Expansive soils

Expansive soils—soils that expand and contract with the gain and loss of water—cause billions of dollars in damage to houses, other lightweight structures, and pavements, exceeding the costs incurred by earthquakes and flooding.

Soil Swelling and Shrinkage

Certain types of soil expand and contract with the gain and loss of water, primarily through seasonal changes. One meter of expansive soil has sufficient power to lift a 35-ton truck 5 centimeters. Many cracks in walls and concrete surfaces are assumed to be caused by settlement, when in fact they are produced by the lifting force of expansion.

Soils vary enormously in their ability to expand, ranging from zero in very sandy soils to highly expansive in montmorillonite clays. By contrast, kaolinite clays do not expand. Some key indicators of expansive soils are a bricklike hardness when dry, including great resistance to crushing, and stickiness and weakness when wet; also, they have a glazed surface when cut with a knife. Soil cracks and popcornlike surface appearance are other key signs. Where soils from volcanic ash produce primarily montmorillonite clays, bentonite is formed. This extremely expansive soil prevents even vegetation from growing and is easily visible from a road, with its barren, popcornlike surface and painted-desert coloration.

The ability of montmorillonite clays to expand comes from their physical and chemical structure. Clay minerals have a molecular structure like sheets of paper. Some bond very strongly from layer to layer and have little swelling potential. Expansive clay minerals, however, have weak bonds between the layers. These layers are negatively charged and repel one another, but are bonded by positively charged ions (cations) such as sodium and calcium dissolved in the soil water. When soils rich in these minerals become wet, water is drawn into the spaces between the clay particles, a process called hydration, causing them to grow up to seven times their original size. The opposite effect, shrinkage, is caused by evapotranspiration.

In climates with great variation in seasonal moisture, especially marginally humid and semiarid lands, large changes occur in the moisture content of the soil, with subsequent swelling and shrinkage. In the midlatitudes and subtropics, winter and spring are the wet seasons, as vegetation is dormant and rain and melting snow are abundant. Summer and fall are the dry seasons, soilwise; evapotranspiration demand is high, and much water is transferred from the soil to the atmosphere. Human activity can also produce localized effects through excessive watering, leaking pipes, poor drainage around buildings, and use of trees and bushes for landscaping. It is these volume changes in the swelling soils beneath structures, caused by the gain and loss of water, that make expansive soils so damaging.

Differential Movement and Variable Loading

Expansive soils do their damage primarily through differential movement and variable loading. If the entire structure is lifted and sinks uniformly, there is little problem. Unfortunately, swelling and shrinkage are seldom uniform. Many buildings are located on sloping land, and, as a result, groundwater is more likely to be encountered on the uphill side during wet seasons. Consequently, more expansive force is exerted on that side, cracking walls and slabs. The construction season can also be a factor on sloping land, as the portion of the building pad cut into the hillside, and the portion built up with fill, tends to respond differently to changes in moisture. Even on level land, deep foundations may be stable while grade beams and slabs resting upon the surface rise and fall with the seasons.

While much attention is given to damaged buildings, damage to roads may actually have a greater impact on the public. The life of a road is measured by the total weight it carries over a period of time, divided by the strength of the roadbed. Many older roads are built directly upon the natural grade, including highly expansive soils. Because these soils are weak when wet, the heavier loading at concentrated points under truck tires causes the pavement to break down more rapidly. Once a cracking pattern is visible, a chuckhole is but a few wetting cycles away, as water can now easily reach and further weaken the soil below. Major roadbed problems may also occur near the boundaries between highly expansive and less expansive soils as a result of differential swelling. Water seepage at concentrated points beneath the roadbed also undermines the strength of the roadbed and contributes to differential movement.

Damage to structures from expansive soils can be categorized as reduced performance, economic loss, and massive failure. Massive failure is rare, but the threat of it is usually enough to demand removal of the danger. High on this list would be leaning chimneys and massively cracked concrete channels. Also, raised portions of patios, sidewalks, and driveways are hazardous to pedestrians. Reduced performance occurs when doors and windows cannot be closed because of misalignment, and unsightly cracks appear on walls and concrete slabs. Economic losses occur from such unsightly cracks when potential buyers are discouraged from purchasing property or property owners have to repair them. Also, higher heating and cooling bills result from energy losses through cracks, especially around windows and doors. For taxpayers, a little-recognized cost is incurred through more frequent road repairs.

Onsite Geotechnical Investigation

Fortunately, the problems caused by expansive soils are reasonably well understood by soils and foundation engineers. Typical procedures include an onsite geotechnical investigation, laboratory testing of samples obtained during the onsite investigation, and design recommendations specific to the anticipated usage of a site. The importance of sound engineering here cannot be overemphasized. The cost of such work is small compared to the cost of the project and is far cheaper than potential remedial work. Inasmuch as the soil ends up underneath the building, roadway, or other structure, it is obviously inaccessible when problems arise; it is far easier and cheaper, therefore, to compact the soil properly and install piers and drains before rather than after.

