Foundation Problems in Clay Soils: Causes and Engineering Risks

Introduction to Clay Soils as Foundation Material

Clay soils are a common foundation material used in residential housing construction throughout the western United States, Central America, and much of South America. Clay has been used in housing construction for millennia, owing to its abundant supply and relative ease of use. However, clay soil presents significant problems when used as a foundation material. The cost of clay relative to pouring concrete is nominal, and it is certainly an attractive option when building a residence in areas where the terrain is relatively flat and the flood plain lies below the foundation level.

A preliminary investigation into clay soil for foundation use raises some immediate concerns. Prior to reviewing this problem from an academic or theoretical perspective, it is useful to consider a professional engineering perspective. Differential movement of building foundations (Professional Engineering Inspections, Inc., 1996) is a common problem witnessed in areas such as southeastern and eastern Texas.

According to Professional Engineering Inspections, Inc., differential movement problems arise "because of the highly expansive clay soil and changing weather conditions. As the building ages, it is probable the foundation will continue to experience differential movement, regardless of how well it was constructed or its present condition. This differential movement does not stop as buildings become older; older structures with a history of minimal differential movement have been known to develop foundation problems in a very short time due to changing conditions at the perimeter of the building foundation." (Professional Engineering Inspections, Inc., 1996)

Differential movement is clearly an issue in foundation development generally. When examining the characteristics of clay soil as a foundation material, it becomes evident that clay is subject to the forces responsible for differential movement and is vulnerable to foundation damage and the need for costly repair. Clay is a particularly expansive soil (Professional Engineering Inspections, Inc., 1996) and is especially susceptible to problems associated with differential movement.

Differential Movement and Clay Shrinkage

The problem is further described by Professional Engineering Inspections, Inc.: "The clay expands or contracts as its moisture content changes with the weather. Depending on the area, the amount of contraction or shrinkage ranges from minimal to upwards of 65% of the total wet volume. The average amount of shrinkage that can be expected in this region is approximately 35%, with wide variation depending on the location. For example, a sample of water-saturated clay will shrink up to an average of 35% when dried completely. This shrinkage accounts for the large cracks that form in the soil after an extended dry period. The more expansive the clay, the larger the cracks." (Professional Engineering Inspections, Inc., 1996)

The magnitude of differential movement, including foundation bed shrinkage, is extreme and poses serious problems given the changes in weather and the shifts in moisture content from saturated to unsaturated states. A regional mean shrinkage rate of 35% is equivalent to the loss of approximately one-third of the total clay foundation volume. This shrinkage is typically most evident during droughts and prolonged dry weather spells that affect the geographical regions described above.

The evidence of these extreme weather events is visible in the cracks that develop in the clay foundation. Striated lines gradually widen into gaps in the foundation bed as a result of moisture loss, constant daytime heat, and the nighttime cold that causes the clay to contract. Moisture acts as a catalyst, either expanding or shrinking the clay bed. When moisture is completely evaporated, the remaining deviation of space between clay particles reflects a cyclical process: moisture introduced into the clay bed evaporates over time, progressively causing the striations that characterize deteriorating clay foundations.

Clay foundation problems present engineers with a variety of complex challenges worldwide. A clay foundation project in Ontario, Canada, provided one such real-world engineering assignment for Morrison Knudsen engineers, who were asked to design a "foundation in soft glacial clay for column loads of up to 2,700 kips." (Hilton, Rager, & Novotny, 1993) According to Hilton, Rager, and Novotny (1993), "Those site constraints included thick soft clays, generally unsuitable for the exceptionally high project loads, up to 2,700 kips. The depth to bedrock was about 120 feet, and there was no definitive vendor data on equipment loads and installation requirements. The natural site soils consist of 15 ft. of desiccated stiff clay overlying soft glacial clay. We determined that the underlying soft clay soils were incapable of supporting a floating or raft-type foundation with or without the desiccated layer." (Hilton, Rager, & Novotny, 1993)

This engineering and construction project illustrates another critical problem in the use of clay foundations. Beyond weather-related perturbations, clay foundations are rendered unsuitable for supporting heavy structures under high project loads of 2,000 kips or more. The large slabs of clay and desiccated clay in the Ontario project provided a clearance of 120 feet to bedrock, underscoring the engineering challenge of supporting substantial loads on a clay foundation.

