The Dirt on Soils in Texas
All soils are not created equal. And when building a foundation, these differences in soil types become critically important.
Some soils tend to act like a sponge, expanding when water is absorbed and shrinking when they dry out. These soils are known by several names – expansive soils, active soils, shrink-swell soils, expandable clay – but the end result is often the same: Expansive soils can cause foundation problems and threaten the structural integrity of any building constructed on a site with this type of soil composition.
The American Society of Civil Engineers (ASCE) estimates that half of all homes in the U.S. are built on expansive soils, and half of those homes will experience some level of expansive soil damage. The estimated damage to buildings, roads and other structures built on expansive soil exceeds $15 billion annually, according to an international engineering study, and the ASCE has stated, historically, that expansive soils account for more home-related damage each year than floods, tornadoes, and hurricanes combined.
Globally, expansive soils create serious engineering problems and economic losses in at least 19 countries. Damage in the U.S. is generally concentrated in certain parts of Texas, Louisiana, Mississippi, Colorado, Nebraska, Wyoming, Montana, and North and South Dakota.
Of the approximate 1.2M annual U.S. housing starts, Tella Firma estimates that over a quarter million housing starts occur in areas with active soils. Texas alone accounts for over 40% of the housing starts with active clay soils.
What’s Underground in Texas?
The Central Texas corridor and coastal areas of Texas have some of the most pronounced regions of expansive soils in the country, and these areas are ripe for enabling foundation damage. Weather itself can pose a problem, with the effects of Texas rain and wind contributing to soil movement and erosion at the base of foundations. Beyond that, rapid population growth, increased urbanization of Texas suburbs, and the expansion into reclaimed farmland are contributing factors to a significant increase in construction where expansive soils are present.
Texas is divided into 21 different Major Land Resource Areas that have similar or related soils, vegetation, topography, climate, and land uses. Here’s what we find in the major metropolitan areas:
- Dallas – The Dallas area is in the Blackland Prairie, with soils comprised of three primary types (Austin chalk, Ozan Marl and Eagle Ford), often referred to as “cracking clays” due to the large deep cracks caused by dry weather. The unique blend of soil, with a higher clay content, is susceptible to significant expansion that can lead to foundation issues.
- Fort Worth – As part of the Grand Prairie region, Fort Worth soils include both Eagle Ford and Woodbine, a composition that is entirely different from that found in Dallas, but still represents a threat to the integrity of foundations.
- Houston – Houston soils are somewhat similar to those found in the Dallas/Fort Worth region, but the presence of sand can create a different type of reaction. Sandy soil presents the unique ability for soil to shift laterally underneath a foundation.
- Austin and San Antonio – Central Texas soil conditions can vary significantly from location to location within a community. For example, the east side of Austin and northeast San Antonio both feature very expansive soils compared to the more stable western areas of both cities, where the Hill Country begins. The Hill Country of Texas has a rockier terrain that generally will provide more stability for a foundation.
The types of foundations used in homes and other buildings vary by region, climate and building size, but the foundation method used most frequently in new home construction in Texas – the concrete slab-on-grade foundation – registers the poorest performance on expansive soils. When soils expand and contract, pressure can create uplift against concrete slabs, causing damages such as cracking, water leaks, broken pipes or water lines, and interior drywall damage.
While a slab-on-grade foundation generally is less expensive than an elevated solution, builders utilizing this method may be sacrificing quality and creating a higher risk of foundation damage in the future. As an alternative, Tella Firma’s solution offers a cost-effective elevated slab option that protects the foundation against soil movement, but represents a much lower cost than a traditional pier-and-beam.
Even in the presence of volatile Texas dirt, the Tella Firma foundation rests on solid ground.