An experienced soils engineer will generally know what to expect within a given part of a city or region. Soils are notoriously diverse, however, and the only sure way to secure the accurate information needed for a sound, cost-effective design is an on-site investigation. Key information is provided by borehole samples, extracted by a drilling rig similar to that used for wells. The selection and number of borehole locations are determined from architectural layouts and the size of the project. One hole may be sufficient for a house, whereas numerous holes may be ordered for a high-rise building. Three kinds of geotechnical information are obtained directly from these holes: a description of the type and thickness of soil and rock; resistance to penetration (influenced by moisture content), which gives a crude measure of soil strength; and, if the hole is deep enough, depth to the water table.

With distressed structures, one of the most important judgments a soils engineer can make is whether remedial work is needed or will be cost-effective. Cracks often appear in new buildings and pavement; though frustrating, they may be of little consequence. By contrast, serious problems may be avoided if timely repairs are made before incipient failures, such as leaning chimneys, reach the critical point.

Tests for Expansive Potential

The most common measure of expansive potential is the Atterberg limits test, run in the laboratory. Though crude, it has proved its value over many years as a predictor of expansive and nonexpansive soil characteristics. The test essentially measures the difference between the resistance of a soil sample to flow as it becomes wetter and wetter, called the liquid limit, and disintegration under rolling as it becomes drier and drier, called the plastic limit. The numbers used are moisture content percentages, obtained by computing the weight of water as a percentage of the dry weight of the soil. The difference between these two percentages is called the plasticity index, referred to as the PI. Variances among plastic limit numbers are generally small, but liquid limit numbers vary enormously. The stickier the clay, the more moisture will be required to make it flow; thus the higher the liquid limit. One characteristic of expansive soils worth remembering is their stickiness when wet. Consequently, the higher the liquid limit, the larger the PI and the greater the swell potential.

The Atterberg limits test is relatively easy and inexpensive to run. It is the simplest way to test large numbers of samples, and such tests are conducted by the thousands in soils laboratories. Where precise and detailed information is needed from key samples, however, a consolidation-swell test is run. This test is much more complex, time-consuming, and expensive, but the information gained is vital for the design of more expensive structures. The test mimics the behavior of soils under the loading applied by a building and their response to swell pressure as water is absorbed. Additional tests, such as confined and unconfined compression, are also commonly run on the more expensive projects. These test results, combined with experience and input from colleagues, form the basis for the recommended design. Good designs anticipate and avoid problems, minimize construction costs, and recommend specifications for quality-assurance testing during construction.

Design Alternatives

Design alternatives include strengthening the structure, stabilizing the soil, using some combination of the first two, or anticipating movements to isolate their impact. If a slab can be strengthened so that it moves as a unit, the effects of differential movement will be mitigated. In larger buildings, reinforced grade beams are commonly suspended on piers, with space deliberately left under the grade beam to allow room for soil expansion. Major attention is given to stabilizing the soil. The goal is to compact the soil in its expanded condition, balancing the need for strength with the need to allow some room for additional expansion. A key problem is to keep the soil from drying out before it is sealed, usually by a concrete slab or asphalt. Drains are commonly installed around the perimeter and covered by concrete, asphalt, or plastic sheeting. Three types of drains are used: peripheral drains around the perimeter; interceptor drains to control subsurface water from uphill sources; and sump drains, which require a pump and are usually located under a basement floor slab.

A combination of increased strength and soil stabilization is often used. Much attention is given to the water content and density of the soil beneath the structure. When feasible, select fill (stable soil having low expansion and good binding qualities) is imported for the top layer. Lime or fly ash may be mixed with the soil to increase both its strength and its stability. This treatment is especially effective under pavement (parking areas and roadbeds). Special equipment has been invented to mix these additives into the soil efficiently. Laboratory tests are commonly run to measure the effectiveness of treatment in terms of PI reduction and strength; a common rule for lime treatment is to reduce the PI to 10 or less. Lime is the easiest to handle and its effectiveness is easiest to test, though fly ash, a by-product of coal burning in electrical power plants, is growing in use because of its low cost. The problem with fly ash is that its use requires meticulous application, beyond the abilities of many inexperienced operators. It often contains heavy metals that may contaminate water supplies. Engineering supervision of the mixing and compaction—with strict attention to moisture and time specifications—is critical to the successful use of fly ash.

Where construction cost is a key concern, it is sometimes feasible to design the structure so that slabs and interior walls can move freely without affecting the foundation or other parts of the building. Trim work can mask such movement so that aesthetic concerns are not a factor. Once structures are completed, the correction of problems is far more difficult. Corrections may range from simple drains and moisture barriers around perimeters to tearing out the pavement and redoing the soil subgrade. An accurate diagnosis is even more essential to success in this case. Consequently, skilled engineers study the problem in detail, and heroic efforts are sometimes made to obtain soil samples. The engineer’s judgment of the seriousness of the problem and the remedial action that follows are especially vital. A skilled engineer can tell much from cracks and other distress patterns.

