundation integrity.

An engineered foundation is defined as one for which design is based on three phases:

  1. Geotechnical Engineering Information

Load conditions are very “area specific.” The Texas State Historical Association’s division of land resources recognizes over 1,300 different types of soil in our state, each with its own distinct attributes and characteristics.

It is important that your structural engineer is familiar with your area’s weather conditions and soil types because every soil has a unique capacity to bear the intensity of a load per unit area.  For example, sand or gravel may have a bearing capacity of 3000-4000psf (lbs/sq/ft) while clay only has a support capacity of around 1500psf.   The higher the bearing capacity of a soil, the smaller the footing you need.

Soil testing is also essential in assuring a foundation’s essential role to act as a barrier keeping water from penetrating the living space.  Certain soils, specifically clay-like soils prevalent in many areas of Texas,  expand and contract more rapidly when exposed to moisture.

  1. The design of the foundation is performed by a licensed engineer.

Structural engineers use science and math based information to understand various relationships between soils, hydrostatic pressure, water flow, and other factors which affect foundation walls’ long-term structural strength.  Areas with expansive soils, organic deposits, landfills, or underground springs require particular attention.  Steep sloping land areas will require extra engineering or specialty foundation designs.

In addition to the soil analysis, foundation engineering calculations will include:

  • Dead load analysis–the weight of the permanent equipment and construction materials in the building.
  • Live load analysis–created by human use of a building such as physical bodies, vehicles, storage, furniture, and construction and maintenance activities.
  • Wind uplift pressure analysis– pressures from wind flow that causes lifting effects.
  • Soil pressure analysis—the pressure exerted from soil mass (backfill) onto a wall in a lateral direction throughout its depth.
  • a well constructed drainage system to address moisture which affects not only the integrity of the foundation but also the piers, beams, joists, and shims supporting the structure.

Your engineer will also check that the plumbing design is solid and consider large tree roots on the site that could lead to severe issues in your foundation.

  1. Construction is observed with written documentation.

Third-party, insurance-backed new-home warranties usually require foundations to be engineered and stipulate that a series of inspections be performed at pre-pour stage and after the concrete pour.  Note that ASTM’s concrete and concrete aggregates committee is developing a proposed international standard that will be used to evaluate the tensile performance of fiber-reinforced concrete (FRC) using cylindrical specimens with a double-punch testing method; this indirect tensile test method shows much less variability and is more time efficient than typical beam-type testing and can be used for investigating various properties of FRCs such as strength, stiffness, ductility, and mix quality.

The compacting soil process is another dire step to evaluate in ensuring foundational integrity to prevent the foundation from sinking under the load bearing weight of the structure.

Next, a framing inspection occurs when framing is complete and after the installation of the electrical, plumbing, mechanical and ventilating systems prior to the application of interior wall coverings such as sheetrock.  And finally, a final grading survey and slab floor elevations survey may also be required to establish that lot grading and drainage, including porches and driveways, directs drain surface water away from the foundation.

 The bottom line:  There is no need to pay for needlessly over-designed foundations, but under-designed, under-performing slabs are a recipe for distress and litigation.  Make sure your team includes essential engineering professionals versed in residential foundation design and that you purchase a third-party warranty to protect against the unexpected.

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