Foundation Problems in Clay Soils

Foundation Problems in Clay Soils

Clay soils are a common source in the use of foundation materials used for residential housing construction throughout the western United States, Central America, and in much of South America. Clay has been used to build housing throughout millennia as the rather abundant supply of clay and for its relative ease in use. However, clay soil does have problems when used as a foundation material. The cost for clay relative to pouring concrete is nominal, and certainly is an attractive option when building a residence in areas where the topography of the geography is relatively flat and to where the flood plain resides below where the foundation is situated.

A preliminary investigation into clay soil for foundation use does establish some immediate concerns. Prior to reviewing this problem from an academic or theoretical perspective, gauging from a professional engineering perspective, differential movement of building foundations (Professional Engineering Inspections, Incorporated, 1996) is an example of a common problem that is witnessed in areas including southeastern/eastern Texas.

According to the Professional Engineering Inspections, Incorporated, differential movement problems are "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, Incorporated, 1996)

Differential movement is clearly an issue in foundation development in general. When speaking to the characteristics of clay soil for the use in foundation material, it becomes evident that clay is subject to the forces responsible for differential movement and is subject to foundation damage and repair. Clay is a rather expansive soil (Professional Engineering Inspections, Incorporated, 1996) and is subject to problems when faced with differential movement.

The problem is further described by the Professional Engineering Inspectors, 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, Incorporated, 1996)

Indeed, the magnitude of differential movement including shrinkage of the foundation bed is rather extreme and certainly will pose a problem given the changes in weather and the shifts in moisture content from saturated to unsaturated. A 35% regional mean shrinkage rate is equivalent to 1/3 the amount of material from the whole clay foundation. Often, the shrinkage of the clay foundation is evident during droughts and other severe dry weather spells that tend to plague the aforementioned geographical regions mentioned in the above paragraph.

The evidence of these extreme weather events is seen in the cracks that develop in the clay foundation. The rather striated lines that do tend to develop into wider and wider gaps in the foundation bed are from the lack of moisture and constant heat from the daytime sun and the night time cold that shrinks the clay material during the night whilst the day time heat seeks to expand the clay from the heat. Moisture acts as a catalyst to either expand the clay bed or to shrink the clay bed. When the moisture is completely evaporated from the clay, what is left is the deviation of space between the clay from where the moisture has been introduced into the clay bed and over time, has evaporated in a cyclical manner having hence caused the striations.

Main Body

Clay foundation problems are a dynamic issue and conceivably offer engineers a variety of quandaries all over the world. A clay foundation project in Ontario, Canada provided a real engineering assignment for Morrison Knudsen engineers as they 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)

The aforementioned engineering and construction project exhibits another relevant problem in the use of a clay foundation. Aside from the weather related perturbations that occur from weather related problems, the issue of using a clay foundation in support of heavy structures as high project loads upwards of 2,000 kips is rendered unsuitable. The large slabs of clay and desiccated clay provided a clearance of material totaling 120 ft. To bedrock. Indeed, the engineering problem as described by the engineering team is in the use of supporting heavy loads on top of a clay foundation.

Clay is considered to be an "expansive soil" (Vaught, Brye, Miller, 2006), which as described earlier has a high potential for differential movement that includes shrinking and expanding up to 1/3 of its original dimensional coverage. According to Vaught, Brye, & 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)

The problem of clay foundation has presented the issue of clay as an expansive soil. Indeed, when the issue of using clay as a foundation material is investigated, one inquisitively asks, why use clay for a foundation? The instability of the material is lucid and therefore to build a structure on a foundation of clay is perhaps tenuous. According to Vaught, Brye, & 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 comprised of finer minerals that are subject to atmospheric conditions at a molecular level. Moisture such as rain and/or humidity can produce these minerals within clay to expand and contract when introduced into the material. Specifically, the phyllosilicate clay minerals possess the interstitial space between the layers, which enable water molecules to enter into and out of these spaces. Soil shrink-swell produces problems that have been described generally as water molecules entering into the interstitial spaces of the clay foundation and then experiencing expansion and contraction at a molecular level.

