Insite Magazine September 2017 | Page 56

BUILDING A STRONG BASE Residential Foundations in the Brazos Valley By DREW DUDLEY DALLAS WACO AUSTIN B/CS HOUSTON SAN ANTONIO HIGH EXPANSIVE CLAY SOIL MAP FOR CENTRAL AND EAST TEXAS T hroughout 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. Foundation Options 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 56 INSITE September 2017 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