Expansive Soil Behavior at the Molecular Level

Clay is classified as an "expansive soil" (Vaught, Brye, & Miller, 2006), which, as described earlier, carries a high potential for differential movement — including shrinking and expanding up to one-third of its original volume. According to Vaught, Brye, and Miller (2006), "An expansive soil is any soil that has a potential for shrinking and swelling under changing moisture conditions. Structural damage to homes (i.e., walls and foundations) due to expansive soils is costly to repair and may be somewhat avoidable if soil properties, such as clay content and the coefficient of linear extensibility (COLE) are investigated." (Vaught, Brye, & Miller, 2006)

When the use of clay as a foundation material is scrutinized in this light, one reasonably asks why clay is chosen at all. The instability of the material is apparent, and building a structure on a clay foundation is therefore inherently precarious. According to Vaught, Brye, and Miller (2006), "Soil shrink-swell behavior is primarily governed by the dominant clay mineralogy (Davidson and Page, 1956; Greene-Kelley, 1974; Nettleton and Brasher, 1983; Erguler and Ulusay, 2003; Kariuki and van der Meer, 2004) and arises from the movement of water into and out of interlayer spaces of the 2:1 phyllosilicate clay minerals (e.g., predominantly montmorillonite and vermiculite) that causes the mineral to expand and contract on a molecular level." (Vaught, Brye, & Miller, 2006)

Clay is composed of fine minerals that are subject to atmospheric conditions at the molecular level. Moisture from rain or humidity causes these minerals within clay to expand and contract upon introduction into the material. Specifically, phyllosilicate clay minerals possess interstitial spaces between their layers that allow water molecules to move in and out. The shrink-swell cycle — water molecules entering and leaving these interstitial spaces — produces expansion and contraction at the molecular level, with visible structural consequences at the foundation scale.

According to Oloyede, Omoogun, and Akinjare (2010), "Different soil types pose varying problems for built foundations and the structural integrity of an entire building. McCarthy (1999) noted that there is therefore a need to carry out soil surveys to ascertain the compressibility or consolidation potentials as well as the bearing strength of the soil of a particular site. Silt deposits are susceptible to collapse if exposed to excessive amounts of water while clays shrink in the dry season only to swell during the wet season or in the constant presence of water." (Oloyede, Omoogun, & Akinjare, 2010) It is implied that these materials — including silt deposits and clay — are often used because they are readily available and cheaply obtained, offering an economical if imperfect foundation option.

Additionally, according to Oloyede, Omoogun, and Akinjare (2010), "The challenge in most cases is the human error of poor monitoring of works on site. Uzokwe (2001) observed that the cause of a building failure is unique to each building but summarized the various causes of building collapse as due to the quality of the blocks used, quality of concrete used, poor compaction and consolidation of foundation soil, weak soil." (Oloyede, Omoogun, & Akinjare, 2010) The cause of foundation failure thus frequently lies in the foundation soil itself, particularly when weak soil conditions prevail. Poor compaction will cause problems with any foundation, regardless of material, but clay and similar materials are especially susceptible to the environmental and meteorological conditions that can compromise a clay-based foundation over time.

Conclusion

The many issues with clay as a foundation material generally center around the fact that clay is an expansive soil. As clay soil tends to expand in heat and contract when exposed to cold, its ability to serve as a sound foundation material is compromised. Subsidence is yet another process that renders clay impractical for many foundation applications. When the ramifications of subsidence and its impact on an underlying clay foundation are taken into account, it becomes clear that differential movement is an inherent and ongoing risk in clay-based construction.

In areas where weather patterns alternate between humid and dry, or hot and cold, foundations of homes and commercial properties that rely on clay as the underlying material are particularly vulnerable to damage. Additionally, where clay is present in the peripheral soil, foundation deviations and cracks can undermine the clay's ability to sustain the loads placed upon it. The use of clay in foundation support is not recommended for modern construction, yet it retains a significant historical place as a critical material in early housing development worldwide.