According to Oloyede, Omoogun, & 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 to include silt deposits and clay are in use as they are often readily available if not cheaply obtained and are seen as providing the most 'bang for the buck' or so to speak.

Additionally, according to Oloyede, Omoogun, & 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) Therefore, ostensibly the cause for foundation failure lays in the foundation soil, which is often a condition of weak soil if the foundation soil is causing a problem. Poor compaction will cause a problem with any foundation, regardless of the material used to provide the foundation support. Clay and materials similar are more readily subject to the environmental and atmospheric meteorological conditions that can impact the sustainability of a clay-based foundation.

Another problem that can cause problems for a foundation subject to weak soil characteristics is "subsidence" (Shabha, Kuhwald, 1995). According to Shabha & Kuhwald (1995), "Subsidence can be defined as a downward movement or a soil on which buildings stand from causes unconnected with loading from the building. Examples are underground mining, clay shrinkage (especially due to the action of tree roots) and erosion due to water passing through the subsoil, but excluding the compaction of made-up ground or infill

." (Shabha, Kuhwald, 1995)

Subsidence is in part a natural process but yet is also in part a man-made process. Throughout millennia, the process of water creating soil erosion has changed the landscape of particles that comprise the rocky granular landscape, such as silica and including clay. The man-made shafts created for the mining process further exacerbates the problem of using clay as a foundation material as there are many mine shafts and other man-made entrances into the earth as well as other entrances created either through natural or animal forces, which reflect the problem of subsidence and can facilitate a foundation instability causing structural collapse.

According to Shabha & Kuhwald (1995), "Clay soils are particularly prone to movement as they can change their volume in response to seasonal weather, or the action of vegetation extracting moisture from the soil. Any reduction of the soil moisture content that leads to subsidence is known as desiccation. At the other extreme, when clay soil holds huge amounts of moisture, this causes the soil to expand and heave." (Shabha, Kuhwald, 1995) As mentioned earlier in the literature review, desiccation is a problem with clay foundations as it causes an increasing level of instability. Desiccation is the process of creating the deviations or striations in the clay soil due to a reduction of the soil moisture content.

The cause of clay shrinkage, according to Shabha & Kuhwald (1995), "The reason for clay shrinkage is water bonded to the surface of the clay particles, rather than filling up the pores between particles. The proportion of clay minerals in the soil has a direct relationship with the potential for shrinkage or expansion. More commonly, a combination of hot weather and vegetation cause the greatest amount of subsidence damage. There are three basic classifications of movement, which involve interaction between trees, vegetation and clay soil. Seasonal. Trees need to obtain a greater amount of moisture during summer months, when precipitation is at its least. The extraction of the water from the soil generates large-pore water suction near the surface, resulting in desiccation, shrinkage and subsidence. The reverse occurs in the winter months, when trees or vegetation will cause the desiccated area to increase, leading to shrinkage and subsidence around the affected area." (Shabha, Kuhwald, 1995)

According to Drazga (1998), "Floors crack, walls crumble, entire houses fall to the ground. A natural disaster? In a sense, it is. All this can be caused by reactive soil. Nationwide, structural damage caused by reactive soil costs $6 million to $10 million annually, according to information provided by Phil Weinert Engineering in Colorodo Springs. The most common type of reactive soil is expansive clay. This clay expands and contracts due to changes in its moisture level. Some expansive clay can swell up to 15 times its original size, causing stress on the surrounding environment. Buildings with foundations anchored in this soil will shift and bend, causing anything from wall cracks to rolling floors, depending on the amount of movement." (Drazga, 1998)

Further evidence of subsidence is evidenced by Douglas Mcleod, vice president of Engineers Incorporated. According to Haywood (2005), "Mcleod said he thinks there could be a water leak under the concrete slab beneath the hallway. He said if water is leaking onto clay-filled soil, the soil will expand, pushing that portion of the concrete foundation up and dragging the walls along with it, which would cause the walls to crack." (Haywood, 2005) However, Mcleod nor Haywood refer to the process as subsidence, the evidence of this phenomena is present and consistent with the description of the process of how subsidence works.