Residential House Foundations on Expansive Soils in Changing Climates

Nafisa Tabassum
Rifat Bulut
School of Civil and Environmental Engineering, Oklahoma State University

Today, communities are vulnerable to extreme weather events, natural disasters, and geologic hazards resulting from changing climates. The U.S. Department of Housing and Urban Development (HUD) aims to provide sustainable, resilient, energy efficient, and healthy homes to its stakeholders. HUD has a great opportunity to invest in climate resiliency to achieve the goal of advancing sustainable communities. Climate change has increased the risk of exacerbating geologic hazards, especially under extreme events like excessive precipitation and drought or shifting patterns of climatic extremes. Lightweight structures with shallow foundations, such as residential houses with slab-on-ground foundations built on expansive soils, are more vulnerable to climate-related challenges than those situated on sites with non-expansive soils (Diaz and Moore, 2017; Mostafiz et al., 2021). Therefore, current design codes and methods need to be improved to mitigate the problems of slab-on-ground foundations of residential houses constructed over expansive soils.

Expansive soils are well-known geologic hazards for residential homes. Even without the exacerbating effects of climate change, they can cause extensive problems to foundations due to their swelling and shrinking characteristics. Expansive soils swell when water enters the soil and shrink when the soil dries out. These swelling and shrinking cycles can lead to severe cracks in foundations and walls and can cause other damages. When foundations are not designed to withstand the movements of expansive soils, houses will begin to show signs of distress in the form of cracks. As the walls move, the cracks begin to appear and, thereafter, progressively, doors start to jam, floors tilt, and structural integrity may diminish. These damages, whether they are at cosmetic levels or more severe but not at a structural failure state, can have multiple consequences, such as reduced service life of the structure, reduction of energy efficiency caused by increased air leakage and heat exchange through cracks or voids in walls and foundations, and loss of durability due to water intrusion. These same consequences can also cause emotional distress, loss of property value with the prospect of costly repairs, and affect occupant health due to impaired indoor environmental and air quality.

In the United States, in addition to many locally supported and practiced design methods, the current Post-Tensioning Institute (PTI) design code DC10.5-19 is widely recognized across many states (Vann and Houston, 2021). PTI DC10.5-19, “Standard Requirements for Design and Analysis of Shallow Post-Tensioned Foundations on Expansive and Stable Soils,” is a more rational method compared with its predecessor versions, and it is based partly on unsaturated soil mechanics principles. However, it still contains some major shortcomings. The use of unsaturated mechanics, climatic and other moisture boundary conditions, soil properties, and soil-structure interface can be used in a more rational way following current knowledge. With these improvements, it is possible to better predict the performance of houses and better manage the risk and potential consequences of building on expansive soils. With the financial support that HUD provided, this research study aimed at developing several design modules that use the principles of unsaturated soil mechanics and soil-structure interaction. These modules reflect the most recent advances and current thought.