Growing Importance of Foundation Engineering

Only since the 1970s has the severe threat represented by swelling soils been recognized by the construction industry. Several changes are responsible for this growth. First, buildings with slab-on-grade floors (concrete resting directly upon soil) have replaced basements and crawl spaces in many areas. Insofar as seasonal moisture changes are restricted to the top 1 meter and essentially disappear at a depth of 3 meters, this change created conditions far more susceptible to expansion and contraction. Second, heavier trucks and increased usage have put greater stress on roadbeds and parking areas. Once the pavement is broken, it is much easier for water to reach the soil below, and failure soon follows. Third, population growth and rising costs of land have encouraged the development of nonfarmland, which increases the odds of encountering expansive soils. Still, this extended use of land is fine, as long as the dangers are understood and taken into account in designing and planning projects.

Expansive soil covers some areas of China, which makes construction of new structures challenging. In 2023, researchers published their findings about the material lignin, which is green and environmentally friendly, and its derivatives, which could be used to change the physical properties of expansive soil. For example, when calcium lignosulfonate was added, the soil's expansion rate decreased. Researchers were optimistic that the results of this study might curb or eliminate the use of inorganic modifiers.

When confronted with any hazard, one must consider whether the problem is serious enough to require expert help. Foundation engineering is a little-recognized but invaluable part of any construction program involving expansive soils. Knowledge of soil conditions prior to new construction is well worth the relatively small cost. Well-informed individuals provide an invaluable service to themselves, employers, and the organizations they serve. As part of the National Cooperative Soil Survey, the US Department of Agriculture and the Soil Conservation Service provide booklets on regional surveys. In map and table format, accompanied by explanatory text, these publications present detailed information about agricultural and engineering soil conditions.

Principal Terms

differential movement: the unequal movement of various parts of a building or pavement in response to swelling or shrinkage of the underlying soil

evapotranspiration: the movement of water from the soil to the atmosphere in response to heat, combining transpiration in plants and evaporation

hydration: a process whereby, when soils become wet, water is sucked into the spaces between the particles, causing them to grow several times their original size

loading: an engineering term used to describe the weight placed on the underlying soil or rock by a structure or traffic

moisture content: the weight of water in the soil divided by the dry weight of the soil, expressed as a percentage

montmorillonites: a group of clay minerals characterized by swelling in water; the primary agent in expansive soils

shrinkage: an effect opposite to hydration, caused by evapotranspiration

soils and foundation engineering: the branch of civil engineering specializing in foundation and soil subgrade design and construction

soil stabilization: engineering measures designed to minimize the opportunity and/or ability of expansive soils to shrink and swell

Bibliography

Al-Rawas, Amer Ali, and Mattheus F. A. Goosen, eds. Expansive Soils: Recent Advances in Characterization and Treatment. New York: Taylor & Francis, 2006.

Buzzi, O., S. Fityus, and D. Sheng, eds. Unsaturated Soils. New York: CRC Press, 2009.

Cai, Yi and Mingxi Ou. "Experimental Study on Expansive Soil Improved by Lignin and Its Derivatives." Sustainability, vol. 15, no. 11, 29 May 2023, doi.org/10.3390/su15118764. Accessed 28 July 2024.

Fenner, Janis L., Debora J. Hamberg, and John D. Nelson. Building on Expansive Soils. Fort Collins: Colorado State University, 1983.

Harpstead, Milo I., Thomas J. Sauer, and William F. Bennett. Soil Science Simplified. 4th ed. Ames: Iowa State University Press, 2001.

Katti, R. K. Behaviour of Saturated Expansive Soil and Control Methods. Rev. ed. Oxford: Taylor & Francis, 2002.

King, Hobart M. "Expansive Soil and Expansive Clay." Geology.com, 2024, geology.com/articles/expansive-soil.shtml#google‗vignette. Accessed 25 July 2024.

Koerner, Robert M. Construction and Geotechnical Methods in Foundation Engineering. New York: McGraw-Hill, 1984.

Nelson, John D, and Debora J. Miller. Expansive Soils: Problems and Practice in Foundation and Pavement Engineering. New York: John Wiley and Sons, 1997.

Newson, Malcolm. Land, Water, and Development: Sustainable and Adaptive Management of Rivers. 3d ed. London: Routledge, 2008.

Rajapakse, Ruwan. Geotechnical Engineering Calculations and Rules of Thumb. Burlington, Mass.: Butterworth-Heinemann, 2008.

Sobolevsky, Dmitry Yu. Strength of Dilating Soil and Load-Holding Capacity of Deep Foundations: Introduction to Theory and Practical Application. Rotterdam, Netherlands: A. A. Balkema, 1995.