By Drew Dudley
Throughout most of the country, residential foundations are given little attention due to a long track record of satisfactory performance and cost effectiveness. However, in parts of Texas and specifically in the Brazos Valley, the soils tend to be highly expansive, and foundations require much more attention.
Locally, clay soil is not the only factor that plays a role in creating the shrink-swell characteristics of expansive soil that make foundation engineering the best solution for long-term foundation performance. The clay’s environment, or climate cycle, contributes greatly. When unsaturated clay is exposed to moisture it will expand, and when the clay begins to lose moisture it will shrink. Using standardized climate measures, the Brazos Valley climate cycle reflects intense dry-moisture cycles. Coupled with the high expansive clay that is predominant throughout the Brazos Valley, you have the perfect recipe for substantial soil movement.
The issues with residential foundations on expansive soils have been well documented since the 1930s. Due to the climate and soil type that predominates in the Brazos Valley, Type III foundations from the Building Research Advisory Board Report #33 are the best residential option. These slabs are intended to behave similarly to a rigid raft floating on water. With this concept, we are accepting the fact that the soil is going to move. To design a foundation for the best long term result, a professional engineer should be consulted.
Standard Practice in the Brazos Valley
Based on my experience and conversations with local builders, approximately half of the residential foundations in the Brazos Valley are engineered. When not engineered, the builders typically utilize a Type III slab that is 4 inches thick with 36-inch deep stiffening beams with reinforcement in the top and bottom of each beam. This exceeds the minimum foundation standards that both Bryan and College Station have added to the building code. Applying a standard foundation design to every house can lead to excessive foundation movement with potential undesirable consequences. It also potentially leaves the owners paying for material and labor that is not necessary.
Locally, the typical cost for foundations is approximately $10 per square foot of the total building cost. For an engineered slab, the owner would need to pay for a geotechnical report and a design fee for the foundation design. Using local cost estimates, in a $450,000 new home construction, the approximate additional cost of an engineered foundation would be around $2,600.
Ultimately, the question becomes whether or not the owner/builder is willing to take the risk of a non-engineered slab that represents approximately 0.6 percent of the total construction cost.
Factors Affecting All Slab-on-Grade Foundations
Regardless of whether the foundation is engineered, non-engineered, or which design method is utilized, there are many other factors that affect the performance of slab-on-grade foundations.
Building Pad: Based on the site-specific geotechnical report, the geotechnical engineer may recommend removing a certain depth of the native high expansive material and replacing it with select fill to create more favorable soil properties. The building pad’s moisture content should also be as close to the average long-term moisture content for the soil in which it is placed. If the building pad is saturated when the foundation is poured, then over its lifetime the potential to swell will be small but the potential to shrink will be very large and lead to greater foundation movement.
Drainage: When a house does not adequately drain water away from the building pad, the ponding of water is generally concentrated on one section of the house creating an imbalance in the moisture content across the building pad, which may cause some sections of the foundation to rise significantly.
Moisture Maintenance: The key is to maintain a uniform moisture level around the entire foundation that is as close as possible to the moisture content of the soil when the foundation was poured. Install gutters that outfall well beyond the footprint of the foundation and actively add moisture to soil during dry periods to maintain a uniform moisture content around the entire foundation.
Landscaping: Trees and large bushes planted within close proximity to the foundation can suck water out of the surrounding soil leaving a moisture imbalance across the foundation. Plant trees a distance equal to half their canopy diameter away from the foundation and/or install a root barrier to help mitigate these effects.
Potential Consequences of Excessive Foundation Movement
Slabs-on-grade are not only going to move but are designed to do so. The purpose of an adequate foundation design is to ensure that these movements are within acceptable limits. Excessive movement can lead to distress phenomena appearing in not only the foundation but the superstructure as well including drywall and brick cracks, sticking doors and separation of walls from the ceiling.
Signs of a bad foundation can send a real estate deal downhill quickly and can lead to stressful and costly repairs for the would-be seller. Consider your next home an opportunity to get it done right the first time – hire a builder and engineer that recognizes your new home as one of the most important investments you will ever make.
Drew Dudley, P.E., leads Dudley Engineering LLC. In his seven years of experience in the industry, Drew has worked closely with architects, owners, and contractors to help deliver marquee projects including the Kyle Field Renovations at Texas A&M University. Email him at firstname.lastname@example.org.